Changing Conditions & Equilibrium (GCSE Chemistry)
Changing Conditions & Equilibrium
Conditions of a Reaction
- Reactant and product amounts are affected by the conditions of a reaction. The conditions of a reaction will affect the relative amounts of reactants and products present at equilibrium. Usually, a balance is maintained and equilibrium remains unaffected.
- When conditions change, the system responds to the change. If the conditions of a reversible reaction at equilibrium change, then the system at equilibrium will respond in order to counteract the change. This is known as Le Chatelier’s principle. For example, if the temperature is increased at equilibrium, then the system will respond by opposing the increase in temperature and favour the reaction which will cause a decrease in temperature.
- Changes to a system in equilibrium can cause the reaction to favour a particular direction. When a reaction reaches equilibrium, and the conditions at equilibrium are changed, the equilibrium can ‘favour the forward reaction’ or ‘favour the reverse reaction’. The direction in which the equilibrium lies will determine whether there is a greater concentration of products or reactants:
If an equilibrium favours the reverse reaction, then there is a higher concentration of reactants than products (due to the backward reaction occurring at a faster rate).
If an equilibrium favours the forward reaction, then there is a higher concentration of products than reactants (due to the forward reaction occurring at a faster rate).
Le Chatelier’s Principle
- Le Chatelier’s principle focuses on maintaining equilibrium. If the conditions of a reaction at equilibrium change, then the system will try to oppose or reverse the change. This principle is called Le Chatelier’s principle.
- Changes are rectified in a predictable manner. When the conditions of an equilibrium are changed, the system will react in a predictable manner in order to oppose the change. This makes it easy for us to use Le Chatelier’s Principle.
Making Qualitative Predictions
In exams, you should be able to make qualitative predictions about the effect of changes on systems at equilibrium. In the next section, we will be looking at the effects of three different changes: changing concentration of a reactant/product, temperature and pressure.
Disturbances to Equilibrium
In a reversible reaction, when concentrations of reactants or products are changed, the system is no longer at equilibrium.
The system will only reach equilibrium when the concentrations of the reactants and products change in order to equalise again.
Changing the Concentrations of Reactants and Products
- When reactant concentration increases, more products will be formed. If an equilibrium is disturbed by increasing the concentration of a reactant, then the system will oppose the increase in concentration and act to decrease the concentration of the reactant. It does this by increasing the rate at which the reactant is used up, and favouring the forward reaction. This increases the rate of formation of the product.
- When product concentration increases, more reactants will be formed. If an equilibrium is disturbed by increasing the concentration of a product, then the system will oppose the increase and act to decrease the concentration of the product. It does this by increasing the rate at which the product turns into reactants, and favouring the reverse reaction, increasing its rate.
Example: The Formation of Ammonia
Let’s look at this example of a reversible reaction, involving the formation of ammonia.
N2 + 3H2 ⇌ 2NH3
If we increase the amount of nitrogen, then the system will act to oppose the increase and act to decrease the amount of nitrogen. The forward reaction is favoured. This causes an increase in formation of ammonia, NH3.
If we increase the amount of ammonia, then the system will act to oppose the increase and act to decrease the amount of ammonia. The reverse reaction is favoured. This is in order to reduce the concentration of ammonia, whilst increasing the amount of nitrogen and hydrogen formed.
Endothermic and Exothermic Directions
As previously mentioned, reversible reactions have two different directions. One direction is exothermic, whilst the other direction is endothermic.
Changing the Temperature of a Reaction
If the temperature of a system is increased, then the equilibrium will shift to decrease the temperature. Therefore, the reaction will proceed in the endothermic direction (as this takes in heat energy from the surroundings).
If the temperature of a system is decreased, then the equilibrium will shift to increase the temperature. Therefore, the reaction will proceed in the exothermic direction (as this gives out heat energy to the surroundings).
Example: The Formation of Ammonia
N2 + 3H2 ⇌ 2NH3
In this instance, the forward reaction is exothermic.
If we increase the temperature in this reaction, then the system will oppose this increase and act to decrease the temperature. The reverse reaction is favoured. This is the endothermic direction, which leads to the formation of N2 and H2.
If we decrease the temperature in this reaction, then the system will oppose the decrease and act to increase the temperature. The forward reaction is favoured. This is the exothermic direction, which leads to the formation of NH3.
Pressure Changes and Equilibrium
When the pressure of a reaction is changed, the position of equilibrium will shift. Gaseous reactions will be affected by changes in pressure, but reactions with solids and liquids will not.
Pressure is affected by the number of molecules. The more molecules/ moles of a gas there are, the higher the pressure.
Changing the Pressure of a Reaction
If the pressure of a system is increased, then the system will act to oppose the increase and decrease the pressure. Therefore, the reaction will proceed in the direction with the least number of molecules, as this will decrease the pressure.
The number of molecules can be calculated by counting the number of each type of gaseous substance present in the reaction.
If the pressure of a system is decreased, then the system will act to oppose the decrease and increase the pressure. Therefore, the reaction will proceed in the direction with the most number of molecules, as this will increase the pressure.
Example: The Formation of Ammonia
N2 + 3H2 ⇌ 2NH3
In this reaction, there are 4 moles of molecules on the left and 2 moles of molecules on the right hand side.
If we increase the pressure of this reaction, then the system will act to oppose this increase and decrease the pressure. The reaction with the fewest number of molecules is favoured, which leads to the formation of NH3. This will reduce the overall pressure.
If we decrease the pressure of this reaction, then the system will act to oppose the decrease and increase the pressure. The reaction with the largest number of molecules is favoured, which leads to the formation of N2 and H2. This means that the overall pressure will increase.
Chemical equilibrium is a state in a reaction where the rate of the forward reaction is equal to the rate of the reverse reaction. This means that the concentration of the reactants and products remain constant over time, even though the reaction is still ongoing.
Chemical equilibrium is important in chemistry because it helps to explain why some reactions reach a point where the concentration of the reactants and products remain constant, even though the reaction is still ongoing. This understanding is important for many areas of chemistry, such as the production of chemicals and the behavior of gases.
The factors that can affect chemical equilibrium include changes in temperature, pressure, and the concentration of reactants and products. When any of these factors change, it can shift the balance between the forward and reverse reactions and affect the overall reaction.
When the conditions of a reaction are changed, such as an increase in temperature or a change in the concentration of reactants and products, it can shift the balance between the forward and reverse reactions. This can lead to a change in the overall reaction, with either more or less of the desired product being produced.
Le Chatelier’s principle is a rule that states that if a chemical system at equilibrium is subjected to a change in conditions, the system will adjust in such a way as to counteract the change and restore equilibrium. This means that when conditions are changed, the system will respond in a way that minimizes the impact of the change and maintains a state of chemical equilibrium.
Le Chatelier’s principle is applied in chemistry by predicting how changes in conditions will affect a chemical system at equilibrium. For example, if the temperature of a reaction is increased, Le Chatelier’s principle can be used to predict how the reaction will adjust to counteract the increase in temperature and maintain chemical equilibrium.
An increase in temperature can have a significant effect on a reaction at equilibrium. According to Le Chatelier’s principle, an increase in temperature will cause the reaction to shift towards the endothermic reaction, which absorbs heat. This will decrease the concentration of the products and increase the concentration of the reactants, reducing the overall reaction.
A decrease in temperature can also have a significant effect on a reaction at equilibrium. According to Le Chatelier’s principle, a decrease in temperature will cause the reaction to shift towards the exothermic reaction, which releases heat. This will increase the concentration of the products and decrease the concentration of the reactants, increasing the overall reaction.
Changes in the concentration of reactants and products can also affect a reaction at equilibrium. If the concentration of a reactant is increased, the reaction will shift towards the side with the product to counteract the increase. If the concentration of a product is increased, the reaction will shift towards the side with the reactant to counteract the increase.
The understanding of chemical equilibrium and Le Chatelier’s principle is important in chemistry because it helps to explain how reactions respond to changes in conditions and how to predict the effects of these changes.
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