Predict how the addition of a catalyst would affect the rate of the reaction below, and explain your prediction h2 (g) + i2 (g) yields 2 hi

A catalyst is a chemical substance that hastens the chemical reaction. This does not participates in the creating the product(s) but allows it to be formed easily. With this, it is now known that the rate of the reaction becomes relatively higher compared to the uncatalyzed reactions.

Therefore, the answer to this item is the rate of the reaction becomes faster. 

The catalyst is defined as a chemical substance that helps increase the rate of the chemical reaction without being changed or affected by the reaction itself (the catalyst is not involved in the reaction, it only aids with increasing its speed).

Based on the above definition, adding a catalyst to the reaction:
H2 + I2 > 2HI
will increase the rate of the reaction. This means that the reaction will proceed faster and the product (HI) will be formed faster that the case where the catalyst is not added.

Hope this helps :)