C5 - Equilibria and The Contact Process Flashcards
What is a reversible reaction? Give an example.
In a reversible reaction, there is a forward reaction and a backward reaction. For example, the reaction between nitrogen and hydrogen to make ammonia is reversible:
nitrogen + hydrogen \ ammonia
N2 + 3H2 \ 2NH3
What is equilibrium?
When a reversible reaction reaches equilibrium, the rate of the forward reaction will be equal to the rate of the backward reaction. The concentrations of the reactants and products do not change.
What is the position of equilibrium?
The position of equilibrium is related to the ratio of the concentration of products to the concentration of reactants:
- It is on the left if the concentration of reactants is greater than the concentration of products.
- It is on the right if the concentration of products is greater than the concentration of reactants.
Describe equilibrium in terms of concentration.
Imagine a reversible reaction starting off with just the substances on the left of the equation. These will react quickly because they are at a high concentration. The rate of the forward reaction will then decrease as these substances are used up.
However, as they are used up, the concentration of the substances on the right of the equation will increase. They will react increasingly quickly as their concentration increases. Eventually, the rate of the forward reaction will equal the rate of the backward reaction, and equilibrium is reached.
How is sulphur trioxide made?
2SO2 + O2 // 2SO3
How does pressure affect the position of equilibrium in reversible reactions involving gases?
In a reversible reaction involving gases at equilibrium, if the pressure is increased the position of equilibrium moves to the side with the lower number of moles of gas molecules.
How does concentration affect the position of equilibrium in reversible reactions?
If the concentration of one of the substances is increased, the position of equilibrium moves to the opposite side.
If the concentration of one of the substances is decreased, the position of equilibrium moves to its side.
How does temperature affect the position of equilibrium in reversible reactions?
If the temperature is increased, the position of equilibrium moves in the direction of the endothermic reaction.
What are some uses of sulfuric acid?
- phosphate fertilisers
- making chemicals
- processing metal ores
- pigments
What are the three raw materials in the contact process?
• sulfur
• air (which provides oxygen)
• water
What is stage 1 of the contact process?
sulfur is burned in air to produce sulfur dioxide gas:
sulfur + oxygen → sulfur dioxide
S + O2 → SO2
What is stage 2 of the contact process?
sulfur dioxide and oxygen react together to produce sulfur trioxide gas:
sulfur dioxide + oxygen // sulfur trioxide
2SO2 +O2 // 2SO
What is stage 3 of the contact process?
the sulfur trioxide reacts with water to produce sulfuric acid:
sulfur trioxide + water → sulfuric acid
SO3 + H2O → H2SO4
Why is a temperature of 450°C used ?
The forward reaction, which produces sulfur trioxide, SO3, is exothermic. This means that a greater yield can be obtained by reducing the temperature.
On the other hand, the rate of reaction will be low if the temperature is too low. So an optimum temperature of around 450 °C is used. It is high enough to give a reasonable rate of reaction without decreasing the yield too much.
Why is atmospheric pressure used?
At atmospheric pressure, the position of equilibrium is far to the right. Almost all the sulfur dioxide is converted into sulfur trioxide. A high pressure would increase the yield of sulfur trioxide. However, the extra cost involved would not be worth it as there would only be a small increase in yield.
