IB Chemistry Power Points Topic 06 Kineticswww.pedagogics.ca Lesson Rates of Reaction Collision Theory
Rate of Reaction – Data analysis exerciseTechnically, the rate of a chemical reaction is ameasure of the change in [concentration] per unittime: [reactant] [product] rate = - rate = t t a decrease in [reactant] per unit time OR an increase in [product] per unit time
Measuring Rate of Reaction Rates of reaction can be determined in different ways. For example: 1. a decrease in mass can be measured if the reaction has a gas product 2.The volume of gas produced can be measured if the reaction has a gas product 3.absorption of light can be measured if there is a color change in the reaction mixture 4.can also measure changes in conductivity or pH
Rate of ReactionGraphs of concentration vs time are plottedto illustrate how reactions proceed.
For example: consider the following reaction A B time [A] rate = - t [B] rate = t
Rate of Reaction – Instantaneous vs Average Rates
Rate of Reaction – Instantaneous vs Average Rates An average rate would be determined by the change in concentration over a longer time interval. This is represented by the slope of the dashed line
Collision TheoryIf a reaction between the particles of two reactantsoccurs:- particles must collide with correct orientation (geometry)- there must be sufficient energy to initiate the reaction E>Ea NOT all collisions result in reactions!
Collision TheoryAccording to collision theory, increasing a reactionrate would result by increasing the number ofSUCCESSFUL collisions per unit of time.To increase a reaction rate- increase the frequency of collisions- increase the energy of collision
Factors affecting the rate of reactionThere are four factors that affect the rate of achemical reaction. Each effect can be explainedusing collision theory. The 4 factors are: Concentration Surface Area Temperature Catalysts
ConcentrationThe more concentrated the reactants are, themore particles there are per unit volume of thereaction medium (the mixture of the reactants).More particles means a greater collisionfrequency (more collisions per unit time) andtherefore a greater reaction rate.
When one reactant is in solid pieces, the other reaction can only occur at the surface. No reaction occursSurface AreaIf the solid pieces aresubdivided, there ismore surface area andmore collisions per unittime. Therefore thereaction occurs faster.
TemperatureIncreasing temperature increases the rate ofreaction for two reasons.1. frequency of collision increases because particles are moving faster.2. increase in energy which increases the proportion of successful collisions – a larger % of particles have E > Ea
Temperature - Maxwell Boltzmann curvesRecall, a temperature measurement representsthe average kinetic energy of a group ofmolecules. As temperature increases, the proportion of molecules with the required velocity (minimum energy) also increases
CatalystsCatalysts increase reaction rates by loweringactivation energy. When a catalyst is present,the proportion of successful collisions increasesbecause more molecules have sufficient energyto react.
Temperature, Concentration and Surface Areaeffects on reaction rates.