Nitrogen & Sulfur

We will talk about some reactions, properties and uses of each element

Nitrogen

Nitrogen is a very inert gas and it is very unreactive. In fact, it is used as an inert gas to store and preserve products

The reason why nitrogen is very unreactive is because due to the presence of the triple bond in N₂. So this requires a lot of energy to break the bond and so it has a very high activation energy. Also as the nitrogen molecule is non polar it is attacked less.

The air is 78% of Nitrogen and so Nitrogen can be obtained by fractional distillation of liquid air. Most reactions of nitrogen are endothermic and so we will see them below

Reactions of Nitrogen

Here are a few reactions of nitrogen

Nitrogen with Oxygen

There are two situations in which oxides of nitrogen can be formed. It can be made from lightning activity because it gives sufficient energy to break the triple bond in N₂

N₂ + 2O₂ → 2NO₂

It can also be produced by internal combustion in engines, where nitrogen and oxygen in air react due to high temperature to form oxides of nitrogen

Oxides of nitrogen can be useful as it does fertilise the soil by adding nitrate ions, however, to much can cause acid rain and photochemical smog. To avoid this, we use a catalytic convertor, which will be discussed later

Formation of Nitrides

When a metal reacts with nitrogen under high temperatures a nitride is formed which is highly unstable

3Mg + N₂ → Mg₃N₂

Haber process

One of the most famous reactions used is the formation of ammonia

N₂ + 3H₂ ⇋ 2NH₃

Its good to refer the equilibrium section to understand how different factors will affect the yield and equilibrium. We will state the conditions for Haber process

Catalyst: Iron filling

Temperature: 450°C

Pressure: 200atm

Another point to remember is that the ammonia has a higher melting point than nitrogen or hydrogen. so it can be obtained by fractional distillation by lowering the temperature

Ammonia

This is a structure of ammonia with a bond angle of 107° ( triangonal pyramidal shape )

Ammonia acts as a weak base due to presence of a lone pair. The lone pair accepts a proton to form the ammonium ion

Reactions of ammonia

There are some reactions of ammonia you need to know:

Reactions with acids

Ammonia acts as an base and so reacts with acids to form an ammonium salt

NH₃ + HCl → NH₄Cl

Reactions with bases

Ammonium salts reacts with bases when heated to form ammonia gas

NH₄Cl + NaOH → NH₃ + NaCl

This is used as a test for the presence of ammonium cations

Reaction with Silver Halides

A complex ion is formed known as a complex ligand

During this reaction the nitrogen does not oxidise at all and the nitrogen forms a dative bonding with the silver cation

Uses of ammonia

Ammonia is

- Used to make fertiliser or can be used as a fertiliser

- Used to produce nitric acid

- Used in household detergents

- Used to make nylon

Production of Nitric acid

This is not important but it is good to know. This process is called ostwald process

NH₃ + O₂ → NO + H₂O

NO + O₂ → NO₂

NO₂ + H₂O → HNO₃

There are many uses of nitric acid. Nitric acid can be used to make nylon and explosives and fertilizers

Ammonia as a fertiliser

Ammonia can be used directly as a fertilisers or it can be reacted to form salts which can be used as a fertilizer. For example, ammonia can be reacted with phosphoric acid to make ammonium phosphate

Fertilizer must contain these:

1. Nitrogen - This could be included in Nitrates or ammonia

2. Phosphate

3. Potassium

That's why we call these fertilizers, NPK fertilisers which includes all of the required nutrient that the plant needs

However, if excess fertilizers are used then this could lead to problems such as eutrophication. When fertilisers are sprayed in excess then this could lead to nutrients leeching from the soil to the lakes which may cause abnormal growth of algae in the water. This will then block the sunlight, and make the lower plants die. This will increase the number of decomposers in the water and reduce the oxygen concentration

Catalytic convertors

Due to the high temperature, Nitrogen oxide ( NO ) is formed in the engine. Also Carbon monoxide will be produced due to incomplete combustion of fossil fuels

NO and CO are pollutants and a very harmful to the enviroment. NO can cause acid rain and photochemical smog and CO is poisonous and toxic to living things as it reduces oxygen flow in blood

Name	Hazards
VOCs or Unburnt Hydrocarbons	Causes cancer and smog
NO	Causes acid rain and smog
CO	Toxic, binds with blood to restrict oxygen flow
CO2	Global warming

These must be coverted to less harmful gases and so we use a catalytic convertor

Due to high temperature the NO reduces to N₂ and the CO oxidises to CO₂. This will still release carbon dioxide which will increase global warming. The equation can be simplified to:

2CO + 2NO → N₂ + CO₂

The catalytic has a honey comb shape to increase the rate of the reaction and to make sure that a high yield of polluntants are filtered. It uses platinum as its catalyst to speed up the reaction

Also unburnt hydrocarbons can also oxidise to form Carbon dioxide and water. This is required because, unburnt hydrocarbons or VOCs are cancerous substances and can cause smog

C_xH_y + O₂ → CO₂ + H₂O

Disadvantages of Catalytic Convertor

1. Catalytic convertors are very expensive as they require precious platinum

2. Catalytic convertors only work when it reaches a high temperature so it is releases harmful gases while it reaches its optimum temperature

3. They may get clogged or inefficient over time so it must be replaced

4. Does not prevent Global warming

Sulfur

Sulfur is a yellow solid at room temperature and it exists in the form of S₈. Sulfur has induced dipole forces between the molecules so it has a relatively low melting point

Sulfur can be produced from many sources, such as combustion of fuels containing sulfur impurities or burning sulfur ores or even from volcano activity

Sulfur has many uses such as:

Production of SO₂

Sulfur dioxide is a powerful reducing agent and it is mainly used as reductant to preserve food and kill bacteria. It is also used to form sulfuric acid

Production of Sulfuric acid

This process is known as contact process and sulfur is used to make sulfuric acid

Here are the step and equations related to contact process

S + O₂ → SO₂

The sulfur reacts or oxidises to form Sulfur dioxide

2S + 3O₂ ⇋ 2SO₃

Sulfur dioxide oxidises further in oxygen to form sulfur trioxide. For this there are some conditions you have to know

Catalyst: Vanadium(V) Oxide

Temperature: 450°C

Pressure: 1 atm

Then the sulfur trioxide is dissolved in concentrated sulfuric acid. They are not allowed to be dissolved directly in water as this will cause acid mist because the reaction is highly exothermic. So instead we dissolve it in sulfuric acid to get oleum

2SO₃ + H₂SO₄ → H₂S₂O₇

Then oleum is dilluted in water to form sulfuric acid

H₂S₂O₇ + H₂O → H₂SO₄

Sulfuric acid is a powerful oxidising agent and also a dehydrating substance and so it can be used for many purposes such as:

1. Fertilisers

2. Oven Cleaner

3. Tanning leather

4. Solvent & dyes & paints

Sulfuric acid is a powerful dehydrating chemical that can be used to dehydrate sugar and even alcohols to corresponding alkenes. This is more of an organic chemistry topic so it will be discussed later

Acid rain

Acid is caused by both sulfur and nitrogen but mostly sulfur. Sulfur dioxide and nitrogen dioxide can also produce acids when dissolved in water but this is not how acid rain is mainly formed. Below are the steps:

Sulfur dioxide produced from engines or volcanos are released to the air

NO from engines are released to the air and oxidises to form NO₂

NO + O₂ → NO₂

The NO₂ reacts with SO₂ to form SO₃. In other words, sulfur dioxide is oxidised by NO₂

NO₂ + SO₂ → SO₃ + NO

As we can see, NO is formed because it has been reduced. Then the SO₃ dissolves in water to form Sulfuric acid

SO₃ +H₂O → H₂SO₄

Sulfuric acid is a strong acid so this is how mainly acid rain is formed. There is another step you need to know

The NO produced then oxidises to form NO₂ again so it can attack another SO₂. This is like a chain reaction and that is why we say that NO is a regenerating catalyst.

NO + O₂ → NO₂

Acid rain does cause many problems so we will see them below:

Damages caused by acid rain

There are so many so we will explain each point in depth:

Leeches nutrients from the soil and kills plants

Acid rain is an acid so it reacts with the nutrients in the soil and make them soluble. When it rains the soluble nutrients are washed away

Kills marine life

This is done by lowering the pH of lakes or by increasing the Al ion concentration in water (caused by leeching). Al³⁺ ions are toxic to fish and kills them

Erodes statues, limestone buildings and Corrodes bridges

As limestone is calcium carbonate it reacts with acids and erodes away. Also bridges are corroded by acid rain and lose their strength over time