Rate Equations Flashcards

Interactive active recall cards for rate expressions, reaction orders, mechanisms, and the Arrhenius equation.

Rate Expressions & Orders: read all 7 questions and answers

What is a rate equation (rate expression)?
An equation that relates the rate of a chemical reaction to the concentration of reactants raised to powers equal to their order of reaction.
Define order of reaction with respect to a reactant.
The power to which the concentration of that particular reactant is raised in the rate equation.
What is the overall order of reaction?
The sum of the individual orders of reaction for all the reactants in the rate equation.
Write the general rate equation for a reaction with reactants A and B.
\(\text{Rate} = k[\text{A}]^x[\text{B}]^y\), where \(k\) is the rate constant, and \(x\) and \(y\) are the orders of reaction with respect to A and B.
State the units of the rate constant (k) for a zero-order reaction.
mol dm⁻³ s⁻¹.
State the units of the rate constant (k) for a first-order reaction.
s⁻¹ (per second).
State the units of the rate constant (k) for a second-order reaction.
mol⁻¹ dm³ s⁻¹.

Determining Orders & Mechanisms: read all 6 questions and answers

What is the rate-determining step?
The slowest step in a multi-step reaction mechanism, which determines the overall rate of the reaction.
How does a reactant's order in the rate equation relate to the rate-determining step?
Any species that appears in the rate equation must be present in the rate-determining step (or in a rapid equilibrium step preceding it).
If doubling [A] quadruples the rate, what is the order with respect to A?
Second order (since 2² = 4).
If doubling [B] has no effect on the rate, what is the order with respect to B?
Zero order (since 2⁰ = 1).
What is the initial rates method?
A method to find reaction orders by measuring the initial rate of reaction in several experiments where the starting concentration of one reactant is varied while keeping others constant.
How does temperature affect the rate constant (k)?
The rate constant (k) increases exponentially as temperature increases because a larger fraction of molecules have energy greater than the activation energy.

The Arrhenius Equation: read all 6 questions and answers

State the Arrhenius equation and define its terms.
\(k = A e^{-E_a / RT}\), where \(k\) is the rate constant, \(A\) is the pre-exponential factor, \(E_a\) is activation energy (J mol⁻¹), \(R\) is the gas constant (8.314 J K⁻¹ mol⁻¹), and \(T\) is temperature (K).
Write the logarithmic form of the Arrhenius equation.
\(\ln k = -\frac{E_a}{RT} + \ln A\) (or \(\ln k = -\frac{E_a}{R}\left(\frac{1}{T}\right) + \ln A\)).
In a plot of ln k against 1/T, what does the gradient represent?
\(\text{Gradient} = -\frac{E_a}{R}\).
In a plot of ln k against 1/T, what does the y-intercept represent?
\(y\text{-intercept} = \ln A\).
What is the pre-exponential factor (A) in the Arrhenius equation?
A constant that represents the frequency of collisions with the correct orientation.
How do you calculate activation energy (Ea) from the gradient of an Arrhenius plot?
\(E_a = -\text{gradient} \times R\) (giving \(E_a\) in J mol⁻¹, which is then divided by 1000 to convert to kJ mol⁻¹).