Doc Brown's GCSE OCR Gateway Science-Chemistry Revision Notes

OCR GCSE Gateway Additional Science Chemistry Module C3 Chemical Economics

Unit-Item C3e Percentage yield and atom economy

1. Percentage yield and atom economy are two important concepts that help the chemical industry make their processes more sustainable and green.

2. In this unit you should learn how to calculate these two quantities and understand their importance to the chemical industry.

3. Practical work and research investigations-activities which you have done which help in revision may have included:

   • Preparation of ammonium sulfate or other salts.

   • Class practical involving the preparation of magnesium sulfate from a variety of starting materials (magnesium, magnesium oxide, magnesium hydroxide or magnesium carbonate) – comparison of percentage yield and atom economy

4. Understand percentage yield as a way of comparing amount of product made (actual yield) to amount expected (predicted yield):

   • a) 100% yield means that no product has been lost

   • b) 0% yield means that no product has been made.

5. Be able to recognise possible reasons (given experimental details) why the percentage yield of a product is less than 100% for example:

   • a) loss in filtration

   • b) loss in evaporation

   • c) loss in transferring liquids

   • d) not all reactants react to make product.

6. Understand atom economy as a way of measuring the amount of atoms that are wasted when manufacturing a chemical:

   • a) 100% atom economy means that all atoms in the reactant have been converted to the desired product

   • b) the higher the atom economy the ‘greener’ the process.

7. Be able to interpretation of simple percentage yield and atom economy data.

8. Know and use the formula:

   • percentage yield = 100 x actual yield / predicted yield

9. HT only: Be able to explain why an industrial process wants as high a percentage yield as possible, to include:

   • a) reducing the reactants wasted

   • b) reducing cost.

10. Know and use the formula:

    • % atom economy = 100 x M_r of desired products / sum of Mr of all products

11. Be able to calculate atom economy when given balanced symbol equation (1:1 molar ratio) and appropriate relative formula masses.

12. HT only: be able to calculate atom economy when given ANY balanced symbol equation and appropriate relative formula masses.

13. HT only: Be able to explain why an industrial process wants as high an atom economy as possible:

    • a) to reduce the production of unwanted products

    • b) to make the process more sustainable.

14. Be able to interpret complex percentage yield and atom economy data.

15. Calculation notes - worked examples:

    • Examples of percent % yield and theoretical yield, atom economy

    • More examples of % percentage & theoretical yield and atom economy

    • Methods of making salts are described