Chemistry in your cupboard

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Bleaches (1 of 2)

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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.

2Na₂CO₃.3H₂O_2(aq) 2Na₂CO_3(aq) + 3H₂O_2(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.

Activity

Question 4

Write an equation for hydrogen peroxide decomposing to oxygen and water

Answer 4a

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.