Equilibria And Reversible Reactions (T4) Flashcards
Describe the reversible reaction of heating copper (II) sulfate crystals..
CuSO4.5H2O(s) CuSO4(s) + 5H2O(l)
CuSO4(s) + 5H2O(l) CuSO4.5H2O(s)
If you heat blue copper (II) sulfate crystals gently, the blue crystals turn to a white powder and water is driven off.
Heating causes the crystals to lose their water of crystallisation, and white anhydrous copper (II) sulfate is formed.
Anhydrous copper (II) sulfate is used to test for the presence of wanter and if you add water to the white solid, it turns blue, and gets very warm.
The original change has been exactly reversed.
Describe the reversible reaction of heating ammonium chloride..
NH4Cl(s) NH3(g) + HCl(g)
NH3(g) + HCl(g) NH4Cl(s)
If you heat ammonium chloride, the white crystals disappear from the bottom of the tube and reappear further up.
Heating ammonium chloride splits it into the colourless gases ammonia and hydrogen chloride.
These gases recombine further up the tube, where it is cooler.
The reaction reverses when the conditions are changed from hot to cool.
Describe reversible reactions under closed conditions and how dynamic equilibrium is achieved (use the blue / yellow squares example)
‘Closed’ conditions means that no substances are added to the reaction mixture and no substances escape from it.
On the other hand, heat may be either given off or absorbed.
Imagine a substance that can exist in two forms, using a grid of squares as an example.
At first, the ‘substance’ is just a grid of blue squares, but after a while some of those squares have turned yellow.
As we are starting with a high concentration of blue squares, to begin with the rate of reaction at which they turn yellow will be relatively high in terms of the number of squares changing colour per second.
The number changing colour per second will fall as the blue squares are gradually used up.
But it is a reversibly reaction so the yellow squares can also turn back to blue again.
At the start, there are no yellow squares so the rate of change from yellow into blue is 0. As their number increases the rate at which they change to blue also increases.
Soon the rates of both reactions would become equal and blue squares are turning into yellow ones at the same rate that the yellow ones are turning blue.
This is an example of dynamic equilibrium.
Taking a general case where A and B react reversibly to give C and D, show the formula and explain the two different reactions..
A + 2B C + D
The reaction between A and B (left to right reaction) is known as the forward reaction.
The reaction between C and D (right to left reaction) is called the back reaction.
In short, what do we mean by the ‘position of equilibrium’?
It is a reference to the proportions of the various things in the equilibrium mixture.
A + 2B C + D
Taking the above equation as an example, if the equilibrium mixture contains high proportions of C and D, what would we say?
That the position of equilibrium lies towards C and D.
That the position of equilibrium lies to the right.
In a sentence what does Le Chatelier’s principle state?
If a chemical system at equilibrium experiences a change in temperature, concentration, volume or partial pressure then the equilibrium shifts to counteract the imposed change and a new equilibrium is established.
A(g) + 2B(g) C(g) + D(g)
Using the above equation as an example, what will happen according to Le Chatelier’s principle if you increase the pressure?
A(g) + 2B(g) C(g) + D(g)
If you increase the pressure, the reaction will respond by reducing it again.
It can reduce the pressure by producing fewer molecules to hit the walls of the container - in this case by creating more C and D.
What direction will increasing the pressure help the reaction go in?
Increasing the pressure will always help the reaction go in the direction which produces the smaller number of molecules.
If there are the same number of molecules on both sides of the equation and you change the pressure, what will happen to the position of equilibrium?
There will be no difference to the position of equilibrium.
A + 2B C + D
Using the above equation, explain what happens if:
- the forward reaction is exothermic
- you decrease the temperature
- you increase the temperature
A + 2B C + D
If the forward reaction is exothermic, the back reaction will be endothermic by exactly the same amount. This is shown in an equation by writing a negative sign in front of the quantity of heat energy.
If you decreased the temperature of the equilibrium, the reaction will respond in such a way as to increase the temperature again. If more C and D is produced, more heat is given out from the exothermic reaction and the extra heat will warm the reaction mixture again.
If you increase the temperature, the reaction will move to get rid of the extra heat by absorbing it in an endothermic change. In this case the back reaction would be favoured.
How does adding a catalyst affect the position of equilibrium?
It doesn’t. In a reversible reaction, it speeds up the forward and back reactions by the same proportion.
Describe an example using Nitrogen Dioxide that illustrates Le Chatelier’s principle, mentioning how changes in temperature and pressure affect it.
NO2 is a dark brown, poisonous gas.
It can join up in pairs (dimerise) to make moleculs of dinitrogen tetroxide (N2O4) which is colourless.
There is a dynamic equilibrium between the two forms:
2NO2(g) N2O4(g)
Enthalpy = -57 kj mol -1
If you increase the pressure, the equilibrium will move to reduce it again by producing fewer molecules. It will produce more dinitrogen tetroxide.
If you lower the pressure, the equilibrium will move to increase it again by producing more molecules.
If you decrease the temperature, the equilibrium will move to produce more heat. At a lower temperature there will be more dinitrogen tetroxide and the colour will fade.
If you increase the temperature, the equilibrium will move to lower it again by favouring the endothermic change. More nitrogen dioxide gas will be formed and the colour will darken.
How is the enthalpy change displayed when there is an exothermic reaction in the forward (left to right) reaction?
With a negative sign before it.
Eg: -57 kj mol-1
