Factors that Affect Equilibrium

In short: Le Chatelier's principle says that when an equilibrium is disturbed, it shifts to partly undo the change. Adding a reactant shifts it towards products, higher pressure shifts it to the side with fewer gas moles, heating shifts it in the endothermic direction, and a catalyst causes no shift.

Le Chatelier's principle

If a system at equilibrium is disturbed, the system adjusts to partly undo the change, and a new equilibrium is reached.

"Shift right" (or "shift forwards") means more products are made. "Shift left" means more reactants are made.

The system only partly undoes the change. It never gets all the way back to where it started.

Changing concentration

• Add a reactant: the system uses some of it up, so it shifts right.
• Remove a product: the system makes more of it, so it shifts right.
• Add a product, or remove a reactant: it shifts left.

Ions can be removed by making a precipitate or by reacting them. Adding $\mathrm{O}{\mathrm{H}}^{-}$ removes ${\mathrm{H}}^{+}$ (they make water). This is a common trick in exam questions. For example, in $\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7^{2-}+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons 2Cr{\mathrm{O}}_4^{2-}+2{\mathrm{H}}^{+}$ (orange to yellow), adding $\mathrm{NaOH}$ removes ${\mathrm{H}}^{+}$, so it shifts right and turns yellow.

Adding water to a solution dilutes every ion. The system shifts to the side with more dissolved particles. For the iron thiocyanate equilibrium (2 ions on the left, 1 on the right), it shifts left and the red fades.

Solids and pure liquids do not appear in the equilibrium expression. Adding more solid carbon or more liquid water (as the solvent) does not shift the equilibrium.

Changing pressure or volume (gases)

Squash a gas mixture into a smaller volume and its pressure goes up. The system shifts to the side with fewer gas moles, which lowers the pressure.

$${\mathrm{N}}_2(\mathrm{g})+3{\mathrm{H}}_2(\mathrm{g})\rightleftharpoons 2N{\mathrm{H}}_3(\mathrm{g})$$

Left side: 4 moles of gas. Right side: 2 moles. Higher pressure shifts right and makes more ammonia.

• Increase the volume: pressure drops, so it shifts to the side with more gas moles.
• If both sides have the same number of gas moles (like ${\mathrm{H}}_2+{\mathrm{I}}_2\rightleftharpoons 2HI$), pressure has no effect on the position.
• Adding an inert gas at constant volume does nothing, because the concentrations of the reacting gases do not change.

Changing temperature

Treat heat like a chemical in the equation.

• Exothermic forward reaction ($\Delta H<0$): heat is a "product". Heating shifts left. Cooling shifts right.
• Endothermic forward reaction ($\Delta H>0$): heat is a "reactant". Heating shifts right.

Temperature is the only change that alters the value of the equilibrium constant $K$ (next chapter).

Adding a catalyst

A catalyst speeds up the forward and reverse reactions equally. The equilibrium is reached faster, but the position and amounts do not change.

Summary table

Reading concentration-time graphs

These graphs often appear in exams. Look at how the lines change at the moment of the disturbance:

• One line jumps up or down suddenly, then the others change smoothly: a chemical was added or removed.
• All lines jump at once (by the same factor): the volume changed.
• No line jumps, but all start changing smoothly: the temperature changed.

Industrial chemistry: the Haber process

Ammonia is made by ${\mathrm{N}}_2+3{\mathrm{H}}_2\rightleftharpoons 2N{\mathrm{H}}_3$, $\Delta H=-92$ kJ.

• High pressure (about 200 atm) shifts right. Very high pressure is costly and dangerous.
• Low temperature gives the best yield, but the reaction would be far too slow. A compromise of about 450 °C is used.
• An iron catalyst speeds it up.
• Ammonia is removed as it forms, so the reaction keeps shifting right.

Quick summary

• Le Chatelier: a disturbed equilibrium shifts to partly undo the change.
• Add reactant or remove product: shift right. Solids and pure liquids: no effect.
• Higher pressure (smaller volume): shift to the side with fewer gas moles.
• Heating shifts in the endothermic direction and changes $K$.
• A catalyst gives no shift; equilibrium is just reached faster.
• On graphs: one line jumps = chemical added; all jump = volume change; smooth change = temperature.

Quiz