Reactivity 1.3.3

Fossil Fuels

Coal, Crude Oil, and Natural Gas.

Viscous crude oil in a transparent container

Crude Oil: A complex mixture of hydrocarbons formed over millions of years.

Specific Energy vs. Energy Density

We compare fuels based on how much energy they release per gram.

Specific Energy

Energy per unit mass

MJ / kg

Energy Density

Energy per unit volume

MJ / L

The Greenhouse Effect

Fossil fuels release molecules ( $CO_2$ , $CH_4$ , $H_2O$ ) that absorb IR radiation reflected from the Earth's surface, trapping heat in the atmosphere.

Coal: Highest Carbon Mass %. Dirtiest.
Natural Gas ( $CH_4$ ): Highest H:C ratio. Cleanest fossil fuel (most energy per gram of $CO_2$ released).

Putting it into Practice

Calculating Specific Energy

Paper 2 Style

Calculate the Specific Energy (in $kJ \: g^{-1}$ ) of Methane ( $CH_4$ ).

Given: $\Delta H_c (CH_4) = -890 \: kJ \: mol^{-1}$ .

Practice: Fuel Comparison

[2 Marks]

Hydrogen gas ( $H_2$ ) has a very high specific energy ( $142 \: MJ \: kg^{-1}$ ) compared to petrol ( $46 \: MJ \: kg^{-1}$ ).

Suggest why Hydrogen is not yet widely used in cars despite this advantage.

Previous1.3.2 Incomplete Combustion Next 1.3.4 Biofuels