5.1 - Exothermic & Endothermic Reactions

Exothermic and endothermic reactions

• In an exothermic reaction, energy is transferred to the surroundings, so the temperature of the surroundings rises.
• In an endothermic reaction, energy is taken in from the surroundings, so the temperature of the surroundings falls.

Energy is conserved

• Energy can't be created or destroyed, only transferred.
• In an exothermic reaction, the products hold less energy than the reactants because energy was released to the surroundings.
• In an endothermic reaction, the products hold more energy than the reactants because energy was taken in from the surroundings.

Sign of the energy change

• Negative (−) - Energy given out (exothermic).
• Positive (+) - Energy taken in (endothermic).

Reaction profiles

A reaction profile (or reaction pathway diagram) shows how the energy stored in the chemicals changes during a reaction.

The axes and levels

• Vertical axis - Energy stored in the chemicals.
• Horizontal axis - Progress of the reaction (from left to right).
• Reactants and products are drawn as horizontal lines.

Reading direction from the levels

• Exothermic - The products are at a lower energy level than the reactants because energy is transferred to the surroundings.
• Endothermic - The products are at a higher energy level than the reactants because energy is taken in from the surroundings.
• The vertical gap between the reactants and products represents the overall energy change.

Activation energy on a profile

• The curve rises from the reactants to a peak before falling to the products.
• The activation energy is the height of the curve measured from the reactants' level to the top of the peak.

Drawing a fully labelled profile

1. Axes - Energy on the vertical axis, progress of reaction on the horizontal axis.
2. Levels - Reactants on the left, products on the right (products lower for exothermic, higher for endothermic).
3. Curve - Starts at reactants, rises to a peak above both levels, then meets the products.
4. Labels - An arrow from the reactants' level to the peak for activation energy, and an arrow between the reactants' and products' levels for the overall energy change.

Worked example - Drawing a reaction profile

A reaction is carried out in which the reactants store more energy than the products. Draw and label a reaction profile, marking the activation energy and the overall energy change, and state whether the reaction is exothermic or endothermic.

Enthalpy change and its sign convention

The enthalpy change ($\mathrm{\Delta }H$) is the energy change of a reaction, measured in kJ or kJ/mol.

The sign convention

• $\mathrm{\Delta }H$ is negative for exothermic reactions - Energy is transferred to the surroundings, so the chemicals lose energy.
• $\mathrm{\Delta }H$ is positive for endothermic reactions - Energy is taken in from the surroundings, so the chemicals gain energy.

Reading and labelling ΔH on a profile

On a reaction profile, the enthalpy change ($\mathrm{\Delta }H$) is the vertical gap between the reactants' level and the products' level.

Direction of the arrow

• Exothermic - The arrow points down from reactants to products ($\mathrm{\Delta }H$ is negative).
• Endothermic - The arrow points up from reactants to products ($\mathrm{\Delta }H$ is positive).

Calculating ΔH from a profile

$$\mathrm{\Delta }H=\text{energy of products}-\text{energy of reactants}$$

• Reactants at 250 kJ, products at 100 kJ:

  $$\begin{array}{cc}\mathrm{\Delta }H& =100-250\\ & =-\mathrm{𝟏𝟓𝟎}\text{ kJ}\text{(exothermic)}\end{array}$$

• Reactants at 100 kJ, products at 250 kJ:

  $$\begin{array}{cc}\mathrm{\Delta }H& =250-100\\ & =+\mathrm{𝟏𝟓𝟎}\text{ kJ}\text{(endothermic)}\end{array}$$