C5f Equilibria

In C1 you learnt that a chemical change is one in which a ________ substance is made and that it is an _____________________ change.

That’s not quite true, some changes are reversible.

In a reversible reaction there is a forward and a backward reaction.

e.g.

In some reversible reactions both forward and backward reactions are

proceeding at the same time.

If the forward reaction happens at the same __________ as the reverse

reaction, then we say the system has reached an _____________________.

(not all reversible reactions do this)

When a system is in equilibrium the _____________________ of the reactants and

products does not change (because the forward and reverse reactions happen at the

same rate)

Reactions that can reach an equilibrium are given a special sign:

A system in equilibrium does not necessarily have 50% reactants and 50%

products.

When the position of equilibrium is on the right the concentration of

__________ is greater than the concentration of ______________.

When the position of equilibrium is on the left the concentration of

___________ is greater than the concentration of _______________.

NOTE: To get a high yield of product the equilibrium needs to be as far towards the

________________ (right) as possible.

Higher

Why do reversible reactions sometimes reach an equilibrium:

Learn this

definition

Fizzy drinks are made fizzy by the dissolved CO2 in them. Consider what happens when you put the lid back on a bottle of drink after you have poured yourself a glass:

Very importantly the system is closed (nothing can get in or out).

1. The dissolved CO2 comes out of solution and into the air quickly at first but then

the rate _______________.

2. Some of the CO2 in the air starts to __________________ into the solution. The rate

_______________ as more CO2 is present in the air.

3. Eventually the forward reaction gets ___________ and the reverse gets _________

until the two reactions happen at the _________ rate.

Summary

• initially rate of forward reaction

decreases

• initially rate of backward reaction

increases

• eventually rate of forward equals rate

of backward reaction.

How do you increase the yield of the product?

A closed system will remain in equilibrium as long as none of the conditions change.

So, if you want to increase the yield of products you need to change the conditions in a

way that moves the equilibrium to the _____________ side of the equation (right).

The conditions that could be changed are:

1.

2.

3.

NOTE: An equilibrium position will move to try and oppose/minimise any changes

to conditions

We are going to use the Haber process to consider how to increase the yield of a reaction in equilibrium (also in C4)

1. Temperature

If you increase the temperature, the equilibrium will try to decrease it by moving in the

______________________ direction

If you decrease the temperature, the equilibrium will try to increase it by moving in the

______________________ direction.

So, for the Haber Process you need to ____________________ the temperature to

increase the yield.

BUT they actually use a high temperature of 450oC, why?

2. Pressure

If you increase the pressure the equilibrium will try to reduce it by moving to the side with

the _______________ number of moles of gas molecules

If you decrease the pressure the equilibrium will try to increase it by moving to the side with

the _______________ number of moles of gas molecules

For the Haber Process there are ____ moles of reactants altogether and only ___ moles

of products so it will move to the ______________ side and _____________ the yield of

the products.

3. Concentration

Removing a product moves the position of equilibrium to the ____________

Adding extra reactant moves the position of equilibrium to the ____________

How do they use this in the Haber Process?

The Contact Process

The Contact Process is used to make ____________________

The raw materials used to make sulphuric acid by the Contact Process:

1.

2.

3.

The production of sulfuric acid by the Contact Process involves the reversible reaction

between sulfur dioxide and oxygen:

the conditions used in the Contact Process:

1. V = vanadium (transition metal)

2.

3. (i.e. no change in pressure)

There are 3 stages in the manufacture of sulfuric acid by the Contact Process:

Stage one:

Stage two: > 95% yield

Stage three:

Explain the conditions used in the Contact Process:

1. high temperature ________________ the yield but ________________ the rate of reaction

so an optimum is used

2. catalyst increases the rate but it does not change position of equilibrium

3. position of equilibrium is already on right (> 95% yield) so a high pressure is not

needed as it is expensive to maintain.

Practical

Reversible reactions between acids and alkalis using an indicator.

Reversible reactions between chromate and dichromate.

Demonstration of the reaction of BiCl3 in concentrated HCl with water.

Fizzy pop

Show a video about Contact Process.