The reaction rate will increase in the following situation:
A when a catalyst is added.
A catalyst lowers the activation energy needed for a reaction, which increases the rate of the reaction without being consumed in the process.
The other options typically result in a decrease in the reaction rate:
- B when the temperature drops: Generally, lowering the temperature slows down the reaction rate.
- C when the pressure is lowered: For reactions involving gases, lowering the pressure often decreases the reaction rate.
- D when the molecules are large and bulky: Larger molecules may hinder the reaction due to steric effects, potentially slowing down the reaction.
Therefore, option A is the correct choice.