Percentage yield & atom economy

Question 1

What do percentage yield and atom economy calculate?

Answer 1

The efficiency of a chemical reaction in terms of the amount of product formed.

Question 2

What is the equation for percentage yield?

Answer 2

actual amount of product (mol) / theoretical amount of product (mol)

Question 3

What assumptions do equations make?

Answer 3

Calculations to work out the amount of product in a reaction assume that all the reactants would be converted to products. This would give a yield of 100%, which in practice would be very difficult to achieve.

Question 4

Why would a yield of 100% be difficult to achieve?

Answer 4

• the reaction may be reversible
• the reactants may contain impurities
• by-products may be formed from unexpected reactions
• reactants or products may be left in apparatus during experiment
• product may be lost through separation/purification e.g. being soaked onto filter paper

Question 5

What is atom economy?

Answer 5

Atom economy considers the efficiency of a reaction by looking at the proportion of products formed that are actually useful.

Question 6

What is an example of a reaction with a 100% atom economy?

Answer 6

Ethene + steam to produce ethanol.

C2H4 (g) + H2O (g) -> C2H5OH (l)

Question 7

What is an example of a reaction without a 100% atom economy?

Answer 7

Calcium carbonate to calcium oxide + CO2. CO2 is a waste product.

CaCO3 (s) -> CaO (s) + CO2 (g)

Question 8

Why are reactions with a high atom economy important?

Answer 8

• they make better use of finite resources.
• they reduce the need to process and safely dispose of waste products.

Question 9

What is the equation for atom economy?

Answer 9

Mr of desired products / Sum of Mrs of all products