Rate Equations and Chemical Kinetics

Flashcards covering the fundamentals of rate equations, experimental methods for determining rates, the Arrhenius equation, and reaction mechanisms.

Rate equation

A mathematical relationship that relates the rate of reaction to the concentration of the reactants, expressed as $r = k[A]^m[B]^n$.

$$r$$

The symbol used for the rate of reaction, typically measured in units of $mol\,dm^{-3}\,s^{-1}$.

Rate constant ($k$)

A coefficient in the rate equation that is independent of concentration and time but constant at a fixed temperature; its value increases as temperature increases.

$[A]$

Notation representing the concentration of reactant $A$, typically measured in units of $mol\,dm^{-3}$.

Reaction orders ($m, n$)

Integers, usually $0$, $1$, or $2$, worked out experimentally to show how the concentration of a reactant affects the rate; they are not the same as stoichiometric coefficients.

Total order

The sum of all individual reaction orders in the rate equation, calculated as $m + n$.

Zero order

A reaction where the concentration of a reactant has no effect on the rate, represented as $r = k[A]^0 = k$.

First order

A reaction where the rate is directly proportional to the concentration of a reactant, represented as $r = k[A]^1$.

Second order

A reaction where the rate is proportional to the square of the concentration of a reactant, represented as $r = k[A]^2$.

Continuous rate method

An experimental approach where one experiment is followed over time to record the change in concentration, with the gradient of the graph representing the rate of reaction.

Initial rate

The rate at the start of the reaction ($time = 0$) where the reaction is fastest; it is calculated from the gradient of a concentration-time graph at time zero.

Pseudo-zero order

A condition where a reactant is kept in large excess so its concentration remains virtually constant, appearing not to affect the reaction rate.

Clock reaction

An experiment where the initial rate is estimated by measuring the time ($t$) taken to reach a fixed concentration, often indicated by a sudden color change when a limited reactant runs out.

Arrhenius equation

An equation relating the rate constant $k$ to temperature and activation energy: $k = Ae^{-\frac{E_A}{RT}}$.

Activation energy ($E_A$)

The minimum energy required for a reaction to occur, which must be converted to $J\,mol^{-1}$ when used in the Arrhenius equation with $R = 8.31\,J\,K^{-1}\,mol^{-1}$.

Arrhenius constant ($A$)

Also known as the pre-exponential factor, this constant has the same units as the rate constant $k$.

Rate-determining step

The slowest step in a multi-step reaction mechanism which controls the overall rate of reaction.

Reaction intermediate

A species formed during a reaction mechanism step that does not appear in the overall balanced equation, such as $NO_3$ in the reaction between $NO_2$ and $CO$.

$$SN1$$

Substitution, Nucleophilic, 1 molecule in the rate-determining step; a mechanism where the rate depends only on the concentration of the halogenoalkane.

$$SN2$$

Substitution, Nucleophilic, 2 molecules in the rate-determining step; a one-step mechanism where the rate depends on the concentrations of both the halogenoalkane and the nucleophile.