Atom economy, percentage yield and gas calculations

• What happens to atoms in a chemical reaction?

  No atoms are gained or lost
• What are by-products in a chemical reaction?

  Atoms that do not form the desired product
• What is produced when methane reacts with steam?

  Hydrogen and carbon monoxide
• What is the chemical equation for the reaction of methane with steam?

  CH4(g) + H2O(g) → 3H2(g) + CO(g)
• What happens to carbon and oxygen atoms in the methane reaction?

  They do not form the useful product
• What is atom economy?

  Measure of starting materials as useful products
• Why is high atom economy important?

  For sustainable development and economic reasons
• How is percentage atom economy calculated?

  • Use the formula:

  $\text{atom economy} =$$\frac{\text{total Mr of desired product}}{\text{total Mr of all reactants}} \times 100$

  • Represents efficiency of a reaction
• What is the highest possible value of atom economy?

  100%
• What does an atom economy of 50% indicate?

  Half the reactant atoms are useful products
• What is the theoretical yield in a chemical reaction?

  Maximum possible mass of product
• How can the theoretical yield be calculated?

  From the balanced equation and limiting reactant
• What factors are needed to calculate theoretical yield?

  Balanced equation, limiting reactant, product mass
• Why is it not always possible to obtain the theoretical yield?

  Because reactions may not go to completion
• What is one reason the mass of product may be less than the theoretical yield?

  Some product may be lost during separation
• What is another reason for a lower mass of product than expected?

  Reactants may react differently than expected
• How is percentage yield calculated?

  mass of product actually made / maximum theoretical mass × 100
• What is the range of percentage yield?

  From 0% to 100%
• What does a percentage yield of 100% indicate?

  No product has been lost
• What does a percentage yield of 0% indicate?

  No product has been made
• What does a reaction pathway describe?

  The sequence of reactions to produce a product
• What factors influence the choice of a reaction pathway?

  Percentage yield, atom economy, rate of reaction
• What is ethanol used for?

  As a fuel
• What are the two methods for manufacturing ethanol?

  1. Fermentation of plant sugars
  2. Hydration of ethene using steam
• What is the chemical equation for the fermentation of glucose?

  C6H12O6(aq) → 2C2H5OH(aq) + 2CO2(g)
• What is the chemical equation for the hydration of ethene?

  C2H4(g) + H2O(g) → C2H5OH(l)
• Compare the features of fermentation and hydration of ethene for ethanol production.

  • Fermentation:
  • Yield: 15%
  • Atom economy: 51.1%
  • Reaction rate: Low
  • Hydration of ethene:
  • Yield: 95%
  • Atom economy: 100%
  • Reaction rate: High
• Why is the hydration of ethene considered a better method for making ethanol?

  It has higher yield and atom economy
• What does the equilibrium position indicate about the hydration of ethene?

  Only about 5% of ethene converts to ethanol
• How is the overall yield of 95% achieved in the hydration of ethene?

  By recirculating unreacted ethene
• What is a by-product of the fermentation of plant sugars?

  Carbon dioxide
• How can carbon dioxide from fermentation be utilized?

  Sold to fizzy drinks manufacturers
• How can the atom economy of fermentation be increased?

  By selling carbon dioxide as a product
• What does Avogadro's law state about gases?

  Equal volumes of gases contain equal molecules
• Under what conditions does Avogadro's law apply?

  When temperature and pressure stay the same
• How many moles of hydrogen gas occupy the same volume as 1 mole of methane gas?

  1 mole
• What can Avogadro's law be used to calculate?

  The volumes of gases in reactions
• What is the reaction equation for hydrogen and chlorine forming hydrogen chloride?

  H2(g) + Cl2(g) → 2HCl(g)
• What is the mole ratio of hydrogen to chlorine in the reaction?

  1:1
• If 1 cm³ of hydrogen reacts with chlorine, how much chlorine is needed?


  1 cm³