A2 Chemistry Definitions

Topics: PH, Acid dissociation constant, Acid Pages: 3 (744 words) Published: April 9, 2011
-1A2 CHEMISTRY MODULE 4 definitions (aqa)

Rate of Reaction
The rate of change of concentration with time (mathematically d[X]/dt). Units are mol.dm-3.s-1 (moles per cubic decimetre per second) Initial Rate
This is the rate at the very beginning of the reaction when all the starting concentrations are known exactly. Rate Equation
Rate Expression
A mathematical expression which relates the rate to the concentrations of substances involved in the reaction Rate Constant, k
This is the multiplying constant in the rate equation, e.g., Rate = k [A].[B]2
It has a constant numerical value as long as temperature does not change. Rate constants increase as temperature increases. Order of Reaction
This is the numerical power to which the concentration of a substance is raised in the rate equation. The overall order is the sum of all the individual powers, e.g., Rate = k [A].[B]2 is 1st order with respect to A, 2nd order with respect to B and 3rd order overall

Homogeneous system
A chemical system where all components in the mixture are in the same phase, e.g., all liquids or all gases Heterogeneous system
A chemical system where components in the mixture are in different phases. Equilibrium Constant, KC
This is the ratio of the product concentrations raised to the powers of their respective stoichiometric coefficients to the reactant concentrations raised to the powers of their respective stoichiometric coefficients. Stoichiometric coefficients are the balancing numbers in the chemical equation. Concentrations must be in units of mol.dm-3 The numerical value of KC changes only with temperature.

Equilibrium Constant, KP
This is similar to KC, except that partial pressures are used instead of concentrations. This applies only for gases (since vapour pressures of solids and liquids are negligible compared to gases). The numerical value of KP changes only with temperature.

Partial Pressure
The sum of the individual partial pressures of gaseous...
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