This question is from a lab on evaluating the rate law for a chemical reaction.

Question

This question is from a lab on evaluating the rate law for a chemical reaction. We added .100M CrCl3, .1M Na2EDTA, .1M NaOH and distilled H2O and recorded the absorbance vs time. Here is the post lab question:

As this expirement was developed, we watned to measure k' for the reaction at two different temperatures and find the activation energy for the reaction. However, when the reactants are cooled in an ice bath, the reaction occurs so slowly that the reaction would not go to completion within the time we have.

a. Write the equation for determining the activation energy from rate constants at two different temperatures.

b. Now look at the equation and decide if this reaction has a high activation energy or a low activation energy. Explain your answer.

Explanation / Answer

(a) Suppose at temperature, T1, rate constant is k1. Now according to Arrhenius equation

lnk1 = lnA - Ea / RT1 ------ (1)

Suppose at temperature, T2 (T1 <T2), rate constant is k2. Now according to Arrhenius equation

lnk2 = lnA - Ea / RT2 ------- (2)

(b) From equation (1) and (2)

ln(k2 / k1) = (Ea / R) x [1/T1 - 1/T2]

=> Ea = ln(k2 / k1) x R xT1xT2 / (T2-T1)

As we decrease the temperature, the rate constant, k1 becomes very small making the reaction very slow. This makes the value of  ln(k2 / k1) very high. Hence the activation energy, Ea is very high.

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