The Haber process - Higher
The The industrial chemical process that makes ammonia by reacting nitrogen and hydrogen together. involves a A chemical reaction which can go both ways. at dynamic In chemical reactions, a situation where the forward and backward reactions happen at the same rate, and the concentrations of the substances stay the same.. The principles covered in Reversible reactions can be applied to explain how the Per unit time or 鈥榩er second鈥. For example, if 2,000 J are transferred over a period of 10 s, then the rate of transfer is 200 J/s or 200 W. This value is the power rating. and The mass of product made in a chemical reaction. The percentage yield is a measure of the yield obtained compared to the maximum possible yield. will be affected by the choice of reaction conditions.
This graph shows that for any specific temperature (ie following any of the coloured curved lines), as the pressure increases, so does the The mass of product made in a chemical reaction. The percentage yield is a measure of the yield obtained compared to the maximum possible yield. of ammonia.
It also shows that at any given pressure (ie following a vertical line up from the x-axis), as the temperature decreases, the yield of ammonia increases. In other words, as the temperature increases, the yield decreases.
The effect of increasing pressure
In a reaction involving gases as reactants and/or products, increasing the Force exerted over an area. The greater the pressure, the greater the force exerted over the same area. of the reaction mixture will cause the A measure of the relative concentrations of substances in an equilibrium, showing if there are more reactants or products at equilibrium. to move to the side with the fewest The amount of substance that contains the same number of particles as there are atoms in 12 g of carbon-12 (contains the Avogadro's constant 6.0 脳10虏鲁 number of particles). of gas, to reduce the pressure.
There are fewer molecules on the right-hand side of the equation for the Haber process:
N2(g) + 3H2(g) 鈬 2NH3(g)
1 + 3 = 4 molecules 鈬 2 molecules
If the pressure is increased, the equilibrium position moves to the right, so the yield of ammonia increases. The rate of reaction also increases because the gas molecules are closer together, so successful collisions are more frequent.
However, the energy costs increase when higher pressures are used and the equipment becomes more expensive. Therefore, the choice of pressure is a compromise between yield and cost.
The effect of increasing temperature
When the temperature is increased, the position of equilibrium moves in the Reaction in which energy is taken in. direction to reduce the temperature.
In the Haber process, the forwards reaction is Reaction in which energy is given out to the surroundings. The surroundings then have more energy than they started with so the temperature increases., so the reverse reaction is endothermic.
N2(g) + 3H2(g) 鈬 2NH3(g) (forwards reaction is exothermic)
This means that as the temperature is increased, the position of equilibrium moves to the left, and the yield of ammonia decreases.
It may seem sensible to use a very low temperature in order to maximise the yield of ammonia but lower temperatures reduce the rate of reaction. The temperature chosen is a compromise between yield and rate.
The effect of using a catalyst
A A chemical or other agent that brings about a change in something else without necessarily changing itself. It can also mean a person or thing that causes an event. speeds up the rate of the forward and reverse reactions equally. This reduces the time taken for the system to reach equilibrium but it does not affect the position of equilibrium or the yield of ammonia.
Using a catalyst in the Haber process means that a lower temperature can be used whilst keeping the rate of reaction high. A lower temperature helps to keep the yield high.
Reducing costs in other ways
Most of the hydrogen and nitrogen which go into the reactor leave unreacted. By recycling them back into the reactor, the cost of making the reactants from raw materials is reduced.
Energy is a significant cost for any chemical industry. Where reactions are exothermic and therefore release energy, this heat is often used to heat up other parts of the process. It can also sometimes be used to generate steam which is passed through a Revolving machine with blades that are turned by wind, water or steam. Turbines in a power station turn the generators. connected to a Device that converts kinetic energy into electrical energy. in order to make electricity.