## 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.

### Question 1a

Write an equation for hydrogen peroxide decomposing to oxygen and water

2H2O2(aq) → 2H2O(l) + O2(g)

### Question 1b

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.

### Question 1c

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

### Question 1d

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.

### Question 1e

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

• hydrogen peroxide
• water
• oxygen?