One mole of acetone requires less heat to vapourise than 1 mol of water. Which of the two liquids has higher e

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Q. One mole of acetone requires less heat to vapourise than 1 mol of water. Which of the two liquids has higher enthalpy of vapourisation?

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Concept understanding — Enthalpy and Types of Enthalpy Change

Enthalpy, $H$ , is a state function defined as $H = U + PV$ , introduced because the overwhelming majority of real chemical reactions take place in open vessels at essentially constant atmospheric pressure, rather than at constant volume. At constant pressure, the change in enthalpy equals the heat exchanged with the surroundings, $\Delta H = q_p$ — precisely the quantity a calorimeter measures for a reaction carried out in an open flask, container, or solution. Enthalpy and internal energy are related, for gas-phase reactions at constant temperature, through $\Delta H = \Delta U + \Delta n_g RT$ , where $\Delta n_g$ is the change in moles of gas across the reaction.

Chemists have defined a family of precisely standardized enthalpy changes, each describing one specific, carefully defined process, denoted by the standard-state superscript $^\circ$ (usually $298\ \text{K}$ , $1\ \text{bar}$ ):

Standard enthalpy of formation ( $\Delta_fH^\circ$ ) — one mole of a compound formed from its elements in their standard states; by convention, an element in its standard state has $\Delta_fH^\circ = 0$ .
Standard enthalpy of combustion ( $\Delta_cH^\circ$ ) — one mole of a substance completely burnt in excess oxygen; always exothermic (negative), and readily measured by bomb calorimetry.
Standard enthalpy of atomization ( $\Delta_aH^\circ$ ) — one mole of a substance completely converted into gaseous atoms; always endothermic (positive).
Bond dissociation enthalpy — the energy to break one mole of a specific covalent bond in a gaseous molecule; dividing an atomization enthalpy by the number of equivalent bonds gives an average bond enthalpy.
Enthalpy of ionization (first, second, …) — the energy to remove one mole of electrons from one mole of gaseous atoms (first) or from the resulting gaseous cations (second, always larger than the first, since removing an electron from an already-positive species is harder). …

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The key idea is that enthalpy of vapourisation is the heat required to vaporise one mole of a substance. Since water needs more heat per mole than acetone, water has the higher enthalpy of vapourisation.

The question gives you a direct comparison: one mole of acetone needs less heat to turn into vapour than one mole of water. That’s the entire experimental fact. The enthalpy of vapourisation, $\Delta H_{\text{vap}}$ , is defined as the heat absorbed when one mole of a liquid vaporises at constant pressure. So if water takes more heat per mole, its $\Delta H_{\text{vap}}$ is larger.

Why does this difference exist? It comes down to intermolecular forces. Water molecules form strong hydrogen bonds with each other — each molecule can participate in up to four hydrogen bonds. Breaking those bonds to escape into the vapour phase requires a lot of energy. Acetone molecules have dipole-dipole interactions (and weaker hydrogen bonding via the carbonyl oxygen), but nothing as extensive as water’s network. So water’s $\Delta H_{\text{vap}}$ is significantly higher.

Let’s walk through the reasoning step by step.

Understand the definition. Enthalpy of vapourisation, $\Delta H_{\text{vap}}$ , is the heat required to convert one mole of a liquid into vapour at constant pressure and temperature. It’s a molar quantity — always per mole.
Interpret the given fact. The problem states: “One mole of acetone requires less heat to vapourise than 1 mol of water.” That is a direct statement about the molar heat of vapourisation. In symbols:

$Q_{\text{acetone}} < Q_{\text{water}}$ for one mole each.
Connect heat to enthalpy. At constant pressure (which is the usual condition for boiling), the heat absorbed equals the change in enthalpy: $Q = \Delta H$ . So the statement becomes:

$\Delta H_{\text{vap, acetone}} < \Delta H_{\text{vap, water}}$ . …

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Q.One mole of acetone requires less heat to vapourise than 1 mol of water. Which of the two liquids has higher enthalpy of vapourisation?

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