Sodium percarbonate: meaning, formula, and chemical behavior
Sodium percarbonate is a solid oxidizing agent widely used as an “oxygen bleach.” Chemically, it is best understood as an addition compound (a crystalline adduct) of sodium carbonate and hydrogen peroxide that releases hydrogen peroxide when dissolved in water.
1) Formula interpretation and composition
Interpreting \(2\,\mathrm{Na_2CO_3}\cdot 3\,\mathrm{H_2O_2}\) as a single “formula unit” for calculations gives the total atom counts:
- \(2\,\mathrm{Na_2CO_3}\) contributes: \(\mathrm{Na_4C_2O_6}\)
- \(3\,\mathrm{H_2O_2}\) contributes: \(\mathrm{H_6O_6}\)
- Total composition: \(\mathrm{Na_4C_2H_6O_{12}}\)
2) Key reactions in water (why it “bleaches”)
Sodium percarbonate does not act as a single covalent molecule in solution. Instead, it dissociates to yield sodium carbonate (a basic salt) and hydrogen peroxide (the oxidizer):
\[ 2\,\mathrm{Na_2CO_3}\cdot 3\,\mathrm{H_2O_2\ (s)} \;\rightarrow\; 2\,\mathrm{Na_2CO_3\ (aq)} + 3\,\mathrm{H_2O_2\ (aq)} \]
Hydrogen peroxide can then undergo decomposition (accelerated by catalysts, higher temperature, or higher pH), producing oxygen gas and water:
\[ 2\,\mathrm{H_2O_2\ (aq)} \;\rightarrow\; 2\,\mathrm{H_2O\ (l)} + \mathrm{O_2\ (g)} \]
The cleaning/bleaching effectiveness comes from peroxide-derived oxidizing species, while \(\mathrm{Na_2CO_3}\) raises pH and improves detergency and stain removal.
3) Molar mass of sodium percarbonate
For a consistent molar-mass calculation, treat one “mole” of sodium percarbonate as one mole of \(2\,\mathrm{Na_2CO_3}\cdot 3\,\mathrm{H_2O_2}\). Using common atomic masses (Na 22.99, C 12.01, O 16.00, H 1.008) gives the following breakdown.
| Component | Atoms per \(2\,\mathrm{Na_2CO_3}\cdot 3\,\mathrm{H_2O_2}\) | Mass contribution (g/mol) |
|---|---|---|
| Na | 4 | \(4 \times 22.99 = 91.96\) |
| C | 2 | \(2 \times 12.01 = 24.02\) |
| O | 12 | \(12 \times 16.00 = 192.00\) |
| H | 6 | \(6 \times 1.008 = 6.048\) |
\[ M\!\left(2\,\mathrm{Na_2CO_3}\cdot 3\,\mathrm{H_2O_2}\right) = 91.96 + 24.02 + 192.00 + 6.048 = 314.028\ \mathrm{g/mol} \approx 314.02\ \mathrm{g/mol} \]
Molar mass ≈ \(314.02\ \mathrm{g/mol}\)
4) Percent hydrogen peroxide and “available oxygen”
Sodium percarbonate contains three moles of hydrogen peroxide per formula unit:
\[ m(\mathrm{H_2O_2}) = 3 \times M(\mathrm{H_2O_2}) = 3 \times \bigl(2 \times 1.008 + 2 \times 16.00\bigr) = 3 \times 34.016 = 102.048\ \mathrm{g} \]
\[ \%\ \mathrm{H_2O_2} = \frac{102.048}{314.028}\times 100 \approx 32.5\% \]
In cleaning chemistry, “available oxygen” (active oxygen) expresses the mass of oxygen equivalent delivered by peroxide. Each \(\mathrm{H_2O_2}\) corresponds to one oxygen atom of oxidizing equivalent, i.e., \(16.00\ \mathrm{g}\) per mole of \(\mathrm{H_2O_2}\). Since there are 3 moles of \(\mathrm{H_2O_2}\), the available oxygen per mole is \(3 \times 16.00 = 48.00\ \mathrm{g}\).
\[ \%\ \text{available oxygen} = \frac{48.00}{314.028}\times 100 \approx 15.3\% \]
5) Practical chemical notes (general chemistry)
- Stability: moisture and heat accelerate decomposition; dry storage improves shelf life.
- Solution chemistry: carbonate makes solutions more basic, which can enhance cleaning and influence peroxide reactivity.
- Redox context: peroxide-derived oxidizing species are responsible for stain bleaching and deodorizing effects.
Sodium percarbonate functions as a convenient solid source of hydrogen peroxide, and its composition and oxidizing capacity follow directly from the formula \(2\,\mathrm{Na_2CO_3}\cdot 3\,\mathrm{H_2O_2}\) and standard molar-mass calculations.