1: Which of the statements about the reaction below are incorrect?
2 PbO(s) + C (s) → 2 Pb (s) + CO2
(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
Ans: (i) (a) and (b)
2: Fe203 + 2Al→Al2+ 2Fe
The above reaction is an example of a
(a) combination reaction. (b) double displacement reaction.
(c) decomposition reaction. (d) displacement reaction.
Ans: (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.
Ans: (a) hydrogen gas and iron chloride are produced.
4: What is a balanced chemical equation? Why should chemical equations be balanced?
Ans: A balanced chemical equation is a symbolic representation of a chemical reaction, which has an equal number of atoms of all the elements on both sides. The law of conservation of mass states that mass can neither be created nor destroyed. Hence, in a chemical reaction, the total mass of reactants should be equal to the total mass of the products. It means that the total number of atoms of each element should be equal on both sides of a chemical equation. This reason that all chemical equations should be balanced.
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.
Ans (a) 3 H2 (g) + N2 (g) → 2 NH3 (g)
(b) 2 H2S (g) + 3O2 (g) → 2H2O (l) + 2 SO2 (g)
(c) 3 BaCl2 (aq) + Al2 (SO4) (aq) → 2 AlCl3 (aq) + 3 BaSO4(s)
(d) 2 K(s) + 2 H2O → 2 KOH(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
Ans:(a) 2 HNO3 + Ca (OH)2 → Ca (NO3)2 + 2 H2O
(b) 2 NaOH + H2SO4 → Na2SO4 + 2 H2O
(c) NaCl + AgNO3 → AgCl + NaNO3
(d) BaCl2 + H2SO4 → BaSO4 + 2 HCl
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
Ans: (a) Ca (OH) 2 + CO2 →CaCO3 +H2O
(b) Zn + 2 AgNO3 → Zn (NO3)2 + 2 Ag
(c) 2 Al + 3 CuCl2 → 2AlCl3 + 3 Cu
(d) BaCl2 + K2SO4 → BaSO4 + 2 Kl
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 bromide(s)
(b) Zinc carbonate(s) → Zinc oxide (s) + Carbon dioxide (g)
(c) Hydrogen (g) + Chlorine (g) → Hydrogen chloride (g)
(d) Magnesium (s) + Hydrochloric acid (aq) → Magnesium chloride (aq) + Hydrogen (g)
Ans: (a) 2 KBr (aq) + BaI2 (aq) → 2 KI (aq) +BaBr2 (s) (double displacement reaction)
(b) ZnCO3(s) → ZnO (s) +CO2 (g) (decomposition reaction)
(c)H2 (g) + Cl2 (g) → 2 HCl (g) (combination reaction)
(d) Mg(s) + 2 HCl (aq) → MgCl (aq) + H2 (g) (displacement reaction)
9: What does one mean by exothermic and endothermic reactions? Give examples.
Ans: Chemical reactions that release energy in the form of heat, light, or sound are called exothermic reactions. Example: mixture of sodium and chlorine to yield salt
2 Na(s) + Cl2 (s) → NaCl (s) + (heat)
Eg. All combination reactions are exothermic.
Reactions that absorb energy or require energy in order to proceed are called endothermic reactions.
Eg. In the process of photosynthesis, plants use the energy from the sun to convert Carbon dioxide and water to Glucose and Oxygen.
6 CO2 (g) + 6 H2O (l) +sunlight → C6 H12 O6 (aq) + 6 O2 (g)
10: Why is respiration considered an exothermic reaction? Explain.
Ans: Energy is required to support life. Energy in our body is obtained from the food we eat. During digestion, large molecules of food are broken down into simpler substances like glucose. Glucose combines with oxygen in the cells and provides energy. The special name of this combustion reaction is respiration. Since energy is released in the whole process, it is an exothermic process.
C6 H12 O6 (aq) + 6 O2 (g) → 6 CO2 (g) + 6 H2O (l) + energy
11: Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Ans: Decomposition reactions are those in which a compound breaks down to form two or more substances. These reactions require a source of energy to proceed. Thus, they are the exact opposite of combination reactions in which two or more substances combine to give a new substance with the release of energy.
Decomposition reaction: AB + energy = A + B
C6 H12 O6 (aq) + 6 O2 (g) → 6 CO2 (g) + 6 H2O (l) + energy
Combination reaction: A+ B = AB + Energy
2 H2 (g) + 2 (g) → 2 H2O (l) + energy
12: Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
Ans: (a) Thermal decomposition
2 FeSO4 (s) →( heat) + Fe2O3 (s) + SO3 (g) + SO2 (g)
(b) Decomposition by light
2 AgCl (s) →( light) + 2Ag (s) + Cl2 (g)
(c) Decomposition by electricity: 2 Al2O3 (aq) → ( electricity) + 4 Al (s) + 3 (O2) (g)
13: What is the difference between displacement and double displacement reactions? Write equations for these reactions.
Ans: In a displacement reaction, a more reactive element replaces a less reactive element from a compound. A+ Bx→Ax+B where A is more reactive than B in a double displacement reaction, Two atoms or a group of atoms switch places to form new compounds. E.g: AB + CD → AD + BC
Displacement reaction: CuSO4 (aq) + Zn (s) → ZnSO4 (aq) + Cu (s)
Double displacement reaction: Na2SO4 (aq)+ BaCl2 (aq) → BaSO4 + 2 NaCl (aq)
14: In the refining of silver, the recovery of silver from silver nitrate solution involves displacement by copper metal. Write down the reaction involved.
Ans: When copper is mixed in silver nitrate solution, it displaces the silver because copper is more reactive than silver.
2 AgNO3 + Cu → Cu(NO3)2 + Ag.
15: What do you mean by a precipitation reaction? Explain by giving examples.
Ans: A reaction in which an insoluble solid (called precipitate) is formed is called a precipitation reaction.
e.g: Na2CO3 (aq) + CaCl2 (aq) → CaCO3 (s) + 2 NaCl (aq). in this reaction, Calcium carbonate is obtained as a precipitate.
Another example of precipitation reaction is:
Na2SO4 (aq) + BaCl2 (aq) → BaSO4 (s) + 2 NaCl (aq)
in this reaction, Barium sulphate is obtained as a precipitate.
16: Explain the following in terms of gain or loss of oxygen with two examples each. (a) oxidation (b) reduction
Ans: (a) Oxidation is the gain of oxygen. e.g.
(i) 2 Mg (s) + O2 (g) → HEAT → 2 MgO (s)
In the above reaction (i) Magnesium is oxidized to Magnesium oxide
(ii) 2 Cu (s) + O2 (g) → HEAT → 2 CuO (s)
In equation (ii) Copper is oxidised to Copper oxide.
(b) Reduction is the loss of oxygen or gain of hydrogen.
(i) CuO + H → HEAT → Cu + H2O
Copper oxide is reduced to Copper.
ZnO + C → HEAT → Zn + CO
(ii) Zinc oxide is reduced to Zinc.
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.
Ans: X is copper (Cu) and the black- coloured compound formed is copper oxide (CuO). The equation of the reaction involved on heating copper is given below.
2 Cu (shiny brown) + O2 → HEAT → 2 Cu (black in colour)
18: Why do we apply paint on iron articles?
Ans: Iron articles are painted because it prevents them from rusting. When painted, the contact of iron articles from moisture and air is cut off. Thus, rusting is prevented.
19: Oil and fat containing food items are flushed with nitrogen. Why?
Ans: Oil and fat containing food items are flushed with nitrogen because nitrogen acts as an antioxidant and it prevents them from being oxidised.
20: Explain the following terms with one example each. (a) corrosion (b) rancidity
Ans: (a) Corrosion is defined as a process where materials, usually metals, deteriorate as a result of a chemical reaction with air, moisture, chemicals, etc.
Eg: (a) rusting of iron : iron reacts with oxygen to form hydrated iron oxide. This hydrated iron oxide is rust.
(b) Rancidity: the process of oxidation of fats and oils that can be easily noticed by the change in taste and smell is known as rancidity.
Eg: the taste and smell of butter that changes when kept out for a long time.