Answer to Question #128558 in Organic Chemistry for Milaa

Question

6.A small piece of platinum is added to a mixture of ethene and hydrogen as shown in the following equation:

H-H-C=C-H-H + H2

a.Identify the mechanism of the reaction
b.Doubling the concentration of hydrogen has no effect on the reaction rate. Doubling the concentration of ethane also has no effect.
i.What is the reaction order of this reaction with respect to ethane? Hydrogen? What is the overall order?
ii.Write the rate equation for this reaction.
iii.Suggest what one might do to accelerate the reaction.

Answer 1

a. It is heterogeneous catalytic hydrogenation of alkene:

Step 1: Ethene molecules adsorbing onto the platinum surface;

Step 2: Hydrogen molecules adsorbing onto the platinum surface;

Step 3: Hydrogen gets attached to the carbon atoms of ethene molecule;

Step 4: Resulted product - ethane - desorbing from the platinum surface.

i. The reaction order of this reaction with respect to ethane is 0. The reaction order with respect to hydrogen is also 0. The overall order is 0.

ii. The rate equation for this reaction:

$v=k$

where k is a rate constant.

iii. To accelerate the reaction we should:

• increase the surface area of platinum on which the reaction takes place;
• increase temperature;
• increase pressure.