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Edexcel Chemistry – Topic 16: Kinetics II

Rates of Reaction

  • The rate of reacion is the change in concentration of a substance per unit timeand is measured in mol dm-3s-1
  • The initial rate of reaction is the rate at the start of the reaction where it is fastest
  • Reaction rates can be calculated from graphs of reactant or product concentration against time by calculating the gradient of the curve

The rate of reaction can be measured by:

  • Measuring a change in mass
  • Measuring a change in volume of gas produced
  • Measuring a change in electrical conductivity
  • Measuring a change in optical activity
  • Colorimetry
  • Titrating samples of reaction mixture

Measuring Changes in Masses and Volumes

Change in Mass

  • This works if there is a gas produced which is allowed to escape. (Works best if the gas is heavy e.g. CO2)
    HCOOH(aq) + Br2(aq) –> 2H+(aq) + 2Br-(aq) + CO2(g)

Change in Volume of Gas

  • This works if there is a change in the number of moles of gas in the reaction.
  • A gas syringe can be used to measure the change in volume of gas produced
    CaCO3(s) + 2 HCl(aq) –> CaCl2(aq) + CO2(g) + H2O(l)

Electrical Conductivity and Optical Activity

Electrical Conductivity

  • Can be used if there is a change in the number of ions present in a reaction

HCOOH(aq) + Br2(aq) –> 2H+(aq) + 2Br-(aq) + CO2(g)

Optical Activity

  • If there is a change in the optical activity (ability to rotate the plane of polarised of plane-polarised light) through the reaction, this could be measured in a polarimeter

CH3CHBrCH3(l) + OH-(aq) –> CH3CH(OH)CH3(l) + Br-(aq)

Colorimetry

  • If one of the reactants of products is coloured then colorimetry can be used to measure the change in colour of the reacting mixtures
  • The time it takes for a precipitate to mask a black cross can also be used to determine the rate of reaction

H2O2(aq) + 2I-(aq) + 2H+(aq) –> 2H2O(l) + I2(aq)

Titrating Reaction Samples

  • Samples of a reaction mixture can be titrated with acid, alkali or sodium thosulphate to determine the rate of reaction
  • The samples have to be quenched first to stop the reaction and then titrated with a suitable reagent

HCOOCH3(aq) + NaOH(aq) –> HCOONa(aq) + CH3OH(aq)
*The NaOH could be titrated with an acid (e.g. HCl)

BrO3-(aq) + 5Br-(aq) + 6H+(aq) –> 3Br2(aq) + 3H2O(l)
*The H+ could be titrated with an alkali (e.g. NaOH)

CH3COCH3(aq) + I2(aq) –> CH3COCH2I(aq)
*The I2 could be titrated with Sodium Thiosulphate

The Rate Equation and The Rate Constant

  • The rate equation relates mathematically the rate of reaction to the concentrationof the reactants
  • The unit for rate is mol dm-3 s-1
  • The rate constant (k) is independent of concentration and time. It is constant at a fixed temperature and increases when temperature increases
  • The units of k must depend on the overall order of reaction.

The Arrheinius Equation

  • Increasing the temperature causes the rate constant (k) to increase.
  • The relationship is given by the Arrheinius Equation
  • The Arrheinius Equation can be rearranged by taking natural logs to create a straight line graph: ln(k) = Constant – Ea/RT
  • k is proportional to the rate of reaction so ln(k) can be replaced by ln(rate)
  • Plotting a graph of ln(Rate) or ln(k) against 1/T (temperature) gives a straight line graph.
    The gradient of the graph is equal to the Activation Energy and is always negative.

Orders of Reaction

  • Orders of reaction are the index or exponent to which the concentration term in the rate equation is raised by.
  • Reaction orders can only be worked out by experiment
  • Orders have nothing to do with the stoichiometric coefficients in balanced equations
  • The total or overall order for a reaction is worked out by calculating the sum of all the orders of a reaction
  • ZERO ORDER means the concentration of a reactant has no effect on the rate of reaction (the rate remains constant)
  • FIRST ORDER means the rate of reaction is directly proportional to the concentration of a reactant (the half-life of the reaction is constant)
  • SECOND ORDER means the rate of reaction is proportional to the concentration of a reactant (the half-life of the reaction rapidly increases over time)
  • The unit of the Rate Constant (k) for a FIRST ORDER reaction is s-1
  • The unit of the Rate Constant (k) for a SECOND ORDER reaction is mol-1dm3 s-1
  • The unit of the Rate Constant (k) for a THIRD ORDER reaction is mol-2dm6 s-1

Clock Reactions

  • Clock reactions produce a sudden colour change after a certain time after a fixed amount of product has been produced
  • The indicator shows when X amount of a product has formed. The time taken for the colour change can be used to calculate an initial rate of reaction

When carrying out a clock reaction, the following assumptions are made:

  • The concentration of each reactant doesn’t change significantly over the time period of a clock reaction
  • The temperature remains constant
  • When an end point has been seen, the reaction hasn’t proceeded too far

Iodine Clock Reaction

H2O2(aq) + 2H+(aq) + 2I-(aq) –> I2(aq) + 2H2O(l)
2[S2O3]2-(aq) + I2(aq) –> 2I-(aq) + [S4O6]2-(aq)
* A small amount of Sodium Thiosulphate and Starch are added to act as an indicator as the Sodium Thiosulphate reacts with any Iodine that forms

  • Rinse a clean pipette with sulphuric acid
  • Transfer a small amount of sulphuric acid with a concentration of 0.25mol dm-3 to a clean beaker
  • Using a clean pipette, add distilled water to the beaker containing the sulphuric acid
  • Using a dropping pipette, add a few drops of starch solution to the same beaker
  • Measure a known amount of potassium iodide solution of known concentrations and pipette this into the beaker
  • Using a pipette rinsed with sodium thisulphate, add a sodium thiosulphate to the beaker and stir the solution so all the solutions are evenly mixed
  • Pipette hydrogen peroxide into the beaker, start a stopwatch and stir the contents
  • Continue to stir until the contents changes from colourless to blue-black and stop the stopwatch
  • Record this time along with all the volumes of reactants used

Mechanisms

  • A mechanism is a series of steps through which a reaction progresses, often forming intermediate compounds.
  • Each step can have a different rate of reaction. The stage with the slowest rate (the Rate-Determining Step) will control the overall rate of the equation
  • Nucleophilic Substitution can occur in two ways:
    SN1 – Tertiary Halogenoalkanes undergo SN1 as they form a more stable carbocation as the methyl group is electron releasing and stabilises the intermediate
    SN2 – Primary Halogenoalkanes undergo SN2 as there is a lot of space around the carbon the halogen is attached to meaning there is space for the hydroxide ion to attack