Rates and equilibrium, Year 11: Le Chatelier's principle

Le Chatelier's principle

HintIt carries the surname of the French chemist who proposed it.

The whyThe principle lets chemists predict which way an equilibrium will shift when the concentration, temperature or pressure is changed. The system never cancels the change completely; it only partly undoes it.

Concentration, temperature and pressure

HintOne for solutions, one read from a thermometer, and one that matters only for gases.

The whyEach of these has its own rule for which way the equilibrium shifts, and all three rules follow from the same principle: the system acts to counteract the change.

No: the relative amounts at equilibrium depend on the conditions

HintA reaction has no single, fixed recipe for its final mixture.

The whyA reaction does not have a single equilibrium mixture. Change the temperature, pressure or concentrations and the system settles at a new equilibrium with different proportions.

It has increased

HintProducts are written on the right-hand side of the equation.

The why'To the right' means towards the products of the forward reaction, so the new equilibrium mixture contains relatively more product and less reactant. 'To the left' means the opposite.

One that lowers it again

HintThe system always pushes back against what was done to it.

The whyWhatever the change, the system shifts in the direction that counteracts it. A rise in pressure is opposed by a shift that lowers the pressure; a rise in temperature is opposed by a shift that takes energy in.

It increases, until equilibrium is reached again

HintThe system acts to use up some of what was added.

The whyRaising the concentration of a reactant makes more product form. The extra nitrogen reacts with some of the hydrogen, so the new equilibrium mixture holds more ammonia than before.

They decrease, as more A and B react

HintThe system acts to replace some of what was taken away.

The whyLowering the concentration of a product makes more reactants react until equilibrium is reached again. This is how removing a product as it forms can pull a reversible reaction forwards.

No: the concentrations all change until equilibrium is reached again

HintAdding something upsets the balance between the two rates.

The whyWith more reactant present, the forward reaction is for a while faster than the reverse reaction. The concentrations of all the substances then change, not just the one that was added, until the two rates are equal once more.

Towards the reactants

HintThe system acts to use up some of what was added.

The whyExtra product is a change the system counteracts by turning some of it back into reactants. The reverse reaction runs faster than the forward one until a new equilibrium is reached.

It becomes a darker red

HintDecide which substance there is more of once the system has responded.

The whyIron(III) ions are a reactant, so raising their concentration makes more of the product form until equilibrium is reached again. More of the dark red product means a deeper colour.

endothermic

HintIt is the direction that takes energy in.

The whyIf the forward reaction is endothermic, heating increases the relative amount of products; if the forward reaction is exothermic, heating decreases it. Cooling does the reverse in each case.

It becomes a darker brown

HintWork out which direction takes energy in, and which gas that direction makes.

The whyA rise in temperature favours the endothermic direction, which here is the forward reaction. More brown NO2 forms, so the mixture darkens; cooling the tube in ice makes it paler again.

It increases

HintCooling favours the direction that gives energy out.

The whyLowering the temperature shifts the equilibrium in the exothermic direction, which here is the forward reaction, so more SO3 is present at equilibrium. The price is that both reactions become slower at the lower temperature.

Exothermic

HintSee what heating does to the amount of product, then ask which direction heating favours.

The whyThe amount of product falls as the temperature rises, so heating is shifting the equilibrium backwards. Heating favours the endothermic direction, so the reverse reaction is endothermic and the forward reaction is exothermic.

The endothermic reaction takes in energy, which counteracts the rise in temperature

HintThe system responds so as to undo part of what was done to it.

The whyHeating is the change, and the system opposes it. Shifting in the direction that absorbs energy uses up some of the energy supplied, so the temperature rises less than it otherwise would.

The side with fewer molecules of gas

HintCount the particles shown to the left and to the right of the arrow; one lot takes up less room.

The whyThe numbers of molecules are read from the balanced symbol equation, using the big numbers in front of the formulae. A decrease in pressure does the opposite and shifts the equilibrium towards the larger number of molecules.

It increases: the right-hand side has fewer molecules (2 against 4)

HintAdd up the big numbers in front of the formulae before the arrow, then after it.

The whyThe left-hand side shows 1 + 3 = 4 molecules of gas and the right-hand side shows 2. Raising the pressure shifts the equilibrium towards the smaller number, so more ammonia is present.

It does not change: there are 2 molecules of gas on each side

HintCompare the totals of the big numbers to the left and to the right of the arrow.

The whyThe left-hand side shows 1 + 1 = 2 molecules and the right-hand side shows 2. With equal numbers, neither direction would lower the pressure, so the system has no way to counteract the change.

To the right, towards NO2

HintLower pressure favours whichever side makes the larger number of particles.

The whyA decrease in pressure shifts the equilibrium towards the side with the larger number of molecules. One molecule of N2O4 becomes two of NO2, so the right-hand side has more.

Fewer gas molecules exert a lower pressure, which counteracts the rise

HintThink of what pushes on the walls of the container, and how the system could reduce it.

The whyGas pressure comes from molecules hitting the walls. Shifting to the side with fewer molecules cuts the number of impacts, so the pressure ends up lower than it would otherwise be: the system has opposed the change.