A student observes two reactions: (i) zinc granules dissolve in dilute sulphuric acid, warming the test tube; (ii) barium hydroxide is mixed with ammonium chloride, cooling the test tube. Both produce new substances, yet they have opposite energy behaviours.
(a) Classify each reaction as exothermic or endothermic and justify.
(b) In reaction (i), identify the type of reaction (other than exothermic) and write its balanced chemical equation.
(c) State one industrial or everyday process that shares the same energy classification as reaction (i).
Generated by claude-sonnet-4-6 · 2026-06-26 00:55 · grounding rag
Model Answer
(a) Classification:
- Reaction (i) — Zn with dilute H₂SO₄ — is exothermic. Heat is released during the reaction, which is why the test tube warms up. Reactions that release heat along with products are called exothermic reactions.
- Reaction (ii) — Ba(OH)₂ with NH₄Cl — is endothermic. Energy is absorbed from the surroundings, causing the test tube to cool down. Reactions in which energy is absorbed are called endothermic reactions.
(b) Type of reaction (i) and balanced equation:
Reaction (i) is a displacement reaction — zinc (a more reactive metal) displaces hydrogen from dilute sulphuric acid.
Balanced equation:
$$\text{Zn}(s) + \text{H}_2\text{SO}_4(aq) \rightarrow \text{ZnSO}_4(aq) + \text{H}_2(g)$$
(c) Everyday exothermic process:
Respiration is an exothermic process. Glucose is broken down in the body to release energy, carbon dioxide and water.
$$\text{C}_6\text{H}_{12}\text{O}_6 + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} + \text{energy}$$
Source: Chapter 1 — Chemical Reactions and Equations
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Explanation
- Part (a) tests the core definitions of exothermic (heat released → surroundings warm) and endothermic (heat absorbed → surroundings cool). Always link the observation (warming/cooling) to the definition.
- Part (b) the question says "other than exothermic," so write displacement (not single-displacement or double-displacement). The balanced equation is directly given in the textbook as Eq. 1.3 — memorise it with state symbols.
- Part (c) any valid exothermic example scores marks — burning of fuels, combustion of natural gas, or respiration are all safe choices. Respiration is explicitly discussed in the NCERT text as an exothermic reaction and is the most expected answer.