Question

which will increase the rate of a chemical reaction?
adding a catalyst
decreasing the pressure
decreasing the temperature
removing a catalyst

Explanation:

Step1: Understand the effect of a catalyst

A catalyst lowers the activation energy of a reaction.
$$\text{Activation energy} \downarrow \implies \text{Rate of reaction} \uparrow$$

Step2: Analyze the effect of decreasing pressure

For reactions involving gases, decreasing pressure reduces the frequency of collisions between reactant molecules.
$$\text{Pressure} \downarrow \implies \text{Collision frequency} \downarrow \implies \text{Rate of reaction} \downarrow$$

Step3: Analyze the effect of decreasing temperature

Decreasing temperature reduces the kinetic energy of molecules.
$$\text{Temperature} \downarrow \implies \text{Kinetic energy} \downarrow \implies \text{Effective collision frequency} \downarrow \implies \text{Rate of reaction} \downarrow$$

Step4: Analyze the effect of removing a catalyst

Removing a catalyst would increase the activation energy (compared to when the catalyst was present).
$$\text{Activation energy (after removal)} \uparrow \implies \text{Rate of reaction} \downarrow$$

Answer:

A. adding a catalyst