C2813 HOW FAR, HOW FAST? ENERGETICS

Energy changes in reactions
In every chemical reaction bonds must be broken in the reactants and new bonds must be formed in the products.

For example in the Haber Process nitrogen reacts with hydrogen to produce ammonia, NH3.

Initially bonds must be broken between the nitrogen atoms in the nitrogen molecule and the hydrogen atoms in the hydrogen molecule.

As energy must be put in to break the bonds and energy is released when the new bonds form this means that most chemical reactions involve energy changes.

When we do reactions in the laboratory the apparatus is usually open to the atmosphere so, if the reaction produces a gas, it is allowed to escape into the air. This means that the reactions are occurring under conditions of constant pressure.

In this case, when a gas is produced in a reaction, it must push back the surrounding air to escape from the apparatus. This uses up some energy. The energy changes we measure under these conditions take account of all these factors and are know as enthalpy changes.
They are given the symbol .

The energy changes are mainly in the form of heat though other forms of energy can be involved (eg. burning of magnesium releases a lot of light energy as well as heat).

Video example of exothermic reaction click here to view.

When energy is lost (or given out) in a reaction it is called an exothermic reaction. As energy is given out in an exothermic reaction the mixture gets hotter and the temperature rises. This can be seen in the reaction between the magnesium and oxygen, as the temperature of the air around the burning magnesium gets very hot.

When energy is taken in by a reaction it is called an endothermic reaction. As energy is taken in the reaction mixture gets colder and its temperature falls.

Sign convention and energy diagrams
Enthalpy changes are usually measured in kilojoules (or joules). The symbol for kilojoules is kj.

In enthalpy changes that are exothermic the energy has been lost in the reaction. It is therefore given a negative sign.

In changes that are endothermic the energy has gained by the reaction mixture so it is given a positive sign.

In energy diagrams the energy changes are shown as follows:

This is an example of an energy diagram showing an exothermic reaction.

Standard enthalpy changes
So that scientists can compare data from experiments a set of standard conditions have been defined. These are

Pressure of 1 atmosphere (or 101kPa or 101,000Pa using SI units)

Temperature of 25oC (or 298K)

All solutions have a concentration of 1M (1 mol.dm-3)
Standard enthalpy changes are ones that carried out under standard conditions. The symbol used for a standard enthalpy change is---

A so-called thermochemical equation summarises all the information for a reaction.

It shows
1. The amounts in moles of the reactants and products.
2. The quantities of energy.
3. The conditions of temperature and pressure.

An example is

If half the amounts are used, then the energy change is halved as well.

It is important to include the state symbols (s) for solids, (1) for liquids, (g) for gases and (aq) for aqueous solutions in all equations connected with energy changes.

Try out interactive exercise 1 and exercise 2