Which of the following best describes what happens to the rate of most chemical reactions as the temperature decreases?

• A. The reaction rate decreases.
• B. The reaction rate first increases, then decreases.
• C. The reaction rate remains unchanged.
• D. The reaction rate increases.

Answer: (A)

Verified step by step guidance

1. Understand that the rate of a chemical reaction depends on the frequency and energy of collisions between reactant molecules.
2. Recall the Arrhenius equation, which relates the reaction rate constant \(k\) to temperature \(T\): \[k = A e^{-\frac{E_a}{RT}}\] where \(A\) is the frequency factor, \(E_a\) is the activation energy, \(R\) is the gas constant, and \(T\) is the temperature in Kelvin.
3. Recognize that as temperature decreases, the exponential term \(e^{-\frac{E_a}{RT}}\) decreases because the denominator \(RT\) becomes smaller, making the negative exponent larger in magnitude, thus reducing \(k\).
4. Since the rate constant \(k\) decreases with decreasing temperature, the overall reaction rate decreases because fewer molecules have enough energy to overcome the activation barrier.
5. Therefore, the best description is that the reaction rate decreases as the temperature decreases.