Transition state and activation energy

The hill every reaction must climb

A reaction happens when reactant molecules collide. But molecules move at different speeds: some have more than the average kinetic energy (the book calls them excited molecules), others less.

Collisions between slow molecules may do nothing. When two excited molecules collide, they can form the transition state: a short-lived arrangement where old bonds are half broken. It then falls back to reactants or goes on to products.

The transition state sits at the top of the energy profile. The energy needed to climb from the reactants to that peak is the activation energy, Ea.

The gap between the reactants' energy and the products' energy is ΔH. If the products end lower, the reaction is exothermic (Fig 5.4); if higher, endothermic (Fig 5.5).

Ea is the climb to the peak; ΔH is the difference between start and finish. They are different heights on the same graph.

Key terms

Excited molecules

The collisions that can succeed.

Transition state

Half-way between reactants and products.

Activation energy (Ea)

Why some reactions do not start on their own.

Energy profile

Showing Ea and ΔH together.

Short questions with model answers

1. Q1. On an energy profile, reactants are at 100 kJ, the peak at 180 kJ and products at 40 kJ. Find Ea and ΔH. (method of Fig 5.4, p. 75)

   • Ea = 180 − 100 = 80 kJ
   • ΔH = 40 − 100 = −60 kJ
   • Products lower: exothermic

   Ea = 80 kJ, ΔH = −60 kJ (exothermic)

2. Q2. Methane and oxygen do not react when mixed, but a burning match starts the reaction, which then keeps going. Explain. (Constructed response iii, p. 82)

   • Too few excited molecules at room temperature
   • The match supplies Ea
   • Heat released keeps it going

   The match provides the activation energy; the heat given out then supplies it for the rest.

Common mistakes

• ✗ Activation energy and ΔH are the same height.

  ✓ Ea is reactants to peak; ΔH is reactants to products.

• ✗ Exothermic reactions need no activation energy.

  ✓ Fig 5.4 still has a peak to climb; methane needs a match.

• ✗ Every collision between reactant molecules makes products.

  ✓ Only collisions with enough energy reach the transition state (p. 75).

MCQs

1. 1. Activation energy is the energy needed to:

   1. (a) Turn reactants into the transition state
   2. (b) Turn products back into reactants only
   3. (c) Melt the container
   4. (d) Measure total enthalpy
   Show answer

   (a) p. 76 and key point 8.

2. 2. The transition state is found:

   1. (a) At the peak of the energy profile
   2. (b) At the reactants' level
   3. (c) At the products' level
   4. (d) Below the products
   Show answer

   (a) p. 75: shown at the peak of the curve.

3. 3. C(diamond) → C(graphite), ΔH = −3 kJ mol⁻¹, does not happen at room conditions because:

   1. (a) Diamond is more stable
   2. (b) The change has a high activation energy
   3. (c) Graphite is denser
   4. (d) It is endothermic
   Show answer

   (b) MCQ (x), p. 81: exothermic, but the hill is too high.

4. 4. In an endothermic energy profile, the products are:

   1. (a) Higher than the reactants
   2. (b) Lower than the reactants
   3. (c) Above the transition state
   4. (d) At zero energy
   Show answer

   (a) Fig 5.5 and key point 10.

5. 5. Collisions between molecules with less than average kinetic energy usually:

   1. (a) Always give products
   2. (b) Produce no result
   3. (c) Form a catalyst
   4. (d) Release Ea
   Show answer

   (b) p. 75: they may not be able to produce any result.

Quick revision

• Only collisions of energetic (excited) molecules reach the transition state.
• Activation energy Ea: reactants up to the transition state at the peak.
• ΔH: reactants to products; products lower is exothermic, higher is endothermic.