Exploring the temperature dependence of reaction rates using the Landolt "iodine clock" reaction

The iodine "clock" reaction is run at a three temperatures to observe the dependence of the reaction time on the temperature of the reaction solution.

Ingredients: sodium bisulfite, potassium iodate, starch

Procedure: A complete recipe follows.

1. Prepare solutions of sodium thiosulfate.

2. Prepare solutions of starch and combine with solution of sodium thiosulfate.

3. Prepare solution of potassium iodate.

4. Add solution of potassium iodate to sodium sulfate and starch solution.

5. Measure reaction time.

6. Repeat reaction for solutions thermally equilibrated in ice water, and heated to near boiling.

Understanding: This reaction is a well known example of the so-called "clock reactions" where a reaction displays a clear "endpoint" that appears after a well-defined amount of time. In the iodine clock reaction, the overall reaction process is described by the following mechanism

IO3-(aq) + 3 HSO3-(aq) → I-(aq) + 3 SO42-(aq) + 3 H+(aq)

IO3-(aq) + 8 I-(aq) + 6 H+(aq) → 3 I3-(aq) + 3 H2O(l)

I3-(aq) + HSO3-(aq) + H2O(l) → 3 I-(aq) + SO42-(aq) + 3 H+(aq)

2 I3-(aq) + starch → starch blue-I5-(aq) + I-(aq)

When any mechanism is proposed, the steps in the mechanism must sum to the overall reaction as required by the law of mass action. For the mechanism above the overall reaction is

2 IO3-(aq) + 3 I-(aq) + 4 HSO3-(aq) → starch blue-I5-(aq) + 2 H2O(l) + 4 SO42-(aq)

representing a conversion of the reactants iodate and hydrogen sulfite to products water, sulfate, and a deep blue starch-pentaiodide complex.

Our demonstration explored the rate of the reaction at three different temperatures: ice water, 0C, room temperature, 22C, and near boiling water, 80C. It was observed that the reaction run at the lowest temperature occurred in the longest time, while the reaction run at the highest temperature occurred in the shortest time.

Rates of reaction show an exponential temperature dependence

We can understand this observation at the molecular level. Kinetic theory provides us with an accurate estimate of the distribution of speeds of the molecules. The average speed of a molecule of mass M at temperature T is

uave = (8RT/πM)1/2

As the temperature is raised, the molecules in the solution will on average move faster. The probability of finding a molecule with a particular kinetic energy, Ekinetic, is

probability of having velocity v = constant x exp[-Ekinetic/RT]

The probability of having a given kinetic energy Ekinetic diminishes exponentially with increasing energy. The probability of observing molecules with high kinetic energy is much lower than the probability of observing molecules with kinetic energy roughly equal to the thermal energy, RT.

Consider an elementary chemical reaction. If the reaction requires energy to occur, an energetic barrier must be overcome. When the energy barrier to reaction is high, the probability of molecules gaining sufficient energy to react can be quite small. It can be show that the rate of reaction for an elementary process will vary as

rate of reaction = constant x exp(-Eact/RT)

where Eact is known as the activation energy that must be overcome for the reaction to occur. This is known as the Arrhenius relation which describes the temperature dependence of the rate of elementary reaction processes. Just as the probability of having a given kinetic energy Ekinetic decreases exponentially as the energy increases, the probability of having adequate energy to react also decreases exponentially as Eact increases.

Experimental determination of the activation energy

As the temperature is increased, the thermal energy "RT" will increase making the ratio of the activation energy to the thermal energy, Eact/RT, smaller. That means that the reaction rate will be larger and the reaction time will be smaller. Increasing the temperature will increase the rate of reaction.

This leads to a useful result. If we measure the reaction rate at two temperatures, k(T2) and k(T1), we can use the result to determine the activation energy of the reaction

ln[ k(T2)/k(T1)] = (Eact/R) (1/T1 - 1/T2)

where we assume that the constant prefactor in the Arrhenius relation does not have a strong dependence on the temperature. This result is equivalent to the van't Hoff relation used to determine the standard state change in enthalpy of a chemical reaction, ΔHo, from the temperature dependence of the equilibrium constant.

Temperature dependence of chemical reaction rates

Question: A chemical reaction is run at 20C, and a reaction time is observed. When the temperature of a reaction solution is increased by 25C, the reaction time is reduced by a factor of two. What is the activation energy of the reaction?

You can check your answers here.