How does an enzyme affect the rate of a reaction?

1. Analyze the question The question asks how an enzyme changes the rate of a chemical reaction. It tests whether you understand what enzymes do as biological catalysts.

2. Explain the concept

• Every reaction needs a minimum amount of energy to get started. This is the activation energy.
• Enzymes are biological catalysts. They speed up reactions without being used up.
• They work by providing an alternative reaction pathway (through the enzyme's active site binding the substrate). This pathway needs less activation energy.
• When less energy is needed to start the reaction, more substrate molecules can react at a given time, so the rate increases.

3. Step-by-step solution

• Does an enzyme raise or lower activation energy? It lowers it. This eliminates B and D.
• Does the rate go up or down? Enzymes are catalysts, so the rate increases. This eliminates C, because lowering activation energy cannot make a reaction slower.
• What remains is A: lowers activation energy and increases the reaction rate.

4. Answer The correct option is A.

5. Exam tip Remember: "Enzymes lower the hill." Picture a hill the reaction must climb. A catalyst makes the hill lower, so the reaction crosses it more easily and faster. Also note that enzymes do not change the final products or the overall energy released. They only change how quickly the reaction gets there.