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Fertilisers

Topic 12 – National 5 Chemistry Summary Notes

Chemistry is extremely important to the future of food production. As the

population of the world increases, more and more efficient ways of producing

food are required.

Healthy Plants

Plants require nutrients from the soil in order to have healthy growth. The 3

main elements found in essential nutrient compounds are:

nitrogen (N)

phosphorous (P)

potassium (K)

Or N, P, K for short. As plants grow, their roots take nutrients from the soil

and this means that as time goes on the level of nutrients present in the soil

decreases. The next year, plants growing in the same place would not have

enough nutrients for healthy growth. Different plants require different

proportions of N, P and K.

Replacing Nutrients in the Soil for Healthy Plant Growth

Nutrient compounds must be soluble if the roots are to take them into the

plant. There are three ways farmers can replace nutrients into over-used soils:

Grow beans or clover for 1 year (this bacterial method of replacing

nitrogen is cheaper than chemical methods)

Add synthetic fertilisers (man-made) or natural fertilisers (manure)

fertilisers

Crop rotation means using the same field to grow different crops from

year-to-year. This allows the field to recover levels of over-used

nutrients.

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LI 3 Fertilisers

A fertiliser is a soluble compound containing at least one of the following

elements - nitrogen, phosphorous and potassium (NPK). Use your data book (page

8) if you wish to find out if a compound is soluble.

Compound Solubility Could be used as a fertiliser?

Ammonium phosphate

Calcium phosphate

Potassium nitrate

Sodium chloride

Both natural and synthetic fertilisers can be used on the soil. Examples are:

Natural Fertilisers Synthetic Fertilisers

Decay of dead plants or animals Ammonia, Ammonium nitrate

Decay of plant or animal waste Potassium phosphate

Unfortunately, soluble fertilisers can be washed away by rainwater into rivers

and lakes. This causes pollution resulting in fish dying and algal blooms (a green

coating) covering water.

There are many useful synthetic fertilisers, examples of these are:

Synthetic Fertiliser Formula

Ammonium salts NH4+__

Potassium salts K+__

Nitrate salts __NO3-

Phosphate salts __PO43-

Using both tables on page 8 of the data book, it can be seen that all salts

containing the above ions:

are very soluble (except some phosphates)

contain the elements N,P and K

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Different crops need fertilisers with different proportions of nitrogen,

phosphorous and potassium. If you go to buy a fertiliser from a shop, you often

find that the fertiliser is classified according to which crop it can improve the

best.

Nitrogen Fixation

Converting nitrogen from the atmosphere into soluble nitrogen compounds is

called nitrogen fixation

For farmers, it is much cheaper to grow peas or clover than it is to use

synthetic fertilisers to replace nitrogen as peas and clover are examples of

plants which have bacteria in their root nodules which can fix nitrogen.

The Haber Process

The Haber process is a process used in industry to manufacture ammonia (NH3)

which is an essential starting point for the making of fertilisers. In order for

ammonia to be made nitrogen and hydrogen need to react together.

Diatomic nitrogen has a triple covalent bond holding the atoms together.

This triple bond requires a large amount of energy to break it, so the direct

reaction of nitrogen to hydrogen is not usually possible. The Haber Process,

shown in the flow chart below, manages to overcome these difficulties.

nitrogen + hydrogen ⇌ ammonia

N2(g) + 3H2(g) ⇌ 2NH3(l)

The double arrow '⇌' tells us that the reaction is reversible. This is a problem

as the ammonia made easily breaks down into nitrogen and hydrogen.

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To make the Haber Process economic and efficient the following conditions

apply:

Condition Reason

Moderately high

temperature

Low temperature – reaction too slow. High temperature

– lower percentage conversion to ammonia

High pressure This increases the yield of ammonia (as it squeezes the

4 volumes of reactants towards 2 volumes of product.)

Iron Catalyst

This iron catalyst lowers the temperature required and

speeds up the production of ammonia, saving energy

costs.

Condenser This cools the reaction down, turning ammonia into a

liquid which is easier to remove.

Recycled reactants Unreacted nitrogen and hydrogen are returned to the

reaction, saving money.

You can remember some of the main point of the Haber Process by remembering

the word HAIR.

H- Haber

A – Ammonia

I – Iron catalyst

R – Reversible reaction

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LI 11 Properties and uses of ammonia

Ammonia is a colourless gas with a very pungent ‘smelling salts’ odour. It is very

soluble in water and dissolves to produce an alkaline solution. Its solubility can

be demonstrated by performing the following experiment.

Ammonia + water ammonium hydroxide

NH3(g) + H2O(l) NH4+(aq) + OH-(aq)

The ammonia in the flask quickly dissolves when water is added from the

syringe. This reduces the gas pressure in the flask and the universal indicator

solution is drawn up to the flask creating a fountain effect. The ammonia forms

ammonia solution when it reacts with water. Ammonia solution is alkaline and

therefore the universal indicator solution turns blue when it reaches the flask.

Ammonia can be used as a fertiliser and for cleaning. It can be used to make

other fertilisers, pharmaceuticals and explosives.

Ammonia Properties Summary

colourless gas

form an alkaline solution i.e. turns damp pH paper blue

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The Ammonia Molecule

Making Ammonia in the Lab

Ammonia can be made by heating any ammonium compound with a metal

hydroxide

Ammonium chloride + calcium hydroxide → calcium chloride + ammonia + water

2NH4Cl(s) + Ca(OH)2(s) → CaCl2(s) + 2NH3(g) + 2H2O(l)

The ammonia gas collected can be tested with damp pH paper to show that it is

alkaline.

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Making Nitric Acid using The Catalytic Oxidation of Ammonia (Ostwald Process)

Due to the unreactive nature of nitrogen, reacting nitrogen and oxygen is not an

economic way of making nitric acid. The German chemist Wilhelm Ostwald

discovered an economical way to react oxygen and ammonia to produce nitric

acid. The Haber process and the Ostwald Process together provide this route

from nitrogen to nitric acid.

Ammonia (from the Haber process) and oxygen (from the air) are passed over a

platinum catalyst at a temperature of 800°C. This creates nitrogen monoxide

which reacts with oxygen to form the brown gas nitrogen dioxide.

Nitric acid (HNO3) is made if the nitrogen dioxide formed is shaken with water

and oxygen.

2H2O + 4NO2 + O2 4HNO3

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Similar to the Haber Process, the higher the temperature, the faster the

reaction rate but the lower the % conversion (due to the breakdown of NO and

water back to reactants.)

A moderately high temperature of 800°C is best. Once the reaction has

started however, it does not need continued heating to keep the reaction

going as it is an exothermic reaction (i.e. one which gives out heat energy).

The catalyst continues to glow once the reaction starts, without further

heating.

You can remember some of the main point of the Ostwald Process by

remembering the word OPEN

The Catalytic Oxidation of Ammonia in the Lab

The catalytic oxidation of Ammonia in the lab reaction is an example of

oxidation as ammonia reacts with oxygen from the air on the platinum catalyst.

When the reaction begins, a colourless gas is formed and collects in the flask.

This gas is called nitrogen monoxide, NO.

4NH3(g) + 5O2(g) 4NO(g) + 6H2O(l)

Nitrogen monoxide reacts with air (oxygen) to form brown nitrogen dioxide.

2NO(g) + O2(g) 2NO2(g)

This brown acidic gas dissolves in water with oxygen to make nitric acid.

2H2O(l) + 4NO2(g) + O2(g) 4HNO3(aq)

O – Ostwald

P – Platinum catalyst

E – Exothermic Reaction

N – Nitric acid

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Importance of Nitric Acid

One of the most important fertilisers is ammonium nitrate, NH4NO3. It is made

by reacting alkaline ammonia gas with nitric acid. Ammonium nitrate is a good

fertiliser as it contains a high % of essential nitrogen and is very soluble.

NH3(g) + HNO3(aq) NH4NO3(aq)

Making Fertilisers

e.g. NH3 + H2SO4 (NH4)2SO4

ammonia + sulphuric acid ammonium sulphate fertiliser

e.g. HNO3 + KOH KNO3 + H2O

nitric acid + potassium hydroxide potassium nitrate + water

Haber Process Ammonia (NH3) Ammonium compounds acids

Ostwald Process Nitric acid (HNO3) Nitrate compounds alkalis

Ammonia

Nitric acid

Oxygen Ostwald

process

Ammonium nitrate

Nitrogen

Hydrogen

Haber

process

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Percentage Composition

The percentage of an element in a compound can be calculated using the following

formula.

Example 1

Calculate the percentage, by mass, of nitrogen in ammonium nitrate,

NH4NO3.

Formula mass of NH4NO3 = 2N, 4H, 3O

= (2 x 14) + (4 x 1) + (3 x 16)

= 80

Mass due to nitrogen = 28

by using the formula above, percentage composition =

x100 = 35%

Example 2

Mercury can be extracted from the ore cinnabar, HgS. Calculate the percentage by

mass of mercury in cinnabar.

Formula mass of HgS = 1Hg, 1S

= 200∙5 + 32

= 232∙5

Mass due to mercury = 200∙5

by using the formula above, percentage composition =

x100 = 86∙2%

Example 3

Calculate the percentage, by mass, of phosphorus in ammonium phosphate,

(NH4)3PO4.

Formula mass of (NH4)3PO4 = 3N, 12H, 1P, 4O

= (3 x 14) + (12 x 1) + (1 x 31) + (4 x 16)

= 42 + 12+ 31+ 64

= 149

Mass due to phosphorus = 31

by using the formula above, percentage composition =

x100 = 20∙8%

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Number

Learning Intention

Success Criteria

1 I will find out that the increasing world population has led to a

need for more efficient food production

I can state that the increasing world population has led to a need for more

efficient food production

2 I will find out that different crops need nutrients containing

different proportions of nitrogen, phosphorus and potassium

I can state that different crops need nutrients containing different

proportions of nitrogen, phosphorus and potassium

3 I will find out that fertilisers are substances which restore

the essential elements for plant growth to the soil (N,P,K)

I can state that fertilisers are substances which restore the essential

elements for plant growth to the soil (N,P,K)

4 I will find out that fertilisers can be natural or synthetic(man

made)

I can state that fertilisers can be natural or synthetic(man made)

5 I will find out about the effect of artificial fertilisers on

lochs and rivers

I can give examples of the effect of artificial fertilisers on lochs and rivers

6 I will find out about why the solubility and essential elements

present determines if the following are useful, synthetic

fertilisers:

• ammonium salts

• potassium salts

• nitrates

• phosphates

I can explain in terms of solubility and essential elements why the following

are useful, synthetic fertilisers:

• ammonium salts

• potassium salts

• nitrates

• phosphates

7 I will find out that nitrogen-fixing bacteria in root nodules of

some plants can convert atmospheric nitrogen into nitrogen

compounds and that bacterial methods of fixing nitrogen are

cheaper than chemical methods

I can state that nitrogen-fixing bacteria in root nodules of some plants can

convert atmospheric nitrogen into nitrogen compounds and that bacterial

methods of fixing nitrogen are cheaper than chemical methods

8 I will find out that nitrogen gas is not very reactive I can state that nitrogen gas is not very reactive

9 I will find out that the Haber process is a reversible reaction

and that not all of the nitrogen and hydrogen are converted to

ammonia

I can state that that the Haber process is a reversible reaction and that

not all of the nitrogen and hydrogen are converted to ammonia

10 I will find out that the industrial manufacture of ammonia

from nitrogen and hydrogen (Haber Process) and explain why

I can state that the industrial manufacture of ammonia from nitrogen and

hydrogen (Haber Process) and explain why the reaction is carried out at

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the reaction is carried out at moderately high temperature moderately high temperature

11 I will find out that ammonia has the following properties:

• is a colourless gas

• has a pungent smell

• is very soluble in water

• turns damp pH paper/universal indicator an alkaline colour

I can state that ammonia has the following properties:

• is a colourless gas

• has a pungent smell

• is very soluble in water

• turns damp pH paper/universal indicator an alkaline colour

12 I will find out that ammonia can be prepared in the laboratory

by the reaction of ammonium compound with alkali

I can state that ammonia can be prepared in the laboratory by the reaction

of ammonium compound with alkali

13 I will find out

that the catalytic oxidation of ammonia to form

nitrogen dioxide is a step in the industrial manufacture

of nitric acid (Ostwald Process)

why the reaction is carried out at a moderately high

temperature.

that the combination of nitrogen and oxygen does not

provide an economic industrial route to nitric acid

I can state

that the catalytic oxidation of ammonia to form nitrogen dioxide is a

step in the industrial manufacture of nitric acid (Ostwald Process)

why the reaction is carried out at a moderately high temperature.

that the combination of nitrogen and oxygen does not provide an

economic industrial route to nitric acid

14 I will find out that nitric acid is formed when nitrogen dioxide,

in the presence of air, dissolves in water

I can state that nitric acid is formed when nitrogen dioxide, in the presence

of air, dissolves in water

15 I will find out that the catalytic oxidation of ammonia can be

carried out in the laboratory

I can state that the catalytic oxidation of ammonia can be carried out in the

laboratory

16 I will find out that ammonia and nitric acid are nitrogen

compounds which are used to make fertilisers

I will find out that ammonia and nitric acid are nitrogen compounds which

are used to make fertilisers

17 I will find out how to work out percentage mass of elements in

compounds from formulae

I can work out percentage mass of elements in compounds from formulae