Factors Affecting Rates (continued)

 

5.                  Temperature

 

·        As mentioned before, temperature increases the number of effective collisions between molecules. This idea is based on the collision theory. It states that reactions only occur when molecules collide with sufficient energy to rearrange atoms into different molecules.

 

·        As we saw in the enzyme example, the angle at which molecules approach themselves is also important, but if we have more collisions at a higher temperature, we also increase the likelihood that molecules will hit each other with the proper orientation.

 

·        What we called Hmax on the “energy versus progress of reaction” is the highest potential energy point in the chemical reaction. That belongs to a temporary, unstable  in-between product known as the activated complex.

 

·        Since temperature is only an average of the kinetic energy of molecules,

not all molecules at a given temperature have the same kinetic energy. Distribution of kinetic energies of molecules graphs show the relative number of molecules that possess different amounts of energy.

 

           

 

(1)               The total area under the curve represents all molecules at a given temperature T1.

(2)               The area under the curve and to the left of the activation energy (vertical line) represents those molecules that will not react at that given temperature.

(3)               The area under the curve and to the right of the activation energy represents those molecules that will react at that given temperature.

 

 

 

 

 

Example 1       Redraw a curve on the same graph for T2 where T2 > T1.

 

 

 

T2
 

 

                                                                        Energy (kJ)

 

Example 2       How would you show the effect of an inhibitor on the same graph? Of a catalyst?

 

 

inhibitor
 

catalyst
 

 

                                                                        Energy (kJ)

 

 

 

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.

 

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

 

3.                       What kind of chemical 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.

6.                        

 

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

b.                  and then estimate the amount at T2 .

c.                   Which temperature is the higher one?