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4. More examples of graphs from 'rate of reaction' experiments

GCSE level Chemistry: More examples of graphical analysis from rate of reaction experiments

Doc Brown's Chemistry KS4 science GCSE/IGCSE Revision Notes - Factors affecting the Speed-Rates of Chemical Reactions Doc Brown's chemistry revision notes: basic school chemistry science GCSE chemistry, IGCSE  chemistry, O level & ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old science students for national examinations in chemistry

Rates of reaction notes INDEX

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4. More examples of interpreting graphical results ('graphing'!)

PLOTTING GRAPHS - PLOTS OF GRAPHS OF DATA AND HOW TO INTERPRET THEM

BUT first, a more detailed look at the single graph shown below

The graph below illustrates how to measure the rate of reaction at a specific time by constructing a tangent through that specific time/

Imagine a reaction evolving a gas, and you measure the volume of the product gas at regular time intervals.

The rate at which the gas is evolved is the measure of the speed of the reaction.

The vertical y scale is the gas volume (2 mm = 1 cm3, 2 mm squares).

The horizontal x scale is time (1 mm = 1 s, 2 mm squares).

You plot the points (deliberately omitted here) and join them up with the 'best curve' as shown!

The diagram shows how to measure and calculate the initial rate at the start of the reaction in cm3/s from the gradient of the first purple triangle on the left at the start of the reaction. Calculation on diagram.

In this case the initial rate of gas evolution is 20 cm3 / 10 s = 2.0 cm3/s

Then, if you wish, to know the speed at any other point, you must draw a tangent at that specific time (e.g. 46 s), construct the second purple triangle to obtain the gradient, carefully read the graph scales and calculate the reaction rate e.g. in cm3/s.

The calculation on diagram for the rate of gas production is:

(39 - 23) cm3 / (74 - 16) s = 16 / 58 = 0.28 cm3/s

Note how the gradient gets smaller and smaller as the reaction proceeds and eventually falls to zero, when the reaction stops, due to one of the reactants being all used up.

It is easy to calculate an average speed over a specific period of time e.g.

Over the first 20 seconds 21 cm3 of gas is evolved, average speed = 21/20 = 1.05 cm3/s

BUT, this is not as accurate as the other initial rate calculation above, which gives nearly twice the speed calculated here (2.0 cm3/s).

The reason average rates are not as accurate is due the speed of reaction constantly changing, as it decreases as the reactant are used up e.g. decrease in concentration or surface area of a solid reactant.

(i) rate of reaction = speed, (ii) see other introductory graphs and notes at the start of this topic

(ii) Graphs 4.1, 4.2 and 4.5 just show the theoretical shape of a graph for a single particular experiment. Graphs 4.3 and 4.4 (temperature), 4.6 and 4.7 (concentration) and 4.8 (several factors illustrated) shows the effect of changing a variable on the rate of the reaction and hence the relative change in the curve-shape of the graph line.

(iii) The rate of reaction may be expressed as the reciprocal of the reaction time (1/time) e.g. for the

time for sulfur formation (to obscure the X)  in the sodium thiosulfate - hydrochloric acid reaction

or where a fixed volume of gas is formed, though in this can also be expressed as gas volume/time too as cm3/s or cm3/min even though the gas volume is the same for a given set of results of changing one variable whether it be concentration or temperature.

If you have detailed data e.g. multiple gas volume readings versus time, the best method for rate analysis is the on the introduction page.

(iv) for detailed observations of gas versus time see individual factor pages, and I've added new data tables and graphs to them, but I've retained the 'simplified graphs' below.

What next? Associated Pages

GCSE Level (~US grade 8-10) School Chemistry Notes (students age ~14-16)

GCSE level 'Rates of Reaction' multiple choice quiz

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