AQA A Level Chemistry复习笔记1.6.2 Energy Level Diagrams

• An energy level diagram is a diagram that shows the energies of the reactants, the transition state(s) and the products of the reaction as the reaction proceeds ( called 'extent of the reaction' below)
• The transition state is a stage during the reaction at which chemical bonds are partially broken and formed
• The transition state is very unstable – a molecule in the transition state cannot be isolated and is higher in energy than the reactants and products
• 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’

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. ∆H is negative because the system (reactants) loses energy to the surroundings.

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. The system (reactant substances) have gained energy so ∆H is positive.

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

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