Reaction Yields & Atom Economy (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about reaction yields and atom economy. Both measure how much of what you put into a reaction ends up as the product you want, and they measure it in different ways: percentage yield from what you actually obtained, atom economy from the balanced equation.

Percentage yield shows how much of a particular product you get from the reactants, compared with the maximum theoretical amount you could get, so it comes from what you actually obtained in the experiment. Atom economy shows how many of the atoms used in the reaction become the desired product, and it is found directly from the balanced equation. Both answer the same question, how much of what goes into a reaction comes out as the product you want, one from the experiment and one from the equation.

We start with percentage yield, and why reactions rarely give you everything the calculation predicts. Then the limiting reagent, which is what sets that maximum product in the first place. Then atom economy, and why a high atom economy matters economically, environmentally and ethically.

In a lot of reactions, not all of the reactants react to form the product you want. Other reactions take place at the same time, the reaction may not go to completion, and reactants or products are lost to the atmosphere.

Percentage yield shows how much of a particular product you get from the reactants, compared with the maximum theoretical amount you could get: the actual yield divided by the theoretical yield, multiplied by a hundred. The actual yield is the number of moles or the mass of product obtained experimentally, and the theoretical yield is the number of moles or the mass obtained by calculation, usually using moles equals mass in grams divided by molar mass. That experimental half is what separates yield from atom economy: it can only be known once the reaction has been done.

When you are working with moles, be clear about the type of particle you are referring to. One mole of calcium fluoride contains one mole of calcium fluoride formula units, but one mole of calcium ions and two moles of fluoride ions.

Sometimes there is an excess of one or more of the reactants. The reactant which is not in excess is the limiting reagent. Once all of the limiting reagent has been used up, the reaction stops, even if there are moles of the other reactants left over.

To work out which reactant is limiting, calculate the number of moles of each one, then take the ratio shown in the balanced equation into account. In two sodium plus sulfur going to sodium sulfide, the ratio of sodium to sulfur is two to one, and that has to be built into the calculation.

The limiting reagent is what fixes the theoretical yield, so it sets the maximum that the actual yield is compared against.

Atom economy shows how many of the atoms used in the reaction become the desired product, and the rest of the atoms, and the rest of the mass, are wasted. It is found directly from the balanced equation: the molecular mass of the desired product, divided by the sum of the molecular masses of all the reactants, multiplied by a hundred. Only the product you want goes on the top of that fraction, and the M r of anything else the reaction produces is not needed. The same calculation can be done using mass instead of M r values. Because it comes from the equation, atom economy is known before the reaction is ever run, and that is what separates it from percentage yield.

In addition reactions the atom economy is always a hundred per cent, because all of the atoms are used to make the desired product. Ethene reacting with bromine is one example. Whenever there is only one product, the atom economy is a hundred per cent.

Efficient processes have high atom economies, and they are important to sustainable development. Economically, there is less waste to separate and dispose of, which lowers costs, and more product is made from the same raw materials, which increases profit. Environmentally, there is less waste and less pollution, and finite raw materials are conserved. Ethically, it is a responsible use of limited resources and it reduces harm to the environment.

Sometimes a question will ask you to show your working when you calculate atom economy. Even if it is an addition reaction with one product, and it is obvious that the atom economy is a hundred per cent, you still need to show that working.

Percentage yield measures how much product you actually obtained against the maximum the equation predicts. The limiting reagent sets that maximum: once it has been used up, the reaction stops, whatever else is left over. Atom economy measures how many of the atoms used become the desired product, and it comes from the balanced equation rather than from the experiment. Both answer the same question, how much of what goes into a reaction comes out as the product you want, and a high atom economy is what makes a process efficient economically, environmentally and ethically.