First Law of Thermodynamics
Definition and meaning of First Law of Thermodynamics in chemistry.
The First Law of Thermodynamics states that energy cannot be created or destroyed, only converted between forms, for a closed system, the change in internal energy equals the heat added to the system plus the work done on it.
In more detail
Expressed mathematically as ΔU = q + w, where q is heat transferred to the system and w is work done on the system, this law is simply conservation of energy applied to thermodynamic processes. It is a foundational postulate (not derived from anything more basic) and underlies calorimetry, reaction energetics, and the definition of enthalpy (H = U + pV), which simplifies the law for processes at constant pressure. Because it tracks energy bookkeeping only, the first law says nothing about which direction a process will spontaneously proceed, that role belongs to the second law.
Key facts
| Field | Physical Chemistry |
|---|---|
| Equation | ΔU = q + w |
| Also known as | Law of conservation of energy (thermodynamic form) |
| Status | Fundamental postulate, not derived from other laws |
A gas confined in a piston absorbs 500 J of heat while expanding and doing 200 J of work on the surroundings. By the first law, ΔU = q + w = 500 J + (−200 J) = 300 J, so the gas's internal energy rises by 300 J.
Frequently asked questions
How does the first law connect to enthalpy?
Enthalpy is defined as H = U + pV. At constant pressure, the heat exchanged in a process equals ΔH, making enthalpy a practical restatement of the first law for reactions run in open vessels.
Does the first law explain why reactions proceed in a particular direction?
No. The first law only accounts for energy conservation; it cannot predict spontaneity. That requires the second law of thermodynamics and the concept of entropy.