Science (086) — AI-generated practice question

(a) Reactions in which heat is released along with the products are called exothermic reactions.
Example: Burning of natural gas —
$$\text{CH}_4(g) + 2\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2\text{H}_2\text{O}(g) + \text{Heat}$$

Reactions in which energy is absorbed are called endothermic reactions.
Example: Decomposition of water by electricity —
$$2\text{H}_2\text{O}(l) \xrightarrow{\text{electricity}} 2\text{H}_2(g) + \text{O}_2(g)$$

(b) When quick lime is added to water, heat is released. This is an exothermic reaction. It is also a combination reaction since two substances form a single product.
$$\text{CaO}(s) + \text{H}_2\text{O}(l) \rightarrow \text{Ca(OH)}_2(aq) + \text{Heat}$$

(c) In the decomposition of water by electricity, energy (electrical energy) is absorbed to break water into hydrogen and oxygen. Since energy is absorbed, it is classified as an endothermic reaction. The type of decomposition involved is electrolytic decomposition.

Source: Chapter 1 — Chemical Reactions and Equations, Section 1.2.1 Combination Reaction

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• (a) Always give the definition + a balanced equation for both. Examiners award 1 mark each for correct definitions and 1 mark for each balanced equation.
• (b) Note that this reaction is both a combination AND an exothermic reaction — mentioning both scores full marks.
• (c) The key logic is: energy is supplied/absorbed → endothermic. The specific type is electrolytic decomposition (energy supplied as electricity). Don't write "electrical decomposition."