Signs, symbols and terminology

The value of a heat change depends upon the conditions under which the change is measured.

The Heat change measured at constant pressure is known as the enthalpy change and is given the symbol DH.

Internationally agreed convention under which enthalpy changes are measured. Under these conditions, an enthalpy change is called a standard enthalpy change.

Temperature of 298K, Pressure of 101,325 Nm^-2 (1 atmosphere). Given the symbol DH^q.

3. Units.

Heat is a form of energy and both are measured in the same units. The SI unit of energy is the Joule (J), although this is too small for most chemical reactions, so the kiloJoule (kJ) is used.

Never use the term calorie, which is for food only, although some books may include calories in questions. 1 cal = 4.184 J.

4. Sign Convention.

In terms of energy (or enthalpy changes), there are two types of chemical reaction.

· Exothermic

Exothermic reactions do the following:

1. Get hot to the touch

2. Liberate heat energy to the surroundings.

3. Result in a general increase in the temperature of the surroundings.

· Endothermic

Endothermic reactions do the following:

1. Get cold to the touch.

2. Absorb heat energy from the surroundings.

3. Result in a general decrease in the temperature of the surroundings.

The sign convention states the following:

DH negative = exothermic change.

DH positive = endothermic change.

A negative DH implies that energy has been lost from the substance involved and given to the surroundings!

An enthalpy change for a reaction can be represented as follows:

C(s) + O₂(g) ® CO₂(g) DH^q= -393 kJ mol^-1

So this equation states in a kind of shorthand:

…one mole of solid carbon reacts with one mole of gaseous oxygen, gaseous carbon dioxide is formed. 393 kJ of heat are evolved under standard conditions.

In practice however we need to be even more precise, there are two different forms of carbon remember!

C(graphite, solid) + O₂(g) ^q= -393 kJ mol^-1

As distinct from:

C(diamond, solid) + O₂(g)