Edexcel Unit 4 Chemistry - Rates - Experimental Methods

Edexcel Unit 4 Chemistry - Rates

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Using Titration to Determine the Rate of a Reactio

Measure out samples to the reactants with known concentrations. Mix them together, start the clock and stir the mixture. At set time intervals take out samples of the reacting mixture and quench.

A mixture can be quenched by adding the solution to ice cold water, or adding the mixture to a solution which will only react with one of the reactants, to prevent further reaction.

Titrate each quenched sample against a standard solution.

The titre is proportional to the concentration of the reactant or product being titrated.

This method can be used when one of the following is a reactant or product:

-acid, as it can be titrated with a standard alkali

-alkali, as it can be titrated with a standard acid

-iodine, as it can be titrated with sodium thiosulphate

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Using Colorimetry to Determine the Rate of a React

If a reactant or product is coloured, the concentration of the coloured species can be measured by a spectrophotometer. This measures the amount of light of a set frequency that is absorbed, as this varies with the concentration of the coloured substance.

After mixing the reactants, the light absorbed is measured at set time intervals.

This will work with reactions such as bromine with methanoic acid, as bromine is orange brown in aqueous solution, and it fades to colourless, as the bromine becomes bromide ions.

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Using Infrared Spectroscopy to Determine rate of R

This works in a similar fashion to colourimetry. The concentration of a reactant or product by measuring the amount of infrared radiation absorbed at a set frequency at regular time intervals.

Oxidation of a secondary alcohol can be measured this way, because the C=O bond is formed over time, and as it increases in concentration, because more of the ketone is being formed, the amount of infrared absorbed at 1700 (the absorbtion frequency due to the stretching of the C=O bond) increases.

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Using Polarimetry to Determine the Rate of a React

When a set of reactants have different optical activity, the reaction can be followed by measuring how much the plane of polarisation of plane polarised light is rotated.

The reaction mixture is placed in a cell in the polarimeter, and the angle of rotation is measured at set time intervals. the angle of rotation is proportional to the concentration of the optically active substance. 

For example, when one of the optical isomers of 2-iodobutane is mixed with aqueous sodium hydroxide solution, hydrolysis occurs, and the product is a racemic mixture, the angle of rotation will gradually decrease to zero.

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Using Volume of Gas Evolved to Determine Rate of R

If a reaction produces a gas, the rate of reaction can be determined by measuring the amount of gas produced at set time intervals. The volume of gas is proportional to the moles of gas, and can be used to measure the concentration of the product.

This can be done by connecting the reaction mixture of a glass syringe, which fills with gas as the reaction progresses. The volume

Or, you can measure the loss of mass as the gas produced escapes from the solution. the reactants are placed in a flask on a balance, and the mass is measured at regular time intervals.However, with this method, the changes in mass are very small, so the percentage error will be high.

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Using pH measurements to Determine the Rate of a R

If one of the reactants or products is an acid or alkali, the change in pH with time can be measured. 

However, this causes problems, because pH is on a logarithmic scale, so there may be little change in a pH when infact a large proportion of the reactants have reacted. A very accurate pH meter would be needed to monitor the change in concentration.

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