Chemistry 9701/13 — May/June 2016

Background Concept

An enthalpy change, $\Delta H$, is the heat energy transferred at constant pressure. A positive $\Delta H$ means the system takes in energy from the surroundings and the change is endothermic. A negative $\Delta H$ means the system releases energy to the surroundings and the change is exothermic.

Bond breaking always requires an input of energy, so it is endothermic and has a positive $\Delta H$. Bond making always releases energy, so it is exothermic and has a negative $\Delta H$. Other common enthalpy changes to remember are that atomisation is endothermic, whereas neutralisation and combustion are exothermic.

Understanding the Question

This item presents four pairs of enthalpy changes and asks which row has both the positive and the negative examples correct. You are not asked to calculate anything: you just need to know the sign convention for each named enthalpy change.

Approach

List the sign of each named change before testing the rows:

• atomisation: positive
• bond breaking: positive
• bond making: negative
• combustion: negative
• neutralisation: negative

Now test each row: both entries in a row must be correct.

Step-by-Step Reasoning

Row A says positive atomisation and negative bond breaking. Atomisation is positive, so the first entry is correct. Bond breaking is positive, not negative, so row A is wrong.

Row B says positive bond breaking and negative neutralisation. Both are correct: bond breaking is endothermic, and neutralisation is a well-known exothermic process (acid + alkali releases heat). Row B is the correct answer.

Row C says positive bond making and negative combustion. Combustion is negative, so the second entry is correct. Bond making releases energy, so it is negative, not positive. Row C is wrong.

Row D says positive combustion and negative bond making. Bond making is negative, so the second entry is correct. Combustion is exothermic, so it is negative, not positive. Row D is wrong.

Key Takeaways

• A positive $\Delta H$ = endothermic; a negative $\Delta H$ = exothermic.
• Bond breaking is always endothermic; bond making is always exothermic.
• Atomisation is endothermic; neutralisation and combustion are exothermic.

Common Mistakes

• Writing that bond breaking releases energy: it actually requires energy, so it is endothermic.
• Treating bond making as endothermic: bond making stabilises the atoms, so energy is released.
• Forgetting the sign of neutralisation: acid–base neutralisation is always exothermic.
• Thinking atomisation could be exothermic; it is endothermic because bonds are broken to produce gaseous atoms.

Things to Be Careful About

• Use the temperature of the surroundings as the clue: if the surroundings get hotter, $\Delta H$ is negative.
• In this question only the sign matters, not any numerical value.
• Do not mix up atomisation with formation. Atomisation is about making gaseous atoms from the standard state and is endothermic.