Chemical Reactions and Equations
NCERT Solutions
Exercises
- Which of the statements about the reaction below are incorrect ?
2 PbO(s) + C(s) → 2Pb (s) + CO2(g)
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced.
(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) All
Answer:
(i) (a) and (b)
- Fe2O3+ 2Al → Al2O3+ 2Fe : This reaction is an example of a
(a) combination reaction
(b) double displacement reaction
(c) decomposition reaction
(d) displacement reaction
Answer:
(d) Displacement reaction.
3. What happens when dilute hydrochloric acid is added to iron filings? Tick the correct answer:
(a) Hydrogen gas and iron chloride are produced.
(b) Chlorine gas and iron hydroxide are produced.
(c) No reaction takes place.
(d) Iron salt and water are produced.
Answer:
(a) Hydrogen gas and iron chloride are produced.
4. What is a balanced chemical equation? Why should chemical equations be balanced?
Answer:
Balanced chemical equation is an equation in which the number of atoms for each element on the reactant side and product side is equal. We need to balance chemical equation because of law of conservation of mass. It states that ‘matter can neither be created nor be destroyed’.
5. Translate the following statements into chemical equations and then balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Answer:
(a) 3H2(g) + N2(g) → 2NH3 (g)
(b) H2S (g) + 3O2 (g) → SO2 (g) + 2H2O(l)
(c) 3BaCl2 (aq) + Al2(SO4)3 (aq) → 2AlCl3 (aq) + 3BaSO4 ↓(s)
(d) 2K (s) + 2H2O (l) → 2KOH (aq) + H2 (g)
6. Balance the following chemical equations :
(a) HNO3+ Ca (OH)2→ Ca (NO3)2 + H2O
(b) NaOH + H2SO4 → Na2SO4 + H2O
(c) NaCl + AgNO3 → AgCl + NaNO3
(d) BaCl2 + H2SO4 → BaSO4 + HCl
Answer:
(a) 2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
(b) 2NaOH + H2SO4 → Na2SO4 + 2H2O
(c) NaCl + AgNO3 → AgCl + NaNO3
(d) BaCl2 + H2SO4 → BaSO4 + 2HCl
7. Write the balanced chemical equations for the following reactions :
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
(c) Aluminium + Copper chloride → Aluminium chloride + Copper
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
Answer:
(a) Ca (OH)2+ CO2→ CaCO3 + H2O
(b) Zn + 2AgNO3 → Zn(NO3)2 + 2 Ag
(c) 2Al + 3 CuCl2 → 2AlCl3 + 3 Cu
(d) BaCl2 + K2SO4 → BaSO4 + 2KCl
8. Write the balanced chemical equation for the following and identify the type of reaction in each case :
(a) Potassium bromide (aq) + Barium iodide (aq) → Potassium iodide (aq) + Barium
(b) Zinc carbonate(s) → Zinc oxide (s) + Carbon dioxide (g) bromide(s)
(c) Hydrogen (g) + Chloride (g) → Hydrogen chloride (g)
(d) Magnesium (s) + Hydrochloric acid (aq) → Magnesium chloride (aq) + Hydrogen(g)
9. What does one mean by exothermic and endothermic reactions ? Give examples.
Answer:
Reactions in which heat is released along with the formation of products are called exothermic chemical reactions. Examples of exothermic Reaction:
a. Formation of calcium hydroxide (slacked lime): C(s) + O_2(g) \rightarrow CO_2(g) + heat
b. C (s)+ O_2 (g) \rightarrow CO_2 (g)+heat
A reaction in which energy is absorbed is called an endothermic reaction. Examples of endothermic Reaction:
a. 2AgBr(s)\xrightarrow{sunlight}2Ag(s) + Br_2(g)
b. 2AgCl(s) \xrightarrow{sunlight}2Ag(s) + Cl_2(g)
10. Why is respiration considered an exothermic reaction? Explain.
Answer:
Respiration is a combination reaction. We, humans, require energy to survive and we receive it from the food we consume. During digestion, food is broken down into simpler substances. Food items like rice, potatoes contain carbohydrates. These are broken down into glucose. This glucose combines with oxygen in the cells of our body and provides energy. This process is called respiration and can be represented as:
C_6H_12O_6(aq) + 6O_2(aq) → 6CO_2(aq) + 6H_2O(l) + energy
11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Answer:
A chemical reaction in which a single compound breaks down into two or more elements or creates a new compound is known as decomposition reaction.
2FeSO_4(s)\xrightarrow{Heat} Fe_2SO_4(s) + SO_2(g) + SO_3(g)
Combination reaction is a reaction in which two or more substances (compound or elements) combine to give a single product.
C(s) + O_2(g) \rightarrow CO_2(g)
12. Write one equation each for the decomposition reactions where energy is supplied in the form of heat, light or electricity.
OR
Decomposition reactions require energy either in the form of heat or light or electricity for breaking down the reactants. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light and electricity. [CBSE 2015 (Delhi)]
CaCO_3 (s) \xrightarrow{Heat} CaO(s)+ CO_2
\\2AgBr(s)\xrightarrow{Light}2Ag(s)+Br_2 (g)
\\2H_2O(l)\xrightarrow{Electricity}2H_2(g)+O_2(g))
13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.
Answer:
In a displacement reaction, a more reactive element reacts and replaces a less reactive element from a compound. Ex: Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s)
A double displacement reaction takes place when two chemicals react and the positive ions (cation) and negative ions (anion) of the two reactants exchange positions. Na_2SO_4(aq) + BaCl_2(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)
14. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
Answer:
2AgNO_3 (aq)+Cu (s) \rightarrow Cu(NO_3)_2+2Ag(s)
15. What do you mean by a precipitation reaction ? Explain by giving examples.
Answer:
A reaction in which an insoluble solid called precipitate is formed that separates from the solution is called a precipitation reaction.
Ex: Na_2SO_4(aq) + BaCl_2(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)
16. Explain the following in terms of gain or loss of oxygen with two examples each:
(a) Oxidation and
(b) Reduction.
Answer:
(a) Oxidation : The addition of oxygen to a substance is called oxidation.
Example:
i. S(s) + O_2(g) → SO_2(g) (Addition of oxygen to sulphur)
ii. 2Mg(s) + O_2 (g) → 2MgO(s) (Addition of oxygen to magnesium)
(b) Reduction : The removal of oxygen from a substance is called reduction.
Example:
i.CuO + H_2 \xrightarrow{Heat} Cu + H_2O: Here, copper oxide is being reduced to copper because oxygen gets removed from copper oxide.
ii. ZnO + C \rightarrow Zn + CO: Here, zinc oxide is being reduced to zinc because oxygen gets removed from zinc oxide.
17. 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:
Element ‘X’ is copper (Cu). The black coloured compound is copper oxide (CuO).
2CuO + O_2 \rightarrow 2CuO
- Why do we apply paint on iron articles ?
Answer:
Painting will prevent the iron articles on getting rust. Pain will not allow the iron articles to come in contact with water or air.
19. Oil and fat containing food items are flushed with nitrogen. Why ?
Answer:
To keep food items fresh and save from getting oxidised.
20. Explain the following terms with one example each (a) Corrosion, (b) Rancidity.
Answer:
CORROSION
- Have you seen that iron articles when bought new are shiny, but as time goes they get coated with reddish brown powder. This is due to the process of rusting of iron.
- When metals come in contact with substances like water or air, the pure metal gets converted to undesirable substances. This process is called corrosion. Other examples of corrosion are the black coating on silver and the green coating on copper.
- Corrosion causes destruction to car bodies, bridges, iron railings, ships and all objects that are made of metals. Iron corrosion is a severe problem as every year a large amount of money is spent to replace damaged iron.
RANCIDITY
- Have you observed that when food containing oil/fat is kept outside for a long time, it gives an unpleasant smell and taste. This is due to rancidity.
- When fats and oils in food undergo oxidation, their smell and taste changes. This process is called rancidity.
- Ways to prevent rancidity
- Antioxidants (substances which prevent oxidation) are added to food.
- Keep food in air tight containers.
Gas like nitrogen can be filled in chips packets to prevent oxidation.
Intext exercise
Pg. No : 6
1. Why should a magnesium ribbon be cleaned before burning in air?
Magnesium is a reactive metal that react with oxygen and results in the formation of a white layer of magnesium oxide (MgO) on its surface. This layer of Magnesium oxide is stable and it prevents the further reaction of magnesium ribbon. So to remove the layer of magnesium oxide from its surface, Magnesium ribbon is cleaned before burning in air.
2. Write the balanced equation for the following chemical reactions.
i) Hydrogen + Chlorine → Hydrogen Chloride
ii) Barium Chloride + Aluminium Sulphate → Barium Sulphate + Aluminium Chloride
iii) Sodium + Water → Sodium Hydroxide + Hydrogen
i) H_2(g)+Cl_2(g) \rightarrow 2HCl(g)
ii) 3BaCl_2(s)+Al_2(SO_4)_3(s)→3BaSO_4(s)+2AlCl_3(s)
iii) 2Na(s)+2H_2O(l)→2NaOH(aq)+H_2(g)
3. Write a balanced chemical equation with state symbols for the following reactions.
i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
i. BaCl_2(aq)+Na_2SO_4(aq) \rightarrow BaSO_4(s)+2NaCl(aq)
ii. NaOH(aq)+HCl(aq)→NaCl(aq)+H_2O(l)
Pg. No : 10
1. A solution of a substance ‘X ‘ is used for white washing. i) Name the substance ‘X’ and write its formula. ii) Write the reaction of the substance ‘X’ named in (i) above with water.
i. The substance ‘X’ that is used for white washing is Calcium Oxide, which is also known as quick lime. Its chemical formula is CaO.
ii. CaO(s)+H_2O(l) \rightarrow Ca(OH)_2(aq)
2. 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.
Water (H2O) consist of two hydrogen and one oxygen atoms. During electrolysis hydrogen goes to one test tube (cathode) and oxygen gas goes to another (Anode). Since the amount of hydrogen and oxygen produced during electrolysis of water is in a ratio of 2:1, the amount of gas collected in one of the test tube is double the amount of other.
Pg. No : 13
1. Why does the color of copper sulphate solution change when an iron nail is dipped in it?
The colour change occurs because of the displacement reaction between iron nails and copper sulphate solution. During this reaction iron displaces the copper from copper sulphate solution (The more reactive metal displaces the less reactive metal). This will results in fading of blue colour of copper sulphate solution and forms a green compound known as ferrous sulphate.
Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s)
2. Give an example of a double displacement reaction other than the one given in Activity 1.10.
A double displacement reaction between Potassium iodide and lead nitrate is shown below. When Potassium iodide and lead nitrate react together we get lead iodide (yellow precipitate) and potassium nitrate.
2KI+Pb(NO_3)_2 \rightarrow PbI_2 + 2KNO_3
3. Identify the substances that are oxidised and the substances that are reduced in the following reactions
Before moving to the answer let’s recall about oxidation and reduction.
Oxidation: Lose of electron
Reduction: Gain of electron
i. 4Na(s)+O_2(g)\rightarrow2Na_2O(s)
Here Sodium (Na) is oxidised and oxygen gets reduced.
ii. CuO(s)+H_2(g)\rightarrow Cu(s)+H_2O(l)
Here Hydrogen is oxidized and Copper oxide is reduced.