RATES OF REACTION Rates of Reaction The rate of a chemical reaction is the speed at which the reaction occurs (i.e. speed at which the reactants are used or products are produced). Rate can be measured by the: 1) change in mass of reactants or products. 2) change in pH 3) change in conductivity (ion production) 4) change in colour (intensity of colour) 5) change in temperature 6) production of a gas The factors that will affect the rate of a reaction are: 1) Temperature 2) Surface Area
3) Concentration 4) Catalyst 5) Chemical Nature of Reactants 6) State of the Reactants Collision Theory The KINETIC MOLECULAR THEORY may be used to explain how the various factors will affect the rate of a chemical reaction. As the particles in the reactants move around, they collide with each other. Most collisions do not result in anything but a few will cause the bonds in the existing molecules to break apart and new bonds will form to make new molecules.
Collisions that result in the formation of products are called EFFECTIVE COLLISIONS. The idea of effective collisions is called the COLLISION MODEL and states that the rate of a reaction is affected by the number of effective collisions between reactant molecules. According to the Collision Model, the rate of a reaction may be increased by increasing the number of total collisions or by increasing the number of effective collisions by decreasing the activation energy.
The Factors Affecting Rate of Reaction and the Collision Theory 1) Concentration (Pressure of Gases) The greater the number of particles the greater the number of total collisions. Since the total number of collisions has increased the number of effective will also increase therefore the rate of the reaction will increase. NOTE: As a reaction proceeds, it tends to slow down. 2) Surface Area (Size of Solid Particles) If the size of a solid particle is decreased, there will be more surface area available for collisions to occur. 3) Temperature of Reactants The temperature of a substance is a measure of the average kinetic energy of the particles. The higher the temperature, the faster the particles are moving
which will increase the chance for collisions. 4) State or Phase of the Reactants Reactions where all reactants are in the same state (homogeneous reactions) will occur at a faster rate than reactions where reactants are in different states (heterogeneous). This is because the reactants will have a greater opportunity of colliding. The state of the reactants will also affect the rate. The relative rates are: Gases fastest Liquids/ Solutions fast Solids slow Note: Stirring increases the rate of reaction 5) Nature of Reactants The type of reactants will determine the activation energy needed to break bonds and form the intermediate state called the ACTIVATED COMPLEX. The higher the
activation energy, the slower the rate of reaction. Endothermic reactions are much slower than exothermic reactions because they tend to have higher activation energies. 6) Presence of a Catalyst A catalyst works by providing an alternative pathway for the reaction which has a lower activation energy. Lowering the activation energy will only increase the number of effective collisions. The number of total collisions will not be affected.
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