5 Kinetics

The Activation Energy is defined as the minimum energy which particles need to collide to start a reaction Maxwell Boltzmann Distribution The Maxwell-Boltzmann energy distribution shows the spread of energies that molecules of a gas or liquid have at a particular temperature Learn this curve carefully The energy distribution should go through the origin because there are no molecules with no energy The energy distribution should never meet the x axis, as there is no maximum energy for molecules The mean energy of the particles is not at the peak of the curve The area under the curve represents the total number of A few have low particles present energies because collisions cause some particles to slow down Only a few particles have energy greater than the EA Most molecules have energies between the two extremes but the distribution is not symmetrical (normal) Q. How can a reaction go to completion if few particles have energy greater than Ea? A. Particles can gain energy through collisions Emp this is the most probable energy (not the same as mean energy) Increasing Temperature As the temperature increases the distribution shifts towards having more molecules with higher energies The total area under the curve should remain constant because the total number of particles is constant At higher temperatures the molecules have a wider range of energies than at lower temperatures. At higher temps both the Emp and mean energy shift to high energy values although the number of molecules with those energies decrease 1.5 Kinetics Ea Collision energy
Fraction of molecules with energy higher temperature Lower temperature Ea Collision energy
Fraction of molecules with energy mean Collision energy
T= 25OC T= 1067OC Fraction of molecules with energy reactants products Activation Energy: EA ∆H Progress of Reaction Energy Reactions can only occur when collisions take place between particles having sufficient energy. The energy is usually needed to break the relevant bonds in one or either of the reactant molecules. This minimum energy is called the Activation Energy 1 Emp N Goalby chemrevise.org Measuring Reaction Rates The rate of reaction is defined as the change in concentration of a substance in unit time Its usual unit is mol dm-3s -1 When a graph of concentration of reactant is plotted vs time, the gradient of the curve is the rate of reaction. The initial rate is the rate at the start of the reaction where it is fastest Reaction rates can be calculated from graphs of concentration of reactants or products Effect of Increasing Concentration and Increasing Pressure At higher concentrations(and pressures) there are more particles per unit volume and so the particles collide with a greater frequency and there will be a higher frequency of effective collisions. If concentration increases, the shape of the energy distribution curves do not change (i.e. the peak is at the same energy) so the Emp and mean energy do not change They curves will be higher, and the area under the curves will be greater because there are more particles More molecules have energy > EA (although not a greater proportion) Comparing rate curves Need to calculate/ compare initial moles of reactants to distinguish between different finishing volumes. e.g. the amount of product is proportional to the moles of reactant Different volumes of the same initial concentrations will have the same initial rate (if other conditions are the same) but will end at different amounts The higher the concentration/ temperature/ surface area the faster the rate (steeper the gradient) In the experiment between sodium thiosulphate and hydrochloric acid we usually measure reaction rate as 1/time where the time is the time taken for a cross placed underneath the reaction mixture to disappear due to the cloudiness of the Sulphur . Na2S2O3 + 2HCl
2NaCl + SO2 + S + H2O This is an approximation for rate of reaction as it does not include concentration. We can use this because we can assume the amount of Sulphur produced is fixed and constant. Higher concentration Ea Collision energy
Number of molecules with energy lower concentration Initial rate = gradient of tangent time concentration Amount of product e.g. Volume of gas Time (secs) A B C D Note: If a question mentions a doubling of concentration/rate then make sure you mention double the number of particles per unit volume and double the frequency of effective collisions. N Goalby chemrevise.org 2 Effect of Increasing Surface area Increasing surface area will cause successful collisions to occur more frequently between the reactant particles and this increases the rate of the reaction. Effect of Catalysts Definition: Catalysts increase reaction rates without getting used up. Explanation: They do this by providing an alternative route or mechanism with a lower activation energy Comparison of the activation energies for an uncatalysed reaction and for the same reaction with a catalyst present. If the activation energy is lower, more particles will have energy > EA, so there will be a higher frequency of effective collisions. The reaction will be faster Effect of Increasing Temperature At higher temperatures the energy of the particles increases. They collide more frequently and more often with energy greater than the activation energy. More collisions result in a reaction As the temperature increases, the graph shows that a significantly bigger proportion of particles have energy greater than the activation energy, so the frequency of successful collisions increases higher temperature Lower temperature Ea Collision energy
Fraction of molecules with energy Ea catalysed Fraction of molecules with energy Collision energy
Ea un catalysed With a lower activation energy more particles have energy greater than the activation energy reactants Activation Energy: uncatalysed ∆H Progress of Reaction EA catalysed 3 products