Chemistry in your cupboard

Bleaches (1 of 2)

Bleaches are used to oxidise coloured substances to colourless ones.

The bleach used is sodium percarbonate. This is a white granular powder of formula 2Na2CO3.3H2O2.

Figure 15: 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 innocuous.

Question 4

Write an equation for hydrogen peroxide decomposing to oxygen and water

2H2O2 2H2O + O2

Question 4b

Use this and the equation above to work out how many moles of oxygen can be obtained from 2 moles of sodium percarbonate.

Answer 4b

2 mol sodium percarbonate produces 3 mol hydrogen peroxide, which would produce 1.5 mol of oxygen molecules.

Question 4c

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

Answer 4c

2 mol sodium percarbonate has a mass of 314 g
1.5 mol oxygen molecules has a mass of 48 g

Question 4d

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

Answer 4d

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