Catalysts only affect the rate of reaction – they do not affect the yield of the reaction. A catalysed reaction produces the same amount of product as an uncatalysed reaction but it produces the product at a faster rate.

Typically, percent yields are understandably less than 100% because of the reasons indicated earlier. However, percent yields greater than 100% are possible if the measured product of the reaction contains impurities that cause its mass to be greater than it actually would be if the product was pure.

Usually, percent yield is lower than 100% because the actual yield is often less than the theoretical value. Reasons for this can include incomplete or competing reactions and loss of sample during recovery. This can happen when other reactions were occurring that also formed the product.

The effect of increasing pressure 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.

How can you increase the yield of product from a reaction?

1. Increase the temperature.
2. Increase the concentration of reactants.
3. Increase the surface area of the reactants.
4. Use a catalyst.
5. Remove the product as it’s formed.

We’ve demonstrated that the increase in pressure shifts the equilibrium to the right because the number of particles in the products is less than that for the reactants. So, therefore, the change in pressure will not affect the equilibrium yield.

If the pressure is increased, the position of equilibrium moves in the direction of the fewest moles of gas. On the left, there are 3 moles of gas (1 + 2), but on the right there is only 1. Therefore, if the pressure is increased, the position of equilibrium will move to the right and more methanol will be produced.

Equilibrium constants aren’t changed if you change the pressure of the system. That means that if you increase the pressure, the position of equilibrium will move in such a way as to decrease the pressure again – if that is possible. It can do this by favouring the reaction which produces the fewer molecules.

Pressure. If the pressure of gaseous reactants is increased, there are more reactant particles for a given volume. There will be more collisions and so the reaction rate is increased. The higher the pressure of reactants, the faster the rate of a reaction will be.

The majority of the rate constants were determined at a pressure of approximately 95 Torr. Given the close agreement between Howard’s low pressure results and our higher pressure values, we conclude that the rate constant does not have a significant pressure dependence for temperatures between 200 and 300 K.

There are four factors that affect the rate (speed) of a chemical reaction:

• temperature.
• concentration.
• particle size.
• use of a catalyst.

Temperature. Usually reactions speed up with increasing temperature. Physical state of reactants. Powders react faster than blocks – greater surface area and since the reaction occurs at the surface we get a faster rate.

It is important to note however that there are reactions involving gases in which a pressure change does not affect the reaction rate. For this reason, the rates of reactions involving gases have to be determined by experiment. Also note that solids and liquids are not affected by pressure changes.

The rate could be expressed in the following alternative ways: d[Z]/dt, –d[A]/dt, –d[B]/dt, dz/dt, −da/dt, −db/dt where t is the time, [A], [B], and [Z] are the concentrations of the substances, and a, b, and z are their amounts. Note that these six expressions are all different from one another but are simply related.

There are 4 methods by which you can increase the rate of a reaction:

1. Increase the concentration of a reactant.
2. Increase the temperature of the reactants.
3. Increase the surface area of a reactant.
4. Add a catalyst to the reaction.

The rate of a reaction can be increased by adding a suitable catalyst. A catalyst is a substance which increases the rate of a chemical reaction but it is not used up (remains chemically unchanged at the end). It provides an alternative reaction pathway of lower activation energy.