Account for the following observations about chemical kinetics. — Kinetics Chemistry Question
Problem Context
Account for the following observations about chemical kinetics.
Reactions involving molecular chlorine often have nonintegral rate laws.
Model Answer
Cl2 often dissociates to Cl atoms, which react individually. If these atoms participate in the rate determining step the overall rate equation is proportional to [Cl•] = [Cl2]1/2
The rates of exothermic reactions increase when their temperatures are increased.
Model Answer
All reactions increase in rate with an increase in temperature due to an increase in the collision frequency and the increase in fraction of species with high velocities. Higher velocity particles impart more energy to collisions in which they participate so those collisions are more likely to exceed Ea. The exothermicity or endothermicity of a reaction has no bearing on its kinetics or the effect of temperature.
Two reactions, A and B, have rate constants that are equal at 25˚C but the rate constant for reaction A is much greater than that for reaction B at 35˚C.
Model Answer
Reactions A and B must have different activation energies. When the log (or ln) of the rate constant for a reaction is plotted versus 1/T the slope is the Ea. Reactions A and B have different slopes which cross at 25 oC. The Ea for reaction A is greater because an increase in temperature affects it’s rate constant more.
The rates of reactions catalyzed by complex molecules, such as enzymes, increase with an increase in temperature up to a certain point above which they decrease again.
Model Answer
Reaction rates increase with higher temperature because of an increase in collision rate and an increase in the fraction of molecules that have the necessary energy to react. At still higher temperatures the enzyme is denatured so that it no longer is an effective catalyst.