Rate Equations Flashcards
What is the general form of a rate equation?
Rate = k [A]^m [B]^n, where k is the rate constant, [A] and [B] are concentrations of reactants, and m and n are their orders.
What does the rate constant (k) represent?
The rate constant k links the rate of reaction to the concentrations of reactants. It is constant at a given temperature.
What is meant by the order of a reaction?
The order of a reaction is the power to which the concentration of a reactant is raised in the rate equation.
How do you determine the overall order of a reaction?
By adding the powers (orders) of all reactants in the rate equation: overall order = m + n.
What are the units of the rate constant k?
Units depend on the overall order of the reaction. They are derived by rearranging the rate equation to make k the subject.
What is the unit of k for a first-order reaction?
s⁻¹
What is the unit of k for a second-order reaction?
mol⁻¹ dm³ s⁻¹
What is the unit of k for a third-order reaction?
mol⁻² dm⁶ s⁻¹
How is the rate of reaction determined experimentally?
By measuring the change in concentration of a reactant or product over time.
How do you determine the order of a reaction from experimental data?
By varying the concentration of one reactant while keeping others constant and measuring the effect on the rate.
What is a concentration–time graph?
A graph that shows how the concentration of a reactant changes with time.
What is a rate–concentration graph?
A graph showing how the rate of reaction changes with changing reactant concentration.
What does a straight line through the origin on a rate–concentration graph show?
First-order with respect to that reactant (rate ∝ [A]).
What does a horizontal line on a rate–concentration graph show?
Zero-order with respect to that reactant (rate is independent of [A]).
What does a curved line on a rate–concentration graph show?
Second-order with respect to that reactant (rate ∝ [A]²).
What is the initial rate of a reaction?
The rate at the very start of a reaction, where t = 0.
How is initial rate determined from a concentration–time graph?
By drawing a tangent at t = 0 and finding its gradient.
What is a clock reaction?
A method of measuring the initial rate by timing how long it takes for a visible change (like color change) to occur.
What assumption is made in a clock reaction?
That the average rate over the time measured is approximately equal to the initial rate.
What is the rate-determining step?
The slowest step in a multi-step reaction mechanism that limits the overall rate.
How can you identify the rate-determining step from a rate equation?
Only species present in the rate equation appear in the rate-determining step.
How can you propose a reaction mechanism from a rate equation?
Use the order of each reactant to deduce which molecules are involved in the rate-determining step.
How does temperature affect the rate constant k?
As temperature increases, k increases exponentially due to more molecules having energy ≥ activation energy.
How is the Arrhenius equation written?
k = Ae^(-Ea/RT), where A is the frequency factor, Ea is activation energy, R is the gas constant, and T is temperature in Kelvin.
How can the Arrhenius equation be used in the form of a straight-line graph?
ln k = -Ea/R × (1/T) + ln A — plot ln k against 1/T to get a straight line with gradient -Ea/R and y-intercept ln A.
What is the significance of the Arrhenius plot?
It allows determination of activation energy and frequency factor from experimental data.
