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Le Chatelier's Principle

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Le Chatelier's Principle

Ammonia, NH3, has a distinct, pungent smell. Despite its off-putting aroma, it is important industrially as it is a fundamental ingredient in many fertilisers, synthetic fibres, and plastics. It is made in a reversible reaction. Under normal atmospheric conditions, the yield of ammonia would be very low. But if we change the conditions, we can increase the yield dramatically, using Le Chatelier’s principle.

Le Chatelier’s principle is an explanation of how systems in dynamic equilibrium respond to changing conditions. It states that if the conditions in a closed system change, the position of the equilibrium will shift to counteract the change.

  • This article is about Le Chatelier's principle in chemistry.
  • We will begin by looking at reversible reactions and dynamic equilibrium, before exploring what Le Chatelier’s principle is and how it can be used.
  • We will then look at how external factors affect equilibria in accordance with Le Chatelier’s principle.
  • Finally, we will investigate common reversible reactions used in industry.

Before we look at Le Chatelier’s principle in more depth, let’s quickly go over reversible reactions and equilibrium.

Equilibrium Le Chatelier's Principle

As you will have gathered, Le Chatelier’s principle depends on reversible reactions and equilibrium. We’ll start by looking at reversible reactions.

Reversible reactions

Reversible reactions are reactions that form products, which under different conditions can react together to form the original reactants again. (Check out Chemical Equilibrium for more information.)

There are a few terms concerning reversible reactions you should know about.

  • The reaction that goes from left to right, or from reactants to products, is called the forward reaction.

  • The reaction that goes from right to left, or from products to reactants, is called the backward reaction.

  • If there is more of the forward reaction than the backward reaction, we say that the forward reaction is favoured and that the equilibrium has shifted to the right.

  • If there is more of the backward reaction than the forward reaction, we say that the backward reaction is favoured and that the equilibrium has shifted to the left.

Le Chatelier’s principle, reversible reaction, StudySmarterA reversible reaction. Anna Brewer, StudySmarter Original

Dynamic equilibrium

When you start a reversible reaction, the concentrations of reactants and products constantly change. But if you leave the reaction in a closed system, eventually the concentrations level off. When this happens, we say that the reaction has reached dynamic equilibrium.

Dynamic equilibrium is a state of a reversible reaction, in which the concentrations of products and reactants remain constant, and the rates of the forward and backward reactions are the same.

Chemical equilibria are examples of dynamic equilibria. This means that both the forward and backward reactions are constantly ongoing. But because they happen at the same rate, their effects cancel each other out; it looks like there is no overall reaction. In a dynamic equilibrium, although products and reactants are constantly being broken down and reformed again, the overall levels of each species don’t change.

Le Chatelier’s principle, dynamic equilibrium graph, StudySmarterDynamic equilibrium graph. Anna Brewer, StudySmarter Original

What if we want more of the products than what the system is currently giving us? We can manipulate the equilibrium using Le Chatelier’s principle. We'll look at this next.

Le Chatelier’s principle definition

Let’s now move on to the main focus of this article: Le Chatelier’s principle. As we explored right at the start, Le Chatelier’s principle explains how equilibria respond to changing conditions. It states that disturbing an equilibrium, such as by changing its conditions or environment, shifts the position of the equilibrium to favour the side that reduces the disturbance and opposes the change.

Imagine you are in charge of letting people into a restaurant. The restaurant can only have a certain number of diners in it at any one time - it is limited by its number of seats. At busy times, this means that some people have to queue outside and wait to get in. However, to maximise profits, you ideally want as many people to be eating inside the restaurant as possible. At 2 o’clock, a big party ends, and lots of diners leave at once. The conditions have changed - suddenly there are far fewer people inside. In order to fill up the empty seats, you open the doors and let a lot of people inside at one go. You've reacted to the ‘disturbance’ caused by many people leaving at once by letting more people in, in order to counteract the change the disturbance caused.

Disturbing the conditions of an equilibrium causes the equilibrium to react in response. There are a few different ways in which we can disturb an equilibrium. These include:

  • Changing the temperature.
  • Changing the concentration of products or reactants.
  • Changing the pressure.

Le Chatelier’s principle and temperature

We'll begin by looking at how temperature affects an equilibrium. Remember that Le Chatelier’s principle states that changing the conditions of an equilibrium causes the reaction to shift to oppose the change. Can you predict what will happen if you increase the temperature of a dynamic equilibrium?

The system will try to decrease the temperature to oppose the change. The only way it can do this is by favouring the endothermic reaction - the one that takes in heat as energy.

Here's an example. Look at this equation for the equilibrium involving nitrogen, hydrogen, and ammonia.

N2(g) + 3H2(g) ⇌ 2NH3(g) ΔH = -92 kJ mol-1

The forward reaction is exothermic, while the backward reaction is endothermic. We can say the following:

  • Increasing the temperature favours the backward endothermic reaction and causes the equilibrium to shift to the left. This will oppose the change by decreasing the temperature.
  • Decreasing the temperature favours the forward exothermic reaction and causes the equilibrium to shift to the right. This will oppose the change by increasing the temperature.

Le Chatelier’s principle and concentration

What happens when you decrease the concentration of a substance? You essentially just have fewer molecules in the same volume. This is what happens when lots of diners leave the restaurant. In order to oppose the disturbance caused by the changing conditions, we need to let more people in, i.e., we need to increase the concentration of that substance. For example, if we decrease the concentration of the products, the equilibrium will shift to favour the forward reaction to bring the concentration of products back up again. If we decrease the concentration of the reactants, the equilibrium will shift to favour the backward reaction, in order to bring the concentration of the reactants back up again.

You can also think about what would happen if you increased one of the concentrations - say, that of the reactants. The forward reaction would increase to try and ‘use up’ some of the extra reactant molecules. You can see this in the diagram below, where the arrow in bold shows the favoured reaction.

Le Chatelier’s Principle, concentration effect, StudySmarterThe effect of changing concentration on an equilibrium reaction. Anna Brewer, StudySmarter Original

Here’s the equation for the equilibrium involving ammonia again:

N2(g) + 3H2(g) ⇌ 2NH3(g) ΔH = -92 kJ mol-1

We can say the following:

  • Increasing the concentration of nitrogen and/or hydrogen will favour the forward reaction and shift the equilibrium to the right. This will oppose the change by using up some of the nitrogen and hydrogen, reducing their concentration.
  • Decreasing the concentration of nitrogen and/or hydrogen will favour the backward reaction and shift the equilibrium to the left. This will oppose the change by forming more nitrogen and hydrogen, increasing their concentration.

Le Chatelier’s principle and pressure

Provided they are at the same temperature and in the same volume container, all gases have the same pressure per mole. Pressure is caused by the gas molecules randomly colliding with the sides of the container. Now imagine that we increase the pressure of the system. To oppose this change, the system will try to reduce the pressure by reducing the number of collisions that occur. The system can’t change the speed of the particles or the frequency of their collisions, but it can reduce the pressure by decreasing the number of gas molecules in the system.

Fewer molecules, fewer collisions - simple, right? Therefore, increasing the pressure favours the reaction that produces fewer moles of gas. On the other hand, decreasing the pressure favours the reaction that produces a greater number of moles of gas.

Take a look at this equation again:

N2(g) + 3H2(g) ⇌ 2NH3(g) ΔH = -92 kJ mol-1

The forward reaction produces two moles of gas. The backward reaction produces four moles of gas. We can say the following:

  • Increasing the pressure will favour the forward reaction and shift the equilibrium to the right. This is because the forward reaction produces fewer moles of gas than the backward reaction. This will oppose the change by reducing the pressure.
  • Decreasing the pressure will favour the backward reaction and shift the equilibrium to the left. This is because the backward reaction produces a greater number of moles of gas than the forward reaction. This will oppose the change by increasing the pressure.

There are two things to note here. Firstly, changing the pressure only affects the equilibrium of gaseous species. You can ignore any moles of solids or liquids in the equation. Secondly, changing the pressure will have no effect on a gaseous equilibrium if both of the reactions produce the same number of moles of gas. There is no way of decreasing the number of moles of gas in the system - no matter which reaction is favoured, the equilibrium won’t change.

Le Chatelier’s principle and catalysts

Catalysts don’t affect the position of equilibrium. This is because they speed up both the forward reaction and backward reaction at the same rate. However, adding a catalyst is useful because it speeds up the time it takes for a system to reach dynamic equilibrium.

Catalysts are substances that increase the rate of reaction without being used up or changed in the process.

Le Chatelier's Principle - Key takeaways

  • Le Chatelier's principle states that changing the conditions of an equilibrium reaction causes the equilibrium to shift to oppose the change.
  • Increasing the temperature of an equilibrium reaction favours the endothermic reaction, whilst decreasing the temperature favours the exothermic reaction.
  • Increasing the pressure of an equilibrium favours the reaction that produces the fewest moles of gas, whilst decreasing the pressure favours the reaction that produces the greater number of moles of gas.
  • Increasing the concentration of the reactants favours the forward reaction, whilst increasing the concentration of the products favours the backwards reaction.
  • Catalysts do not affect the position of equilibrium, as they speed up both the forward and backward reactions at the same rate. However, adding a catalyst is useful because it speeds up the time it takes for a system to reach dynamic equilibrium.

Frequently Asked Questions about Le Chatelier's Principle

Le Chatelier’s principle is an explanation of how systems in dynamic equilibrium respond to changing conditions. It states that if the conditions in a closed system change, the position of the equilibrium will shift to counteract the change.

An example of Le Chatelier’s principle is the Haber process, used to make ammonia. If we increase the pressure, this favours the forward reaction and increases the yield of ammonia.

Le Chatelier’s principle is an explanation of how systems in dynamic equilibrium respond to changing conditions. It states that if the conditions in a closed system change, the position of the equilibrium will shift to counteract the change. It’s important because it allows us to manipulate the conditions of an equilibrium reaction in order to increase or decrease the yield.

To solve problems involving Le Chatelier’s principle, you need to consider the effect of the change on the equilibrium. Ask yourself the following questions - which reaction is exothermic? Which reaction produces the greatest number of moles of gas? Remember that according to Le Chatelier’s principle, the position of the equilibrium will always shift to counteract the change in conditions. For example, if you increase the temperature of the system, the endothermic reaction will be favoured to take in some of the excess heat. If you increase the pressure, the reaction that produces the fewest moles of gas will be favoured. Analysing the equilibrium in this way should help you solve Le Chatelier’s principle problems.

Decreasing the pressure of a system at equilibrium favours the reaction that produces the fewest moles of gas. This is because one mole of any gas always takes up the same volume at a given temperature and pressure, so reducing the number of moles of gas reduces the overall pressure. 

Final Le Chatelier's Principle Quiz

Question

What is a reversible reaction?

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Answer

Reversible reactions are reactions that form products, which under different conditions can react together to form the original reactants again.

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Question

What is the definition of dynamic equilibrium?

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Answer

 Dynamic equilibrium is a state of a reversible reaction, in which the concentrations of products and reactants remain constant and the rates of the forward and backward reactions are the same. 

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Question

State Le Chatelier’s principle. 


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Answer

Le Chatelier's principle states that changing the conditions of an equilibrium causes the position of the equilibrium to shift to oppose the change.

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Question

Which of the following affect the position of an equilibrium?


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Answer

Temperature

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Question

Increasing the temperature of an equilibrium favours the ____ reaction.


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Answer

Endothermic

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Question

Increasing the pressure of an equilibrium favours the reaction that produces ____ number of moles of gas


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Answer

The smallest

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Question

Increasing the concentration of the reactants in an equilibrium favours the _____ reaction.


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Answer

Forward

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Question

In which direction would the equilibrium shift if we were to add a catalyst to the reaction below?


Cl2(g) + H2(g) ⇌  2HCl(g)


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Answer

The position of the equilibrium would not shift as catalysts speed up both the forward reaction and reverse reaction by the same amount.

Show question

Question

In which direction would the equilibrium shift if we were to increase the pressure for the reaction below?


Cl2(g)  + H2(g) ⇌  2HCl(g)

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Answer

The equilibrium would not shift as there is an equal number of moles of gas on both sides of the equation.

Show question

Question

Why does adding a catalyst have no effect on the position of an equilibrium?

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Answer

Adding a catalyst speeds up the rates of both reactions equally. It does not favour one reaction over the other.

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Question

What symbol do you use to show that the forward reaction is favoured in a reversible reaction?

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Answer

Trick question - we don't use a symbol to show this.

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Question

What are compromise conditions?

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Answer

Compromise conditions are conditions that don’t necessarily give the greatest yield of the product, but are the most economical when it comes to balancing factors like cost and rate of reaction.

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Question

Name the reactants used to make methanol.

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Answer

Synthesis gas (a mixture of carbon monoxide and hydrogen)

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Question

Give the reactants used to make ethanol via hydration.

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Answer

Ethene (C2H4) and steam (H2O).

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Question

Give the reactants used to make sulphuric acid in the Contact process.

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Answer

Sulphur dioxide (SO2), oxygen (O2) and a small amount of sulphuric acid (H2SO4).

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Question

Give the reactants used to make ammonia in the Haber process.

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Answer

Nitrogen (N2) and hydrogen (H2).

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Question

Many industrial reversible reactions are exothermic. Explain how compromise temperatures are used to increase the yield. 

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Answer

The forward reaction is exothermic, so decreasing the temperature favours the forward reaction. However, decreasing the temperature too much decreases the overall rate of reaction, and so a compromise temperature that is somewhere in the middle is used instead.

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Question

Many industrial reversible reactions are favoured by a high pressure. Explain how compromise temperatures are used in these cases.

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Answer

Increasing the pressure favours the forward reaction and so increases the yield. However, maintaining a high pressure is expensive and so a compromise pressure that is somewhere in the middle is used instead.

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Question

Why are catalysts used in industrial reversible reactions?

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Answer

They increase the rate of reaction and so increase yield.

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