5.7 - 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).

Combustion is exothermic

• Combustion occurs when a substance reacts with oxygen and releases energy.
• Burning fuels (like hydrocarbons and alcohols) in a good supply of oxygen transfers a large amount of energy to the surroundings.

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.

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:

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