Chemical Kinetics: Rate Laws & Reaction Mechanisms

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Questions and Answers

Explain how increasing the temperature affects the distribution of molecular energies in a gas, according to the Maxwell-Boltzmann distribution, and how this relates to reaction rate.

Increasing temperature broadens the Maxwell-Boltzmann distribution, shifting it to higher energies. This means more molecules possess the activation energy, leading to a higher reaction rate.

How does the presence of a catalyst affect the activation energy of a reaction, and what is the consequence of this change on the reaction rate?

A catalyst lowers the activation energy by providing an alternate reaction pathway. This allows a greater proportion of molecules to react, increasing the reaction rate.

For the reaction A + B → C, the experimental rate equation is Rate = k[A]^2[B]. What can be inferred about the reaction mechanism from this rate equation?

The rate-determining step involves two molecules of A and one molecule of B. Any steps occurring after this one do not affect the overall rate.

What is the significance of the pre-exponential factor (A) in the Arrhenius equation, and how does it relate to the success of molecular collisions?

The pre-exponential factor (A) represents the frequency of collisions and the probability that the collisions have the correct orientation for a reaction to occur. Signup and view all the answers

Describe how you would use the method of initial rates to determine the rate equation for a reaction involving two reactants, A and B.

Conduct several experiments varying the initial concentrations of A and B. Measure the initial rate for each. Compare how changes in [A] and [B] affect the initial rate to determine the order of reaction with respect to each reactant. Signup and view all the answers

Explain why increasing the surface area of a solid reactant generally increases the reaction rate.

A larger surface area provides more contact points between the solid reactant and other reactants, increasing the frequency of successful collisions and thus the reaction rate. Signup and view all the answers

For a first-order reaction, how is the half-life related to the rate constant, and what does this imply about the time it takes for the reaction to reach completion?

For a first-order reaction, $t_{1/2} = 0.693/k$. This implies that the time taken for each successive halving of the reactant concentration is constant. Signup and view all the answers

How can plotting ln(k) vs. 1/T be used to determine the activation energy of a reaction?

Plotting ln(k) versus 1/T yields a straight line with a slope of -Ea/R. Multiplying the slope by -R (the gas constant) gives the activation energy, Ea. Signup and view all the answers

Distinguish between a homogenous and a heterogenous catalyst, giving an example of each.

A homogenous catalyst is in the same phase as the reactants (e.g., acid catalysis in aqueous solution). A heterogenous catalyst is in a different phase (e.g., a solid catalyst in a gas-phase reaction). Signup and view all the answers

Explain what a rate-determining step is in a reaction mechanism, and why it is important in understanding the overall rate of a reaction.

The rate-determining step is the slowest step in a multi-step reaction mechanism. Since it is the slowest, it limits the rate at which the overall reaction can proceed. Signup and view all the answers

Flashcards

Rate of Reaction

Change in concentration of reactants or products per unit time, measured in mol dm⁻³ s⁻¹.

Catalysis

Catalysts increase reaction rates by providing an alternative pathway with a lower activation energy; they are not consumed in the reaction.

Activation Energy (Ea)

The minimum energy required for a reaction to occur; catalysts lower this energy requirement.

Rate Equation

Expresses the relationship between the rate of a reaction and the concentrations of reactants: Rate = k[A]^m[B]^n.