OCR GCSE Chemistry B (J258) — Making useful chemicals
In industrial chemical synthesis, efficiency is measured by how effectively a process uses its starting materials. Atom economy is defined as the percentage of the mass of the starting materials that ends up as the desired product.
To evaluate atom economy, chemists use the relative formula mass () of the reactants. This measure is based on the Law of Conservation of Mass, which states that no atoms are created or destroyed during a chemical reaction. Because the total mass of the starting materials must equal the total mass of the products, any mass that does not end up in the desired product is instead found in unwanted by-products.
Evaluating atom economy is a central principle of Green Chemistry. It allows industrial chemists to assess the sustainability and economic viability of different chemical pathways.
One way industry improves overall efficiency is by finding a commercial use for a by-product. If a substance previously considered "waste" can be sold or used in another process (such as selling carbon dioxide to the drinks industry), it is reclassified as a useful product. This effectively increases the atom economy of the manufacturing process.
In 'Define' questions, ensure you use the exact phrase: 'the percentage of the mass of the starting materials that ends up as the desired product'. Avoid using 'weight'.
When explaining the significance of atom economy, focus on three pillars: sustainability (resource conservation), environmental protection (less waste), and profit (lower raw material and disposal costs).
A common exam scenario involves 'reclassifying' a by-product. If a company finds a way to sell a by-product, the atom economy is considered to have increased because more of the starting atoms are now part of a useful product.
Atom economy
The percentage of the mass of the starting materials that ends up as the desired product.
Desired product
The specific chemical substance that a reaction is intended to manufacture.
Reactants
The starting materials in a chemical reaction that are used to create products.
By-product
Any additional substance produced during a chemical reaction that is not the primary desired product.
Relative formula mass (Mᵣ)
The sum of the relative atomic masses of all the atoms shown in a chemical formula.
Green Chemistry
The design of chemical processes and products that reduce the generation of hazardous substances and minimise waste.
Law of Conservation of Mass
A scientific law stating that the total mass of the reactants is always equal to the total mass of the products in a chemical reaction.
Put your knowledge into practice — try past paper questions for Chemistry B
Atom economy
The percentage of the mass of the starting materials that ends up as the desired product.
Desired product
The specific chemical substance that a reaction is intended to manufacture.
Reactants
The starting materials in a chemical reaction that are used to create products.
By-product
Any additional substance produced during a chemical reaction that is not the primary desired product.
Relative formula mass (Mᵣ)
The sum of the relative atomic masses of all the atoms shown in a chemical formula.
Green Chemistry
The design of chemical processes and products that reduce the generation of hazardous substances and minimise waste.
Law of Conservation of Mass
A scientific law stating that the total mass of the reactants is always equal to the total mass of the products in a chemical reaction.
