⇒ THE RATE OF CHEMICAL REACTIONS Rate of Reactions is affected by: Surface Area of a Solid Reactant Temperature Pressure of gaseous reactants Catalyst Concentration of a reactant in Solution DEFINITION OF A RATE OF A CHEMICAL REACTION; change in concentration of a substance time Unit: moldm-3s-1 NB: the use of square brackets denotes concentration e.g. [HCl] Measuring Rate of Reactions Some techniques for doing this are: 1.Gas syringe. 2.Volumetric analysis - samples are removed at regular intervals. 3.Measuring changes in pressure (for gas reactions) 4.Colorimeter 5.A conductivity meter The rate at a particular instant in a reaction can be determined by finding the gradient of a tangent on a concentration versus time curve at that time. The gradient at t=0 is called the initial rate and is used to calculate k. THE RATE EXPRESSION The rate equation, the rate constant and the orders of reaction can only be worked out experimentally. Consider the chemical equation A + B → products Rate expression: rate α [A]m[B]n GENERAL RATE EQUATION: rate = k [A]m[B]n The individual order is the power to which a concentration is raised in the rate The overall order of a reaction is the sum of all the individual orders in the rate equation eg = m + n k is the rate constant - it’s unique to each reaction and conditions, units depends on order of reaction. The rate constant, k, increases exponentially with increasing temperature.