Question Bank — Chemical Reactions and Equations

Section A — MCQs (1 mark each)

1. Which of the following is not an indication that a chemical reaction has taken place?
   (a) Change in colour
   (b) Evolution of a gas
   (c) Change in physical state without any new substance formed
   (d) Change in temperature

2. The reaction represented by the equation
   \(2\mathrm{FeSO}_4(s) \xrightarrow{\text{Heat}} \mathrm{Fe}_2\mathrm{O}_3(s) + \mathrm{SO}_2(g) + \mathrm{SO}_3(g)\)
   is an example of
   (a) combination reaction
   (b) decomposition reaction
   (c) displacement reaction
   (d) double displacement reaction

3. In the reaction \(\mathrm{Zn}(s) + \mathrm{CuSO}_4(aq) \to \mathrm{ZnSO}_4(aq) + \mathrm{Cu}(s)\), zinc acts as
   (a) an oxidising agent
   (b) a reducing agent
   (c) a catalyst
   (d) a precipitate

4. Which of the following reactions is endothermic?
   (a) Burning of natural gas
   (b) Respiration
   (c) Electrolysis of water
   (d) Reaction of quicklime with water

5. Assertion (A): A chemical equation must always be balanced.
   Reason (R): Mass can neither be created nor destroyed in a chemical reaction.
   (a) Both A and R are true and R is the correct explanation of A.
   (b) Both A and R are true but R is not the correct explanation of A.
   (c) A is true but R is false.
   (d) A is false but R is true.

6. When a shiny brown element ‘X’ is heated in air, it becomes black. The black substance formed is
   (a) copper oxide
   (b) zinc oxide
   (c) iron oxide
   (d) silver chloride

7. The reaction \(\mathrm{CaO}(s) + \mathrm{H}_2\mathrm{O}(l) \to \mathrm{Ca(OH)}_2(aq) + \text{Heat}\) is
   (a) endothermic combination
   (b) exothermic combination
   (c) decomposition
   (d) double displacement

8. Assertion (A): Silver chloride turns grey in sunlight.
   Reason (R): Silver chloride decomposes into silver and chlorine in the presence of sunlight.
   (a) Both A and R are true and R is the correct explanation of A.
   (b) Both A and R are true but R is not the correct explanation of A.
   (c) A is true but R is false.
   (d) A is false but R is true.

9. In the balanced equation \(3\mathrm{Fe}(s) + 4\mathrm{H}_2\mathrm{O}(g) \to \mathrm{Fe}_3\mathrm{O}_4(s) + 4\mathrm{H}_2(g)\), the physical state of water is shown as (g) because
   (a) water is in liquid form
   (b) water is used in the form of steam
   (c) water is an aqueous solution
   (d) water is a reactant only

10. Which of the following prevents rancidity in chips packets?
    (a) Flushing with oxygen
    (b) Flushing with nitrogen
    (c) Adding water
    (d) Exposing to sunlight

Section B — Very Short Answer (2 marks each)

1. State any two observations that help us conclude that a chemical reaction has taken place when zinc granules react with dilute sulphuric acid.
2. Write the balanced chemical equation with physical states for the reaction of barium chloride solution with sodium sulphate solution.
3. Differentiate between a combination reaction and a decomposition reaction with one example each from the chapter.
4. What is meant by a precipitation reaction? Give one example.
5. Why is respiration considered an exothermic reaction? Write the relevant equation.
6. Name the type of reaction in which one element displaces another element from its compound. Give one example equation.

Section C — Short Answer (3 marks each)

1. Balance the following skeletal equation by the hit-and-trial method and identify the type of reaction:
   \(\mathrm{Fe} + \mathrm{H}_2\mathrm{O} \to \mathrm{Fe}_3\mathrm{O}_4 + \mathrm{H}_2\).
2. (a) What happens when lead nitrate is heated? Write the balanced equation.
   (b) Name the type of reaction and the brown gas evolved.
3. Explain with a balanced equation why the blue colour of copper sulphate solution fades when an iron nail is dipped in it. Identify the type of reaction.
4. Distinguish between oxidation and reduction with reference to gain or loss of oxygen. Give one example of each from the chapter.
5. Write balanced chemical equations for:
   (i) Burning of magnesium ribbon in air
   (ii) Reaction of quicklime with water
   (iii) Electrolysis of water.

Section D — Long Answer (5 marks each)

1. (a) What is a double displacement reaction? How is it different from a displacement reaction?
   (b) Write balanced equations for one example of each.
   (c) Why is the reaction between sodium sulphate and barium chloride called a precipitation reaction?

2. Describe with a labelled diagram the electrolysis of water. Write the balanced chemical equation, name the gases evolved at each electrode and state the observation when a burning candle is brought near each gas. (Diagram-based)

3. (a) What is corrosion? Explain with an example.
   (b) Why should we apply paint on iron articles?
   (c) What is rancidity? How do manufacturers prevent it in food items containing fats and oils?

Section E — Case/Source-Based (4 marks each)

Case 1

A student performs an activity in which he takes about 2 g of ferrous sulphate crystals in a dry boiling tube and heats it strongly. The green crystals change colour and a characteristic odour is noticed.

(i) Name the type of reaction and write the balanced equation.
(ii) What is the colour of the residue left?
(iii) Name the gases evolved and the substance responsible for the odour.
(iv) Why is this reaction called thermal decomposition?

Case 2

In daily life we observe that iron articles lose their shine and get coated with a reddish-brown powder when left exposed to moist air for some time. Similarly, silver articles turn black and copper articles develop a green coating.

(i) Name the phenomenon and the reddish-brown powder formed on iron.
(ii) Write the chemical equation for rusting of iron.
(iii) How does this process cause economic loss?
(iv) Suggest one method to prevent this phenomenon on iron railings.

Section F — HOTS and Application (3 marks each)

1. Predict-and-justify: What would happen if the chemical equation for the burning of magnesium were not balanced? Justify your answer using the law of conservation of mass.
2. Analyse anomalous observation: In the electrolysis of water, the volume of gas collected in one test tube is double that collected in the other. Identify the gases and explain why the volumes differ.
3. Apply to unfamiliar real-life situation: A farmer stores harvested potatoes in an airtight container. After a few days the potatoes begin to rot and produce heat. Which type of reaction is responsible and why does the container feel warm?
4. Compare two situations: Compare the reaction of iron nails with copper sulphate solution and the reaction of silver chloride with sunlight. In which case is a displacement reaction occurring and why does the outcome differ in the two situations?