Submitted by on 14 October 2021
We have had tried this prac, it gave us tint green precipitate instead. what happened to this reaction?
Saying the sodium sulfite and copper chloride are aged stock.
Submitted by sat on 27 May 2015
Answer reviewed 23 February 2023
This reaction is a typical example of a double displacement reaction whereby an aqueous solution of sodium sulfide reacts with an aqueous solution of copper (II) chloride to form copper (II) sulfide (black shiny crystals) and aqueous sodium chloride (a colourless solution). Solid sodium sulfide is a yellow, yellow–pink or white solid flake with a sulfurous (rotten egg) smell. It is hydroscopic and absorbs moisture from air to form hydrogen sulfide. Discolouration of the solid occurs with time. When sodium sulfide is dissolved in water, hydrolysis occurs and the sulfide solution develops the characteristic rotten-egg odour. Dissolving an aging sodium sulfide solid in water will result in more hydrolysis of the anion (-ve ion) compared to a fresh sample. Instead of colourless, a yellow aqueous solution of sodium sulfide is observed.
When an aging sodium sulfide solution is made to react with copper (II) chloride solution, a black precipitate (copper (II) sulfide) in a yellow solution is formed. Filtration results in a yellow filtrate, which contains aqueous sodium chloride and sulfide/polysulfide ions in equilibrium.
Recommendations
Science ASSIST strongly recommends precipitating out the sulfide ions from the filtrate before performing evaporation. These ions can be removed through reaction with aqueous silver nitrate solution:
To the filtrate, add 0.1 M silver nitrate solution drop wise, until no further black precipitate is formed, filter the mixture and evaporate the filtrate to get the sodium chloride crystals.
Warning: If the filtrate contains sulfide ions and is heated, the sulfide ions will be oxidized to sulfur dioxide, a very toxic and corrosive gas.
Science ASSIST also recommends
- using a fume hood and appropriate PPE while preparing the copper (II) chloride and sodium sulfide solution,
- storing sodium sulfide in a tightly closed secondary containment container in the fridge,
- ensuring proper disposal of all chemicals involved in this experiment.
Disposal of waste chemicals
- The sulfides of copper and silver should be kept in the heavy metal wastes (solid) disposal bottle, which then should be disposed of by a licensed waste disposal contractor.
- Old degradable samples of solid sodium sulfide should also be disposed of by a licensed waste disposal contractor.
Alternative reactions
Due to the toxic and corrosive nature of sodium sulfide, other safer chemicals can be used to illustrate “Making Ionic compounds” such as:
- Reaction between aqueous solutions of copper (II) chloride and sodium phosphate.
- Reaction between aqueous solution of copper (II) chloride and sodium hydroxide.
- Reaction between aqueous copper (II) chloride and potassium carbonate.
References
Chem-Supply. (2018). Sodium sulfide, Safety Data Sheet. Search https://shop.chemsupply.com.au/ to source the latest Safety Data Sheet via the product information page.
Chem-Supply. (2018). Copper chloride, Safety Data Sheet. Search https://shop.chemsupply.com.au/ to source the latest Safety Data Sheet via the product information page.
Wikibooks (2022, September) ‘Inorganic chemistry/Qualitative analysis/test for anions’ Retrieved from the Wikibooks website: https://en.wikibooks.org/wiki/Inorganic_Chemistry/Qualitative_Analysis/T...
