- A reversible reaction is one that can go both ways; occuring in different directions.
E.g. Haber Process: N2 + 3H2 <-> 2NH3
- This means that the reaction is in equilbrium.
- Dynamic equilibrium requires:
-System must be closed.
-Rate of forward reaction must = rate of backward reaction.
-Macroscopic (what you can see) properties must remain the same.
- Le Chatilier's principle states: "when any of the conditions affecting the position of equilbrium change, then the position changes to minimise that change".
- CH3COOH + CH3CH2 <-> CH3COOCH2CH2CH3 + H2O
-Increasing concentration of ethanoic acid (CH3COOH), will make the equilibrium shift to the right to mimise the change; so more H2O (etc).
-BUT reducing concentration of H2O will make the equilibrium shift to the right again to minimise the change; so more products.
- Only applies to equilibria with gases (check state symbols).
2SO2(g) + O2(g) <-> 2SO3(g)
-Direction depends on number of moles of gases (3 on the left here, 2 on the right).
-So increasing pressure here will make it shift to the right, as there are fewer moles.
-And decreasing will make it shift to the left, as there are more moles.
- INCREASING TEMP ALWAYS FAVOURS THE
ENDOTHERMIC REACTION DH = positive
- DECREASING TEMP ALWAYS FAVOURS THE
EXOTHERMIC REACTION DH = negative
E.g. A + B <-> C DH = -190 kJmol –1
DH = + 190 kJmol-1 <---------
-So increasing temperature will cause the equilibrium to shift to the left, creating more A + B.
- Adding a catalyst will increase rate of reaction, BUT
- Will not affect the position of equilibrium.
- N2(g) + 3H2(g) <-> 2NH3(g) DH = -92kJmol-
-Increasing the pressure will make it shift to the right to minimise change.
-And decreasing temperature will make it shift to the right also; exothermic.
-Low temperature = slow reaction.
-High pressure = dangerous/expensive.
-Comromise on both pressure and temperature.
-Use Fe (catalyst).