Enthalpy and enthalpy change

The heat stored in a system

Enthalpy, H, is the total heat content of a system: the thermal energy stored in its substances. It is measured in kJ mol⁻¹.

Nobody can measure the total H of a system directly. What we can measure is the change, ΔH, when a reaction happens.

When the system absorbs energy its enthalpy rises, so ΔH is positive. When it gives energy out its enthalpy falls, so ΔH is negative. Burning carbon monoxide: 2CO + O₂ gives 2CO₂ with ΔH° of −566.0 kJ, given to the surrounding.

Heat is different. Heat is energy on the move, flowing from a hot body to a cold one because of a temperature difference; it comes and goes. Enthalpy belongs to the system and depends on how many molecules it has, what they are made of and their structure. At constant pressure, ΔH equals the heat given out or taken in.

Think of H as money in an account you cannot see, and ΔH as the deposit or withdrawal you can.

Key terms

Enthalpy (H)

Describing stored thermal energy.

Enthalpy change (ΔH)

Measuring the energy of a reaction.

ΔH°

Comparing reactions fairly.

Heat

The transfer that changes H.

Short questions with model answers

1. Q1. 2CO(g) + O₂(g) → 2CO₂(g), ΔH° = −566.0 kJ. Does the enthalpy of the system rise or fall? (p. 69)

   • ΔH° < 0
   • Energy leaves the system
   • H of products is 566.0 kJ lower

   It falls by 566.0 kJ; the reaction is exothermic.

2. Q2. Why can we not calculate the total enthalpy of a system? (Exercise 2, p. 70)

   • H depends on every molecule and bond
   • No zero point to start from
   • Measure the heat in or out

   Only the change ΔH can be measured, not the total H.

Common mistakes

• ✗ Heat and enthalpy are the same thing.

  ✓ Heat is energy in transfer; enthalpy is stored in the system. Heat changes the enthalpy.

• ✗ A calorimeter measures the total H of a substance.

  ✓ Only ΔH can be measured; total H cannot (p. 69).

• ✗ ΔH = −566 kJ means the system gained 566 kJ.

  ✓ Negative ΔH: the system lost 566 kJ to the surrounding.

MCQs

1. 1. The symbol for a change in enthalpy is:

   1. (a) H
   2. (b) ΔH
   3. (c) E
   4. (d) Q
   Show answer

   (b) p. 69: H is enthalpy, ΔH is its change.

2. 2. Which can be measured directly?

   1. (a) Total enthalpy H
   2. (b) Enthalpy change ΔH
   3. (c) Both
   4. (d) Neither
   Show answer

   (b) p. 69: H cannot be measured directly; ΔH can.

3. 3. Heat flows:

   1. (a) From a cold body to a hot body
   2. (b) From a hot body to a cold body
   3. (c) Only inside a system
   4. (d) Only at 0 °C
   Show answer

   (b) p. 70: because of a difference in temperature.

4. 4. When a system absorbs energy, its enthalpy:

   1. (a) Increases
   2. (b) Decreases
   3. (c) Stays the same
   4. (d) Becomes zero
   Show answer

   (a) p. 68: absorbed energy raises the total enthalpy.

5. 5. Enthalpy of a system depends on:

   1. (a) Only the temperature of the room
   2. (b) The number of molecules, composition and structure
   3. (c) Only the container
   4. (d) Nothing; it is always zero
   Show answer

   (b) p. 70 lists these three.

Quick revision

• Enthalpy H is the total heat content of a system; only its change ΔH can be measured.
• Energy in: H rises, ΔH positive. Energy out: H falls, ΔH negative.
• Heat is energy in transfer; at constant pressure it equals ΔH.