Learning path

- open/closed/isolated systems
- Reversibility
- dynamic (vs static) equilibria
- steady state
- macroscopic/microscopic changes
- representing reversibility and equilibria
- Le Chatelier's principle
- equilibrium constant for solutions; Kc (temperature dependent)
- partial pressure calculations
- equilibrium constant for gases; Kp (temperature and volume dependent)
- concentration/time graphs
- rate/time graphs

We only discuss equilibria in closed systems where no reactant or product can be added or removed. We can, however, affect changes in temperature and/or pressure (for gaseous systems via pressure changes).

Static (mechanical) equilbrium in chemistry can only occur when there is no microscopic change from reactant to product. This can only happen for irreversible changes like combustion. At equilibrium, the rate of the forward reaction is zero. The rate of the reverse reaction (by definition because there isn't one) is also zero. Hence, it's very rare to use the term 'static' equilbrium in chemistry.

Dynamic equilibrium, however, is very relevant.

Examples

H2(g) + I2(g) ⇌ 2HI(g) ΔH = +52.1 kJmol-1
CH3COOC2H5(l) + H2O(l) ⇌ CH3COOH(l) + C2H5OH(l) ΔH = +480kJmol-1
N2(g) + 3H2(g) ⇌ 2NH3(g) ΔH = -46kJmol-1
3Fe(s) + 4H2O(g) ⇌ Fe3O4(s) + 4H2(g)


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For those of you who are interested, these symbols are not called 'arrows' in the character set, they are called 'harpoons' https://graphemica.com/search?q=harpoon