Practical Work for Learning

The effect of concentration on the rate of a reaction © Nuffield Foundation 2013

Chemical reactions can be ... fast… © Nuffield Foundation 2013

…or slow.

Learning outcomes
You will be able to ...
• evaluate different models for explaining the rates of reactions • use experimental data to identify how the concentration of each reactant affects the rate of a reaction © Nuffield Foundation 2013

What you already know
Organise what you know about the reaction of marble chips with hydrochloric acid into three categories.
Practical techniques equipment and methods
Observations / data / summaries of data including from previous experimental work, e.g. graphs, or statements summarising a trend/pattern
Explanations
inferences, models, type of reaction, equations

© Nuffield Foundation 2013

What is rate of reaction?
In a chemical reaction, reactants are used up and products form.
A + B  products
In this reaction A and B are the reactants. They are used up.
Rate of reaction is the rate at which reactants are converted to products. © Nuffield Foundation 2013

Models of rate
Collision theory can make qualitative predictions.
It can predict how the rate may change with changing conditions – but not by how much.
A mathematical model can make quantitative predictions.
A rate equation is a mathematical model.
(This is different from the chemical equation for the reaction).
What is the relationship between rate and the concentration of acid in the reaction with marble chips?
© Nuffield Foundation 2013

A mathematical model
The rate could be proportional to the concentration of acid: rHCl ∝[HCl] or rHCl = k[HCl] rHCl = rate of change of concentration of HCl
(mol dm–3 s –1) k = a constant (the rate constant)
[HCl] = concentration of HCl (mol dm-3)
If this was the mathematical relationship between rate and concentration, what would the graph look like?

© Nuffield Foundation 2013

A mathematical model rHCl = k[HCl]
If the rate is proportional to the concentration of acid…
…the graph will look like this.
Double the concentration, and the rate doubles.

© Nuffield Foundation 2013

A mathematical model
The rate could be proportional to the square of the concentration of acid:

rHCl ∝[HCl]2 or rHCl = k[HCl]2
If this was the mathematical relationship between rate and concentration, what would the graph look like?
© Nuffield Foundation 2013

A mathematical model rHCl = k[HCl]2
If the rate is proportional to the square of the concentration of acid…
…the graph will look like this.
Double the concentration, and the rate goes up 4 times (22).
© Nuffield Foundation 2013

A mathematical model
Maybe the rate is constant and is not affected the concentration of acid at all:

rHCl = k or rHCl = k[HCl]0
If this was the mathematical relationship between rate and concentration, what would the graph look like?

© Nuffield Foundation 2013

A mathematical model rHCl = k

If the rate is not affected by the concentration of acid…
…the graph will look like this.
Double the concentration, and the rate stays exactly the same.
© Nuffield Foundation 2013

Rate and concentration
So there are three possible models: rHCl = k

This is called zero order ([HCl]0)

rHCl = k[HCl]

This is called first order ([HCl]1)

rHCl = k[HCl]2

This is called second order ([HCl]2)

To find the order of reaction for marble and acid we need to collect some data …
© Nuffield Foundation 2013

Rate and concentration
So, how can you measure rate?
Rate cannot be measured directly. So what can we measure?
• Time
• Concentration
• Volume of gas
• Change in mass
© Nuffield Foundation 2013

Rate and concentration
Rate of change of concentration cannot be measured directly.
It can be found by analysing measurements of concentration over time. How fast is the concentration falling at any one time? The gradient of the concentration–time graph tells us about the rate.
A steep gradient means a fast rate.
A shallow gradient means a slow rate.
© Nuffield