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Science

Chemical calculations - higher

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Atom economy

The atom economy of a chemical reaction is a measure of the amount of starting materials that become useful products. Inefficient, wasteful processes have low atom economies. Efficient processes have high atom economies, and are important for sustainable development, as they use fewer natural resources and create less waste.

The atom economy of a reaction can be calculated:

The value of the atom economy expressed as a percentage is equal to 100 x mass of the desired product from the equation divded by the total mass of products from the equation

Note that, because the total mass of products equals the total mass of reactants, you can put that into the bottom of the fraction in the calculation like this:

The value of the atom economy expressed as a percentage is equal to 100 x mass of the desired product from the equation divded by the total mass of reactants from the equation

For example, what is the atom economy for making hydrogen by reacting coal with steam?

Write the balanced equation:

C(s) + 2H2O(g)    →    CO2(g) + 2H2(g)

Write out the Ar and Mr values underneath:

C(s) + 2H2O(g)    →    CO2(g) + 2H2(g)

12       2 × 18                44           2 × 2

Remember that the Ar or Mr in grams is one mole, so:

  • total mass of products = 44 + 4 = 48g (note that this is the same as the reactants: 12 + 36 = 48g)
  • mass of desired product (H2) = 4g

% atom economy = 448 × 100 = 8.3%

This process has a low atom economy and is therefore an inefficient way to make hydrogen. It also uses a non-renewable resource: coal.

Summary

This illustrated podcast explains how to work out a balanced equation for a chemical reaction.

This illustrated podcast shows how to work with moles in chemical calculations.

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