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OCR GCSE Chemistry B · J258
OCR J258 Check the specification (PDF) (opens in a new tab)
Define atom economy.
A chemical reaction rearranges atoms: it does not create or destroy them. However, not every atom supplied in the reactants necessarily becomes part of the substance that the chemist wants to make. Some atoms may enter other products.
Atom economy is a measure of the proportion of the atoms in the reactants that become part of the desired product. It is usually expressed as a percentage. The desired product is the substance the process is intended to produce.
Imagine a reaction that forms a useful salt as well as an unwanted by-product. The atoms incorporated into the useful salt contribute to the atom economy; those incorporated into the unwanted product do not. A reaction has a high atom economy when most of the reactant atoms enter the desired product, and a low atom economy when a substantial proportion enters other products.
Atom economy is a theoretical value based on the balanced reaction equation. It describes how atoms would be distributed between the products according to that equation. It is therefore possible to compare alternative reaction pathways for making the same substance: a route producing fewer unwanted by-products will often have the higher atom economy.
The identity of the desired product matters. If a reaction forms several products, atom economy is judged with respect to the particular product that the manufacturer wants.
A high-atom-economy reaction makes better use of its starting materials and produces less material as unwanted by-products. This can reduce the resources needed and the waste that must be separated, treated or disposed of. OCR therefore treats atom economy as one indicator used when evaluating industrial processes, and reactions with high atom economy are generally more sustainable.
It is not a complete measure of sustainability. The renewability of raw materials, energy requirements and the nature of any waste also matter.
Atom economy and percentage yield answer different questions:
A reaction can have a high atom economy but a low yield if much of the reactant fails to form the desired product. Conversely, a reaction may give a high yield while still having a low atom economy because its equation also produces substantial by-products.
Atom economy is the proportion of reactant atoms that become part of the desired product, usually expressed as a percentage.
Atom economy describes the distribution of atoms shown by the equation. Percentage yield describes how much product is actually obtained compared with the theoretical amount.
High atom economy generally improves resource use and reduces waste, but sustainability also depends on factors such as energy use, raw materials and the nature of the waste produced.
In a definition question, state that atom economy concerns the proportion of reactant atoms that become the desired product—not merely the amount of product made.
Do not confuse atom economy with percentage yield: atom economy comes from the reaction equation, whereas yield compares the product actually obtained with the theoretical amount.
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Atom economy
A measure of the proportion of atoms from the reactants that become part of the desired product in a chemical reaction.
Desired product
The substance that a chemical process is intended to make.
By-product
A product formed alongside the desired product in a reaction.
Theoretical value
A value predicted from a model or equation rather than obtained by measuring the result of an actual experiment.
Put your knowledge into practice — try past paper questions for Chemistry B
Atom economy
A measure of the proportion of atoms from the reactants that become part of the desired product in a chemical reaction.
Desired product
The substance that a chemical process is intended to make.
By-product
A product formed alongside the desired product in a reaction.
Theoretical value
A value predicted from a model or equation rather than obtained by measuring the result of an actual experiment.