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*Revision Materials and Past Papers* 2.1.1 Atomic structure and isotopes 2.1.2 Compounds, formulae and equations 2.1.3 Amount of substance 2.1.4 Acids 2.1.5 Redox 2.2.1 Electron structure 2.2.2 Bonding and structure 3.1.1 Periodicity 3.1.2 Group 2 3.1.3 The halogens 3.1.4 Qualitative analysis 3.2.1 Enthalpy 3.2.2 Reaction Rates 3.2.3 Chemical equilibrium 4.1 Basic concepts and hydrocarbons 4.1.2 Alkanes 4.1.3 Alkenes 4.2.1 Alcohols 4.2.2 Haloalkanes 4.2.3 Organic synthesis 4.2.4 Analytical techniques 5.1.1 How fast? 5.1.2 How far? 5.1.3 Acids, bases and buffers 5.2.1 Lattice enthalpy 5.2.2 Enthalpy and entropy 5.2.3 Redox and electrode potentials 5.3.1 Transition elements 5.3.2 Qualitative analysis 6.1.1 Aromatic compounds 6.1.2 Carbonyl compounds 6.1.3 Carboxylic acids and esters 6.2.1 Amines 6.2.2 Amino acids, amides and chirality 6.2.3 Polyesters and polyamides 6.2.4 Carbon–carbon bond formation 6.2.5 Organic synthesis 6.3.1 Chromatography and qualitative analysis 6.3.2 Spectroscopy Required Practicals

2.1.3 Amount of substance

Determination of formulaePercentage yields and atom economyThe Mole and Calculation of reacting masses, gas volumes and mole concentrations

Amount of Substance, Percentage Yields and Atom Economy

Specification Reference 2.1.3 (h)–(j)

Quick Notes

  • Atom Economy
    • Atom economy = (mass of desired product ÷ total mass of products) × 100
  • Percentage Yield
    • % yield = (actual ÷ theoretical) × 100

Full Notes

Percentage Yield

Percentage yield compares the actual amount of product obtained to the maximum possible amount (theoretical yield). Unlike atom economy, percentage yield is calculated using experimental (actual) data.

Formula for Percentage Yield

OCR (A) A-Level Chemistry formula diagram showing percentage yield as actual yield divided by theoretical yield times 100.

Actual yield = Mass of product obtained in an experiment.
Theoretical yield = Maximum amount of product predicted by the balanced equation.

Steps to Calculate Percentage Yield

  1. Write the balanced chemical equation.
  2. Calculate the theoretical yield (use mole ratios).
  3. Compare actual and theoretical yields.
  4. Use the percentage yield formula.
Worked Example: Simple Percentage Yield Calculation

In the reaction: Mg + 2HCl → MgCl2 + H2, if 2.40 g of Mg is reacted with excess HCl, and only 5.80 g of MgCl2 is obtained, what is the percentage yield?

OCR (A) A-Level Chemistry worked example of percentage yield calculation for magnesium reacting with hydrochloric acid.

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Matt’s Exam Tip

In reality, percentage yields won't be 100% because of incomplete or reversible reactions, side reactions, or product lost during isolation and purification (i.e. filtration, transfering, and evaporation).

Atom Economy

Atom economy is a measure of a reaction’s efficiency – how much of the reactants end up as the desired product rather than unwanted compounds. Atom economy is theoretical and based only on the balanced reaction equation, not experimental data.

Formula for Atom Economy

OCR (A) A-Level Chemistry formula diagram showing atom economy as desired product mass divided by total product mass times 100.

Higher atom economy = more efficient process.
Low atom economy = more waste, less 'green'.

Worked Example: Calculating Atom Economy

Find the percentage atom economy for the production of Iron (Fe) from the reaction between Iron(III) Oxide and Carbon Monoxide.

OCR (A) A-Level Chemistry worked example showing calculation of atom economy for Fe production from Fe2O3 and CO.

If there is only one product in the reaction, then the atom economy must be 100%.

Worked Example: Calculating Atom Economy

Find the percentage atom economy for the production of ethanol (CH3CH2OH) from the reaction between ethene (CH2CH2) and water (H2O)

OCR (A) A-Level Chemistry example showing atom economy of 100% for production of ethanol from ethene and water.

This reaction is 100% atom efficient because all reactants are converted into the desired product.

Experimental Techniques

Amounts of substance can be measured in different ways, depending on the state of a substance or the intended unit.

Sustainability and Green Chemistry

Designing reactions with high atom economy:

Summary