Rate of Reactions Fast or Slow Reactions • Extremely slow reactions – Iron rusting – Limestone weathering • Extremely fast reactions – Explosion Measuring Rate of Reactions • Some rate of reactions have detectable change with respect to time • Changes that are observable like – When a volume of gas is given off – When this is a change in mass during the reaction – When there are temperature changes – When there are colour changes – When a precipitate forms – When there are pH changes Collision Theory • For a reaction to occur – The reacting particles must collide into each other – The reacting particles must possess enough activation energy • Once products are formed, effective collisions have occurred Factors affecting Rate of Reactions • Temperature of the substances used (reactants) • Concentration of the substances used (reactants) • Pressure on the reaction • Particle size (surface area) of the substances used (reactants) • Presence of catalyst Temperature • Rate of reaction increases with increasing temperature • High temperature, particles have greater heat energy • Particles move faster with greater kinetic energy • Leading to more collisions between particles • Increased probability of effective collision • Reactions take place faster • Speed of reaction doubles when the temperature rises by 10 °C Temperature Amt of product formed Higher temperature Lower temperature Time Concentration • Rate of reaction increases with increasing concentration • Higher concentration, more reacting particles are present • Greater probability of an effective collision • Faster rate of reaction Concentration – Same no. of moles Amt of product formed Higher concentration Lower concentration Time/s Concentration – Different no. of moles Amt of product formed Higher concentration Lower concentration Time/s Pressure • Rate of reaction increases with increasing pressure • Higher pressures, reacting particles are closer together • Increasing concentration per unit volume • Greater probability of an effective collision • Faster rate of reaction Pressure Amt of product formed Higher pressure Lower pressure Time/s Particle Size • Rate of reaction increases when particle size decreases • Smaller particles has greater surface area than larger particles of the same mass • Greater surface area for collision by another reacting particle • Greater probability of an effective collision • Faster rate of reaction Particle size Amt of product formed Smaller particle size Larger particle size Time/s Catalyst • Presence of catalyst increases rate of reaction • (Presence of inhibitors decreases rate of reaction) • Catalysts lower activation energy of reactants • Aids the formation of unstable intermediate products • Increases probability of formation of products • Faster rate of reaction Catalyst Amt of product formed Use of catalyst Absence of catalyst Time/s Catalyst Energy Absence of catalyst Ea Ea Use of catalyst Time/s Catalyst Definition: A substance which increases the rate of a chemical reaction by providing an alternative pathway with a lower activation energy but remains unchanged at the end of the reaction
