Test: Chemical Kinetics - MCAT MCQ

The rate of a chemical reaction can be influenced by temperature, pressure, and the concentration of reactants. Increasing temperature generally increases the rate of reaction, while pressure and reactant concentration can also impact the rate by affecting the frequency of effective collisions between reactant particles.

The overall order of a reaction is the sum of the individual reaction orders.

The following reaction is first order for C₆H₅N₂Cl and zero order for H₂O

To find the overall order you would add the order of C₆H₅N₂Cl and H₂O, or 1+0 = 1.

Activation energy is the minimum amount of energy required for a chemical reaction to occur. It represents the energy barrier that reactant molecules must overcome to transform into products. Activation energy is specific to each reaction and depends on the nature of the reactants and the reaction pathway.

The overall rate law is determined by the slowest step of a reaction, not the fastest.

The higher the temperature of a reaction, the faster the reaction rate.

Normally, the greater the concentration of reactants, the faster the reaction rate will occur. However this is not true of zero order reactions! Based on what you know about zero order reactions, see if you can convince yourself of this!

A catalyst will decrease the activation energy, and any decrease in activation energy will increase the reaction rate, therefore II and III are correct.

As can be seen in the Arrhenius equation, a decrease in activation energy will result in an increase in reaction rate, while a decrease in temperature will result in a decrease in reaction rate.

Don’t get tricked by the numbers! Always pay attention to the units of the given values.

Since we have a decrease of 20kJ, that is the same as a decrease of 20,000J, which with the negative sign, results in a proportional increase of reaction rate.

While the 20kJ is a proportional increase, meanwhile the decrease of 20 K in temperature would be extremely small relative to this proportional increase in rate, therefore the overall reaction rate would increase.

A catalyst does not affect the thermodynamics of a reaction, only the kinetics. Therefore ΔG will not change.

The addition of a catalyst will increase the rate of both the forward and reverse reactions.

The equilibrium of a reaction is not changed upon the addition of the catalyst, only the rate at which the reaction reaches equilibrium is changed.

‘Activated’ complex is an alternate term for the reactants in transition state.

The activated complex is formed upon reaching the required activation energy level. This required energy is what is decreased upon the addition of a catalyst.

This question is testing the difference between an intermediate and a transition state. An intermediate differs from a transition state in that the intermediate has a discrete lifetime, whereas a transition state is simply the transition to a new molecule, and is then the higher points on the reaction coordinate.

A transition state (both points A+C) is the transition to a new molecule, it is simply a very high energy state that cannot be isolated during a reaction.

An intermediate is a short-lived unstable molecule in a reaction. It shows a slight reduction in energy on the reaction coordinate (point B), and can be isolated during a reaction.

The rate determining step is the slowest step of a reaction.

The reaction with the slowest rate will have the highest activation energy, or largest energy difference between the reactant and transition state.

The step between C and D has the largest difference of energy between reactant (B) and the transition state and therefore this step has highest activation energy. This is the rate-determining step.

In a zero-order reaction, the rate of reaction is independent of the concentration of the reactant. Radioactive decay follows zero-order kinetics because the rate of decay remains constant over time, regardless of the amount of radioactive substance present.

Increasing the volume of a gaseous reaction system would decrease the pressure. According to Le Chatelier's principle, the system would shift towards the side with more moles of gas to alleviate the pressure change. This would increase the rate of the reaction if there are more moles of gaseous reactants than products.