Oxygen-based bleaches (1 of 2)
Oxygen bleaches are based on sodium percarbonate, Figure 5. This is a white powder of formula 2Na2CO3.3H2O2.
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.
Write an equation for hydrogen peroxide decomposing to oxygen and water
2H2O2(aq) → 2H2O(l) + O2(g)
Use this and the equation
2Na2CO3.3H2O2(aq) 2Na2CO3(aq) + 3H2O2(aq)
to work out how many moles of oxygen can be obtained from 2 moles of sodium percarbonate.
2 mol sodium percarbonate produces 3 mol hydrogen peroxide, which would produce 3/2 mol oxygen.
Now work out these quantities in terms of grams of sodium percarbonate and oxygen.
2 mol sodium percarbonate has a mass of 314 g
3/2 mol oxygen has a mass of 48 g
One commercial specification for sodium percarbonate guarantees 'not less than 13% active oxygen'. From your calculation above, is this realistic?
(48/ 314) x 100 = 15.3%, so the specification is realistic.
What is the oxidation number (oxidation state) of the oxygen atoms in
- hydrogen peroxide
What type of reaction is this?
It is a disproportionation reaction as the oxidation state of some atoms of oxygen goes up and that of others goes down.