Chemical Energy

Chemical energy is defined as the form of potential (stored) energy within the structure of atoms and molecules. It is the energy stored within chemical bonds, but it's also the energy of the electron arrangement of ions and atoms. It is the energy stored within the chemical makeup of the substance.

It is determined by the bonding within the substance and the form it takes - its state - whether it is crystalline or not. (Eg. ice at O^OC has less energy than water at O^OC - the difference is the latent (hidden) heat of fusion.).

Chemical energy can be observed and measured only when a chemical reaction occurs or matter changes form. Energy is either absorbed or released when chemical energy changes happen, or a substance changes state or form.

Any matter that is 'a fuel' contains a store of chemical energy. That energy can be released into the surroundings or absorbed from them (exothermic or endothermic). For example, combustion releases more energy than is needed to initiate the reaction. but the process of photosynthesis absorbs more energy than it releases.

In a chemical reaction, chemical bonds that store chemical energy are broken and new ones are formed, changing 'products' into 'reactants'. When breaking bonds releases more chemical energy than forming new bonds absorbs, then the reaction is exothermic and heat is released. But, sometimes it takes more energy than breaking bonds in the reactants release to form chemical bonds to make the products. This type of chemical reaction absorbs heat or other energy and is endothermic. Both exothermic and endothermic reactions involve chemical energy because energy is converted into other forms by a chemical reaction.

In physics questions we mainly see chemical energy as a store for energy in food, fuel, batteries etc. - useful objects that 'store' energy which is then released by our'use' of them.

Examples

Fossil fuels

Coal: The combustion reaction converts chemical energy into light and heat.

coal (carbon) + air (the oxygen in it!) = carbon dioxide + heat energy

Oil: This is broken into many forms all of which may be burned to release light and heat and changed into another form such as kinetic energy (petrol, propane - lighter fuel etc)

Natural gas (methane):Combustion converts chemical energy into light and heat.

Biomass

Wood: Combustion converts chemical energy into light and heat.

Food (both animal and vegetable): Digestion converts chemical energy into other forms of energy used by cells.

Photosynthesis: Photosynthesis transforms light (solar energy) into chemical energy (the sugar glucose) which is stored in all vegetation.

Chemical batteries: Batteries store chemical energy to be changed into electricity.

Air bags: Air bags contain the compound sodium azide, which is ignited when the bag is activated. The reaction produces nitrogen gas, which fills the air bag, converting chemical energy into kinetic energy.

Cold packs: Chemical energy is absorbed in a reaction.

Hot packs: Chemical reaction produces heat or thermal energy.

Matches: Striking a match converts the chemicals on the match head into other compounds, releasing light and heat.

Cellular respiration: Cellular respiration is a set of reactions that changes the chemical energy in glucose into chemical energy in ATP, a form our bodies can use.