18.5 Rate constants and temperature

1. Sunday, 24 April 2016 18.5 Rate constants and temperature

2. Temperature and rate constants • As temperature increases, the rate increases and so does the value of the rate constant, k. • For many reactions an increase in temperature of 10°C will double the rate constant and double the rate of reaction

3. Factors that affect the rate constant • When temperature increases there are two factors that contribute to the increased rate and rate constant: • Increasing temperature shifts the Boltzmann distribution to the right increasing the proportion of particles that exceed the activation energy Ea. • As temperature increases particles move faster and collide more frequently. • With increasing temperature the first factor is greater than the second.

4. The Arrhenius equation There is an exponential relationship between the rate constant k and temperature T. This relationship can be expressed as:

5. The Arrhenius equation The exponential factor e-Ea/RT represents the proportion of molecules that exceed Ea and have sufficient energy for a reaction to take place. The pre-exponential term (frequency factor) A takes in to account the frequency of collision with the correct orientation. This term is essentially a constant (over a small temperature range). The frequency factor would give the rate of reaction if there was no activation energy.

6. Logarithmic form of the Arrhenius equation Applying natural logarithms to this equation it can be rewritten as: ln is the opposite function of e