Within this unit, we have observed changes in thermal energy by observing changes in temperature within a closed system. We saw this in a temperature decrease with ionic solids dissolving in water in lessons 2 and 3. We also saw this with a temperature increase through the synthesis of an ionic compound in lesson 4.

The thermal energy transferred when bonds are broken or formed is referred to as the enthalpy change (∆H). For our purposes, energy transfer will always occur within a closed system.

Because any energy transfer discussed in this class will occur within a closed system, the following relationship will be true for these energy transfers:

Enthalpy change = heat energy

∆H = q

In other words, as long as the energy is transferred within a closed system the enthalpy change (∆H) will be equal to the heat energy (q) that is transferred:

q = m c ∆T

∆H = m c ∆T

where:

q = heat energy in joules (J), so ∆H = energy transferred in joules (J)

m = mass of sample in grams (g)

c = specific heat (J/g^OC)

ΔT = change in temperature (final temperature - initial temperature)

Specific heat, (c), is an intensive physical property that has a different, characteristic value for every substance. For example, the specific heat for water is 4.18 J/g^OC. This would mean that it will take 4.18 joules of energy to raise the temperature of 1 gram of water 1^OC.