Chemical Reaction: The transformation of chemical substance into another chemical substance is known as Chemical Reaction. For example: rusting of iron, setting of milk into curd, digestion of food, respiration, etc.
In chemical reaction new substance is formed which is completely different in properties from the original substance, so in a chemical reaction chemical change takes place.
Signs of Chemical Reaction
Change of state of substance
Change of colour of substance
Evolution of heat
Absorption of heat
Evolution of gas
Evolution of light
Evolution of sound, etc.
Reactant and Product:
Reactant: Substances which take part in a chemical reaction are called reactants.
Product: New substance formed after a chemical reaction is called product.
Chemical Equation: Representation of chemical reaction using symbols of substances is called chemical equation.
Example: A + B ⇨ C + D
In this equation, A and B are called reactants and C and D are called the products. Arrow shows the direction of chemical reaction. Condition, if any, is written generally above the arrow.
When hydrogen reacts with oxygen, it gives water. This reaction can be represented by following chemical equation.
Hydrogen + Oxygen ⇨ Water
H2 + O2 ⇨ H2O
In first equation words are used and in second symbols of substances are used to write the chemical equation. For convenience, symbol of substance is used to represent chemical equations.
Chemical Equation is a way to represent the chemical reaction in concise and informative way.
Chemical equation can be divided into two types – Balanced Chemical Equation and Unbalanced Chemical Equation.
Balanced Chemical Equation: A balanced chemical equation has number atoms of each element equal on both sides.
Example: Zn + H2SO4 ⇨ ZnSO4 + H2
In this equation, numbers of zinc, hydrogen and sulphate are equal on both sides, so it is a balanced chemical equation.
According to Law of Conservation of Mass, mass can neither be created nor destroyed in a chemical reaction. To obey this law, the total mass of elements present in reactants must be equal to the total mass of elements present in products.
Unbalanced Chemical Equation: If the number of atoms of each element in reactants is not equal to the number of atoms of each element present in product, then the chemical equation is called unbalanced chemical equation.
Example: Fe + H2O ⇨ Fe3O4 + H2
In this example number atoms of elements are not equal on two sides of the reaction. For example, on the left hand side only one iron atom is present, while three iron atoms are present on the right hand side. Therefore, it is an unbalanced chemical equation.Balancing a chemical equation:
To balance the given or any chemical equation, follow these steps:
Fe + H2O ⇨ Fe3O4 + H2
Write the number of atoms of elements present in reactants and in products in a table; as shown here.
Name of atom No. of atoms in reactant No. of atoms in product
Iron 1 3
Hydrogen 2 2
Oxygen 1 4
Balance the atom which is the maximum in number; on either side of chemical equation.
In this equation, the number of oxygen atom is the maximum on the RHS.
To balance the oxygen one needs to multiply the oxygen on the LHS by 4; so that the number of oxygen atoms becomes equal on both sides.
Fe + 4 x H2O ⇨ Fe3O4 + H2
Now, the number of hydrogen atoms becomes 8 on the LHS; which is more than that on the RHS. To balance it, one needs to multiply the hydrogen on the RHS by 4.
Fe + 4 x H2O ⇨ Fe3O4 + 4 x H2
After that number of oxygen and hydrogen atoms becomes equal on both sides. The number of iron is one on the LHS, while it is three on the RHS. To balance it, multiply the iron on the LHS by 3.
3 x Fe + 4 x H2O ⇨ Fe3O4 + 4 x H2
Now the number of atoms of each element becomes equal on both sides. Thus, this equation becomes a balanced equation.
Name of atom No. of atoms in reactant No. of atoms in product
Iron 3 3
Hydrogen 8 8
Oxygen 4 4
After balancing, the above equation can be written as follows.
3Fe + 4H2O ⇨ Fe3O4 + 4H2
Balance the given equations.
H2 + O2 ⇨ H2O
Answer: 2H2 + O2 ⇨ 2H2O
Fe + H2O ⇨ Fe2O3 + H2
Answer: 2Fe + 3H2O ⇨ Fe2O3 + 3H2
CO2 + H2O ⇨ C6H12O6 + O2
Answer: 6CO2 + 6H2O ⇨ C6H12O6 + 6O2
Fe2O3 + C ⇨ Fe + CO2
Answer: 2Fe2O3 + 3C ⇨ 4Fe + 3CO2
Writing the symbols of Physical States of substances in Chemical equation:
By writing the physical states of substances a chemical equation becomes more informative.
Gaseous state is represented by symbol ‘g’
Liquid state is represented by symbol ‘l’
Solid state is written by symbol ‘s’
Aqueous solution is written by symbol ‘aq’
Writing the condition in which reaction takes place: The condition is generally written above and/or below the arrow of a chemical equation.
Thus, by writing the symbols of physical state of substances and condition under which reaction takes place, a chemical equation can be made more informative.
Double Displacement Reaction:Reactions in which ions are exchanged between two reactants forming new compounds are called double displacement reactions.
AB + CD ⇨ AC + BD
Example: When solution of barium chloride reacts with the solution of sodium sulphate, white precipitate of barium sulphate is formed along with sodium chloride.
BaCl2 + Na2SO4 ⇨ BaSO4 + 2NaCl
When sodium hydroxide (a base) reacts with hydrochloric acid, sodium chloride and water are formed.
NaOH + HCl ⇨ NaCl + H2O
Double displacement reaction, in which precipitate is formed, is also known as precipitation reaction. Neutralisation reactions are also examples of double displacement reaction.
Exothermic and Endothermic Reaction
Reactions which produce energy are called exothermic reaction. On the other hand, reactions which absorb energy are called endothermic reaction. Most of the combination reactions are endothermic. Most of the decomposition reactions are exothermic. Respiration is a decomposition reaction in which energy is released. When quick lime (calcium carbonate) is added to water, it decomposes and releases energy. Cooking involves chemical reactions which are endothermic as cooking is possible because of heating.
Oxidation and Reduction Reaction:
Oxidation: Addition of oxygen or non-metallic element or removal of hydrogen or metallic element from a compound is known as oxidation.
Elements or compounds in which oxygen or non-metallic element is added or hydrogen or metallic element is removed are called to be oxidized.
Oxidizing agent: Compounds which can add oxygen or a non-metallic compound or remove hydrogen or metallic element are known as oxidizing agents.
Reduction: Addition of hydrogen or metallic element or removal of oxygen or non-metallic element from a compound is called reduction. The compound or element which goes under reduction is called to be reduced.
Reducing agent: Compounds or elements which can cause reduction are called reducing agents.
In a chemical reaction oxidation and reduction both take place simultaneously and such reactions are also known as REDOX REACTIONS. In the word REDOX, ‘Red’ stands for reduction and ‘Ox’ stands for oxidation.
Example: When iron reacts with air, it forms iron oxide (rust)
4Fe + 3O2 ⇨ 2Fe2O3
In this reaction, oxygen is added to iron, thus, iron is oxidized. Here oxygen is oxidizing agent.
When cupric oxide reacts with hydrogen, it gives copper and water.
CuO + H2 ⇨ Cu + H2O
In this reaction, oxygen is removed from copper and oxygen is added to hydrogen. So, cupric oxide is reduced to copper and hydrogen is oxidized to water. Cupric oxide is oxidizing agent and hydrogen is reducing agent.
When sodium hydroxide reacts with hydrochloric acid, it gives sodium chloride and water.
NaOH + HCl ⇨ NaCl + H2O
In this reaction, sodium hydroxide is reduced to sodium chloride since hydrogen is removed from sodium hydroxide. Hydrochloric acid is oxidized to water, since oxygen is added to hydrogen chloride and non-metallic element chloride is removed. Sodium hydroxide is oxidising agent and hydrochloric acid is reducing agent.
In this reaction oxidation and reduction both takes place simultaneously, thus it is an example of redox reaction.
Significance of Oxidation Reduction in Everyday Life:
Respiration is oxidation reaction in which food is oxidized to produce energy.
Iron gets oxidized to form rust; which leads to corrosion of iron in the long run.
Most of the metals react with atmospheric oxygen and it leads to formation of a layer on the metal article. The metal gets corroded in the long run.
Rusting of iron can be prevented by painting the iron article. This can also be prevented by applying a layer of zinc over iron article. This process is known as galvanization.
Fried food gets oxidized when exposed to air. This spoils the taste of the food and the food becomes unfit for consumption. The spoiling of fried food because of oxidation is called rancidity. Fried food is often packed in airtight packets to prevent rancidity.
We are able to utilize various types of fuel because of oxidation. Oxidation of fuel helps in producing energy.
Combination Reaction: Reactions in which two or more reactants combine to form one product are called COMBINATION REACTION.
A general combination reaction can be represented by the chemical equation given here.
A + B ⇨ AB
Example: When magnesium is burnt in air (oxygen), magnesium oxide is formed. In this reaction, magnesium is combined with oxygen.
Mg + O2 ⇨ 2MgO
Magnesium + Oxygen ⇨ Magnesium oxide
When carbon is burnt in oxygen (air), carbon dioxide is formed. In this reaction, carbon is combined with oxygen.
C + O2 ⇨ CO2
Carbon + Oxygen ⇨ Carbon dioxide
When hydrogen reacts with chlorine, hydrogen chloride is formed.
H2 + Cl2 ⇨ 2HCl
Hydrogen + Chlorine ⇨ Hydrogen chloride
When calcium oxide reacts with water, calcium hydroxide is formed
CaO + H2O ⇨ Ca(OH)2
Calcium oxide + Water ⇨ Calcium hydroxide
When carbon monoxide reacts with oxygen, carbon dioxide is formed.
2CO + O2 ⇨ 2CO2
Carbon monoxide + Oxygen ⇨ Carbon dioxide
Decomposition Reaction: Reactions in which one compound decomposes in two or more compounds or element are known as DECOMPOSITION REACTION. Decomposition reaction is just opposite of combination reaction.
A general decomposition reaction can be represented as follows:
AB ⇨ A + B
Example: When calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide
CaCO3 ⇨ CaO + CO2
Calcium carbonate ⇨ Calcium oxide + Carbon dioxide
When ferric hydroxide is heated, it decomposes into ferric oxide and water
2Fe(OH)3 ⇨ Fe2O3 + 3H2O
Ferric hydroxide ⇨ Ferric oxide + Water
When lead nitrate is heated, it decomposes into lead oxide, nitrogen dioxide and oxygen.
2Pb(NO3)2 ⇨ 2PbO + 4NO2 + O2
Lead nitrate ⇨ Lead oxide + Nitrogen oxide + Oxygen
In above examples, compound is decomposed because of heating, so, these reactions are called THERMAL DECOMPOSITION REACTION.
Electrolytic Decomposition:
Reactions in which compounds decompose into simpler compounds because of passing of electricity, are known as ELECTROLYTIC DECOMPOSITION. This is also known as ELECTROLYSIS.
Example: When electricity is passed in water, it decomposes into hydrogen and oxygen.
2H2O ⇨ 2H2 + O2
Photolysis or Photo Decomposition Reaction:
Reactions in which a compound decomposes because of sunlight are known as PHOTOLYSIS or PHOTO DECOMPOSITION REACTION.
Example: When silver chloride is put in sunlight, it decomposes into silver metal and chlorine gas.
2AgCl ⇨ 2Ag + Cl2
Similarly, when silver bromide is put under sunlight, it decomposes into silver metal and bromine gas.
2AgBr ⇨ 2Ag + Br2
Photographic paper has coat of silver chloride, which turns into grey when exposed to sunlight. It happens because silver chloride is colourless while silver is a grey metal.
Displacement Reaction
Reactions in which atoms or ions move from one compound to other to form new compound are known as DISPLACEMENT REACTION. Displacement reaction is also known as Substitution Reaction or Single displacement /Replacement Reaction.
A general displacement reaction can be represented using chemical equation as follows:
A + BC ⇨ AC + B
Displacement reaction takes place only when ‘A’ is more reactive than B. If ‘B’ is more reactive than ‘A’, then ‘A’ will not displace ‘C’ from ‘BC’ and reaction will not be taken place.
Example: When zinc reacts with hydrochloric acid, it gives hydrogen gas and zinc chloride.
Zn + 2HCl ⇨ ZnCl2 + H2
When zinc reacts with copper sulphate, it forms zinc sulphate and copper metal.
Zn + CuSO4 ⇨ ZnSO4 + Cu
When silver metal is dipped in copper nitrate, no reaction takes place because silver metal is less reactive than copper.
Ag + Cu(NO3 )2 ⇨ No reaction takes place
Solutions to In Text Questions NCERT Book
Write the balanced equation for the following chemical reactions.
Hydrogen + Chlorine ⇨ Hydrogen Chloride
Answer:
H2 + Cl2 ⇨ 2HCl
Barium chloride + Aluminium sulphate ⇨ Barium sulphate + Aluminium chloride
Answer:
3BaCl+ Al2(SO4 )3 ⇨ 3BaSO4 + 2AlCl3
Sodium + Water ⇨ Sodium hydroxide + Hydrogen
Answer:
2Na + 2H2O ⇨ 2NaOH + H2
Write a balanced chemical equation with state symbols for the following reactions.
Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
Answer:
BaCl2 (aq) + Na2SO4 ⇨ BaSO4 (s) + 2NaCl (aq)
Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
Answer:
NaOH (aq) + HCl ⇨ NaCl (aq) + H2O (l)
A solution of a substance ‘X’ is used for white washing.
Name the substance ‘X’ and write its formula.
Answer: The chemical name of substance, which is used for white washing is Calcium oxide. Its common name is lime or quick lime. The chemical formula of Calcium Oxide is CaO.
Write the reaction of the substance ‘X’ named in (i) above with water.
Answer:
CaO + H2O ⇨ Ca(OH)2
When calcium oxide reacts with water it forms calcium hydroxide. Calcium hydroxide is also known as slaked lime.
Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.
Answer: (Hydrolysis of water has been performed in the activity 1.7 in the text book.) When electric current is passed through water it decomposes into its constituent components, i.e. hydrogen and oxygen gases. During electrolysis of water hydrogen gas collected in one test tube and has double amount than oxygen collected in another test tube. This is because a water molecule is formed by the combination of hydrogen and oxygen in the ratio of 2:1.
Answer the following questions:
Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Answer: The colour of copper sulphate solution is blue. When an iron nail is dipped in the solution of copper sulphate, it forms ferrous sulphate solution and copper metal by the displacement of copper from the solution. The colour of ferrous sulphate is green. That’s why the solution becomes green.
Give an example of a double displacement reaction other than the one given in Activity 1.10.
Answer: When lead nitrate reacts with potassium iodide, potassium nitrate and lead iodide is formed. In this reaction the both the reactants exchanges their irons after reaction. This type of reaction is known as double displacement reaction.
Pb(NO3 )2 (aq) + KI (aq) ⇨ KNO3 + PbI2
Identify the substances that are oxidised and the substances that are reduced in the following reactions.
4Na (s) + O2 (g) ⇨ 2NaO (s)
Answer: In this reaction sodium is combined with oxygen, i.e. gains oxygen and hence oxidized. Consequently oxygen is reduced.
CuO (s) + H2 (g) ⇨ Cu (s) + H2O (l)
Answer: In this reaction Hydrogen gains oxygen and forms water, hence it is oxidised. On the other hand copper loses oxygen and hence, reduced.
Solutions to NCERT Questions
What is a balanced chemical equation? Why should chemical equations be balanced?
Answer: When the number of atoms of reactants is equal to the number of atoms of the products, the reaction is called a balanced chemical equation.
According to the Law of conservation of mass, total mass of the elements present in the reactants must be equal to the total mass of the elements present in products. That’s why a chemical equations should be balanced always.
Answer the following questions:
Which of the statements about the reaction below are incorrect?
2PbO (s) + C (s) ⇨ 2PbS (s) + CO2 (g)
Lead is getting reduced.
Carbon dioxide is getting oxidised.
Carbon is getting oxidised.
Lead oxide is getting reduced.
a and b
a and c
a, b and c
All
Answer: (1) a and b
The following reaction is an example of:
Fe2O3 + 2Al ⇨ Al2O3 + 2Fe
Combination reaction
Double displacement reaction
Decomposition reaction
Displacement reaction
Answer: (d) Displacement reaction
What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.
Hydrogen gas and iron chloride are produced
Chlorine gas and iron hydroxide are produced
No reaction takes place
Iron salt and water are produced
Answer: (a) Hydrogen gas and iron chloride are produced
Translate the following statements into chemical equations and then balance them.
Hydrogen gas combines with nitrogen to form ammonia.
Hydrogen sulphide gas burns in air to give water and sulpur dioxide.
Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Answer:
(a) 3H2 + N2 ⇨ 2NH3
(b) 2H2S + 3O2 ⇨ 2H2O + 2SO2
(c) 3BaCl2 + Al2(SO4 )3 ⇨ 2AlCl3 + 3BaSO4
(d) 3K + 2H2O ⇨ 2KOH + H2
Write the balanced chemical equation for the following and identify the type of reaction in each case.
Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)
Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)
Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)
Answer:
(a) 2KBr (aq) + BaI2 (aq) ⇨ 2KI (aq) + BaBr2 (s)
It is a double displacement reaction.
(b) ZnCO3 (s) ⇨ ZnO (s) + CO2 (g)
It is a decomposition reaction.
(c) H2 (g) + Cl2 (g) ⇨ 2HCl
It is a combination reaction.
(d) Mg (s) + 2HCl (aq) ⇨ MgCl2 (aq) + H2 (g)
It is a displacement reaction.
What does one mean by exothermic and endothermic reactions? Give examples.
Answer: Exothermic reaction: A reaction in which heat is released is called the exothermic reaction.
Example: Burning of fuel is an example of exothermic reaction. When methane is burnt it gives heat along with carbon dioxide and water.
CH4 (g) + 2O2 (g)⇨ CO2 (g) + 2H2O
Endothermic reaction: A reaction in which heat is absorbed is called endothermic reaction.
Example: When silver chloride is left in the sunlight, it absorbs heat and turns grey because of formation of silver metal.
2AgCl (s) ⇨ 2Ag (s) + Cl2 (g)
Why is respiration considered an exothermic reaction? Explain.
Answer: In the course of respiration glucose is broken into carbon dioxide and energy is released. That’s why it is considered as an exothermic reaction.
C6H12O6 (aq) + 6O2 (g) ⇨ 6CO2 (aq) + 6H2O (l) + Energy
Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Answer: In a combination reaction two or more substances are combined to form new substance while in the decomposition reaction one substance decomposes into two or more substances. Hence, the decomposition reactions are called opposite of combination reactions.
CaCO3 (s) ⇨ CaO (s) + CO2 (g) Decomposition reaction
H2 (g) + Cl2 ⇨ 2HCl (g) Combination reaction
Answer the following questions:
Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
Answer:
CaCO3 (s) ⇨ CaO (s) + CO2 (g)
In this reaction energy is supplied in the form of heat.
2AgCl (s) + sunlight ⇨ 2Ag (s) + Cl2 (g)
In this reaction energy is supplied in the form of light,i.e.sunlight
2H2O (l) + electricity ⇨ 2H2 + O2
In this reaction energy is supplied in the form of electricity.
What is the difference between displacement and double displacement reactions? Write equations for these reactions.
Answer: When a more reactive element displaces a less reactive element from its salt solution, then it is called displacement reaction. While there is an exchange of ions between the reactants to produce new substances, it is called double displacement reaction.
In displacement reaction one displacement takes place. While in double displacement reaction two displacements takes place. Following are examples of displacement reaction and double displacement reaction.
Mg (s) + 2HCl (aq) ⇨ MgCl2 + H2 (g)
2KBr (aq) + BaCl2 ⇨ 2KCl (aq) + BaBr2 (s)
In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
Answer:
2AgNO3 (aq) + Cu (s) ⇨ Cu(NO3 )2 (l) + 2Ag (s)
What do you mean by a precipitation reaction? Explain by giving examples.
Answer: Reactions in which precipitate is formed are called precipitation reactions. Generally in a precipitation reaction insoluble salts are settled down as precipitate.
Na2SO4 (aq) + BaCl2 (aq) ⇨ 2NaCl (aq) + BaSO4 (s)
In the above reaction, when sodium sulphate reacts with barium chloride, the white precipitate of barium sulphate is formed along with the sodium chloride.
Explain the following: (a) Oxidation (b) Reduction
Answer: Oxidation: When a substance gains oxygen or loses hydrogen during a reaction, it is called that the substance is oxidized. This phenomenon is called oxidation. In the following reaction, copper gets oxygen and hence is oxidised.
2Cu + O2 + Heat ⇨ 2CuO
CuO + H2 + Hear ⇨ Cu + H2O
In the above reaction hydrogen gains oxygen, and hence oxidized.
Reduction: If a substance loses oxygen or gains hydrogen during a reaction, it is that the substance is reduced. This phenomenon is called reduction. In the following reaction zinc loses oxygen, and hence called is reduced.
ZnO + C ⇨ ZnCO
MnO2 + 4HCl ⇨ MnCl2 + 2H2O + Cl2
In this reaction Mangnese dioxide loses oxygen and hence is called readuced.
A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.
Answer: Copper is a brown coloured shiny element, when heated in air, it becomes black in colour.
2Cu + O2 + Heat ⇨ 2CuO
Hence, the element ‘X’ is copper and black coloured compound formed is copper oxide.
Why do we apply paint on iron articles?
Answer: When articles made of iron come in contact with the moisture present in air, it forms iron oxide, which is known as rust. Hence, to prevent an iron article to come in contact with moisture present in air paint is applied. Paint prevents the iron to get rusted.
Oil and fat containing food items are flushed with nitrogen. Why?
Answer: When fats and oils are oxidised, they become rancid and their smell and taste change. Hence, food items containing oil and fat are flushed with nitrogen which prevents them to get oxidized and becoming rancid.
Explain the following terms with one example each: (a) Corrosion (b) Rancidity
Answer: Corrosion - Metals react with oxygen which is present in the atmospheric moisture. This leads to the formation of metal oxides. In due course of time, the metal keeps on changing into its oxide and finally the whole metal is lost due to oxidation. This process is called corrosion.
Example: Iron articles; like iron gates or bridges tend to rust because of oxidation by atmospheric moisture. We know that rust is nothing but iron oxide. Conversion of iron into rust leads to corrosion of the iron articles. Due to this, the iron articles weaken and finally wither away.
Rancidity: When fats and oils are oxidised, their smell and taste change. This process is called rancidity. Oily food often become rancid and start giving out obnoxious smell. The taste also becomes bad. Such oily food is not fit for eating.
Example: When packets of potato chips or other oily snacks are kept open for a long time; their taste and smell become bad. The oily food is no longer safe to eat.
In chemical reaction new substance is formed which is completely different in properties from the original substance, so in a chemical reaction chemical change takes place.
Signs of Chemical Reaction
Change of state of substance
Change of colour of substance
Evolution of heat
Absorption of heat
Evolution of gas
Evolution of light
Evolution of sound, etc.
Reactant and Product:
Reactant: Substances which take part in a chemical reaction are called reactants.
Product: New substance formed after a chemical reaction is called product.
Chemical Equation: Representation of chemical reaction using symbols of substances is called chemical equation.
Example: A + B ⇨ C + D
In this equation, A and B are called reactants and C and D are called the products. Arrow shows the direction of chemical reaction. Condition, if any, is written generally above the arrow.
When hydrogen reacts with oxygen, it gives water. This reaction can be represented by following chemical equation.
Hydrogen + Oxygen ⇨ Water
H2 + O2 ⇨ H2O
In first equation words are used and in second symbols of substances are used to write the chemical equation. For convenience, symbol of substance is used to represent chemical equations.
Chemical Equation is a way to represent the chemical reaction in concise and informative way.
Chemical equation can be divided into two types – Balanced Chemical Equation and Unbalanced Chemical Equation.
Balanced Chemical Equation: A balanced chemical equation has number atoms of each element equal on both sides.
Example: Zn + H2SO4 ⇨ ZnSO4 + H2
In this equation, numbers of zinc, hydrogen and sulphate are equal on both sides, so it is a balanced chemical equation.
According to Law of Conservation of Mass, mass can neither be created nor destroyed in a chemical reaction. To obey this law, the total mass of elements present in reactants must be equal to the total mass of elements present in products.
Unbalanced Chemical Equation: If the number of atoms of each element in reactants is not equal to the number of atoms of each element present in product, then the chemical equation is called unbalanced chemical equation.
Example: Fe + H2O ⇨ Fe3O4 + H2
In this example number atoms of elements are not equal on two sides of the reaction. For example, on the left hand side only one iron atom is present, while three iron atoms are present on the right hand side. Therefore, it is an unbalanced chemical equation.Balancing a chemical equation:
To balance the given or any chemical equation, follow these steps:
Fe + H2O ⇨ Fe3O4 + H2
Write the number of atoms of elements present in reactants and in products in a table; as shown here.
Name of atom No. of atoms in reactant No. of atoms in product
Iron 1 3
Hydrogen 2 2
Oxygen 1 4
Balance the atom which is the maximum in number; on either side of chemical equation.
In this equation, the number of oxygen atom is the maximum on the RHS.
To balance the oxygen one needs to multiply the oxygen on the LHS by 4; so that the number of oxygen atoms becomes equal on both sides.
Fe + 4 x H2O ⇨ Fe3O4 + H2
Now, the number of hydrogen atoms becomes 8 on the LHS; which is more than that on the RHS. To balance it, one needs to multiply the hydrogen on the RHS by 4.
Fe + 4 x H2O ⇨ Fe3O4 + 4 x H2
After that number of oxygen and hydrogen atoms becomes equal on both sides. The number of iron is one on the LHS, while it is three on the RHS. To balance it, multiply the iron on the LHS by 3.
3 x Fe + 4 x H2O ⇨ Fe3O4 + 4 x H2
Now the number of atoms of each element becomes equal on both sides. Thus, this equation becomes a balanced equation.
Name of atom No. of atoms in reactant No. of atoms in product
Iron 3 3
Hydrogen 8 8
Oxygen 4 4
After balancing, the above equation can be written as follows.
3Fe + 4H2O ⇨ Fe3O4 + 4H2
Balance the given equations.
H2 + O2 ⇨ H2O
Answer: 2H2 + O2 ⇨ 2H2O
Fe + H2O ⇨ Fe2O3 + H2
Answer: 2Fe + 3H2O ⇨ Fe2O3 + 3H2
CO2 + H2O ⇨ C6H12O6 + O2
Answer: 6CO2 + 6H2O ⇨ C6H12O6 + 6O2
Fe2O3 + C ⇨ Fe + CO2
Answer: 2Fe2O3 + 3C ⇨ 4Fe + 3CO2
Writing the symbols of Physical States of substances in Chemical equation:
By writing the physical states of substances a chemical equation becomes more informative.
Gaseous state is represented by symbol ‘g’
Liquid state is represented by symbol ‘l’
Solid state is written by symbol ‘s’
Aqueous solution is written by symbol ‘aq’
Writing the condition in which reaction takes place: The condition is generally written above and/or below the arrow of a chemical equation.
Thus, by writing the symbols of physical state of substances and condition under which reaction takes place, a chemical equation can be made more informative.
Double Displacement Reaction:Reactions in which ions are exchanged between two reactants forming new compounds are called double displacement reactions.
AB + CD ⇨ AC + BD
Example: When solution of barium chloride reacts with the solution of sodium sulphate, white precipitate of barium sulphate is formed along with sodium chloride.
BaCl2 + Na2SO4 ⇨ BaSO4 + 2NaCl
When sodium hydroxide (a base) reacts with hydrochloric acid, sodium chloride and water are formed.
NaOH + HCl ⇨ NaCl + H2O
Double displacement reaction, in which precipitate is formed, is also known as precipitation reaction. Neutralisation reactions are also examples of double displacement reaction.
Exothermic and Endothermic Reaction
Reactions which produce energy are called exothermic reaction. On the other hand, reactions which absorb energy are called endothermic reaction. Most of the combination reactions are endothermic. Most of the decomposition reactions are exothermic. Respiration is a decomposition reaction in which energy is released. When quick lime (calcium carbonate) is added to water, it decomposes and releases energy. Cooking involves chemical reactions which are endothermic as cooking is possible because of heating.
Oxidation and Reduction Reaction:
Oxidation: Addition of oxygen or non-metallic element or removal of hydrogen or metallic element from a compound is known as oxidation.
Elements or compounds in which oxygen or non-metallic element is added or hydrogen or metallic element is removed are called to be oxidized.
Oxidizing agent: Compounds which can add oxygen or a non-metallic compound or remove hydrogen or metallic element are known as oxidizing agents.
Reduction: Addition of hydrogen or metallic element or removal of oxygen or non-metallic element from a compound is called reduction. The compound or element which goes under reduction is called to be reduced.
Reducing agent: Compounds or elements which can cause reduction are called reducing agents.
In a chemical reaction oxidation and reduction both take place simultaneously and such reactions are also known as REDOX REACTIONS. In the word REDOX, ‘Red’ stands for reduction and ‘Ox’ stands for oxidation.
Example: When iron reacts with air, it forms iron oxide (rust)
4Fe + 3O2 ⇨ 2Fe2O3
In this reaction, oxygen is added to iron, thus, iron is oxidized. Here oxygen is oxidizing agent.
When cupric oxide reacts with hydrogen, it gives copper and water.
CuO + H2 ⇨ Cu + H2O
In this reaction, oxygen is removed from copper and oxygen is added to hydrogen. So, cupric oxide is reduced to copper and hydrogen is oxidized to water. Cupric oxide is oxidizing agent and hydrogen is reducing agent.
When sodium hydroxide reacts with hydrochloric acid, it gives sodium chloride and water.
NaOH + HCl ⇨ NaCl + H2O
In this reaction, sodium hydroxide is reduced to sodium chloride since hydrogen is removed from sodium hydroxide. Hydrochloric acid is oxidized to water, since oxygen is added to hydrogen chloride and non-metallic element chloride is removed. Sodium hydroxide is oxidising agent and hydrochloric acid is reducing agent.
In this reaction oxidation and reduction both takes place simultaneously, thus it is an example of redox reaction.
Significance of Oxidation Reduction in Everyday Life:
Respiration is oxidation reaction in which food is oxidized to produce energy.
Iron gets oxidized to form rust; which leads to corrosion of iron in the long run.
Most of the metals react with atmospheric oxygen and it leads to formation of a layer on the metal article. The metal gets corroded in the long run.
Rusting of iron can be prevented by painting the iron article. This can also be prevented by applying a layer of zinc over iron article. This process is known as galvanization.
Fried food gets oxidized when exposed to air. This spoils the taste of the food and the food becomes unfit for consumption. The spoiling of fried food because of oxidation is called rancidity. Fried food is often packed in airtight packets to prevent rancidity.
We are able to utilize various types of fuel because of oxidation. Oxidation of fuel helps in producing energy.
Combination Reaction: Reactions in which two or more reactants combine to form one product are called COMBINATION REACTION.
A general combination reaction can be represented by the chemical equation given here.
A + B ⇨ AB
Example: When magnesium is burnt in air (oxygen), magnesium oxide is formed. In this reaction, magnesium is combined with oxygen.
Mg + O2 ⇨ 2MgO
Magnesium + Oxygen ⇨ Magnesium oxide
When carbon is burnt in oxygen (air), carbon dioxide is formed. In this reaction, carbon is combined with oxygen.
C + O2 ⇨ CO2
Carbon + Oxygen ⇨ Carbon dioxide
When hydrogen reacts with chlorine, hydrogen chloride is formed.
H2 + Cl2 ⇨ 2HCl
Hydrogen + Chlorine ⇨ Hydrogen chloride
When calcium oxide reacts with water, calcium hydroxide is formed
CaO + H2O ⇨ Ca(OH)2
Calcium oxide + Water ⇨ Calcium hydroxide
When carbon monoxide reacts with oxygen, carbon dioxide is formed.
2CO + O2 ⇨ 2CO2
Carbon monoxide + Oxygen ⇨ Carbon dioxide
Decomposition Reaction: Reactions in which one compound decomposes in two or more compounds or element are known as DECOMPOSITION REACTION. Decomposition reaction is just opposite of combination reaction.
A general decomposition reaction can be represented as follows:
AB ⇨ A + B
Example: When calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide
CaCO3 ⇨ CaO + CO2
Calcium carbonate ⇨ Calcium oxide + Carbon dioxide
When ferric hydroxide is heated, it decomposes into ferric oxide and water
2Fe(OH)3 ⇨ Fe2O3 + 3H2O
Ferric hydroxide ⇨ Ferric oxide + Water
When lead nitrate is heated, it decomposes into lead oxide, nitrogen dioxide and oxygen.
2Pb(NO3)2 ⇨ 2PbO + 4NO2 + O2
Lead nitrate ⇨ Lead oxide + Nitrogen oxide + Oxygen
In above examples, compound is decomposed because of heating, so, these reactions are called THERMAL DECOMPOSITION REACTION.
Electrolytic Decomposition:
Reactions in which compounds decompose into simpler compounds because of passing of electricity, are known as ELECTROLYTIC DECOMPOSITION. This is also known as ELECTROLYSIS.
Example: When electricity is passed in water, it decomposes into hydrogen and oxygen.
2H2O ⇨ 2H2 + O2
Photolysis or Photo Decomposition Reaction:
Reactions in which a compound decomposes because of sunlight are known as PHOTOLYSIS or PHOTO DECOMPOSITION REACTION.
Example: When silver chloride is put in sunlight, it decomposes into silver metal and chlorine gas.
2AgCl ⇨ 2Ag + Cl2
Similarly, when silver bromide is put under sunlight, it decomposes into silver metal and bromine gas.
2AgBr ⇨ 2Ag + Br2
Photographic paper has coat of silver chloride, which turns into grey when exposed to sunlight. It happens because silver chloride is colourless while silver is a grey metal.
Displacement Reaction
Reactions in which atoms or ions move from one compound to other to form new compound are known as DISPLACEMENT REACTION. Displacement reaction is also known as Substitution Reaction or Single displacement /Replacement Reaction.
A general displacement reaction can be represented using chemical equation as follows:
A + BC ⇨ AC + B
Displacement reaction takes place only when ‘A’ is more reactive than B. If ‘B’ is more reactive than ‘A’, then ‘A’ will not displace ‘C’ from ‘BC’ and reaction will not be taken place.
Example: When zinc reacts with hydrochloric acid, it gives hydrogen gas and zinc chloride.
Zn + 2HCl ⇨ ZnCl2 + H2
When zinc reacts with copper sulphate, it forms zinc sulphate and copper metal.
Zn + CuSO4 ⇨ ZnSO4 + Cu
When silver metal is dipped in copper nitrate, no reaction takes place because silver metal is less reactive than copper.
Ag + Cu(NO3 )2 ⇨ No reaction takes place
Solutions to In Text Questions NCERT Book
Write the balanced equation for the following chemical reactions.
Hydrogen + Chlorine ⇨ Hydrogen Chloride
Answer:
H2 + Cl2 ⇨ 2HCl
Barium chloride + Aluminium sulphate ⇨ Barium sulphate + Aluminium chloride
Answer:
3BaCl+ Al2(SO4 )3 ⇨ 3BaSO4 + 2AlCl3
Sodium + Water ⇨ Sodium hydroxide + Hydrogen
Answer:
2Na + 2H2O ⇨ 2NaOH + H2
Write a balanced chemical equation with state symbols for the following reactions.
Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
Answer:
BaCl2 (aq) + Na2SO4 ⇨ BaSO4 (s) + 2NaCl (aq)
Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
Answer:
NaOH (aq) + HCl ⇨ NaCl (aq) + H2O (l)
A solution of a substance ‘X’ is used for white washing.
Name the substance ‘X’ and write its formula.
Answer: The chemical name of substance, which is used for white washing is Calcium oxide. Its common name is lime or quick lime. The chemical formula of Calcium Oxide is CaO.
Write the reaction of the substance ‘X’ named in (i) above with water.
Answer:
CaO + H2O ⇨ Ca(OH)2
When calcium oxide reacts with water it forms calcium hydroxide. Calcium hydroxide is also known as slaked lime.
Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.
Answer: (Hydrolysis of water has been performed in the activity 1.7 in the text book.) When electric current is passed through water it decomposes into its constituent components, i.e. hydrogen and oxygen gases. During electrolysis of water hydrogen gas collected in one test tube and has double amount than oxygen collected in another test tube. This is because a water molecule is formed by the combination of hydrogen and oxygen in the ratio of 2:1.
Answer the following questions:
Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Answer: The colour of copper sulphate solution is blue. When an iron nail is dipped in the solution of copper sulphate, it forms ferrous sulphate solution and copper metal by the displacement of copper from the solution. The colour of ferrous sulphate is green. That’s why the solution becomes green.
Give an example of a double displacement reaction other than the one given in Activity 1.10.
Answer: When lead nitrate reacts with potassium iodide, potassium nitrate and lead iodide is formed. In this reaction the both the reactants exchanges their irons after reaction. This type of reaction is known as double displacement reaction.
Pb(NO3 )2 (aq) + KI (aq) ⇨ KNO3 + PbI2
Identify the substances that are oxidised and the substances that are reduced in the following reactions.
4Na (s) + O2 (g) ⇨ 2NaO (s)
Answer: In this reaction sodium is combined with oxygen, i.e. gains oxygen and hence oxidized. Consequently oxygen is reduced.
CuO (s) + H2 (g) ⇨ Cu (s) + H2O (l)
Answer: In this reaction Hydrogen gains oxygen and forms water, hence it is oxidised. On the other hand copper loses oxygen and hence, reduced.
Solutions to NCERT Questions
What is a balanced chemical equation? Why should chemical equations be balanced?
Answer: When the number of atoms of reactants is equal to the number of atoms of the products, the reaction is called a balanced chemical equation.
According to the Law of conservation of mass, total mass of the elements present in the reactants must be equal to the total mass of the elements present in products. That’s why a chemical equations should be balanced always.
Answer the following questions:
Which of the statements about the reaction below are incorrect?
2PbO (s) + C (s) ⇨ 2PbS (s) + CO2 (g)
Lead is getting reduced.
Carbon dioxide is getting oxidised.
Carbon is getting oxidised.
Lead oxide is getting reduced.
a and b
a and c
a, b and c
All
Answer: (1) a and b
The following reaction is an example of:
Fe2O3 + 2Al ⇨ Al2O3 + 2Fe
Combination reaction
Double displacement reaction
Decomposition reaction
Displacement reaction
Answer: (d) Displacement reaction
What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.
Hydrogen gas and iron chloride are produced
Chlorine gas and iron hydroxide are produced
No reaction takes place
Iron salt and water are produced
Answer: (a) Hydrogen gas and iron chloride are produced
Translate the following statements into chemical equations and then balance them.
Hydrogen gas combines with nitrogen to form ammonia.
Hydrogen sulphide gas burns in air to give water and sulpur dioxide.
Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Answer:
(a) 3H2 + N2 ⇨ 2NH3
(b) 2H2S + 3O2 ⇨ 2H2O + 2SO2
(c) 3BaCl2 + Al2(SO4 )3 ⇨ 2AlCl3 + 3BaSO4
(d) 3K + 2H2O ⇨ 2KOH + H2
Write the balanced chemical equation for the following and identify the type of reaction in each case.
Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)
Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)
Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)
Answer:
(a) 2KBr (aq) + BaI2 (aq) ⇨ 2KI (aq) + BaBr2 (s)
It is a double displacement reaction.
(b) ZnCO3 (s) ⇨ ZnO (s) + CO2 (g)
It is a decomposition reaction.
(c) H2 (g) + Cl2 (g) ⇨ 2HCl
It is a combination reaction.
(d) Mg (s) + 2HCl (aq) ⇨ MgCl2 (aq) + H2 (g)
It is a displacement reaction.
What does one mean by exothermic and endothermic reactions? Give examples.
Answer: Exothermic reaction: A reaction in which heat is released is called the exothermic reaction.
Example: Burning of fuel is an example of exothermic reaction. When methane is burnt it gives heat along with carbon dioxide and water.
CH4 (g) + 2O2 (g)⇨ CO2 (g) + 2H2O
Endothermic reaction: A reaction in which heat is absorbed is called endothermic reaction.
Example: When silver chloride is left in the sunlight, it absorbs heat and turns grey because of formation of silver metal.
2AgCl (s) ⇨ 2Ag (s) + Cl2 (g)
Why is respiration considered an exothermic reaction? Explain.
Answer: In the course of respiration glucose is broken into carbon dioxide and energy is released. That’s why it is considered as an exothermic reaction.
C6H12O6 (aq) + 6O2 (g) ⇨ 6CO2 (aq) + 6H2O (l) + Energy
Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Answer: In a combination reaction two or more substances are combined to form new substance while in the decomposition reaction one substance decomposes into two or more substances. Hence, the decomposition reactions are called opposite of combination reactions.
CaCO3 (s) ⇨ CaO (s) + CO2 (g) Decomposition reaction
H2 (g) + Cl2 ⇨ 2HCl (g) Combination reaction
Answer the following questions:
Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
Answer:
CaCO3 (s) ⇨ CaO (s) + CO2 (g)
In this reaction energy is supplied in the form of heat.
2AgCl (s) + sunlight ⇨ 2Ag (s) + Cl2 (g)
In this reaction energy is supplied in the form of light,i.e.sunlight
2H2O (l) + electricity ⇨ 2H2 + O2
In this reaction energy is supplied in the form of electricity.
What is the difference between displacement and double displacement reactions? Write equations for these reactions.
Answer: When a more reactive element displaces a less reactive element from its salt solution, then it is called displacement reaction. While there is an exchange of ions between the reactants to produce new substances, it is called double displacement reaction.
In displacement reaction one displacement takes place. While in double displacement reaction two displacements takes place. Following are examples of displacement reaction and double displacement reaction.
Mg (s) + 2HCl (aq) ⇨ MgCl2 + H2 (g)
2KBr (aq) + BaCl2 ⇨ 2KCl (aq) + BaBr2 (s)
In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
Answer:
2AgNO3 (aq) + Cu (s) ⇨ Cu(NO3 )2 (l) + 2Ag (s)
What do you mean by a precipitation reaction? Explain by giving examples.
Answer: Reactions in which precipitate is formed are called precipitation reactions. Generally in a precipitation reaction insoluble salts are settled down as precipitate.
Na2SO4 (aq) + BaCl2 (aq) ⇨ 2NaCl (aq) + BaSO4 (s)
In the above reaction, when sodium sulphate reacts with barium chloride, the white precipitate of barium sulphate is formed along with the sodium chloride.
Explain the following: (a) Oxidation (b) Reduction
Answer: Oxidation: When a substance gains oxygen or loses hydrogen during a reaction, it is called that the substance is oxidized. This phenomenon is called oxidation. In the following reaction, copper gets oxygen and hence is oxidised.
2Cu + O2 + Heat ⇨ 2CuO
CuO + H2 + Hear ⇨ Cu + H2O
In the above reaction hydrogen gains oxygen, and hence oxidized.
Reduction: If a substance loses oxygen or gains hydrogen during a reaction, it is that the substance is reduced. This phenomenon is called reduction. In the following reaction zinc loses oxygen, and hence called is reduced.
ZnO + C ⇨ ZnCO
MnO2 + 4HCl ⇨ MnCl2 + 2H2O + Cl2
In this reaction Mangnese dioxide loses oxygen and hence is called readuced.
A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.
Answer: Copper is a brown coloured shiny element, when heated in air, it becomes black in colour.
2Cu + O2 + Heat ⇨ 2CuO
Hence, the element ‘X’ is copper and black coloured compound formed is copper oxide.
Why do we apply paint on iron articles?
Answer: When articles made of iron come in contact with the moisture present in air, it forms iron oxide, which is known as rust. Hence, to prevent an iron article to come in contact with moisture present in air paint is applied. Paint prevents the iron to get rusted.
Oil and fat containing food items are flushed with nitrogen. Why?
Answer: When fats and oils are oxidised, they become rancid and their smell and taste change. Hence, food items containing oil and fat are flushed with nitrogen which prevents them to get oxidized and becoming rancid.
Explain the following terms with one example each: (a) Corrosion (b) Rancidity
Answer: Corrosion - Metals react with oxygen which is present in the atmospheric moisture. This leads to the formation of metal oxides. In due course of time, the metal keeps on changing into its oxide and finally the whole metal is lost due to oxidation. This process is called corrosion.
Example: Iron articles; like iron gates or bridges tend to rust because of oxidation by atmospheric moisture. We know that rust is nothing but iron oxide. Conversion of iron into rust leads to corrosion of the iron articles. Due to this, the iron articles weaken and finally wither away.
Rancidity: When fats and oils are oxidised, their smell and taste change. This process is called rancidity. Oily food often become rancid and start giving out obnoxious smell. The taste also becomes bad. Such oily food is not fit for eating.
Example: When packets of potato chips or other oily snacks are kept open for a long time; their taste and smell become bad. The oily food is no longer safe to eat.
No comments:
Post a Comment