Energy Changes in Reactions

This section explains energy changes in reactions covering, exothermic and endothermic reactions, reaction profiles and energy change of reactions.

Exothermic and Endothermic Reactions

In chemical reactions, energy is either released or absorbed. These energy changes are fundamental in determining the type of reaction, and they can affect the temperature of the surroundings.

• Exothermic reactions release energy to the surroundings, usually in the form of heat, and they cause the temperature of the surroundings to increase.
• Endothermic reactions absorb energy from the surroundings, usually in the form of heat, and they cause the temperature of the surroundings to decrease.

Exothermic Reactions

An exothermic reaction is one that releases energy, usually as heat or light, to the surroundings. This type of reaction is common in combustion, respiration, and neutralisation.

Examples of Exothermic Reactions:

• Combustion of fuels (e.g. burning of methane): 

$$CH_4 (g) + 2O_2 (g) \rightarrow CO_2 (g) + 2H_2O (g)$$ 

• Neutralisation between hydrochloric acid and sodium hydroxide:

$$HCl (aq) + NaOH (aq) \rightarrow NaCl (aq) + H_2O (l)$$ 

In these reactions, energy is released to the surroundings, which causes the temperature to rise.

Endothermic Reactions

An endothermic reaction is one that absorbs energy from the surroundings, usually in the form of heat. This causes a decrease in the temperature of the surroundings.

Examples of Endothermic Reactions:

• Photosynthesis in plants: 

$$6CO_2 (g) + 6H_2O (l) \xrightarrow{light} C_6H_{12}O_6 (aq) + 6O_2 (g)$$

•  of calcium carbonate: 

$$CaCO_3 (s) \xrightarrow{\text{heat}} CaO (s) + CO_2 (g)$$

In these reactions, energy is absorbed from the surroundings, and the temperature of the surroundings decreases.

Energy Changes in Reactions

The energy change in a reaction depends on the energy required to break the bonds in the reactants and the energy released when new bonds are formed in the products.

• In exothermic reactions, the energy released when bonds are formed in the products is greater than the energy needed to break the bonds in the reactants. This results in a net release of energy.
• In endothermic reactions, the energy required to break bonds in the reactants is greater than the energy released when new bonds are formed, so the reaction absorbs energy overall.

Table: Exothermic and Endothermic Reactions

Reaction Profiles

A reaction profile is a graphical representation of the energy changes that occur during a chemical reaction. It shows the energy of the reactants and products and the energy required to break the bonds of the reactants (activation energy).

Exothermic reaction profile: The energy of the products is lower than the energy of the reactants, indicating that energy is released during the reaction.

• The activation energy is the energy needed to start the reaction. After this, the energy of the system decreases as the reaction progresses, and energy is released to the surroundings.

Example: In a reaction where energy is released, the graph will show the reactants at a higher energy level, and the products at a lower energy level.

Endothermic reaction profile: The energy of the products is higher than the energy of the reactants, indicating that energy is absorbed during the reaction.

• The activation energy is still required to start the reaction, but after this, energy is absorbed from the surroundings, leading to an increase in the energy of the products.

Example: In a reaction where energy is absorbed, the graph will show the reactants at a lower energy level, and the products at a higher energy level.

Energy Change of Reactions

The energy change (ΔH) of a reaction can be calculated by considering the energy required to break bonds in the reactants and the energy released when bonds are formed in the products. The difference between these two energies is the energy change:

$$\Delta H = \text{Energy of bonds broken} - \text{Energy of bonds formed}$$ 

• For exothermic reactions, ΔH is negative (energy is released).
• For endothermic reactions, ΔH is positive (energy is absorbed).

Summary of Key Points:

• Exothermic reactions release energy, causing the temperature of the surroundings to increase. Common examples include combustion and neutralisation.
• Endothermic reactions absorb energy, causing the temperature of the surroundings to decrease. Common examples include photosynthesis and thermal decomposition.
• The energy change of a reaction can be determined by comparing the energy required to break bonds in the reactants and the energy released when bonds are formed in the products.
• Reaction profiles graphically represent the energy changes in a reaction, showing the activation energy and the energy difference between reactants and products.

This guide will help you understand the key concepts of energy changes in reactions and how they affect the surroundings in terms of temperature.