3.2.2 (e) Measuring a Rate of Reaction
Syllabus
(e) the techniques and procedures used to investigate reaction rates including the measurement of mass, gas volumes and time
What does this mean?
The definition for rate of reaction is the change of in concentration of a reactant (or product) in a unit time.
In effect this means change of concentration/time.
So any method of following how a reactant/product is changing will do.
Gas collection
The most familiar is probably collecting gas.
And the best way to do this is to collect it in a gas syringe.
Collecting over water in a measuring cylinder or a burette is also possible but the limitations are that the scale is less easy to read upside down.
Importantly, if the gas is very soluble like Carbon Dioxide, then this method is inaccurate as not all the gas will make it into the cylinder - rendering your results worthless.
So this method would be best avoided if a gas-syringe is available.
Mass loss
Allowing the gas to escape and simply measuring the mass loss is a more accurate method.
The cotton wool "bung" is important as it allows gas out but traps any tiny droplets of water that might otherwise be thrown through the mouth of the conical flask when bubbles burst.
This means that only the gas-loss affects the mass-loss.
One advantage of this method is that it lends itself to data-logging which can collect data from a balance more accurately than you can.
But few balances have the option of data-logging ports.
It's important to realise that measuring a loss of mass will produce a different shaped graph to measuring a gain of gas.
It will produce the inverse graph.
This may seem like a problem but at any given time the tangents to the two lines should be the same.
Except that one will be positive (gas-collection)
And the other will be equal but negative (mass loss)
Other methods
Opacity
If your reaction does not make gas then you need to follow its progress some other way.
If it makes a solid then this will form in very small particles.
And these will slowly block out light until the reaction mixture becomes opaque.
If we change the concentration of reactant or temperature of the mixture this will affect the speed at which the mixture becomes opaque.
We can measure this with a simple cross on a piece of paper below the flask.
Staring at the cross and timing how long it takes to become invisible gives surprisingly good results.
We don't actually measure a quantity of reactant or product with this method but we can say that the amount of solid it takes to mask the cross is the same each time.
And that Rate ∝ 1/time - and we simply call the amount of solid needed 1 (because it is some constant we don't know)
A data-logger could be used to measure the intensity of light passing through the mixture.
Colorimetry
A light source passes through a filter to select the appropriate wavelength of light.
Some of which is then absorbed by the solution.
The amount of light absorbed depends on the concentration of the solution.
- This is compared to what is absorbed by pure water.
- As the reaction proceeds the concentration of the absorbing product increases and the absorbance increases with it.
pH
Some reactions produce acidic or alkaline products.
Some neutralise an acidic or alkaline reactant.
Either way the pH of the reaction mixture will change.
This can be easily followed with a pH meter.
And these are commonly attached to data-loggers.
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