Reaction Profiles

Reaction Profile Diagram

A reaction profile is a graph that shows how the energy of the reactants and products changes during a chemical reaction. The x-axis represents the reaction progress (from reactants to products), and the y-axis shows the energy of the system.

Key features of a reaction profile diagram:

• Reactants: The substances you start with, shown at the beginning of the graph.
• Products: The substances formed, shown at the end of the graph.
• Activation energy peak: The highest point on the graph, representing the energy barrier that must be overcome for the reaction to proceed.
• Energy change: The difference in energy between reactants and products, indicating whether the reaction is exothermic or endothermic.

The energy change is shown by the vertical difference between reactants and products:

• If products are at a lower energy level than reactants, the reaction releases energy (exothermic).
• If products are at a higher energy level than reactants, the reaction absorbs energy (endothermic).

For example, a reaction profile diagram might look like this:

• Y-axis: Energy
• X-axis: Reaction progress
• Reactants start at a certain energy level.
• Energy rises to a peak (activation energy) before falling to the energy level of products.

Exothermic Reactions

Exothermic reactions release energy to the surroundings, usually as heat, causing the temperature of the surroundings to rise.

In a reaction profile for an exothermic reaction:

• The products have a lower energy than the reactants.
• The energy change ($\Delta H$) is negative because energy is given out.
• The graph shows a net fall in energy from reactants to products.

Common examples of exothermic reactions include:

• Combustion: Burning fuels like petrol or wood releases heat and light.
• Respiration: The process in cells where glucose reacts with oxygen to release energy.

For instance, in combustion of methane:

Methane reacts with oxygen to produce carbon dioxide and water, releasing energy:

$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} + \text{energy}$

The energy released warms the surroundings, which is why combustion is exothermic.

Endothermic Reactions

Endothermic reactions absorb energy from the surroundings, usually as heat, causing the temperature of the surroundings to fall.

In a reaction profile for an endothermic reaction:

• The products have a higher energy than the reactants.
• The energy change ($\Delta H$) is positive because energy is taken in.
• The graph shows a net rise in energy from reactants to products.

Examples of endothermic reactions include:

• Photosynthesis: Plants absorb sunlight energy to convert carbon dioxide and water into glucose and oxygen.
• Thermal decomposition: Breaking down compounds using heat, such as heating calcium carbonate to produce calcium oxide and carbon dioxide.

For example, photosynthesis can be summarised as:

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

Energy is absorbed from sunlight, making this an endothermic reaction.

Activation Energy

Activation energy is the minimum amount of energy needed to start a chemical reaction. It is the energy barrier that reactants must overcome to form products.

On a reaction profile diagram, activation energy is shown as the height of the peak above the energy level of the reactants.

The size of the activation energy affects the rate of reaction:

• A higher activation energy means fewer particles have enough energy to react, so the reaction is slower.
• A lower activation energy means more particles can react, so the reaction is faster.

Catalysts lower the activation energy, making reactions faster without being used up. See the separate topic on Catalysts and Activation Energy for more details.

Inline example: If reactants are at 250 kJ and the peak is at 400 kJ, the activation energy is $400 - 250 = 150$ kJ.

Example: Interpreting a Reaction Profile

A reaction profile shows reactants at 200 kJ, products at 100 kJ, and the peak at 300 kJ.

Is the reaction exothermic or endothermic? What is the activation energy?

Since products (100 kJ) are lower than reactants (200 kJ), the reaction releases energy and is exothermic.

Activation energy = peak energy 00reactants energy = $300 - 200 = 100$ kJ.