Decomposition Reactions - UNSOLVED PRACTICE SET

Q7. Define a decomposition reaction. Write one example each of: (i) thermal decomposition, (ii) electrolytic decomposition, and (iii) photochemical decomposition.

Q8. Write balanced chemical equations for the following decomposition reactions:

(a) Heating of calcium carbonate.

(b) Electrolysis of water.

(c) Decomposition of silver bromide in sunlight.

Q9. Your mother uses a pressure cooker to cook rice faster. She explains that water boils at a higher temperature under pressure. How is this related to the decomposition of calcium carbonate in a lime kiln, where high temperature is required?

Q10. Why is the decomposition of silver chloride (AgCl) in sunlight used in black and white photography? Explain the chemical reaction involved.

Q11. Distinguish between thermal decomposition and electrolytic decomposition with one example of each.

Q12. When green ferrous sulphate crystals are heated in a test tube, the colour changes and gases are evolved. Describe the observations and write the balanced chemical equation for this reaction.

Q13. Define decomposition reaction and explain its types with examples. Describe the decomposition of: (i) calcium carbonate by heat, (ii) water by electricity, and (iii) silver bromide by light. Write balanced chemical equations for each and explain the energy changes involved.

Q14. A student performs three experiments in the laboratory:

Experiment 1: Heating copper carbonate in a hard glass test tube

Experiment 2: Passing electricity through acidulated water

Experiment 3: Exposing silver chloride to sunlight

(a) Identify the type of decomposition in each experiment.

(b) Write balanced chemical equations for all three reactions.

(c) Describe the observable changes in each experiment.

(d) Explain why Experiment 2 is called electrolytic decomposition and not thermal decomposition.

(e) What practical application does Experiment 3 have in everyday life?

Q15. "Decomposition reactions are the opposite of combination reactions, and both are essential for the cycle of matter in nature." Discuss this statement by explaining: (i) how decomposition reactions break down complex substances, (ii) their role in the carbon cycle and oxygen cycle, (iii) industrial applications such as the decomposition of limestone in cement manufacture, and (iv) how the energy requirements of decomposition reactions relate to endothermic processes.

Q16. A cement factory decomposes limestone (calcium carbonate) to produce quicklime (calcium oxide) for cement manufacture:

CaCO₃ → CaO + CO₂

(a) If the factory processes 500 tonnes of limestone per day, calculate the mass of quicklime produced. (Molar masses: CaCO₃ = 100 g/mol, CaO = 56 g/mol)

(b) Calculate the volume of CO₂ produced at STP. (1 mole of gas at STP = 22.4 L)

(c) This reaction requires high temperature (about 900°C). Calculate the energy required if the decomposition needs 178 kJ per mole of CaCO₃.

(d) Why is this decomposition reaction important for the construction industry in India?

(e) If the factory operates at 80% efficiency, how much limestone is actually needed to produce 280 tonnes of quicklime?

Q17. In a school laboratory, a student performs the electrolysis of water using a Hofmann apparatus:

2H₂O → 2H₂ + O₂

(a) If the student collects 112 mL of hydrogen gas at STP, calculate the mass of water decomposed. (Molar masses: H₂O = 18 g/mol, H₂ = 2 g/mol; 1 mole gas at STP = 22.4 L)

(b) What volume of oxygen gas is collected at STP?

(c) The ratio of hydrogen to oxygen volumes is 2:1. Does this support the chemical equation? Explain.

(d) If acidulated water is not used, the reaction is very slow. Why is dilute acid added to water before electrolysis?

(e) Calculate the number of molecules of hydrogen produced when 112 mL is collected at STP.

Q18. Photographic films contain silver bromide (AgBr), which decomposes in light:

2AgBr → 2Ag + Br₂

(a) Is this a photochemical decomposition? Give a reason.

(b) If a photographic film contains 0.376 g of AgBr, calculate the mass of silver deposited when completely decomposed. (Molar masses: AgBr = 188 g/mol, Ag = 108 g/mol)

(c) Why is photographic film stored in dark containers?

(d) Modern digital cameras do not use film. What chemical principle do they use instead?

(e) If a photo studio processes 100 films per day, and each film contains 0.376 g of AgBr, calculate the total mass of silver that can be recovered annually (365 days).