Chemistry 9701/12 — October/November 2018

Background Concept

An enthalpy change, $\Delta H$, is the heat energy change of a reaction at constant pressure. When a reaction releases heat to the surroundings it is exothermic and $\Delta H$ is negative; when it absorbs heat it is endothermic and $\Delta H$ is positive. A standard enthalpy change is defined for a specific process, and each type has a characteristic sign because of the physical process it describes:

• Enthalpy change of atomisation, $\Delta H_{at}$: the enthalpy change when one mole of gaseous atoms is formed from the element in its standard state. Breaking bonds or separating atoms always requires energy, so atomisation is always endothermic ($\Delta H$ positive).
• Enthalpy change of combustion, $\Delta H_c$: the enthalpy change when one mole of a substance is completely burned in oxygen. Combustion releases heat, so it is always exothermic ($\Delta H$ negative).
• Enthalpy change of formation, $\Delta H_f$: the enthalpy change when one mole of a compound is formed from its elements in their standard states. This can be exothermic or endothermic depending on the compound (e.g. formation of $\text{CO}_2$ is exothermic; formation of $\text{NO}$ is endothermic).
• Enthalpy change of neutralisation, $\Delta H_{neut}$: the enthalpy change when one mole of water is formed by neutralising an acid with a base. The reaction $\text{H}^+(\text{aq}) + \text{OH}^-(\text{aq}) \rightarrow \text{H}_2\text{O}(\text{l})$ always releases energy, so neutralisation is always exothermic ($\Delta H$ negative).

Understanding the Question

This is a recall question that asks which of four statements about the sign (positive or negative) of different enthalpy changes is correct. The word "always" is key — the correct statement must be true for every example of that type of reaction. The question tests whether you know the characteristic sign of atomisation, combustion, formation, and neutralisation.

Approach

Recall the definition of each type of enthalpy change and the sign it always has. Then test each statement: if the statement claims a sign that contradicts the characteristic sign, it is false. Only one statement will be consistent with the chemistry.

Step-by-Step Reasoning

Option A: "Enthalpy changes of atomisation are always negative." Atomisation involves converting an element into gaseous atoms, which requires breaking bonds or overcoming metallic/giant covalent interactions — this always requires energy input, so $\Delta H_{at}$ is always positive. Statement A is false.

Option B: "Enthalpy changes of combustion are always positive." Combustion is the burning of a substance in oxygen, which always releases heat — $\Delta H_c$ is always negative. Statement B is false.

Option C: "Enthalpy changes of formation are always positive." Formation of a compound from its elements can be exothermic or endothermic. For example, the formation of carbon dioxide is exothermic, while the formation of nitrogen monoxide is endothermic. So formation is not always positive. Statement C is false.

Option D: "Enthalpy changes of neutralisation are always negative." Neutralisation is the reaction of an acid with a base to form water, and the net ionic process $\text{H}^+(\text{aq}) + \text{OH}^-(\text{aq}) \rightarrow \text{H}_2\text{O}(\text{l})$ always releases energy. Thus $\Delta H_{neut}$ is always negative. Statement D is correct.

Key Takeaways

• Each standard enthalpy change has a characteristic sign determined by the physical process.
• Atomisation is always endothermic (positive); combustion and neutralisation are always exothermic (negative); formation can be either.
• The word "always" in such questions requires you to check whether the sign is universal for that process.

Common Mistakes

• Thinking atomisation is negative: atomisation requires energy to break bonds, so it is always positive.
• Thinking combustion is positive: combustion is the classic exothermic reaction, so it is always negative.
• Thinking formation is always positive: this is false — many compounds form exothermically.
• Confusing the sign of neutralisation: it is always exothermic (negative), not positive.

Things to Be Careful About

• The word "always" is decisive — a statement is only correct if the sign holds for every example.
• Be precise about definitions: atomisation forms gaseous atoms from the element in its standard state; combustion involves one mole of substance burned in excess oxygen; neutralisation forms one mole of water.