Signs of Enthalpy Changes in Key Processes

Steps:

• Define enthalpy of atomisation as ΔH for converting an element in standard state to gaseous atoms.
• Recognize this process breaks interatomic bonds, absorbing energy, so ΔH > 0 always.
• Contrast with bond breaking in molecules, where context like strain can affect sign.
• Contrast with neutralisation, an acid-base reaction releasing energy, but varying by acid/base strength.

Why A is correct:

• Enthalpy of atomisation requires energy to overcome attractive forces between atoms in the element, making ΔH positive by definition (endothermic process).

Why the others are wrong:

• B: Breaking a C-C bond in strained systems like cyclopropane releases strain energy, making ΔH negative for the process.
• C: Neutralisation of a weak acid with a weak base involves competing ion solvation effects, resulting in ΔH not always negative (can be near zero or positive).
• D: Not enough information.

Final answer: A