The effect of concentration on rate

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  • Created by: J.E.C.
  • Created on: 13-04-14 15:23

Measuring rate of reaction

Rate of reaction = rate at which the reactants are converted into products

Monitoring rate of reaction:

> Measure volumes of gases evolved eg collect CO2 in a syringe

> Measure mass changes eg record mass lost from reaction (keep flask on scales)

> pH measurement: monitor acid concentration as reaction proceeds

> Colorimetry: measure the change in colour of a reaction

Chemical analysis and titration

> Take samples of reaction mixture at regular intervals and stop reaction in the sample = quenching before analysis

> For acid-catalysed reactions - add reagent that will neutralise acid-catalyst

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Investigating how rate depends on concentration

Graph of volume of O2 produced against time:

> Graph is steep at first: when the concentration of reactants is high.

> Graph gradually flattens out: reactants used up - lower concentration - slower reaction

Initial rate = rate of reaction at the start = find this by drawing a tangent to the curve at t=0 and measure the gradient of the tangent

Rate = [A]m[B]n                          m & n are order of reaction with respect to A & B

Overall order of reaction =(m+n)

The rate constant,k, increases with increasing temperature

Half-life = time taken for the concentration of a reactant to decrease to one half of its intial value.

For a first order reaction, the half-life is constant, whatever the starting amount. eg Radioactive decay

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Finding the order of reaction

Can carry out several experiments and measure the initial rate, keeping the concentration of one of the reactants constant.

Or can carry out the experiment with an excess amount of the reactant so that over the course of the experiment, its concentration does not change significantly.

The progress curve method

Shows how the concentration of a reactant/product changes as the reaction proceeds.

Draw tangents to the curve at particular concentrations > gradient gives rate of reaction for that concentration > then find the order with the initial rates method.

Initial rates method: draw tangents at the origin of different progress curves > then draw graph of intial rates against concentration.

  > Straight line = first order reaction

  > If graph of intial rate against (concentration)² is a straight line = second order reaction

  > If rate does not change with changing concentration = zero order reaction

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Finding the order of reaction

Using the reciprocal of the reaction time as a measure of the rate

Find intial rate: how long the reaction takes to produce a small fixed amount of one of the products

Average rate is proportional to 1/reaction time

Plot graph of 1/t against concentration of that reactant = straight line = first order with respect to that reactant.

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Rate equations, mechanisms & rate-determining step

Rate-determining step = slowest step in the reaction

Substances that do not appear in rate equation do not appear in rate-determining step

Those that appear in the rate equation are involved in the rate-determining step

The order of the substances involved tells you the relative number of moles of each substance involved in the rate-determining step.

Different steps of a reaction can have different energy barriers so some occur more quickly than others.

In any reaction with several steps, the rate-determining step will always be the one with the highest activation enthalpy.

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