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Oxygen-based bleaches (1 of 2)

Oxygen bleaches are based on sodium percarbonate, Figure 5. This is a white powder of formula 2Na2CO3.3H2O2.

beta carotene diagram

Figure 5: Structure of sodium percarbonate

In water it breaks down into sodium carbonate and hydrogen peroxide.

2Na2CO3.3H2O2(aq) → 2Na2CO3(aq) + 3H2O2(aq)

Hydrogen peroxide is the active oxidising agent as it in turn breaks down to oxygen and water. The beauty of this system is that the starting material is a relatively stable powder (although it obviously must be kept dry) and the by-products (sodium carbonate and water) are harmless.

Activity

Question 1a

Write an equation for hydrogen peroxide decomposing to oxygen and water

Answer 1a

2H2O2(aq) → 2H2O(l) + O2(g)

Question 1b

Use this and the equation

2Na2CO3.3H2O2(aq) arrow 2Na2CO3(aq) + 3H2O2(aq)

to work out how many moles of oxygen can be obtained from 2 moles of sodium percarbonate.

Answer 1b

2 mol sodium percarbonate produces 3 mol hydrogen peroxide, which would produce 3/2 mol oxygen.

Question 1c

Now work out these quantities in terms of grams of sodium percarbonate and oxygen.

Answer 1c

2 mol sodium percarbonate has a mass of 314 g

3/2 mol oxygen has a mass of 48 g

Question 1d

One commercial specification for sodium percarbonate guarantees 'not less than 13% active oxygen'. From your calculation above, is this realistic?

Answer 1d

(48/ 314) x 100 = 15.3%, so the specification is realistic.

Question 1e

What is the oxidation number (oxidation state) of the oxygen atoms in

  • hydrogen peroxide
  • water
  • oxygen?

Answer 1e

  • -1
  • -2
  • 0

Question 1f

What type of reaction is this?

Answer 1f

It is a disproportionation reaction as the oxidation state of some atoms of oxygen goes up and that of others goes down.