After Completing This Worksheet, You Should Be Able To

Atom economy

Learning outcomes

After completing this worksheet, you should be able to:

· calculate the atom economy for a given chemical reaction

· explain why using reactions with high atom economy is important.

The equation we use to work out the atom economy of a reaction is:

Worked example

The metal tungsten (W) can be extracted from its oxide (tungsten oxide, WO3) by reduction with hydrogen at a high temperature:

WO3 + 3H2 ® W + 3H2O

Calculate the percentage atom economy for this reaction.

Relative atomic masses (Ar values): W = 184, O = 16, H = 1

Step 1

First, we calculate the relative formula mass of the desired product.

In this case, the desired product is the element tungsten so it is the relative atomic mass of tungsten that we need, which is 184.

Step 2

Next, we calculate the sum of the relative formula masses of the reactants:

reactants = WO3 + 3 H2

sum of relative formula masses of the reactants:

= 1 ´ Mr(WO3) + 3 ´ Mr(H2)

= (1 ´ W) + (3 ´ O) + [3 ´ (2 ´ H)]

= (1 ´ 184) + (3 ´ 16) + [3 ´ (2 ´ 1)]

= 184 + 48 + 6

= 238

Step 3

Finally, we substitute these values into the equation:

= 77.3%

Questions

1 Copper can be extracted from copper carbonate by heating with carbon. The equation for the reaction is:

2CuCO3 + C ® 2Cu + 3CO2

Chemical / Relative atomic or formula mass
CuCO3 / 123.5
C / 12
Cu / 63.5
CO2 / 44

a Calculate the mass of all the atoms in the desired product. (1 mark)

b Calculate the mass of all of the atoms in all of the reactants. (1 mark)

c Calculate the atom economy of this reaction for producing copper. (1 mark)

2 Titanium can be extracted from titanium chloride by reaction with sodium.

The equation for the reaction is: TiCl4 + 4Na ® Ti + 4NaCl (Ar): Ti = 48; Cl = 35.5; Na = 23

a Calculate the mass of all the atoms in the desired product. (1 mark)

b Calculate the mass of all of the atoms in all of the reactants. (1 mark)

c Calculate the atom economy of this reaction for producing titanium. (1 mark)

3 Aspirin, C9H8O4, is a common painkiller. Pharmaceutical companies make aspirin by reacting salicylic acid, C7H6O3, with one of two chemicals:

· salicylic acid can be reacted with ethanoyl chloride, C2H3OCl

C7H6O3 + C2H3OCl → C9H8O4 + HCl

· or salicylic acid can be reacted with ethanoic anhydride, C4H6O3

C7H6O3 + C4H6O3 → C9H8O4 + C2H4O2

Look at the table of relative formula masses:

Substance / Relative formula mass, Mr
C7H6O3 / 138
C2H3OCl / 78.5
C4H6O3 / 102
HCl / 36.5
C2H4O2 / 60
C9H8O4 / 180

a Calculate the atom economy of making aspirin

i from salicyclic acid and ethanoyl chloride (2 marks)

ii from salicyclic acid and ethanoic anhydride. (2 marks)

b State which method of production has the higher atom economy. (1 mark)

c Atom economy is not the only factor that must be considered when choosing the most appropriate chemical reaction for producing a product.

State two other factors that you might consider. (2 marks)

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