woodchurch science

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this podcast gives the user an introduction to year 11 chemistry coursework for GCSE.
the presentation lasts approximately 5 minutes.

1. what do we mean by rate and how is it measured?

The phrase rate of reaction means how fast is the reaction or the speed of the reaction. It can be measured as the rate of formation of product or the rate of removal of reactant. The speeds of reactions are varied.
for example:
(1) rusting is a slow reaction, you hardly see any change looking at it!
(2) the weathering of rocks is a very slow reaction.
(3) the fermentation of sugar to alcohol is quite slow but you can see the carbon dioxide bubbles forming in the froth!
(4) a fast reaction would be magnesium dissolving in hydrochloric acid or sodium with water.
(5) explosions and burning/combustion reactions would be described as very fast!

2. Why is rates of reaction knowledge important?

time is money in industry, the faster the reaction can be done, the more economic it is.
you need to know how long reactions are likely to take.
hence the great importance of catalysts e.g. transition metals or enzymes which reduce time and save money.

3. how do we measure rate?

a reaction will continue until one of the reactants is used up.
to measure the speed or rate of a reaction depends on what the reaction is, and if what is formed can be measured as the reaction proceeds?

experiment 1.
marble chips (calcium carbonate) reacting with hydrochloric acid.

here a gas is formed from the solid reacting with an acid, which can be collected in a gas syringe.
in this experiment carbon dioxide is formed and collected.
a graph of cm3 of gas versus time is plotted. the slope or gradient of the graph shows that the reaction is fast at the start, but if the reaction is allowed to go on, you can usually measure the final maximum volume of gas and the time at which the reaction stops. however, this is a very poor measure of rate, because the reaction just goes slower and slower as the reactant amounts or concentrations are decreasing.

the shape of the graph is also quite characteristic.
the reaction is fastest at the start when the reactants are at a maximum (i.e. the steepest slope). the gradient becomes progressively less as reactants are used up and the reaction slows down. finally the graph levels out when one of the reactants is used up and the reaction stops.
the amount of product depends on the amount of reactants used.
the initial rate of reaction is obtained by measuring the gradient at the start of the reaction.
a tangent line is drawn through the first part of the graph, which is usually reasonably linear from the origin.
this gives you an initial rate of reaction in cm3 gas per minute.

the rate of a reaction that produces a gas can also be measured by following the mass loss as the gas is formed and escapes from the reaction flask.
the method is ok for reactions producing carbon dioxide or oxygen, but not very accurate for reactions giving hydrogen (here the mass lost is too low for accuracy).

experiment 2.
sodium thio-sulphate and hydrochloric acid.

when sodium thio-sulphate reacts with acid, a yellow precipitate of sulphur is formed and forms the basis of a good project for assessment.
to follow this reaction in your investigation you can measure how long it takes for a certain amount of sulphur to form.
you do this by observing the reaction down through a conical flask, viewing a black cross on white paper.
the x is eventually obscured by the sulphur precipitate and the time is noted.

by using the same flask and paper with x drawn on it you can obtain a relative measure of the speed of the reaction in forming the same amount of sulphur.
the speed or rate of reaction can expressed as x amount of sulphur/time, so the rate is proportional to 1/time for a particular run of the experiment.
you can investigate the effects of
(i) the hydrochloric acid or sodium thiosulphate concentration
(ii) the temperature of the reactants.
to show the effects of changing one of the variables you can plot graphs of:
(i) reaction time versus temperature or concentration, or
(ii) rate of reaction (1/reaction time) versus temperature or concentration.

you could also measure the speed of this reaction by using a light gate to detect the precipitate formation. the system consists of a light beam emitter and sensor connected to a computer and the reaction vessel is placed between the emitter and sensor. the light reading falls as the sulphur precipitate forms.

thanks for listening - look out for more podcasts from woodchurch science

This entry was posted on Thursday, September 7th, 2006 at 6:36 pm and is filed under chemistry, coursework, ks4, podcasts, revision, warren, y11. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.