Percentage yields and atom economy

Question 1

Why is it difficult to achieve the maximum theoretical yield?

Answer 1

• The reaction may not have gone to completion.
• Side reactions may have taken place alongside the main reaction.
• Purification of the product may result in loss of some product.

Question 2

Percentage yield eqn

Answer 2

Actual yield / Theoretical yield x 100%

Question 3

Limiting reagent

Answer 3

• The reactant that is not in excess will be completely will be completely used up first and stop the reaction - it is called the limiting reagent.
• If equal amounts of hydrogen and oxygen are allowed to react, hydrogen will be used up first, and half the oxygen will be unreacted. The limiting reagent is hydrogen.

Question 4

Atom economy

Answer 4

• The atom economy of a chemical reaction is a measure of how well atoms have been utilised.
  Reactions with high atom economies:
• Produce large proportion of desired products and few unwanted waste products.
• are important for sustainability as they make the best use of natural resources.

Question 5

Atom economy eqn

Answer 5

Sum of Mr of desired / sum of Mr of all x 100

Question 6

What does improving atom economy do?

Answer 6

• Improving atom economy makes industrial processes more efficient
• Preserves raw materials
• Reduces waste

Question 7

Atom economy in industry

Answer 7

• A low atom economy means there’s lots of waste produced.
• It costs to separate the desired product from waste product and it also costs to dispose of waste products safely without harming the environment.
• Waste of money if high proportion of reactants companies buy end up as useless products.