[INTRODUCTION]

Chemical equilibrium is of great importance both in the laboratory and for the chemical industry as it determines the maximum amount of product you can end up with for any reaction.
For example the reaction between hydrogen and nitrogen gases to produce ammonia for crop fertilizers.
For this reaction an understanding of chemical equilibrium aids the choice of reaction conditions at every stage. (thumbnail graphics of activities appear)
In this activity, you will investigate several aspects of chemical equilibrium.
In the first section, you'll see a simulation of molecules in a gaseous reaction and discover what equilibrium means at the molecular level.
In the second and third sections you'll investigate the mixture of compounds present at equilibrium.
The second section deals with the effect of changing pressure and the third with the effect of changing temperature on the composition of the mixture.
Finally we turn to equilibrium in solution.
In this section, you'll see how you can shift the balance between products and reactants by increasing the concentration of one of the species involved in the reaction.

[Forward reaction - introduction]
Second screen (bouncing particles, starting with reactants)
You are going to start by thinking about molecules reacting in the gas phase.
On the left of your screen is a box which you can think of as a container for gas molecules.
In the box are eight blue spheres representing molecules of a compound that we shall call reactant 1.
The molecules are moving in random directions, continually colliding with each other and the walls of the container.
In the panel on the right of the screen, you will find two boxes labelled plus and minus.
By clicking on these boxes you can adjust the speed of the molecules.
Alter the speed so you find it comfortable to view.
When you have done this you should click on the forward arrow at the bottom right hand corner of the screen to continue.

[Forward reaction - add molecules]

I now want you to investigate what happens when these molecules take part in a chemical reaction.
Firstly, click eight times on the box labelled reactant 2, to add eight molecules of the second reactant, represented by yellow spheres.
As you add molecules of reactant 2 you should notice that sometimes when a molecule of reactant 1, blue, collides with a molecule of reactant 2, yellow, reaction occurs.
In this case, two products are formed, product 1 and product 2 shown as red and green spheres.

[Forward reaction - build-up]

You'll see that as well as the reactants forming products, collisions between product molecules sometimes lead to formation of reactants.
You'll find it helpful to focus on the fate of a particular molecule as it collides and reacts.
A blue sphere, for example, will collide with a yellow sphere, producing a red and a green sphere.
The red sphere on collision with another green sphere will then give a blue sphere and a yellow sphere.

[Forward reaction - static?]

As you can see reaction is still occurring, but are the numbers of reactant and product molecules still changing?
Click on the stop button and count the number of molecules of product 1, the red ones.
Write down the number of product 1 molecules in a table in your notebook.
Then click on the start button to start the reaction again.

[Forward reaction - study file]

Now I want you to see if the number of product 1 molecules is changing with time by stopping and restarting the reaction ten times.
Each time note down, the number of molecules of product 1 as before.

[Forward reaction - summary]

You should have found that there were about four red spheres every time you stopped the reaction.
Sometimes you'll have found only three and sometimes five but the average was probably four.
A balance has been reached in which the rate at which reactant molecules form product is the same as the rate at which the product molecules react to form reactants.
Immediately following the reaction of two molecules, there may be a temporary increase in the number of product or reactant molecules, but this is soon balanced by the reaction of another pair of molecules.
In a real sample of gas there would be many billions of molecules and such fluctuations would not be noticed, but we can't fit billions of molecules onto the screen.
This state where reaction is continually occurring but the overall proportions of reactants and products remain fixed is chemical equilibrium.

[Back reaction - introduction]
Third screen (bouncing particles, starting with products)
Now I want you to look at the same reaction again but this time in reverse.
We start with eight red spheres representing product 1.
Click eight times on the box labelled product 2 to add eight molecules of product 2, the green spheres.
You now have the same total number of molecules as you had when you started with reactants. Observe what happens.

[Back reaction - comparison with forward reaction]

Now I want you to see if starting from the products has an effect on the number of each type of molecule present at equilibrium.
Stop and start the reaction a few times and each time count the number of product 1 molecules, red spheres, and make a note of the numbers again in your table.
Compare the numbers of product 1 molecules with the numbers that you counted earlier when you started the reaction with reactant molecules.
Click on the forward arrow when you are ready to continue.

[Back reaction - summary]

You've looked at one example of a reaction at chemical equilibrium under one set of conditions.
Would there still be the same number of product molecules at equilibrium if the pressure in the box was changed, for example by squashing it to reduce the volume?
In the next section you'll investigate the effect of changing pressure on the equilibrium mixture for several reactions.

[THE EFFECT OF PRESSURE]
Introduction
So far we have represented reactions with a snapshot view of a few molecules; you are now going to look at equilibrium mixtures on a laboratory or industrial scale.
We'll use pie-charts to illustrate the proportions of products and reactants at equilibrium and you will investigate the effect of changing pressure on these proportions as shown in the pie-chart for several chemical reactions.
Once you have investigated a few reactions you will answer a set of questions which will help you tie together the results of your investigations.
In the reaction on your screen, one mole of nitrogen, N2, reacts with three moles of hydrogen, H2, to form two moles of ammonia, NH3.
Nitrogen, hydrogen and ammonia are all present in the equilibrium mixture.
The proportion of each substance is given by the area of the corresponding coloured sector on the pie-chart - blue for nitrogen, yellow for hydrogen and red for ammonia.
What I want you to do now is to find out what happens when the pressure is increased.
On the right of your screen there are two boxes next to a pressure gauge labelled increase and decrease.
Click and hold on the increase box to increase the pressure and note whether the proportion of ammonia, represented by the area of the red sector increases or decreases.
Write your result again in a table in your notebook. Click on the forward arrow when you are ready to continue.

[Pressure - red sector became larger]
(after you click on the forward arrow that appears)
As you increased the pressure, the red sector became larger, that is, the proportion of ammonia increased.
The extra ammonia comes from the reaction of the nitrogen and hydrogen and so the blue and yellow sectors become smaller.
Now I'd like you to try out the effect of pressure on some other reactions.
Does the product always increase as you increase the pressure?
Are all reactions affected by changes in pressure?
Click on the change box under the current chemical equation and a menu will appear listing some other reactions that you can investigate.

[Pressure - three sets of reactions]
(when you view the list of reactions)
The reactions are divided into three sets labelled A, B and C.
To select a reaction you need to click on its chemical equation and then the 'done' box.
I want you to try another reaction from set A, two from set B and two from set C.
Each time record any changes in the proportion of products when you change the pressure by making a note of changes in the area of the red sector again in your table.
When you have completed all your observations click on the forward arrow to continue.

[Pressure notebook - introduction]
Notebook
This screen contains a record of which reactions you have studied.
For some of these reactions, the proportion of product increases with increasing pressure, for some the proportion of product decreases and for some it remains the same.
What causes these differing behaviours?
You are going to answer some questions that will help you sort this out.
These questions ask you to think about the change in the number of moles that takes place as a result of chemical reaction.
Since the pressure of a gas depends on the number of moles present, it is reasonable that the response to a change of pressure is connected to a change in the number of moles.
The answers to the questions will tell you what this connection is.
Look at your table of results and find a reaction for which the amount of product increased when you increased the pressure.
Then click on the chemical equation for that reaction and then the 'done' box to continue.

[Pressure notebook - fewer moles of product molecules]
(when you select an appropriate reaction)
For the reaction you just selected there were fewer moles of product molecules than of reactant molecules
and this led to an increase in the proportion of product when the pressure was increased.
This is true for all the reactions in set A. (when it presents the full list of reactions again)
Now select a reaction for which the amount of product decreased when you increased the pressure and click on its chemical equation as before.

[Pressure notebook - Le Chatelier's Principle]
(when you select an appropriate reaction)
The effect of changing pressure on the equilibrium mixture is an example of Le Chatelier's principle. Le Chatelier's principle states that:
"When a system in equilibrium is subject to an external constraint, the system responds in a way that tends to oppose the effect of this constraint"
In this case the system is the reaction and the external constraint is a change in pressure.
When you increase the pressure the reaction opposes the change by reducing the number of moles.
For example, if there are more moles of reactant than product, when the forward reaction occurs the number of moles decreases and hence the pressure is reduced.
If the pressure in the container is increased, for example by reducing the volume, more product will be formed to try and undo this increase.
A new state of equilibrium results in which there is some increase in pressure and an increase in the proportion of product. Summary
You have now completed this section on the effects of changing pressure. Click on the forward arrow to go to the next section where you will investigate the effects of changes in temperature.

[THE EFFECT OF TEMPERATURE]
Introduction
You are now going to investigate the effect of changing temperature on the composition of equilibrium mixtures.
We start again with the reaction of hydrogen and nitrogen to form ammonia.
This reaction is at equilibrium at the temperature shown on the thermometer.
As in the section on pressure, the areas of the sectors represent the proportions of the products and reactants present.
By clicking on the 'increase' and 'decrease' boxes next to the thermometer you can change the temperature.
I want you to change the temperature for this reaction now and watch how the proportion of product, ammonia, - the red sector - changes.
Note your results in a table in your notebook and then click on the 'change' box.

[Temperature - choose further reactions]
(when you view the list of reactions)
You are going to choose further reactions to study from the list on screen.
Note that there are only two sets this time, A and B.
You should study at least two reactions from set A and two from set B.
As before, you can select a reaction by clicking on its chemical equation.
For each reaction, follow the change in the proportion of a product - the red sector - as you raise the temperature.
Record your observations in your table and when you've completed all your investigations, click on the forward arrow to continue.

[Temperature notebook - introduction]
Notebook
This time we have given you thermochemical equations which tell you whether the reactions are exothermic or endothermic.
An exothermic reaction is one in which energy is released, or one in which heat exits the reaction.
An endothermic reaction is one in which energy is absorbed - or one in which heat enters the reaction.
Choose one of the reactions in set A that you have studied and click on the thermochemical equation.

[Temperature - summary]
Summary -
Well done, you have completed the section on the effect of temperature.
You have seen that for exothermic reactions, set A, with delta H negative, increasing the temperature reduces the proportion of product.
Heat is released when product are formed for these reactions so that when we put heat in to raise the temperature, more reactants are formed absorbing some of the heat.
For the endothermic reactions, with positive delta H values, forming products absorbs heat and so raising the temperature increases the proportion of product.

[THE EFFECT OF CONCENTRATION]
Introduction
In this final section, you are going to study equilibrium between ions in solution.
For reactions of gases, you studied the effect of changing pressure, but liquids are not very squashable and changing pressure has little effect on reactions in solution.
We can however increase the number of moles per unit volume by increasing the concentration,
and you are going to investigate the effect of increasing the concentration of both reactants and products on the equilibrium mixture.
There are two ways in which your investigations in this section differ from those for reactions of gases.
First, you are adding reactant or product to the solution so that the amount of material changes not just the proportions of reactants and products.
To reflect this we are using bar charts rather than a pie chart.
Secondly, and more importantly, you are going to explore a quantitative relationship between the concentrations of products and reactants at equilibrium.
You will find it much easier to see what this relationship is if you plot a graph.
The data for this graph will be generated from your results in the first part of this activity.
On the left of your screen is a set of bars whose lengths represent the equilibrium concentration for the ion indicated.
The chemical equation for the reaction is also given below in the box.
I want you to see what happens if you increase the concentration of one of the ions.
To do this you have to add a solid or an acid containing the ion.
Think for a second or two why we have not asked you just to add ions.
Well, you cannot get a bottle of, say, chloride ions; you have to add a solid that is electrically neutral.
So for chloride ions we ask you to add potassium chloride, KCl, and the yellow spot next to potassium chloride indicates that this is your source of chloride ions,
the concentration of chloride ions being given by the yellow bar. The chloride ions will react but what happens to the potassium ions?
When potassium chloride dissolves in water, the chloride ions and potassium ions act independently.
So while the chloride ions are taking part in the reaction shown, the potassium ions are just wandering about in the solution.
Ions such as the potassium ions which are present but do not play a part in the reaction are known as spectator ions.
Now, I want you to increase the concentration of a reactant ion and see what effect this has on the composition of the equilibrium mixture.
Click on the box next to one of the reactants indicated by blue or yellow.
Each click adds a set amount of that chemical.
Make sure that you make at least five additions of one of the reactant ions for this reaction.
Observe the changes in the concentrations of all the ions and then click on the forward arrow in the bottom right hand corner of the screen to continue.

[Concentration - products increase]

As you increase the concentration of reactant ion, the concentration of the products, the length of the red and green bars, increases.
This is another manifestation of Le Chatelier's principle.
The constraint is increasing the concentration of the reactant ion and to oppose this effect some of the excess reactant ions form products.
A new equilibrium mixture is then reached containing higher concentrations of product ions.
The concentration of the second reactant ion decreases because it is used up in forming products.
Now click on "reset" to restore the initial equilibrium mixture.

[Concentration - try other reactant]

I now want you to increase the concentration of the second reactant and show that this also increases the concentration of products.
When you have done this click on reset.

[Concentration - what about product ions?]

You have seen the effect of increasing reactant ion concentration,
what do you think would be the effect of increasing the product ion concentration?
Investigate this yourself by adding a compound containing a product ion.
Click on the forward arrow at the bottom right hand corner of the screen when you have done this.

[Concentration - reactants increase]

Increasing the concentration of a product ion leads to a new equilibrium mixture with increased concentrations of reactants.
The concentration of the other product ion decreases as it is used up in forming reactants.
To summarise the effects of increasing concentration:
If the concentration of a reactant is increased, then the concentrations of the products in the equilibrium mixture rise.
If the concentration of a product is increased, then the concentrations of the reactants in the equilibrium mixture rise.
Check for yourself that these conclusions hold for other reactions.
Click on the 'change' box under the present chemical equation to obtain the menu of available reactions.

[Concentration - select a reaction]

Reactions can be selected by clicking on the chemical equations.
Try increasing the concentrations of reactants and products for two or three different reactions.
Make sure that you choose at least one reaction with only one product and that you make at least five additions of one of the reactant ions for this reaction.
You will be using the results from this reaction to plot a graph in the next part of this activity.
When you have finished all your investigations, click the forward arrow at the bottom right hand corner of the screen to continue.

[Concentration - notebook]

On your screen is a list of the available reactions. Click on a reaction that only has one product.
On the screen you should have a table of data for the reaction you selected.
The numbers in this table are the concentrations in moles per decimetre cubed of the various ions in the equilibrium mixtures for the reaction you selected.
This particular set are the results you obtained when you increased the concentration of reactant 1.
Note that the numbers in the column headed reactant 1 increase as you go down the column.
The numbers in the reactant 2 column decrease as reactant 2 is used up and the numbers in the product 1 column increase as more product forms.
Although you can see these general trends by looking at the figures, you can't exactly see how the increases and decreases are related.
You need to plot a graph to see this and this is what you are going to do now.
To plot the graph, you first need to multiply together the entries in the two reactant ion columns.
Do this by pressing the calculate button in the column headed 'reactant 1 times reactant 2 slash moles squared decimetres to the minus 6'.

You now have a fourth column of data containing the product of the two reactant ion concentrations for each equilibrium mixture.
In the next column labelled 'product 1 times product 2 slash moles squared decimetres to the minus 3, press the calculate button.
This multiplies the entries in product columns, but here we only have one product so it just transfers the entries in that one column.
This is the case for this particular reaction but as you will see later if you continue with plotting graphs for other reactions from the menu,
some may have two products and two reactants, or even 1 reactant and 2 products, which are dealt with in the same way.

Now you are going to plot a graph that shows how the concentration of product varies as you increase the concentration of reactant.
The concentration of product is plotted on the vertical or y axis.
On the horizontal axis you will plot the new column you calculated where you multiplied the two reactants together.
To do this press the 'Graph' button at the bottom of the data table.

You should now have a graph with the concentration of product, labelled product, on the vertical axis
and the result of multiplying together the reactant concentrations, labelled reactants, on the horizontal axis.
Your points should lie on a straight line through the origin.
To see this get the computer to fit a straight line through your points by pressing the 'fit' button at the top of the graph.

A straight line through the origin like this means that the quantity you plotted on the vertical axis is proportional to the quantity on the horizontal axis.
That is the concentration of product is proportional to the concentrations of the reactants multiplied together. This is the relationship you are looking for.
The concentrations in an equilibrium mixture for this reaction will always obey this relationship.
The relationship can be written as an equation which can be used to work out concentrations of ions in solution.
The equation will have the form concentration of products equals a constant times concentration of reactant 1 times concentration of reactant 2.
The concentrations you have plotted all refer to equilibrium mixtures and so the constant in this equation is called the equilibrium constant.
This is given a special symbol, capital K.
The value of K, the equilibrium constant, for this reaction is the gradient of the line the computer fitted through the points of the graph.
The value of K is displayed near to the line on the screen, so now record this value in your notebook next to the appropriate chemical reaction.
There is a straight line relationship between the concentrations of reactants and products at equilibrium for every chemical reaction.
The exact equation for this relationship depends on the number of moles of reactant and product in the chemical equation.
For the reaction you studied, there was one product and two reactants. You had to multiply the concentrations of the reactants together.
If there had been two products you would have had to multiply their concentrations together.
In general you have to multiply the concentrations of all the reactants and the concentrations of all the products.
The gradient of the straight line is the equilibrium constant, K. The value of K that you recorded is for the reaction you chose only.
Every chemical reaction has its own equilibrium constant whose value is unique to that reaction.
You may finish this section here and go to the summary where the equation for the equilibrium constant for a reaction involving two reactants and two products is given.
If you have time you can go on and plot the graphs for some other reactions and go to the summary when you have finished these.
To finish now click on the forward arrow or press the done box on the graph to return to the data table to select another reaction.

Select another reaction by clicking the 'change' button under the current reaction equation at the bottom of the table.
This will give you the list of reactions again that you have investigated, click on one of the equations from the list to obtain the data table of the results.
If there are two reactants multiply their concentrations together using the calculate button in the reactants column as before.
If there are two products multiply their concentrations together likewise.
Again if you have only one reactant and two products then press the calculate in the 'reactants' column
which transfers the concentration numbers of the one reactant and then calculate the products as before to plot your new graph.
You should obtain a straight line through the origin, again using the fit button on the graph.
Read off the equilibrium constant for the reaction you have chosen and write this in your note book as before.
Repeat the process if you wish for other reactions in the menu and each time note the value of the equilibrium constant for that reaction.
When you have investigated as many reactions as you like click on the forward arrow to end this activity.