5.1.1 (h) Techniques to investigate reaction rates

Syllabus

the techniques and procedures used to investigate reaction rates by the initial rates method and by continuous monitoring, including use of colorimetry

What does this mean?

If you haven't already figured this out, the initial rates method looks at the effect of altering the initial conditions of a reaction and seeing what effect this has on the initial rate of reaction.

It is obvious that the initial rate of reaction is what we should use since the rate will change during all reactions.

Comparing initial rates is the only way to make a sensible comparison of the effect of any change of conditions.

This means comparing slopes of the Concentration-Time graphs at t= 0 seconds.

But this requires a method to follow the changes in concentration, preferably by continuously monitoring it.

There are various methods of doing this:

Colorimetry

If there is a change of colour during a reaction then colorimetry is a relatively easy way to log continuous data.

Light of one frequency is sent through the reaction mixture and detected at the other side with a photo-cell.

Some of this light will be absorbed and the amount of absorbance will depend on the contents of the reaction vessel.

Possibly the absorbance will increase with the falling concentration of colourful reactant.

Or the the absorbance will decrease with the rising concentration of a colourful product.

Either way it is important to calibrate the colorimeter - which is generally done by passing the same frequency of light through a reaction vessel containing nothing but water.

Even this won't transmit all the light since some will always be scattered.

pH and Conductivity

If there is a change of pH during a reaction (either neutralising a reactant, or forming an acidic or alkaline product) then the pH will constantly change.

Clearly we could follow this reaction with a pH meter attached to a data-logger.

But we could do much the same even when the pH does not change if there is a change in concentration of ions.

Any reaction in which more ions are produced increases the conductivity of the reaction mixture.

Any reaction in which ion concentrations decrease decreases the conductivity of the reaction mixture.

So we could follow the reaction with a conductivity meter and a data logger.

Mass Loss Method

You will be familiar with the idea of Mass-Loss for reactions that produce gases.

You'll probably have done the reaction with a stop-clock and an electronic balance.

This gives fairly good results and generally produces the kind of nice smooth curve that we would expect.

But humans can only collect data every few seconds at best.

For a better set of results we could use a data-logger attached to the balance to continuously gather the same data.

Which is also a good deal easier.

Collecting Gas

Just because you collected gas in a gas-syringe at GCSE does make this method invalid.

It is difficult to collect the data continuously but collecting data every few seconds still gives quite convincingly smoothly curved graphs.

But it would be difficult to use a data-logger to improve the method.

Sampling

It is possible to take samples of a reaction mixture at regular intervals and to study the concentrations of reactants or products.

The problem with this is that the reaction continues while you gather your sample and while you are measuring the concentrations.

This clearly affects the accuracy of the data gathered.

The best we can do is to dry to stop the reaction immediately by instantly cooling it to the point where the rate decreases to zero.

Or if an acid (or alkaline catalyst) was needed we could neutralise the catalyst to stop the reaction.

Even then the data is not being gathered continuously.

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