Energy Profiles (DP IB Chemistry): Revision Note
Energy profiles
An energy profile shows how the energy of a chemical system changes during a reaction
It includes the energy of the reactants, products, and the transition state:
The transition state is a stage during the reaction at which chemical bonds are partially broken and formed
The transition state is unstable and cannot be isolated
It is the highest energy point along the reaction pathway
The activation energy (Ea) is the energy needed to reach the transition state
We can define the activation energy as
‘the minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction’
Energy profile diagram
Exothermic reaction
In an exothermic reaction, the reactants are higher in energy than the products
The reactants are therefore closer in energy to the transition state
This means that exothermic reactions have a lower activation energy compared to endothermic reactions
In summary:
The products have less energy than the reactants
Energy is released to the surroundings
The temperature of the surroundings increases
Diagram to show an energy profile for an exothermic reaction
Endothermic reaction
In an endothermic reaction, the reactants are lower in energy than the products
The reactants are therefore further away in energy to the transition state
This means that endothermic reactions have a higher activation energy compared to exothermic reactions
In summary:
The products have more energy than the reactants
Energy is absorbed from the surroundings
The temperature of the surroundings decreases
Diagram to show an energy profile for an endothermic reaction
Worked Example
The enthalpy of combustion for methane is -890 kJ mol-1 and the activation energy is +2653 kJ mol-1.
Draw a labelled energy level diagram for this reaction.
Answer:
Step 1: The chemical equation for the complete combustion of methane is:
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l)
Step 2: Combustion reactions are always exothermic (ΔH is negative) so the reactants should be drawn higher in energy than the products
Step 3: Draw the curve in the energy level diagram clearly showing the transition state
Step 4: Draw arrows to show the Ea and ΔH including their values
Worked Example
Use the energy level diagram below to identify the activation energy, Ea, for the reaction.
The reaction pathway diagram for a reversible reaction
Answer:
The Ea is the energy difference from the energy level of the reactants to the top of the ‘hump’
Ea (forward reaction) = (+70 kJ mol-1) + (+ 20 kJ mol-1 ) = +90 kJ mol-1
Examiner Tips and Tricks
The activation energy is the energy difference from reactants to transition state. The enthalpy change of the reaction is the energy difference from reactants to products. Remember to label the axis of the energy level diagrams!
You've read 0 of your 5 free revision notes this week
Unlock more, it's free!
Did this page help you?
