Glossary
Elementary Reaction
A reaction that occurs in a single step, involving the collision of one or more molecules exactly as written in the stoichiometric equation for that step.
Example:
The collision of two NO molecules to form N₂O₂ is an elementary reaction if it happens in one direct step.
First Order
A reaction is first order with respect to a reactant if its rate is directly proportional to the concentration of that reactant (exponent of 1 in the rate law).
Example:
If a decomposition reaction's rate doubles when the reactant's concentration doubles, it is first order with respect to that reactant.
Half-life (t₁/₂)
The time required for the concentration of a reactant to decrease to half of its initial value. For first-order reactions, this value is constant.
Example:
If a radioactive isotope has a half-life of 10 years, then after 10 years, half of the original sample will have decayed.
Initial Rate
The instantaneous rate of a reaction at the very beginning of the reaction, before significant amounts of reactants have been consumed.
Example:
In an experiment to determine a rate law, the initial rate is measured by observing the change in concentration over the first few seconds.
Order (of the reaction)
The exponent to which a reactant's concentration is raised in the rate law, indicating how the reaction rate is affected by changes in that reactant's concentration. It is determined experimentally.
Example:
If doubling a reactant's concentration quadruples the rate, the order with respect to that reactant is 2.
Rate Constant (k)
A proportionality constant in the rate law that relates the reaction rate to the concentrations of reactants, specific to a given reaction at a particular temperature.
Example:
In the rate law R = k[X], a rate constant of 0.050 s⁻¹ means the reaction proceeds at 0.050 times the concentration of X.
Rate Law
A mathematical expression that describes how the rate of a chemical reaction depends on the concentration of its reactants.
Example:
For the reaction A + B → C, a common rate law might be R = k[A]¹[B]², indicating the reaction is first order in A and second order in B.
Reaction Mechanism
A series of elementary steps that describe the pathway by which an overall chemical reaction occurs, showing the sequence of bond breaking and forming.
Example:
The overall reaction 2NO + O₂ → 2NO₂ might proceed through a two-step reaction mechanism involving an intermediate like N₂O₂.
Reaction Rate
The speed at which reactants are consumed and products are formed in a chemical reaction, typically expressed as a change in concentration over time.
Example:
If a chemical process consumes 0.02 M of a reactant every second, its reaction rate is 0.02 M/s.
Second Order
A reaction is second order with respect to a reactant if its rate is proportional to the square of the concentration of that reactant (exponent of 2 in the rate law).
Example:
If a reaction's rate quadruples when a reactant's concentration doubles, it is second order with respect to that reactant.
Third Order
A reaction is third order with respect to a reactant if its rate is proportional to the cube of the concentration of that reactant (exponent of 3 in the rate law).
Example:
If a reaction's rate octuples when a reactant's concentration doubles, it is third order with respect to that reactant.