Briefly comment on the effect of a catalyst on each of the following.

(a) activation energy
A catalyst lowers the activation energy by changing the reaction mechanism.
A catalyst lowers the activation energy by changing the rate.
A catalyst raises the activation energy by changing the reaction mechanism.
A catalyst does not change the activation energy.
A catalyst raises the activation energy by changing the rate.

(b) reaction mechanism
A catalyst always simplifies a reaction mechanism.
A catalyst does not change the reaction mechanism.
A catalyst always makes a reaction mechanism more complicated.
A catalyst changes the reaction mechanism.

(c) enthalpy of reaction
A catalyst always raises the enthalpy of reaction.
A catalyst does not change the enthalpy of reaction.
A catalyst always lowers the enthalpy of reaction.
A catalyst changes the enthalpy of reaction, either raising or lowering it.

(d) rate of forward step
A catalyst decreases the rate of the forward reaction.
A catalyst does not change the rate of the forward reaction.
A catalyst increases the rate of the forward reaction.

(e) rate of reverse step
A catalyst does not change the rate of the reverse reaction.
A catalyst decreases the rate of the reverse reaction.
A catalyst increases the rate of the reverse reaction.