Energy and change of state
Energy in reactions
Every substance contains energy, which varies according to the elements involved and its physical state. Reactions occur because of the energy provided by the reactants. Knowing how and how fast this energy will be released or used helps in understanding how to control reactions and recognise how certain chemical reactions benefit people in their daily lives.
Exothermic reactions release heat during a reaction. Occasionally a little heat is needed to energise the reactants, but once the reaction starts, the substances release heat of their own. When you light a match, the friction energy between the match and the striking surface generates enough heat to continue the reaction. See image 1.
Conversely, endothermic reactions absorb heat throughout the reaction process. This lowers the temperature around the reaction. An example of this process is when ammonium nitrate mixes with water such as in an 'instant' ice pack. Although this example is not a chemical reaction (because it does not create a new substance), it shows that a substance can absorb heat to reduce the temperature from its environment.
A reaction that does not release or absorb heat is thermoneutral. For the most part, reactions will usually involve some form of thermal energy gained or lost.
See animation 1.
Occasionally, a reaction will release or absorb other types of energy such as light or electricity. Glow sticks contain hydrogen peroxide, a fluorescent dye and an ester. When the hydrogen peroxide mixes with the ester it excites the dye, which then releases excess light energy in the form of a glow. See image 2.
Common household batteries contain chemical energy released as electricity. In this case, the reactants are not in contact with each other but placed close enough so that electron transfer can take place. This transfer produces a current, which is used as electrical power. The reactants do eventually start to form products on both sides, which is why batteries eventually run out of power.
Change of state
Sometimes it is necessary to indicate the states of the reactants and products. To write a chemical formula, write the state of any molecule in subscript after the molecular formula using the following notations:
- Molecule (s) = solid
- Molecule (l) = (pure) liquid
- Molecule (g) = gas
- Molecule (aq) = aqueous - the substance is dissolved in water but is not a pure liquid
An example using this notation:
Mg(s) + 2HCl(aq) 
MgCl2 (aq) + H2 (g)
This more detailed equation shows that when solid magnesium meets dilute hydrochloric acid, the result is magnesium chloride in an aqueous solution, and hydrogen gas.
