Endothermic Reactions

Definition of Endothermic Reactions

Endothermic reactions are chemical reactions that absorb energy from their surroundings. This energy intake causes the temperature of the surroundings to decrease during the reaction. The energy absorbed is usually in the form of heat, which is taken in to break bonds in the reactants; although new bonds form, the overall energy absorbed is greater than energy released.

Because energy is absorbed, the surroundings feel colder. This is a key sign that a reaction is endothermic.

Energy Changes in Endothermic Reactions

In an endothermic reaction, energy is required to break the bonds in the reactants. This energy input is greater than the energy released when new bonds form in the products. As a result, there is a net absorption of energy.

The energy change can be shown on an energy profile diagram, where the products have more energy than the reactants. The curve goes upwards, indicating energy is taken in.

For example, if the energy needed to break bonds is 500 kJ and the energy released when new bonds form is 300 kJ, the overall energy absorbed is:

$$\text{Energy absorbed} = 500\, \text{kJ} - 300\, \text{kJ} = 200\, \text{kJ}$$

This means 200 kJ of energy is taken in from the surroundings, causing a temperature drop.

Examples of Endothermic Reactions

Several important chemical processes are endothermic. These include:

• Thermal decomposition: A compound breaks down into simpler substances when heated, absorbing heat energy. For example, heating calcium carbonate (limestone) breaks it down into calcium oxide and carbon dioxide:

$$\text{CaCO}_3 (s) \rightarrow \text{CaO} (s) + \text{CO}_2 (g)$$

This reaction requires continuous heat input to proceed.

• Photosynthesis: Plants absorb energy from sunlight to convert carbon dioxide and water into glucose and oxygen:

$$\text{6CO}_2 + \text{6H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + \text{6O}_2$$

This is an endothermic process because energy from light is absorbed to build chemical bonds in glucose.

• Electrolysis: Electrical energy is used to break chemical bonds in compounds, such as splitting water into hydrogen and oxygen gases:

$$\text{2H}_2\text{O} (l) \rightarrow 2\text{H}_2 (g) + \text{O}_2 (g)$$

Energy must be supplied continuously for the reaction to occur.

Practical Investigations

Endothermic reactions can be investigated by measuring temperature changes during the reaction. A common required practical involves mixing reactants and recording the temperature drop.

For example, dissolving ammonium chloride in water is an endothermic process:

$$\text{NH}_4\text{Cl} (s) \rightarrow \text{NH}_4^+ (aq) + \text{Cl}^- (aq)$$

When ammonium chloride dissolves, it absorbs heat from the water, causing the temperature to fall.

To carry out this practical safely:

• Wear safety goggles to protect eyes from splashes.
• Use a thermometer to measure temperature accurately. Stir the solution gently to ensure even temperature distribution.
• Handle chemicals carefully and follow instructions.
• Work in a well-ventilated area.

The temperature drop confirms that energy is absorbed by the reaction.

For instance, if the temperature of water falls from 20.0°C to 15.5°C after dissolving ammonium chloride, the reaction is endothermic.