The Rate Equation

Vocabulary practice flashcards covering chemical kinetics, collision theory, rate equations, reaction orders, graphical analysis, and reaction mechanisms.

Activation Energy

The minimum amount of energy that reacting particles must possess before a collision can successfully lead to a chemical reaction.

Steric Effect

The requirement in collision theory that reacting particles must collide with a specific relative orientation to result in a reaction.

Rate of Reaction

How much the concentration of a reactant or product changes with time; described as the chemical equivalent of velocity.

Initial Rate

The rate of a chemical reaction at the very start ($t = 0$), determined from the slope or gradient of the tangent to the concentration-time curve at the beginning.

Rate Equation

An experimentally determined mathematical relationship that links the rate of reaction to the concentrations of the reactants (for example, $r = k[A][B]^2$).

Rate Constant ($k$)

The proportionality constant in the rate equation linking rate to reactant concentrations, whose numerical value and units depend on the overall order of the reaction.

Individual Order

The power to which a specific reactant's concentration is raised in the rate equation, indicating how much that reactant affects the rate.

Overall Order of Reaction

The sum of all the individual orders of each reactant appearing in the rate equation.

Zero Order

A reaction order with respect to a reactant where changing its concentration has no effect on the rate of reaction, producing a horizontal line when rate is plotted against concentration.

First Order

A reaction order where the rate is directly proportional to the concentration of the reactant ($rate \times 2$ when concentration is doubled), yielding a straight line with a constant gradient on a rate-concentration graph.

Second Order

A reaction order where the rate is proportional to the square of the reactant concentration ($rate \times 4$ when concentration is doubled), resulting in an upwardly sloping curve on a rate-concentration graph.

Initial Rates Method

A procedure for finding the rate equation by running a series of experiments at the same temperature, varying the concentration of one reactant while holding all others constant, and measuring the initial gradient of each concentration-time curve.

Half-Life ($t_{1/2}$)

The time required for the concentration of a reactant to drop to half of its initial value; in a first-order reaction, it is constant and independent of concentration, satisfying $k t_{1/2} = \text{loge } 2 = 0.693$.

Rate Determining Step

The slowest step in a multi-step reaction mechanism that governs the overall rate of the chemical process and determines which species appear in the rate equation.

Autocatalysis

A phenomenon where a chemical reaction accelerates for a period of time because one of the products formed acts as a catalyst (such as $Mn^{2+}(aq)$ catalyzing the reaction of $MnO_4^-(aq)$).

Molecularity

The total number of individual reacting particles (molecules, ions, or atoms) that take part in the rate-determining step of a reaction.