• An energy profile or energy level diagram is a diagram that shows the energies of the reactants, the?transition state(s)?and the products of the reaction with time
  • The?transition state?is a stage during the reaction at which chemical bonds are partially broken and formed
  • The?transition state?is very unstable – it cannot be isolated and is higher in energy than the reactants and products
• The?activation energy (E_a)?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’

The energy level diagram for the reaction of hydrogen with chlorine to form hydrogen chloride gas

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

The energy level diagram for exothermic reactions

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

The energy level diagram for endothermic reactions

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?E_a?and ΔH??including their values

Answer:

• • The?E_a??is the energy difference from the energy level of the reactants to the top of the ‘hump’
  • E_a?(forward reaction) = (+70 kJ mol^-1) + (+ 20 kJ mol^-1?) =?+90 kJ mol^-1