Exercise on Distribution of Kinetic Energies

1.                   Why is the activation energy in a “Distribution of Kinetic Energies" graph sometimes referred to as the threshold energy? Threshold = lowest limit at which something can be observed.

Any molecules with less than the activation energy will not react; they are below the threshold level.

2.                   What does increasing the temperature do to the number of molecules whose energy exceeds the activation energy?

It increases the percent of molecules with energy beyond the activation energy.

3.                   What kind of chemical lowers the threshold energy?

A catalyst lowers the threshold energy.

4.                   Draw a “Distribution of Kinetic Energies" graph for the decomposition of hydrogen peroxide at room temperature (very slow reaction). Using a broken line, use the same graph to show what happens when the powder from a dry cell battery (MnO2) catalyzes the reaction.



5.                   When you cut an apple and leave it on your counter, the phenolic compounds in an apple oxidize with the help of a catalyst, and the apple turns brown. However if you add CuSO4, this will inhibit the reaction, and the apple will not change colour. Again represent both situations on the same graph, using a broken line for the inhibited reaction.

The inhibited reaction (with CuSO4) will have a vertical line representing activation energy that will be to the right of the line representing the activation energy for the uninhibited reaction.


a.                Estimate the percent of molecules that are reacting at T1.

25.5/77 = 33% You have to count the squares!

b.               and then estimate the amount at T2 .

8/77 = 10%

c.                Which temperature is the higher one?

T1 > T2

7.         a)         tallest curve actually has the lowest average speed = 20 K

                        Middle curve = 100 K

                        Curve to the right = 1000 K

b)               the area is constant = same number of molecules

8.               Ineffective