Reversible reactions and equilibria

Question 1

Many chemical reactions are irreversible but some can be reversed, so the products can change back into the ………….. Why is it important to be able to control these reactions In industrial processes?.

Answer 1

Many chemical reactions are irreversible but some can be reversed, so the products can change back into the reactants. In industrial processes, it is important to be able to control these reactions to make the most product as quickly and cheaply as possible.

Question 2

When a chemical reaction is reversible, do we use the usual symbol instead of an arrow?

Answer 2

NO

Question 3

For example: H2 + I2 2HI

What does this mean?

Answer 3

This means that while H2 and I2 can react together to form HI molecules, the HI molecules can also react with each other to make H2 and I2 again

Question 4

What is Dynamic equilibrium?

Answer 4

Dynamic equilibrium

If they take place within a closed container, reversible reactions will eventually reach a state of dynamic equilibrium. This is when the forwards reaction and the reverse reaction are taking place at the same rate (speed).

This means that even though there will be some products and some reactants in the mixture, the amount of product in the reaction mixture remains constant over time.

Question 5

What is the Haber Process?

Answer 5

Process for making ammonia. The Haber process is an artificial way of ‘fixing’ unreactive nitrogen from the air and turning it into useful chemicals that plants can absorb and use when they build protein molecules in their cells.

Question 6

Why is Ammonia, NH3, is a very important chemical? It is used in the manufacture of fertilisers, explosives, dyes, medicines and a variety of other chemicals.

It has been estimated that one third of the world’s population rely on food grown using synthetic fertilisers. Without ammonia, millions of people would starve. Some naturally-occurring bacteria use enzymes to ‘fix’ nitrogen from the air, but these natural processes are not sufficient to sustain the world’s population

Answer 6

Ammonia, NH3, is a very important chemical. It is used in the manufacture of fertilisers, explosives, dyes, medicines and a variety of other chemicals.

It has been estimated that one third of the world’s population rely on food grown using synthetic fertilisers. Without ammonia, millions of people would starve. Some naturally-occurring bacteria use enzymes to ‘fix’ nitrogen from the air, but these natural processes are not sufficient to sustain the world’s population

Question 7

How is Ammonia produced in the Haber process?

Answer 7

Ammonia is produced in the Haber process by reacting hydrogen with nitrogen.

Question 8

Why is the haber process difficult?

Answer 8

This is difficult because nitrogen is such an unreactive gas.:

Question 9

What are the chemical equations for the Haber process?

Answer 9

The chemical equations are:

nitrogen + hydrogen ammonia
N2(g) + 3H2(g) 2NH3(g)

Question 10

What are the Raw materials in the Haber process?

The nitrogen comes from the air. The hydrogen is produced in a reaction between methane (natural gas) and steam.

Answer 10

Raw materials: The nitrogen comes from the air. The hydrogen is produced in a reaction between methane (natural gas) and steam.

Question 11

The conditions chosen for the Haber process aim to do what?

Answer 11

The conditions chosen for the Haber process aim to maximise the yield of ammonia in the shortest time and at the lowest cost.

Question 12

In the Haber process , when the pressure is increased, what happens to the yield ? When the temperature is increased, what happens to the yield?

Answer 12

maximise the yield of ammonia in the shortest time and at the lowest cost.

Pressure and temperature

When the pressure is increased, the yield increases. When the temperature is increased, the yield decreases

Question 13

How is the yield further increased in the Haber process by recycling ?

Answer 13

The three gases (nitrogen, hydrogen and ammonia) do not actually stay in the reactor for long enough to reach equilibrium.

But the yield is further increased by recycling the unreacted hydrogen and nitrogen after they have been separated from the ammonia (when the mixture leaves the reactor).

Question 14

What catalyst is used to increase the rate of reaction?

Answer 14

Using a catalyst

An iron catalyst is used to increase the rate of the reaction. The catalyst does not affect the yield of ammonia.

Scientists are constantly trying to find new catalysts that will speed up the reaction even more, and therefore make the Haber process more efficient

Question 15

What do ‘nitrogen-fixing’ bacteria need to be able to ‘fix’ nitrogen from the air, at room temperature and pressure? If this could be carried out and controlled at a large scale why would this be good?

Answer 15

enzymes….better for the environment that the Haber process?

Question 16

Making ammonia using the Haber process requires a lot of energy, which usually involves burning fossil fuels. What are the negatice environmental impact of the Haber process?

Answer 16

This releases carbon dioxide which causes global warming. Oxides of nitrogen are also emitted during the process.

The widespread use of fertilisers can cause a type of pollution called eutrophication.

This is when fertilisers are washed off fields and into rivers and lakes, causing algae to grow. The algae die off when they have exhausted the nutrients. The algae are then decomposed by aerobic bacteria, which use up all the oxygen - causing the aquatic ecosystem to collapse.

Question 17

A reversible reaction will reach a state of dynamic equilibrium if it takes place in a …………………………… ………………and is left for long enough at …………… ……………….(eg temperature and pressure).

Answer 17

A reversible reaction will reach a state of dynamic equilibrium if it takes place in a sealed container and is left for long enough at constant conditions (eg temperature and pressure).

Question 18

A dynamic equilibrium is when the amount of products and reactants in the reaction mixture remain ……….. over time.

Answer 18

A dynamic equilibrium is when the amount of products and reactants in the reaction mixture remain constant over time.

When a system is at dynamic equilibrium, it does not mean that the reaction has finished. In fact, both the forward and reverse reactions are still taking place.

.

Question 19

When a system is at dynamic equilibrium, it does not mean that the reaction has finished. In fact, both the forward and reverse reactions are still taking place.
What is happening to the reactants and products?

Answer 19

Because they are taking place at the same rate (speed), the reactants are turning into products at the same speed as the products are turning back into reactants.

Note that the fact that a system is at dynamic equilibrium does not tell us about the yield. The yield depends on factors like pressure and temperature.

Question 20

In the Haber process, the conditions chosen are a compromise. Why is the pressure chosen a compromise between rate, cost, and safety?

Answer 20

Increasing the pressure causes the yield to increase. However, the energy costs also increase and the equipment used for the process must be strong to withstand the high pressure, which makes it expensive.

Question 21

Why is the temperature chosen a compromise between rate and yield?

Answer 21

Increasing the temperature decreases the yield but increases the rate.

Question 22

Selecting the catalyst - The catalyst can be expensive to develop and to buy - but it does not need to be ………………,

Answer 22

Selecting the catalyst

The catalyst can be expensive to develop and to buy - but it does not need to be replaced, because catalysts are not used up or chemically changed in a reaction. Care must be taken to ensure that the catalyst is not poisoned by impurities in the reactants.

Question 23

The catalyst does not affect the yield, but it does increase the rate of the reaction. What are the benefits of this ?

Answer 23

This allows a lower temperature to be used (increasing temperature is one way of speeding up a reaction) and using a lower temperature allows the yield to be maximised as well as saving money on energy costs