The reaction equation for sodium thiosulfate, particularly when it reacts with dilute hydrochloric acid, is as follows:
Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + S(s) + H₂O(l) + SO₂(g)
This chemical reaction is a common demonstration in chemistry due to its visible products and is often referred to as the "disappearing cross experiment" because the cloudy sulfur precipitate obscures a marked cross beneath the reaction vessel.
Understanding the Reaction
When sodium thiosulfate (Na₂S₂O₃) solution is mixed with hydrochloric acid (HCl), a series of changes occur, leading to the formation of several distinct products. This is an example of a decomposition reaction where sodium thiosulfate disproportionates, meaning sulfur in the thiosulfate ion (S₂O₃²⁻) is both oxidized and reduced.
Reactants and Products
The table below summarizes the substances involved in this reaction:
| Component | Chemical Formula | State | Description |
|---|---|---|---|
| Reactants | |||
| Sodium Thiosulfate | Na₂S₂O₃ | Aqueous | A clear, colorless solution. |
| Hydrochloric Acid | HCl | Aqueous | A clear, colorless, strong acid solution. |
| Products | |||
| Sodium Chloride | NaCl | Aqueous | Dissolved salt, remains in solution. |
| Sulfur | S | Solid | Fine yellow precipitate, causing cloudiness. |
| Water | H₂O | Liquid | A clear liquid, part of the solution. |
| Sulfur Dioxide | SO₂ | Gas | Pungent, colorless gas. |
Key Observations and Properties of Products
- Formation of Solid Sulfur (S): The most noticeable product is elemental sulfur, which precipitates out of the solution as a fine, yellow, cloudy solid. This cloudiness increases over time, eventually making the solution opaque.
- Release of Sulfur Dioxide Gas (SO₂): Sulfur dioxide is a gas with a characteristic pungent, choking odor. It is responsible for the strong smell often associated with this reaction and can be irritating to the respiratory system.
- Sodium Chloride Solution (NaCl): This is a spectator ion product, meaning it remains dissolved in the solution and does not participate in the core chemical change involving sulfur.
- Water (H₂O): Water is also produced as a liquid.
This reaction is highly dependent on temperature; increasing the temperature speeds up the reaction, causing the sulfur precipitate to form more quickly.
