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# Chemical energy

## Chemical energy definition

Chemical energy is the energy stored in the bonds of molecules.

Chemical reactions involve the breaking and/or formation of chemical bonds. Chemical energy therefore always changes during a reaction.

In reactions, the overall chemical energy is either taken in or given out. Chemical energy can be given out and converted into other forms of energy (usually heat).

A wood fire can be a source of heat in the home during cold weather.

When wood is burned in air, some of the chemical energy stored in the bonds of wood molecules is converted to heat.

Chemical energy is taken in when other forms of energy are converted into chemical energy.

During photosynthesis, the energy from sunlight is converted to chemical energy stored in the bonds of glucose (sugar) molecules.

## Enthalpy change

Enthalpy change ($\Delta H$) measures the change in chemical energy in controlled chemical reactions.

If the $\Delta H$ of a reaction is positive, then there is an increase in chemical energy. If $\Delta H$ is negative, then there is a decrease in chemical energy.

Photosynthesis (shown below) has a positive enthalpy change because chemical energy increases (mainly due to the formation of glucose).

$$\ce{6CO2 {(g)} + 6H2O {(l)} ->T[light] C6H12O6 {(s)} + 6O2 {(g)}}$$

The combustion of glucose (shown below) has a negative enthalpy change because chemical energy decreases (chemical energy in glucose is converted to heat).

$$\ce{C6H12O6 {(s)} + O2 {(g)} -> 6CO2 {(g)} + 6H2O {(g)}}$$

When the pressure is constant, enthalpy change ($\Delta H$) is equal to the heat ($Q$) added to a reaction.

$$\Delta H = Q$$

## Endothermic and exothermic reactions

A reaction is classified as either endothermic or exothermic based on the enthalpy change ($\Delta H$) during the reaction.

An endothermic reaction absorbs heat at constant pressure and has a positive $\Delta H$. In such a reaction, heat is converted to chemical energy stored in bonds.

$$\ce{N2 {(g)} -> 2N {(g)}}\hspace{5pt}\Delta H = 472.7\text{ kJ/mol}$$

The splitting of nitrogen gas ($\ce{N2}$) into nitrogen atoms ($\ce{N}$) is endothermic and requires $472.7\text{ kJ/mol}$ of heat. The $\Delta H$ is $472.7\text{ kJ/mol}$.

An exothermic reaction releases heat and has a negative $\Delta H$. In such a reaction, chemical energy from bonds is converted to heat.

$$\ce{C3H8 + 5O2 -> 3CO2 + 4H2O}\hspace{5pt}\Delta H = -2,220\text{ kJ/mol}$$

The combustion of propane ($\ce{C3H8}$) is exothermic and releases $2,220\text{ kJ/mol}$ of heat. The $\Delta H$ is $-2,220\text{ kJ/mol}$.

## Bond breaking and bond formation

The breaking of chemical bonds is endothermic ($\Delta H$ is positive).

Energy must be applied to separate two magnets bonded together. The person separating the magnets must apply energy to overcome the attractive forces.

In the same manner, energy must be applied to remove atoms bonded together in molecules. The breaking of bonds is endothermic because energy is added (chemical energy increases).

Energy must be added to separate the carbon and hydrogen atoms held together in this molecule of methane.

The formation of chemical bonds is exothermic ($\Delta H$ is negative).

Energy does not have to be supplied to bring two magnets together. The magnets naturally pull themselves together by applying energy on the person.

In the same manner, energy is released when atoms form new bonds. The formation of chemical bonds is exothermic because energy is released (chemical energy decreases).

## Bond energies

The enthalpy change ($\Delta H$) of a reaction is related to the bond energies.

Every bond in a molecule has a set bond energy. A bond with a higher energy is a stronger bond.

The total bond energy of the reactants is found by adding the bond energies of all bonds in the reactants.

Likewise, the total bond energy of the products is found by adding the bond energies of all the bonds in the products.

$\Delta H$ is equal to the difference between the total bond energy of the reactants and the total bond energy of the products. Essentially:

$\Delta H$ = Energy of reactant bonds broken $-$ Energy of product bonds formed

A reaction is therefore endothermic ($\Delta H$ is positive) if the bonds broken are stronger than the bonds formed.

A reaction is exothermic ($\Delta H$ is negative) if the bonds formed are stronger than the bonds broken.

## Fuel cells

A fuel cell is a device that uses exothermic reactions to generate electricity for automobiles and other devices.

The chemical energy released in these exothermic reactions is converted to electrical energy, which helps power devices.

Hydrogen fuel cells use the formation of water to generate electricity.

The formation of water from hydrogen and oxygen gas involves a release of energy. This reaction can therefore produce electricity.

$$\ce{H2 {(g)} + 1/2 O2 {(g)} -> H2O {(l)}}\hspace{5pt}\Delta H = -285.83\text{ kJ/mol}$$

Fuel cells are increasingly used to generate electricity for mechanical devices. They could one day become the main source of power for vehicles.

A fuel cell generates electricity from the reaction of hydrogen and oxygen to form water.

## Phase changes

Changes in state, also called phase changes can be endothermic or exothermic.

Molecules are attracted to each other because of intermolecular forces.

Moving molecules farther apart requires an input of energy. Moving molecules together releases energy because they are naturally attracted to one another.

Melting, boiling and sublimation involve moving molecules away from each other by adding energy (usually in the form of heat).

These three processes are therefore endothermic (enthalpy increases).

Condensation, freezing and deposition involve moving molecules closer together, which releases energy (usually in the form of heat).

These three processes are therefore exothermic (enthalpy decreases).