Question

Explain why:

A) doubling the concentration of a reactant does not always double the rate of the reaction

B) decreasing the container size for a gas phase reaction usually increases the rate for a gas phase reaction

C) adding a catalyst increases the rate of the reaction

Answer 1

A) rate of a reaction is guided by order of the reaction. It is only for a 1st order(order wrt reactant = 1) reaction the rate doubles when the concentration of the reactant doubles, this is because the rate is directly proportional to the concentration of the reactants. However, doubling the concentration of a reactant does not doubles the rate for 2^nd and 3^rd order reaction. This can be clearly visualised from the rate law epression,

  A → product

1^st order rate expression, Rate = k [A]

2^ndorder rate expression, Rate = k [A]²

3^rdorder rate expression, Rate = k [A]³