Energy From Fuels

Key Ideas

• System: the chemical reaction
• Surroundings: the environment which the chemical reaction is taking place in
• Latent heat: the heat released or absorbed by a chemical substance during a change of state

Energy

Chemical Energy $$=$$ Potential Energy (Stored Energy) $$+$$ Kinetic Energy (Energy of Movement)

• Attractions between electrons and protons
• Repulsion between nuclei
• Repulsion between electrons
• Movement of electrons
• Vibration and rotation of bonds

Enthalpy

Enthalpy refers to the chemical energy or heat content of a substance. The exchange of heat between a system and its surroundings under constant pressure is referred to as the enthalpy change

• $$\Delta H=H_P\,(Products)\,-\,H_R\,(Reactants)$$

Exothermic Reactions

Exothermic reactions, e.g. combustion release energy into the surroundings. Therefore, the energy of the reactants is higher than the products and $$\Delta H=H<0$$

Exothermic Reaction

Endothermic Reactions

Endothermic reactions, e.g. cold packs, absorb energy from the surroundings. Therefore, the energy of the products is higher than the reactants and $$\Delta H=H>0$$

Endothermic Reaction

Activation Energy

Activation energy is the energy required to break the bonds of the reactants so that the reaction can proceed.

Thermochemical Equations

These chemical reactions are merely those which include the $$\Delta H$$ value and must include states.

For example: $$CH_4(g)+O_2(g)\rightarrow CO_2(g)+H_2O(l)\,\,\Delta H=882kJ\,mol^{-1}$$

Specific Heat Capacity

Specific heat capacity is the amount of energy needed to increase the temperature of a substance by $$1^oC$$
$$q=m\times c\times \Delta T$$

• $$q=\text{energy}$$
• $$m=\text{mass in kg}$$
• $$c=\text{Heat Capacity in } J\,g^{-1} \degree C$$
• $$T=\text{Temperature in} \degree C$$

Latent Heat

The heat released or absorbed by a chemical substance during a change of state

• Latent heat of fusion is the conversion of a liquid to a solid
• Latent heat of vaporisation is the conversion of a solid to a gas

$$q=n\times L$$

• $$q=\text{Energy}$$
• $$n=\text{mol}$$
• $$L=\text{Latent Heat}$$

Combustion

• Complete combustion occurs when oxygen is plentiful, with the products being $$CO_2$$ and $$H_2O$$
• Incomplete combustion occurs when oxygen is lacking, with the products being $$CO$$ and $$H_2O$$

Heat of Combustion

The heat combustion of a fuel is defined as the enthalpy change that occurs when a fuel burns completely in oxygen. However, as many fuels are mixtures of chemicals that donâ€™t have a specific molar mass, they cannot be expressed in kJ/mol.
$$q=n\times \Delta H_C$$

• $$q=\text{Energy}$$
• $$n=\text{mol}$$
• $$\Delta H_C=\text{Heat of Combustion}$$