Important Compounds of Silver (Ag) - Revision Guide
Introduction to the Major Compounds of Silver
Silver (Ag) is a transition metal well-known for its excellent electrical conductivity, malleability, and ductility. In high school chemistry, several silver compounds are crucial due to their distinct properties, laboratory preparations, and applications. This guide focuses on the most frequently tested compounds: Silver Nitrate (AgNO₃), Silver Chloride (AgCl), and Silver Oxide (Ag₂O).
Silver Nitrate (AgNO₃)
Chemical Formula
AgNO₃
Common Name
Lunar caustic, Silver nitrate.
Laboratory Preparation
Silver nitrate is typically prepared by reacting silver metal with dilute nitric acid.
Balanced Equation:
3Ag(s) + 4HNO₃(dilute) → 3AgNO₃(aq) + 2NO(g) + 2H₂O(l)
(Note: With concentrated nitric acid, NO₂ gas is produced instead of NO.)
Ag(s) + 2HNO₃(conc.) → AgNO₃(aq) + NO₂(g) + H₂O(l)
Properties
- Appearance: Colourless, transparent, crystalline solid.
- Solubility: Highly soluble in water and ethanol.
- Stability: Stable at room temperature but decomposes on heating. It slowly darkens upon exposure to light due to reduction to silver metal, especially in the presence of organic matter.
- Oxidizing Nature: A strong oxidizing agent, particularly in solution.
Exam-Relevant Reactions
-
Thermal Decomposition:
2AgNO₃(s) --(heat)--> 2Ag(s) + 2NO₂(g) + O₂(g)(Solid silver nitrate decomposes into silver metal, nitrogen dioxide gas, and oxygen gas.) -
Precipitation of Silver Halides (e.g., with NaCl):
AgNO₃(aq) + NaCl(aq) → AgCl(s)↓ + NaNO₃(aq)(Forms a white curdy precipitate of silver chloride.) -
Reaction with reducing agents (e.g., Glucose in alkaline medium - Tollens’ Test):
2AgNO₃(aq) + 2NH₃(aq) + H₂O(l) → Ag₂O(s)↓ + 2NH₄NO₃(aq)(Followed by dissolution in excess NH₃ to form Tollens' reagent: Ag₂O(s) + 4NH₃(aq) + H₂O(l) → 2[Ag(NH₃)₂]OH(aq))2[Ag(NH₃)₂]OH(aq) + RCHO(aq) → 2Ag(s)↓ + RCOONH₄(aq) + 3NH₃(aq) + H₂O(l)(The reduction of Tollens’ reagent by aldehydes forms a “silver mirror”.)
Silver Chloride (AgCl)
Chemical Formula
AgCl
Common Name
Silver chloride.
Laboratory Preparation
Silver chloride is prepared by the precipitation reaction between a soluble silver salt (like silver nitrate) and a soluble chloride salt (like sodium chloride or hydrochloric acid).
Balanced Equation:
AgNO₃(aq) + NaCl(aq) → AgCl(s)↓ + NaNO₃(aq)
(Alternatively, using hydrochloric acid):
AgNO₃(aq) + HCl(aq) → AgCl(s)↓ + HNO₃(aq)
Properties
- Appearance: White, curdy precipitate.
- Solubility: Practically insoluble in water and dilute acids.
- Solubility in Ammonia: Readily dissolves in aqueous ammonium hydroxide (ammonia water) to form a soluble complex ion.
- Light Sensitivity: Highly sensitive to light. It turns grey then black upon exposure to light (especially sunlight) due to photodecomposition into silver metal and chlorine gas.
Exam-Relevant Reactions
-
Solubility in Aqueous Ammonia:
AgCl(s) + 2NH₃(aq) → [Ag(NH₃)₂]Cl(aq)(Forms diamminesilver(I) chloride, a soluble complex.) -
Photodecomposition:
2AgCl(s) --(light)--> 2Ag(s) + Cl₂(g)(This reaction is the basis for black-and-white photography and photographic paper.) -
Reaction with Potassium Cyanide:
AgCl(s) + 2KCN(aq) → K[Ag(CN)₂](aq) + KCl(aq)(Forms potassium dicyanoargentate(I), a soluble complex.)
Silver Oxide (Ag₂O)
Chemical Formula
Ag₂O
Common Name
Silver(I) oxide.
Laboratory Preparation
Silver oxide is prepared by adding an alkali (like sodium hydroxide) to an aqueous solution of silver nitrate. Initially, silver hydroxide (AgOH) is thought to form, which immediately decomposes to silver oxide and water.
Balanced Equation:
2AgNO₃(aq) + 2NaOH(aq) → Ag₂O(s)↓ + 2NaNO₃(aq) + H₂O(l)
Properties
- Appearance: Dark brown or black solid.
- Solubility: Sparingly soluble in water, giving a slightly basic solution (due to formation of AgOH). Soluble in acids.
- Stability: Decomposes on heating.
- Nature: It is a basic oxide.
- Oxidizing Agent: Acts as a mild oxidizing agent.
Exam-Relevant Reactions
-
Thermal Decomposition:
2Ag₂O(s) --(heat)--> 4Ag(s) + O₂(g)(Decomposes at relatively low temperatures (around 200 °C) into silver metal and oxygen gas.) -
Reaction with Acids:
Ag₂O(s) + 2HNO₃(aq) → 2AgNO₃(aq) + H₂O(l)(Reacts with acids to form the corresponding silver salt and water, demonstrating its basic nature.) -
Oxidation of Hydrogen Sulfide:
Ag₂O(s) + H₂S(g) → Ag₂S(s) + H₂O(l)(Silver oxide can oxidize hydrogen sulfide, forming black silver sulfide.)
Comparative Properties of Important Silver Compounds
| Property | Silver Nitrate (AgNO₃) | Silver Chloride (AgCl) | Silver Oxide (Ag₂O) |
|---|---|---|---|
| Appearance | Colourless crystalline solid | White curdy precipitate | Dark brown/black solid |
| Solubility in Water | Highly soluble | Practically insoluble | Sparingly soluble |
| Solubility in Ammonia | Soluble | Soluble (forms [Ag(NH₃)₂]⁺ complex) | Insoluble (but reacts with conc. NH₃ over time) |
| Reaction to Light | Slow decomposition/darkening | Decomposes rapidly (turns grey/black) | Stable, but prolonged exposure can cause slight changes |
| Thermal Decomposition | Decomposes to Ag, NO₂, O₂ | Stable, but melts without decomposition | Decomposes to Ag, O₂ at ~200 °C |
| Nature | Salt (forms neutral solution) | Insoluble salt | Basic oxide |
| Key Use/Relevance | Precursor for other Ag salts, photography, antiseptic, analytical reagent | Photography, qualitative analysis (halide test) | Batteries, mild oxidizing agent |