The first law of thermodynamics, which is a restatement of the law of conservation of energy, states that the total amount of energy in the universe is constant. When a system undergoes any chemical or physical change, the accompanying change in its internal energy, ΔE, is given by the heat added to the system, q, plus the work done by the system, W.

ΔE = q + W

Thermodynamic quantities always consist of two parts:

• A number plus units, which gives the magnitude of the change; and
• A sign that indicates the direction of the flow.