2.8d: demonstrate an understanding of the concept of activation energy and its qualitative relationship to the effect of temperature changes on the rate of reaction 2.8e: demonstrate an understanding of the role of catalysts in providing alternative reaction routes of lower activation energy and draw the reaction profile of a catalysed reaction including the energy level of the intermediate formed with the catalyst 2.8f: carry out simple experiments to demonstrate the factors that influence the rate of chemical reactions, eg the decomposition of hydrogen peroxide

Rate of reaction: the increase in concentration of products or the decrease in concentration of reactants per unit of time

• rate of reaction = change in concentration of reactant/time taken
• rate of reaction = change in concentration of product/time taken
• measured in mol dm-3 s-1

Measuring rate of reaction:

• change in volume of gas produced
• can be measured using a gas syringe, or displacement of water from an inverted measuring cylinder (cannot be used when the gas being collected is soluble in water, eg. CO2, HCl)
• change in mass
• change in absorption of light
• eg. I2(aq) + S2O32-(aq) → 2I-(aq) + S4O62-(aq)
• the iodine would start off as brown and gradually become paler until it was colourless, so the amount of light absorption would decrease
• the progression of the reaction and concentration of iodine could be measured in a colorimeter calibrated with known concentrations of iodine solution
• titration
• samples are withdrawn from the reaction at regular intervals and “quenched” with a suitable chemical, to stop the reaction instantly
• could be quenched with sodium carbonate (a mild base), cooled to a suitable temperature, or more solvent added (this only slows the reaction, it does not stop it)
• change in electrical conductivity
• eg. BrO3-(aq) + 5Br-(aq) + 6H+(aq) → 3Br2(g) + 3H2O(l)
• there are 12 ions on the reactant side and none on the product side, so electrical conductivity will decrease as the reaction progresses
• clock reactions
• eg. Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
• time (with a stopwatch) how long it takes for the magnesium to disappear
• eg. Na2S2O3(aq) + 2HCl(aq) → 2NaCl(aq) + SO2(aq) + H2O(l) + S(s)
• time (with a stopwatch) how long it takes for the solid formed to cover a cross drawn under the flask