Topic 6 · page 1 of 15

Enthalpy change and its sign

ΔH is the heat energy change at constant pressure, measured under standard conditions. A negative ΔH means heat leaves the chemicals (exothermic); a positive ΔH means heat enters them (endothermic).

11 min read6.16.2

Enthalpy change 6.1

Magnesium dropped into hydrochloric acid warms the cup. Sodium hydrogencarbonate stirred into citric acid solution cools it. Both are reactions, and neither creates or destroys energy. The heat moves between the chemicals and their surroundings.

The reason is bonds. Breaking bonds in the reactants takes in energy. Making bonds in the products gives energy out. The balance of the two is the enthalpy change, ΔH.

Constant pressure is the everyday case. A reaction in an open beaker or cup stays at atmospheric pressure, so the heat you measure with a thermometer is the enthalpy change.

You cannot measure the enthalpy of a substance on its own. You only ever measure the change when reactants become products:

ΔH=H(products)−H(reactants) \Delta H = H(\text{products}) - H(\text{reactants})

ΔH is quoted in kJ mol⁻¹: kilojoules for the amounts of substance in the balanced equation, as written. The next two pages come back to what "per mole" means.

Standard conditions 6.1

An enthalpy change depends on the pressure, the temperature and the physical states of the chemicals. To compare reactions fairly, chemists fix the conditions and mark the result with a standard symbol, as in ΔrH⊖\Delta_\text{r}H^{\ominus}.

If a different temperature is used, the question or the data must say so. A standard enthalpy change is quoted in kJ mol⁻¹ and refers to chemicals in their normal physical states under those conditions.

Exothermic and endothermic 6.2

The sign of ΔH tells you which way the heat goes. The convention is always written from the point of view of the chemicals, not the surroundings.

Two beakers with thermometers. In the exothermic beaker the mercury is high and heat arrows point out of the beaker into the surroundings. In the endothermic beaker the mercury is low and heat arrows point into the beaker from the surroundings.
Exothermic: heat leaves the chemicals and the thermometer rises. Endothermic: heat enters the chemicals and the thermometer falls.
Exothermic Endothermic
Heat moves from the chemicals to the surroundings from the surroundings into the chemicals
Temperature of the mixture rises falls
Enthalpy of the products lower than the reactants higher than the reactants
Sign of ΔH negative positive
Examples combustion, neutralisation, magnesium with acid thermal decomposition of calcium carbonate, sodium hydrogencarbonate with citric acid

Worked exampleDeciding the sign from a temperature change

A reaction mixture cools from 21.0 °C to 17.5 °C. Is the reaction exothermic or endothermic, and what is the sign of ΔH?

  1. The temperature of the mixture falls, so the mixture is taking in heat from its surroundings.
  2. Heat enters the chemicals, so the reaction is endothermic.
  3. ΔH is positive.

Check yourself

  1. State the standard conditions used for enthalpy changes.
  2. Define enthalpy change, ΔH.
  3. A student mixes two solutions and the thermometer reading rises from 19.0 °C to 24.5 °C. State whether the reaction is exothermic or endothermic and give the sign of ΔH.
  4. A student writes "ΔH = +890 kJ mol⁻¹" for the combustion of methane. Explain what is wrong.
  5. Explain why a reaction in which the surroundings get hotter has a negative ΔH.
  6. Classify each as exothermic or endothermic: (a) magnesium burning in air; (b) calcium carbonate being heated until it decomposes; (c) a candle burning.

Answers cover these until you have tried the questions

  1. A pressure of 100 kPa and a specified temperature, usually 298 K.
  2. The heat energy change measured at constant pressure.
  3. The temperature of the mixture rises, so heat has left the chemicals. The reaction is exothermic and ΔH is negative.
  4. Burning is exothermic, so the sign must be negative: ΔH = −890 kJ mol⁻¹. A positive value would mean the products have more enthalpy than the reactants.
  5. The surroundings gain heat energy, so the chemicals must have lost it. Their enthalpy goes down, so ΔH = H(products) − H(reactants) is negative.
  6. (a) Exothermic: heat and light are given out. (b) Endothermic: it keeps taking in heat from the flame. (c) Exothermic: combustion always releases heat.