Obtaining Rate Data (Edexcel International A Level Chemistry): Revision Note
Obtaining Rate Data
To measure the rate of a reaction, we need to be able to measure either how quickly the reactants are used up or how quickly the products are formed
The method used for measuring depends on the substances involved
There are a number of ways to measure a reaction rate in the lab; they all depend on some property that changes during the course of the reaction
That property is taken to be proportional to the concentration of the reactant or product, e.g., colour, mass, volume
Some reaction rates can be measured as the reaction proceeds (this generates more data);
faster reactions can be easier to measure when the reaction is over, by averaging a collected measurement over the course of the reaction
Some commonly used techniques are:
titration
colorimetry
mass loss
gas production
Changes in colour
A colorimeter measures the amount of light that passes through a solution
Colorimetry Set Up
If a solution changes colour during a reaction this can be used to measure the rate
The intensity of light reaching the detector is measured every few seconds and the data is plotted to show how the concentration of the reactants or products changes with time
The light intensity is related to the concentration, so the graph represents a graph of concentration of products or reactants against time
Sketch graph of colour intensity against time (the coloured species is a reactant in this case)
Note that colorimetry cannot be used to monitor the formation of coloured precipitates as the light will be scattered or blocked by the precipitate
Changes in mass
When a gas is produced in a reaction it usually escapes from the reaction vessel, so the mass decreases
This can be used to measure the rate of reaction
For example, the reaction of calcium carbonate with hydrochloric acid produces CO2
The mass is measured every few seconds and change in mass over time is plotted as the CO2 escapes
Measuring changes in mass using a balance
The mass loss provides a measure of the amount of reactant, so the graph is the same as a graph of amount of reactant against time
Mass loss of a product against time
However, one limitation of this method is the gas must be sufficiently dense or the change in mass is too small to measure on a 2 or 3 dp balance
So carbon dioxide would be suitable (Mr = 44) but hydrogen would not (Mr = 2)
Volumes of gases
When a gas is produced in a reaction, it can be trapped and its volume measured over time
This can be used to measure the rate of reaction.
For example, the reaction of magnesium with hydrochloric acid produces hydrogen
Collecting gases experimental set up
An alternative gas collection set up involves collecting a gas through water using an inverted measuring cylinder ( as long as the gas is not water soluble)
Alternative gas collection set up
The volume can be measured every few seconds and plotted to show how the volume of gas varies with time
The volume provides a measure of the amount of product, so the graph is a graph of amount of product against time
Graph of gas volume evolved against time
Measuring concentration changes
Measuring concentration changes during a reaction is not easy; the act of taking a sample and analysing it by titration can affect the rate of reaction (unless the reaction is deliberately stopped- this is called quenching).
Often it is more convenient to ‘stop the clock’ when a specific (visible) point in the reaction is reached
For example when a piece of magnesium dissolves completely in hydrochloric acid
Another common rate experiment is the reaction between sodium thiosulfate and hydrochloric acid which slowly produces a yellow precipitate of sulfur that obscures a cross when viewed through the solution:
Na2S2O3 (aq) + 2HCl (aq) → 2NaCl aq) + SO2 (g) + H2O (l) + S(s)
The disappearing cross experiment
The main limitation here is that often it only generates one piece of data for analysis
Examiner Tips and Tricks
You should be familiar with the interpretation of graphs of changes in concentration, volume or mass against time and be able to calculate a rate from a tangent to the graph
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