First law of thermodynamics

Definition

The first law of thermodynamics states that energy cannot be created or destroyed, only transferred or converted from one form to another. In chemistry, it is often formulated as $\Delta U = Q - W$, where $\Delta U$ is the change in internal energy, $Q$ is the heat added to the system, and $W$ is the work done by the system.

5 Must Know Facts For Your Next Test

1. The first law of thermodynamics is also known as the law of energy conservation.
2. In a closed system, any increase in internal energy must come from heat added to the system or work done on the system.
3. For an ideal gas undergoing an isothermal process, $\Delta U = 0$ and thus $Q = W$.
4. If a process occurs at constant volume (isochoric), no work is done ($W=0$), so $\Delta U = Q_v$, where $Q_v$ is heat at constant volume.
5. For a cyclic process, where the system returns to its initial state, the net change in internal energy over one cycle ($\Delta U_{cycle}$) is zero.

Review Questions

• What does the first law of thermodynamics state about energy?
• How can you express the first law of thermodynamics mathematically?
• What happens to internal energy during an isochoric process?

Related terms

Internal Energy: The total energy contained within a system due to both kinetic and potential energies of its particles.

Enthalpy:

A thermodynamic quantity equivalent to the total heat content of a system. It is equal to the internal energy plus the product of pressure and volume: $H = U + PV$.

Isothermal Process: A process that occurs at constant temperature.