Simple rate equations, orders of reaction and rate constants
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Define the terms 'rate equation' and 'rate constant'.
The rate equation expresses the relationship between the rate of reaction and the concentration of reactants raised to appropriate powers (orders). The rate constant (k) is the proportionality constant in the rate equation; its value reflects the rate of the reaction at a specific temperature.
What is the 'order of reaction' with respect to a specific reactant, and how does it relate to the rate equation?
The order of reaction with respect to a reactant is the power to which the reactant's concentration is raised in the rate equation. It indicates how the rate changes with changing concentration of that specific reactant;
Explain the meaning of 'overall order of reaction'.
The overall order of reaction is the sum of the individual orders of reaction with respect to each reactant in the rate equation.
Describe how to determine the numerical value of a rate constant (k) using initial rates and the rate equation.
Given initial rates for several experiments with varying reactant concentrations, and a known rate equation, substitute the initial rates and concentrations into the rate equation. Solve the resulting equation for 'k'.
Describe how to calculate the rate constant (k) using the half-life (t½) for a first-order reaction.
For a first-order reaction, the rate constant k can be calculated using the equation: k = 0.693 / t½, where t½ is the half-life of the reaction. Make sure time units are consistent.
What is a 'rate-determining step' in a multi-step reaction mechanism?
The rate-determining step is the slowest step in a multi-step reaction mechanism. Because it is the slowest, it determines the overall rate of the reaction, as it acts as a bottleneck.
What is a reaction 'intermediate'?
A reaction intermediate is a species that is formed in one step of a reaction mechanism and consumed in a subsequent step. It does not appear in the overall balanced chemical equation.
Outline the qualitative effect of temperature change on the rate constant (k) and the rate of a reaction.
Increasing the temperature generally increases the rate constant (k) and, consequently, the rate of reaction. This is because a higher temperature provides more molecules with the activation energy needed to react.
Given a multi-step reaction mechanism, how can you identify a catalyst?
A catalyst participates in a reaction mechanism, but is both consumed in an early step and regenerated in a later step. Therefore, a catalyst does not appear in the overall balanced equation.
Explain how a proposed reaction mechanism can be validated (or refuted) by experimental data.
A proposed mechanism is valid if the rate equation derived from the rate-determining step matches the experimentally determined rate equation. If they don't match, the mechanism is incorrect and needs to be revised.
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