# TP: Thermodynamic Equilibrium

### Thermodynamic Equilibrium

##### Definition:

Equilibrium is the condition whereby there are no changes with time nor any tendency to change with time.

##### Note:

By no tendency to change with time, we mean that all driving forces in the system are balanced.

##### Note:

This means all driving forces are balanced.

Equilibrium implies:

• Mechanical equilibrium: $\sum F = 0$ or $P_{sys} = P_{surr}$.
• Thermal equilibrium: $T_{sys} = T_{surr}$.
• Chemical (and chemical reaction) equilibrium: no tendency to change phase or react (we will yield an equation to mathematically state this later).
• Phase equilibrium .....
##### Outcome:

Distinguish between equilibrium and steady state

##### Test Yourself:

Explain why only closed systems may be in equilibrium.

##### Test Yourself:

Give examples of systems at steady state but not in equilibrium.

Note that this necessitates a closed system because an open system -- where material flows across the boundaries -- would require that there is a tendency of the material to change position with time (also there is an out of balance driving force that creates the movement).

A stable equilibrium is one that a system will return to when perturbed.