Haber process

The Haber process is a process used to produce the useful substance ammonia from nitrogen and hydrogen.

Sources of gases

Hydrogen

Hydrogen is only found in the air in the form of water vapour, as if there was more hydorgen gas, it would react with the oxygen, forming water. However, the reverse reaction can be used to form hydrogen: the electrolysis of water. Also, a fuel processor can be used to extract the hydrogen from methane (natural gas).

Electrolysis of water

Pure water is a poor conductor of electricity, so often a soluble ionic compound is added, such as an acid, base or salt. Sulphuric acid (H₂SO₄) is often used because it is fully dissociated when dissolved in water, and is difficult to oxidise, so oxygen gas will form at the anode..

Only some water molecules form an oxide and a hydroxide ion, meaning that water is only partially ionised and hence a poor conductor of electricity.

H₂O _(l) → H⁺ + OH^-

Sulphuric acid, on the other hand, is fully ionised when dissolved in water:

H₂SO_{4 (aq)} → 2H⁺ + SO₄^2-

Once electrolysis has begun, the hydrogen ions move towards the cathose where they are reduced to form hydrogen gas:

2H⁺ + 2e^- → H_{2 (g)}

At the anode, the water splits into two oxygen ions which form a covalent bond, and are relased as oxygen gas. Two hydrogen ions are also produced:

H₂O → O^2- + 2H⁺ + 2e^-

2O^2- → O_{2 (g)}

For every two electrons passed, 2 hydrogen ions form a molecule of hydrogen gas at the cathode, but another 2 hydrogen ions are formed at the anode. The sulphate ions stay in solution throughout the reaction, meaning that overall, the amount of sulphuric acid remains constant, and it is the water that is electrolysed:

4H⁺ + 2H₂O _(l) → 2H_{2 (g)} + O_{2 (g)} + 4H⁺

Or, more simply:

2H₂O _(l) → 2H_{2 (g)} + O_{2 (g)}

Fuel processor

Also known as a fuel reformer, a fuel processor extracts hydrogen from hydrocarbons, such as methanol and methane (natural gas).

Reforming methanol

The reforming of methanol involves mixing liquid methanol with water, and then using a catalyst to help break down the methanol molecules into carbon monoxide and hydrogen. The water than reacts with the carbon monoxide to produce carbon dioxide and more hydrogen:

CH₃OH _(l) → CO _(g) + 2H_{2 (g)}

CO _(g) + H₂O _(g) → CO_{2 (g)} + H_{2 (g)}

So, overall:

CH₃OH _(l) + H₂O _(l) → CO_{2 (g)} + 3H₂

Reforming methane

Methane is reacted with water to form carbon dioxide and hydrogen:

CH_{4 (g)} + H₂O _(l) → CO + H_{2 (g)} + H_{2 (g)}

Just as with reforming methanol, the carbon monoxide produced reacts with water to form carbon monoxide and more hydrogen:

CO _(g) + H₂O → CO_{2 (g)} + H₂O _(l)

So, overall:

CH_{4 (g)} + 2H₂O _(l) → CO_{2 (g)} + 4H₂O _(l)

Reaction

The reaction between nitrogen and hydgrogen gases is reversible , meaning that some ammonium will be formed, but not all with react. The yield of ammonia depends upon the conditions: temperature, pressure and the presence of a catalyst.

N_{2 (g)} + H_{2 (g)} ↔ NH_{3 (g)}

Conditions used

Le Chatelier's principle explains the effects of changing the temperature and pressure on a reversible reaction, as well as showing the effects of a catalyst.

Temperature

Increasing temperature pushes the equilibrium to the side with the most molecules, and also speeds up the rate of reaction.

Pressure

Increasing pressure pushes the equilibrium to the side with fewer molecules, and also speeds up the rate of reaction. However, it is worth noting that high pressures are extremely expensive to maintain.

Catalyst

A catalyst has no effect on the yield, but causes the rate of reaction to increase.

Industry

The Haber process is still used to produce ammonia.

Uses of ammonia

• Manufacture of nitric acid
• Manufacture of ammonium nitrate, a fertilizer
• Explosives

See also

• Ammonia
• Ammonium
• Hydrogen
• Le Chatelier
• Nitric acid
• Nitrogen
• Reversible reaction