Yield and Atom EconomyAQA GCSE Chemistry: Revision notes

Theoretical yield is the maximum mass of product that could be obtained from a reaction if all the limiting reactant is converted to product and no product is lost. It is calculated using stoichiometry from the balanced equation and molar masses.

Actual yield is the mass of product that is actually obtained when the reaction is carried out in the laboratory.

In practice, actual yield is always less than theoretical yield because:

• The reaction may not go to completion (reversible reactions)
• Some product may be lost during separation and purification
• Side reactions may occur, producing unwanted products
• Not all reactant may be converted due to practical limitations

Calculating percentage yield allows chemists to evaluate the efficiency of a reaction and identify where improvements can be made.

Percentage yield is calculated using the formula:

Percentage yield = (actual yield ÷ theoretical yield) × 100

This tells you what fraction of the theoretical maximum was actually achieved as a percentage.

Step-by-step calculation process:

1. Calculate the theoretical yield using the balanced equation and molar masses
2. Obtain the actual yield from experimental results
3. Divide actual by theoretical
4. Multiply by 100 to convert to a percentage
5. Always include units (%) and round to appropriate significant figures

Example worked calculation:

In the reaction: 2Cu + O₂ → 2CuO

• Molar mass Cu = 64, O = 16, so CuO = 80
• 4.0 g Cu is used (theoretical mass of CuO = 5.0 g)
• Actual yield = 4.4 g CuO
• Percentage yield = (4.4 ÷ 5.0) × 100 = 88%

A percentage yield of 88% means 12% of the product was lost to incomplete reaction, side reactions, or during purification.

In real laboratory conditions, percentage yield is almost always less than 100% due to practical and chemical reasons:

Incomplete reactions

• Many reactions are reversible – they do not go to completion
• The reaction reaches equilibrium before all reactants are used
• Example: dissolving salt in water reaches saturation

Side reactions

• Unwanted reactions can occur alongside the desired reaction
• These use up some reactants to form different products
• Example: when heating copper, some Cu reacts with nitrogen in air to form CuₓN instead of just CuO

Losses during purification and separation

• Product may stick to apparatus (beakers, filters, evaporating basins)
• Some product is lost as spray during heating or transfer
• Solid product trapped in filter paper is not fully recovered
• Solvent may not fully evaporate if conditions are not ideal

Other practical factors

• Impurities in reactants reduce the amount of pure reactant available
• Temperature and pressure conditions are not always optimal
• Not all reactant may be fully converted due to kinetic factors

Understanding these reasons helps chemists design better processes and improve yields through experimental refinement.

Atom economy measures the proportion of reactants that end up as the desired product in a chemical reaction. It is calculated using:

Atom economy = (relative formula mass of desired product ÷ sum of relative formula masses of all products) × 100

Key points:

• Atom economy is expressed as a percentage
• It ranges from 0% to 100%
• High atom economy (close to 100%) means little waste is produced
• Low atom economy means many unwanted by-products are formed
• Atom economy is calculated from the balanced equation – it is a theoretical value independent of actual yield

Example calculation:

Reaction: 2Na + Cl₂ → 2NaCl

• Relative formula mass of NaCl = 58.5 (desired product)
• Relative formula masses of all products = 2 × 58.5 = 117
• Atom economy = (117 ÷ 117) × 100 = 100%

Contrast with: Reaction: CH₄ + 2O₂ → CO₂ + 2H₂O

• If CO₂ is the desired product: only 44 out of (44 + 36) = 80 g total products
• Atom economy = (44 ÷ 80) × 100 = 55% (low – water is wasted)

High atom economy is crucial for both sustainability and economic reasons:

Sustainability benefits

• Reduces waste – fewer by-products mean less material going to landfill
• Conserves raw materials – more of the starting material becomes useful product rather than waste
• Lowers environmental impact – disposal of by-products can be costly and harmful
• Reduces energy use – less waste to process and separate

Economic benefits

• Reduces costs – less money spent on disposing of unwanted by-products
• Increases profit – more of the reactant is converted to sellable product
• Improves efficiency – the process becomes more economically viable
• Meets regulations – many environmental regulations favour processes with high atom economy

Real-world examples:

Chemists actively work to design green chemistry processes with high atom economy to balance profit with environmental responsibility.

You can assess atom economy directly from balanced equations without calculating numerical values:

Reactions with HIGH atom economy (≥80%):

• Synthesis reactions where one product forms
  • Example: N₂ + 3H₂ → 2NH₃ (all atoms form the desired product)
• Simple decomposition producing only desired products
  • Example: 2H₂O₂ → 2H₂O + O₂ (if both products are wanted)
• Addition reactions where two reactants form one product
  • Example: C₂H₄ + H₂O → C₂H₅OH (100% atom economy)

Reactions with LOW atom economy (<50%):

• Substitution reactions producing multiple unwanted by-products
  • Example: CH₃Cl + KOH → CH₃OH + KCl (if only CH₃OH is desired, KCl is waste)
• Reactions producing water or gases as waste
  • Example: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂ (water and CO₂ are often discarded)
• Processes with large molar mass by-products
  • Example: C₆H₅Cl + 2Na → C₆H₆ + NaCl (NaCl waste has significant mass)

Key observation rules:

1. Count the number of products – fewer products often means higher atom economy
2. Check the relative formula masses – if desired product is much lighter than by-products, atom economy will be low
3. Look for industrial importance – well-established industrial reactions usually have high atom economy (they were designed to be economical)

Quick identification checklist:

• One main product formed? → Likely high atom economy
• Multiple products, some are gases or water? → Likely low atom economy
• Used on industrial scale? → Probably high atom economy