Other Chemical Fuels (Leaving Cert Chemistry): Revision Notes

Other Chemical Fuels

In this section, we explore two important chemical fuels: ethyne (acetylene) and hydrogen, examining their preparation, combustion, and industrial applications. Additionally, we will discuss the potential of hydrogen as a clean energy source and the processes used to produce it, including electrolysis and steam reforming.

Ethyne (Acetylene)

Preparation of Ethyne

Ethyne (C₂H₂) can be prepared in the lab by adding calcium carbide (CaC₂) to water:

$$CaC_2 + 2H_2O \rightarrow C_2H_2 + Ca(OH)_2$$

This reaction produces ethyne gas and calcium hydroxide as a byproduct.

Combustion of Ethyne

• Ethyne burns with a luminous, smoky flame and gives off a large amount of heat.
• The flame is sooty because of incomplete combustion, producing carbon particles.

Test for Unsaturation

Ethyne is an unsaturated hydrocarbon, meaning it contains a carbon-carbon triple bond. To test for unsaturation:

• Bromine water test: When ethyne is bubbled through bromine water, the orange colour disappears, indicating the presence of double or triple bonds (unsaturation).
• Acidified potassium manganate(VII) test: Ethyne decolorizes purple acidified potassium manganate(VII) solution, another indicator of unsaturation.

Oxyacetylene Welding and Cutting

Ethyne is widely used in oxyacetylene torches for welding and cutting metals. When ethyne is burned in pure oxygen, it produces an extremely hot flame (over 3000°C), which can melt metals.

Hydrogen

Manufacture of Hydrogen

Hydrogen is an important fuel and chemical feedstock. It can be manufactured by two main methods:

1. Electrolysis of Water

Electrolysis involves passing an electric current through water, causing it to split into hydrogen and oxygen gases:

$$2H_2O(l) \rightarrow 2H_2(g) + O_2(g)$$

This process is simple but requires a significant amount of energy.

2. Steam Reforming of Natural Gas

In this industrial process, natural gas (mainly methane) reacts with steam at high temperatures to produce hydrogen:

$$CH_4 + H_2O \rightarrow CO + 3H_2$$

The carbon monoxide can further react with steam in a process called a water-gas shift reaction to produce more hydrogen and carbon dioxide.

Industrial Uses of Hydrogen

• Ammonia production: Hydrogen is a key reactant in the Haber process for producing ammonia, which is essential for fertilisers.
• Hydrogenation: Used to convert unsaturated oils into solid fats (e.g., margarine production).
• Fuel cells: Hydrogen can be used in fuel cells to generate electricity, with water as the only byproduct.

Potential of Hydrogen as a Fuel

Hydrogen is considered a clean fuel because it produces only water when burned:

$$2H_2 + O_2 \rightarrow 2H_2O$$

• Advantages: No carbon emissions, and high energy content.
• Challenges: Storage and transport are difficult due to hydrogen's low density and the need for high-pressure containers.