Chemistry: Unit F322: Chains, Energy and Resources



Chemistry: Unit F322: Chains, Energy and Resources

Equilibrium

Le Chatelier states that;

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Dynamic equilibrium occurs in a reversible reaction where;

▪ the system is closed

▪ the rate of the forwards reaction is equal to the rate of the reverse reaction

▪ there is a constancy of macroscopic properties (no change in temperature, pressure, concentration or colour)

When a system in dynamic equilibrium is subjected to a change, equilibrium will shift to minimise the effect of the change.

Changes in Temperature;

Increasing the temperature will favour the endothermic reaction; the system will try to reduce the temperature again to minimise the change, so the endothermic reaction will become dominant and equilibrium will shift towards the reaction which is endothermic.

Decreasing the temperature will favour the exothermic reaction; the system will try to increase the temperature again to minimise the change, so the exothermic reaction will become dominant and equilibrium will shift towards the reaction which is exothermic.

Changes in Pressure;

Increasing the pressure will shift equilibrium towards the fewest number of moles. In this example, equilibrium will shift left towards the reactants, because there are only 3 moles on the left, compared to 4 moles on the right.

Decreasing the pressure will shift equilibrium towards to most number of moles. In this example, equilibrium would shift right, towards the products, because there are 4 moles on the right and only 3 on the left.

A + B C + D ”H+ve

Temperature increasey 3 on the left.

A + B C + D ΔH+ve

Temperature increased: equilibrium shifts left to favour the reverse endothermic reaction

Temperature decreased: equilibrium shifts right to favour the forwards exothermic reaction.

2A + B 3C + D

Pressure increased: equilibrium shifts left, to the side with the fewest number of moles

Pressure decreased: equilibrium shifts right, towards the side with the most number of moles.

Pressure:

High pressure would favour the forwards reaction because there are more moles of product; high pressure increases the rate of reaction because there are more molecules in the same space, so there are more frequent successful collisions and the rate of reaction is increased. High pressure however is expensive to generate and is dangerous.

200 atmospheres

A + B C + D Reactant conc. Increased: equilibrium shifts right to favour the formation of products.

Products conc. Increased: equilibrium shifts left to favour the formation of reactants.

Adding a catalyst;

A catalyst does not effect the position of equilibrium; it speeds up both the forwards and the reverse reactions at equal rates, and so the position of equilibrium is reached more quickly. The rate of reaction is increased.

Changes in Concentration;

Increasing the concentration of reactants will favour the forwards reaction (the formation of products). This causes equilibrium to shift to the right, in order to lower the concentration of reactants, and oppose the change.

Decreasing the concentration of reactants will favour the backwards reaction (the formation of reactants). This causes equilibrium to shift left, in order to increase the concentration of reactants and oppose the change.

Increasing the concentration of products will favour the backwards reaction (the formation of reactants). This causes equilibrium to shift to the left, in order to lower the concentration of products, and oppose the change.

Decreasing the concentration of products will favour the forwards reaction (the formation of products). This causes equilibrium to shift right, in order to increase the concentration of products and oppose the change.

Temperature:

Low temperatures favour the forwards reaction as it is exothermic. However, low temperatures reduce the rate of reaction, because particles do not have energy exceeding the activation energy and so do not have enough energy to react.

400-450˚c yield is sacrificed for a faster rate of reaction

Catalyst: An iron catalyst is used in the reaction. The rate of reaction increases, but the position of equilibrium is unaffected. The catalyst lowers the activation energy, which means that the process requires lower temperatures and less energy is required, saving fossil fuels. Iron

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