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Syllabus
Syllabus
The atom economy (atom utilisation) is a measure of the amount of starting materials that end up as useful products.
It is important for sustainable development and for economic reasons to use reactions with high atom economy.
The percentage atom economy of a reaction is calculated using the balanced equation for the reaction as follows:
(Mr of desired product from equation / Sum of Mrs of all reactants from equation) × 100
Students should be able to:
calculate the atom economy of a reaction to form a desired product from the balanced equation
(HT only) explain why a particular reaction pathway is chosen to produce a specified product given appropriate data such as atom economy (if not calculated), yield, rate, equilibrium position and usefulness of by-products.
What does this mean?
What does this mean?
What is Atom Economy?
What is Atom Economy?
Atom economy is not the same as percentage yield.
Rather than measuring whether the reaction is working efficiently (% yield), it tries to measure whether we should do the reaction at all based on ecological principle (whether it is bad for the Earth).
Most reactions don't just make one product, the other products are thought of as waste and have to be disposed of.
Some of these may be toxic.
Even if they are not, throwing away significant amounts of what you started with is wasteful.
Even a reaction with 99% yield might produce lots of waste and be bad for the environment, so we measure the atom economy of a reaction.
Atom economy = (Mr of desired product/Sum of Mrs of all reactants) × 100
Atom economy = (Mr of desired product/Sum of Mrs of all reactants) × 100
Example 1
Example 1
Lime for agriculture and industry is made in the equation shown left:
The Total Mr of the useful product = 56 (the CO2 was waste)
There's only one reactant so the Sum of Reactant Mrs is 100
Atom Economy = 56/100 x 100 =56%
Example 2
Example 2
No one makes water deliberately by burning Methane - but we could.
If so then the CO2 would be waste.
The Chemist works out the Mr of both reactants { Mr (CH4) = 16 & Mr (O2) = 32)
Then works out the sum of all the reactant Mrs, taking note that there are two lots of water in the equation.
Sum Reactant Mrs = 16 = ( 2 x 32) = 80
The Mr of the product (water) is 18
But two lots were made so the total Mr of product = 36
And Atom economy = 36/80 x 100
Example 3
Example 3
This reaction only has one product.
By definition there can't be any waste.
So, Atom economy must be 100%
Example 4
Example 4
Chemists sometimes need to make Hydrogen
One way we might do this is:
C(s) + H2O(g) --> CO(g) + H2(g)
They wouldn't throw the Carbon Monoxide away because it is also useful (and also toxic).
So we could work out the atom economy of this reaction with respect to Hydrogen and it would be very low:
( 2/30 x 100 = 6.67%)
But this would be misleading since we'd actually be using the 93.33% "waste".
Meaning that it isn't waste at all.
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