Physical - Chemical Equilibria

Question 1

Give an example of a reversible reaction.

Answer 1

When you heat blue hydrated copper sulphate, it becomes white anhydrous copper sulphate as the water of crystallisation is driven off. The white copper sulphate returns to blue if you add water.

Question 2

What happens in a closed system? What is an equilibrium mixture?

Answer 2

As soon as the products are formed, they react together and form the reactants again, so that instead of reactants or products, you get a mixture of both. This mixture is called an equilibrium mixture if the concentrations of reactants and products stay constant.

As the reactants get used up, the forward reaction slows down. As more product is formed, the reserve reaction speeds up.

After a while, the forward reaction will be going at exactly the same rate as the backwards reaction so the amounts of reactants and products won’t be changing any more.

This is called dynamic equilibrium and can only happen in a closed system. The proportions and concentrations of all components remain constant.

Question 3

How is dynamic equilibrium achieved in the evaporation of water?

Answer 3

Physical process - evaporation of water.

In a closed container, eventually the rate of evaporation and the rate of condensation will become equal. The level of the liquid water will then stay exactly the same and so will the number of molecules in the vapour and in the liquid. The evaporation and condensation are still going on but at the same rate. This situation is called a dynamic equilibrium.

Question 4

What happens at equilibrium?

Answer 4

Both forward and backward reactions occur at the same rate so the concentrations of all the reactants and products remain constant.

Question 5

What conditions are needed for equilibrium?

Answer 5

• Equilibrium can only be reached in a closed system (one where the reactants and products can’t escape). The system does not have to be sealed.
• Equilibrium can be approached from either direction and the final equilibrium position will be the same (as long as conditions, such as temperature and pressure, stay the same).
• Equilibrium is a dynamic process. It is reached when the rates of two opposing processes, which are going on all the time, are the same.
• You know that equilibrium has been reached when the macroscopic properties of the system do not change with time. These are properties like density, concentration, colour, and pressure (properties that do not depend on the total quantity of matter).

Question 6

What is chemical equilibria?

Answer 6

The same principles for a physical change apply to chemical equilibria.

• Imagine starting with A and B only. At the start of the reaction, the forward reaction is fast because A and B are plentiful. There is no reverse reaction because there is no C and D.
• Then as the concentrations of C and D build up, the reverse reaction speeds up. At the same time, the concentrations of A and B decrease so the forward reaction slows down.
• A point is reached where exactly the same number of particles are changing from A + B to C + D as are changing from C + D to A + B. Equilibrium has been reached.

Question 7

Do the proportions of reactants and products have to be the same?

Answer 7

An equilibrium mixture can have any proportions of reactants and products. It is not necessarily half reactants and half products, though it could be. The proportions may be changed depending on the conditions of the reaction, such as temperature, pressure, and concentration. But at any given constant conditions, the proportions of reactants and products do not change.

Question 8

How do you increase the yield in an equilibrium mixture?

Answer 8

It is possible to change the proportion of reactants to products in an equilibrium mixture. In this way, you are able to obtain a greater yield of the products. This is called changing the position of equilibrium.

• If the proportion of products in the equilibrium mixture is to be increased, the equilibrium is moved to the right, or in the forward reaction.
• If the proportion of reactants in the equilibrium mixture is to be increased, the equilibrium is moved to the left, or in the backward direction.

You can often move the equilibrium position to the left or right by varying conditions like temperature, the concentration of species involved, or the pressure (in the case of reactions involving gases).

Question 9

What is Le Chatelier’s principle?

Answer 9

Le Chatelier’s principle is useful because it gives us a rule. It tells us whether the equilibrium moves to the right or to the left when the conditions of an equilibrium mixture are changed.

It states:
If a system at equilibrium is disturbed, the equilibrium moves in the direction that tends to reduce the disturbance.

If any factor is changed which affects the equilibrium mixture, the position of equilibrium will shift so as to oppose the change.

Le Chatelier’s principle does not tell us how far the equilibrium moves so you cannot predict the quantities involved.

Question 10

How does changing concentrations affect equilibrium?

Answer 10

If you increase the concentration of one of the reactants, Le Chatelier’s principle says that the equilibrium will shift in the direction that tends to reduce the concentration of this reactant.

You could add in more products or take away more reactants.

Question 11

How does changing the overall pressure affect equilibrium?

Answer 11

Pressure changes only affect reactions involving gases. Changing the overall pressure will only change the position of equilibrium of a gaseous reaction if there are a different number of molecules on either side of the equation.

Increasing the pressure of a gas means that there are more molecules of it in a given volume - it is equivalent to increasing the concentration of a solution.

If you increase the pressure on this system, Le Chatelier’s principle tells ut that the position of equilibrium will move to decrease the pressure.

Question 12

What does increasing the pressure have an effect on?

Answer 12

Increasing the pressure or decreasing the volume of a mixture of gases increases the concentration of all reactants and products by the same amount, not just one of them.

Question 13

If the number of moles of gases on both sides of the equation is the same, what effect does increasing the pressure have?

Answer 13

If there is the same number of moles of gases on both sides of the equation, then pressure has no effect on the equilibrium position.

The rate at which equilibrium is reached will be speeded up by increasing the pressure, as there will be more collisions in a given time.

Question 14

How does changing temperature affect equilibrium?

Answer 14

Reversible reactions that are exothermic (give out heat) in one direction are endothermic (take in heat) in the other direction. The size of the enthalpy is the same in both directions but the sign changes.

Question 15

What does move forwards and move backwards mean in regards to equilibrium?

Answer 15

• The term move forwards and move to the right mean the same thing in this context.
• This term move backwards and move to the left mean the same thing in this context.

Question 16

What effect does increasing the temperature have?

Answer 16

Le Chatelier’s principle tells us that if you increase the temperature, the equilibrium moves in the direction that cools the system down. To do this it will move in the direction which absorbs heat (favouring the endothermic - positive enthalpy change - reaction).

Question 17

What effect does decreasing the temperature have?

Answer 17

Le Chatelier’s principle tells us that if you decrease the temperature, the equilibrium moves in the direction that counters that change. To do this it will move in the direction which releases heat (favouring the exothermic - negative enthalpy change - reaction).

Question 18

What effect do catalysts have on equilibrium?

Answer 18

Catalysts have no effect on the amount of yield produced or the position of equilibrium so they do not alter the composition of the equilibrium mixture. They work by producing an alternative route for the reaction, which has a lower activation energy of the reaction. This affects the forwards and back reactions equally.

Although catalysts have no effect on the position of equilibrium (the yield of the reaction), they do allow equilibrium to be reached more quickly and are therefore important in industry.

Question 19

Which type of equilibria does Le Chatelier’s principle apply to?

Answer 19

Homogeneous equilibria - reactions where every species is in the same physical state.

Question 20

What is the equilibrium constant Kc?

Answer 20

For any reaction that reaches an equilibrium, we can write the equation in the form:

aA + bB +cC xX + yY + zZ

Then the expression:

[X}^x[Y]^y[X]^x
___________
[A]^a[B]^b[C]^c

is constant, provided the temperature is constant. We call this constant, Kc. This expression can be applied to any reversible reaction. Kc is called the equilibrium constant and is different for different reactions. It changes with temperature.

Question 21

How do you work out the units for Kc?

Answer 21

By cancelling out the units for each term.

Question 22

What does the subscript eqm mean?

Answer 22

That the concentrations have been measured when equilibrium has been reached.

Question 23

What does the equilibrium constant indicate?

Answer 23

Kc indicates the position of equilibrium for a reaction at a certain temperature.

Question 24

Under what conditions does the equilibrium constant change value?

Answer 24

Kc for a reaction always has the same value unless the reaction conditions and therefore the position of equilibrium is changed.

The value of Kc is not affected by concentration change or use of a catalyst, however, it is affected by changing the reaction temperature as the equilibrium position change results in different concentrations of reactants and products. Whether Kc increases or decreases depends on whether the reaction is exothermic or endothermic.

Question 25

How do you find the value of Kc?

Answer 25

In order to find the value of Kc, the concentrations of the substances in the reaction at the equilibrium position needs to be known. It is equal to the concentration of the products divided by the concentration of the reactants. Any variation in the number of moles raises that substance to a power with the same value as the number of moles.

Question 26

How does equilibria change when the temperature decreases in an endothermic reaction?

Answer 26

type of reaction: endothermic
temperature change: decrease
effect on Kc: decrease
effect on products: decrease
effect on reactants: increase
direction of change of equilibrium: moves left

Question 27

How does equilibria change when the temperature increases in an endothermic reaction?

Answer 27

type of reaction: endothermic
temperature change: increase
effect on Kc: increase
effect on products: increase
effect on reactants: decrease
direction of change of equilibrium: moves right

Question 28

How does equilibria change when the temperature increases in an exothermic reaction?

Answer 28

type of reaction: exothermic
temperature change: increase
effect on Kc: decrease
effect on products: decrease
effect on reactants: increase
direction of change of equilibrium: moves left

Question 29

How does equilibria change when the temperature decreases in an exothermic reaction?

Answer 29

type of reaction: exothermic
temperature change: decrease
effect on Kc: increase
effect on products: increase
effect on reactants: decrease
direction of change of equilibrium: moves right

Question 30

What happens when Kc increases and decreases?

Answer 30

If the equilibrium constant Kc increases in value, the equilibrium moves to the right (forward direction) to produce more products.

If it decreases in value, the equilibrium moves to the left (backward direction) to produce less product.

Question 31

What is the general rule linking the type of reaction and the equilibrium constant?

Answer 31

• For an exothermic reaction (enthalpy is negative), increasing the temperature decreases the equilibrium constant, moving the equilibrium to the left.
• For an endothermic reaction (enthalpy is positive), increasing the temperature increases the equilibrium constant, moving the equilibrium to the right.

Question 32

How can the size of the equilibrium constant Kc tell us about the composition of the equilibrium mixture?

Answer 32

The equilibrium expression is always (products / reactants).

• If Kc is much greater than 1, products predominate over reactants and the equilibrium position is over to the right.
• If Kc is much less than 1, reactants predominate and the equilibrium position is over the left.

Question 33

What happens when the equilibrium constant is greater than 10^10 or less than 10^-10?

Answer 33

Reactions where the equilibrium constant is greater than 10^10 are usually regarded as going to completion, whilst those with an equilibrium constant of less than 10^-10 are regarded as not taking place at all.

Question 34

How do catalysts affect the value of Kc?

Answer 34

Catalysts have no effect on the value of Kc and therefore the position of equilibrium. This is because they affect the rates of both forward and back reactions equally. They do this by reducing the activation energy for the reactions.

They do however affect the rate at which equilibrium is attained (reached faster) - this is important in industrial processes.

Question 35

What is gaseous equilibria?

Answer 35

Reversible reactions may take place in the gas phase as well as in solution. These include many reactions of industrial importance such as the manufacture of ammonia by the Haber process and a key stage of the Contact process for making sulphuric acid. Gaseous equilibria also obey the equilibrium law, but usually their concentrations are expressed in a different way using partial pressures rather than concentrations.