Every card in Rates and equilibrium, Year 11: Le Chatelier's principle
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- The principle that a system at equilibrium responds to counteract any change made to its conditions
Le Chatelier's principle
HintIt carries the surname of the French chemist who proposed it.
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.
- Which three conditions of a reaction at equilibrium can be changed to alter the relative amounts of the substances present?
Concentration, temperature and pressure
HintOne for solutions, one read from a thermometer, and one that matters only for gases.
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.
- The same reversible reaction is brought to equilibrium in two sealed containers, one at 200 °C and one at 400 °C. Would you expect the two equilibrium mixtures to contain the same proportions of reactants and products?
No: the relative amounts at equilibrium depend on the conditions
HintA reaction has no single, fixed recipe for its final mixture.
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.
- A chemist says that the position of an equilibrium has 'shifted to the right'. What has happened to the relative amount of products in the mixture?
It has increased
HintProducts are written on the right-hand side of the equation.
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.
- The pressure on a mixture of gases at equilibrium is suddenly raised. According to Le Chatelier's principle, does the system respond in a way that raises the pressure further or one that lowers it again?
One that lowers it again
HintThe system always pushes back against what was done to it.
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.
- Nitrogen and hydrogen react reversibly to form ammonia: N2(g) + 3H2(g) ⇌ 2NH3(g). The mixture is at equilibrium in a sealed container, and extra nitrogen is pumped in. What happens to the amount of ammonia?
It increases, until equilibrium is reached again
HintThe system acts to use up some of what was added.
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.
- A reaction A + B ⇌ C + D is at equilibrium in solution. Some of the product D is removed. What happens to the amounts of A and B?
They decrease, as more A and B react
HintThe system acts to replace some of what was taken away.
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.
- Extra reactant is added to a reaction mixture that was at equilibrium. In the moments just afterwards, is the mixture still at equilibrium?
No: the concentrations all change until equilibrium is reached again
HintAdding something upsets the balance between the two rates.
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.
- A reversible reaction is at equilibrium, and the concentration of one of its products is increased. Does the equilibrium position shift towards the reactants or towards the products?
Towards the reactants
HintThe system acts to use up some of what was added.
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.
- In solution, pale yellow iron(III) ions react reversibly with colourless thiocyanate ions to form a dark red product. The mixture is at equilibrium and looks orange. More iron(III) ions are added. What happens to the colour?
It becomes a darker red
HintDecide which substance there is more of once the system has responded.
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.
- Raising the temperature of a system at equilibrium shifts the equilibrium in the ____ direction.
endothermic
HintIt is the direction that takes energy in.
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.
- In a sealed tube, colourless N2O4 gas is in equilibrium with brown NO2 gas: N2O4(g) ⇌ 2NO2(g). The forward reaction is endothermic. The tube is placed in hot water. What happens to the colour of the gas?
It becomes a darker brown
HintWork out which direction takes energy in, and which gas that direction makes.
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.
- Sulfur dioxide reacts reversibly with oxygen: 2SO2(g) + O2(g) ⇌ 2SO3(g). The forward reaction is exothermic. The temperature is lowered. What happens to the relative amount of SO3 at equilibrium?
It increases
HintCooling favours the direction that gives energy out.
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.
- For a reversible reaction, the percentage of product in the equilibrium mixture is 60% at 200 °C, 25% at 400 °C and 8% at 600 °C. Is the forward reaction exothermic or endothermic? (one word)
Exothermic
HintSee what heating does to the amount of product, then ask which direction heating favours.
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.
- Use Le Chatelier's principle to explain why heating an equilibrium mixture favours the endothermic direction.
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.
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.
- For a reaction between gases at equilibrium, an increase in pressure shifts the equilibrium position towards which side of the equation?
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.
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.
- N2(g) + 3H2(g) ⇌ 2NH3(g) is at equilibrium. The pressure is increased. What happens to the amount of ammonia at equilibrium, and why?
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.
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.
- H2(g) + I2(g) ⇌ 2HI(g) is at equilibrium. The pressure is increased. What happens to the position of equilibrium?
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.
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.
- N2O4(g) ⇌ 2NO2(g) is at equilibrium in a gas syringe. The plunger is pulled out, lowering the pressure. Which way does the equilibrium position shift?
To the right, towards NO2
HintLower pressure favours whichever side makes the larger number of particles.
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.
- Why does a system of gases at equilibrium shift towards the side with fewer molecules when the pressure is raised?
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.
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.
1★ GCSE-CHEM-RTE-0040
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20★ GCSE-CHEM-RTE-0059
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