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 don’t have a specific molar mass, they cannot be expressed in kJ/mol.
$$q=n\times \Delta H_C$$

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