Chemistry In Society #2: Fertilisers

Question 1

What do plants do when they grow?

Answer 1

They extract nutrients from the soil via their roots

Question 2

What happens to plant soil over time?

Answer 2

The level of nutrients decreases and they need to be replaced

Question 3

What do fertilisers, both natural and synthetic, do?

Answer 3

They allow farmers to increase the crop yield and farm intensely

Question 4

What elements are essential for healthy plant growth?

Answer 4

Nitrogen, phosphorus, potassium

Question 5

Nitrogen in fertilisers give

Answer 5

Nitrates and ammonium salts

Question 6

Phosphorus compounds in fertilisers give

Answer 6

Phosphates

Question 7

Potassium compounds in fertilisers give

Answer 7

Potassium salts

Question 8

What is the Haber Process?

Answer 8

The industrial production of ammonia from nitrogen and hydrogen using an iron catalyst

Question 9

Why can’t true equilibrium be achieved in the Haber Process?

Answer 9

It is a continuous process and an open system

Question 10

What does the catalyst do in the Haber Process?

Answer 10

It provides a fast reaction rate and reasonable yield at lower temperature

Question 11

Effect of temperature on the haber process

Answer 11

• at lower temperatures, the forward reaction is too slow to be economical
• at higher temperatures the rate of reaction increases but favours the reverse reaction

Question 12

Effect of pressure on Haber Process

Answer 12

• high pressure favours ammonia production but the required equipment is costly to build & maintain
• compromises must be reached between the competing factors

Question 13

How in the Haber Process made effective, and the temperature and pressure problems resolved?

Answer 13

• maximum yield in the Haber Process requires high pressure + low temperature
• the process is made effective using 200 atmospheres, 400 °c and an iron catalyst, giving about 15% yield
• the ammonia produced is condensed and the unreacted gases are recycled reacting in and overall yield on 98%

Question 14

What does maximum yield in the Haber process require?

Answer 14

High pressure and low temperature

Question 15

How is the Haber Process made cost effective?

Answer 15

Using 200 atmospheres, 400 °c and an iron catalyst
= 15% yield

Ammonia produced is condensed and unreacted gases recycled
= 98% overall yield

Question 16

Haber Process Equation

Answer 16

N2(g) + 3H2(g) ⇌ 2NH3(g)

ΔH = -92kJ (exothermic therefore -)

Question 17

What are the starting materials for the commercial manufacturing of nitric acid? (Ostwald Process)

Answer 17

Ammonia (Haber) and Oxygen

Question 18

What catalyst is used in the Ostwald Process? To give what yield?

Answer 18

Platinum, 98%

Question 19

Products of the Ostwald Process

Answer 19

Nitrogen monoxide, nitrogen dioxide, nitric acid

Question 20

Ammonia and nitric acid are important compounds used to produce what?

Answer 20

Soluble, nitrogen containing salts

Question 21

What happens to the backward reaction in Haber?

Answer 21

It is essentially cancelled as the unreacted gases are recycled

Question 22

Fertilisers are made by what reactions?

Answer 22

Neutralisation

Question 23

Ammonia + nitric acid -> ammonia nitrate

Answer 23

NH3 + HNO3 -> NH4NO3

Question 24

Ammonia + phosphoric acid -> ammonia phosphate

Answer 24

3NH3 + H3PO4 -> (NH4)3PO4

Question 25

Potassium hydroxide + nitric acid -> potassium nitrate + water

Answer 25

KOH + HNO3 -> KNO3 + H2O

Question 26

Describe one environmental problem that can result from the overuse of fertilisers and explain how it happens

Answer 26

- Overuse can cause eutrophication - This happens when fertiliser washes into rivers/lakes, causing algae to grow, blocking sunlight and reducing oxygen

Question 27

What element is commonly added to fertilisers to promote healthy leaf growth?

Answer 27

Nitrogen

Question 28

Name the two elements used in the Haber process and give their sources

Answer 28

- Nitrogen (from air) - Hydrogen (from natural gas/steam reforming)