Explain why the rate of reaction slows down above the enzyme's optimum temperature.

Chemical reactions occur faster at higher temperatures since at higher temperatures the enzyme is more likely to bump into the substrate more often and with a higher energy than at a lower temperature. The temperature that the enzyme works the fastest is known as its optimum temperature.
The rate of reaction however, starts to slow down above the enzyme's optimum temperature. This is because the enzyme starts to lose its shape which leads to the active site no longer fitting with the substrate. The enzyme can no longer catalyse the reaction and is said to be denatured.