Revision Guide: Important Gold (Au) Compounds for High School

Introduction to Major Compounds of Gold

Gold (Au), a noble transition metal, is renowned for its chemical inertness. Its compounds are less common and often less stable than those of more reactive metals. In high school chemistry, the focus is primarily on gold’s general properties, its noble character, and its characteristic dissolution in aqua regia. Gold commonly exhibits oxidation states of +1 (aurous) and +3 (auric), with the +3 state being generally more stable in aqueous solutions.

Key Oxidation States of Gold:

• +1 (Aurous): Found in compounds such as gold(I) chloride (AuCl).
• +3 (Auric): The more prevalent and stable oxidation state, found in compounds like gold(III) chloride (AuCl₃) and gold(III) oxide (Au₂O₃).

Gold(III) Chloride (Auric Chloride), AuCl₃

Chemical Formula and Common Name

• Chemical Formula: AuCl₃
• Common Name: Gold trichloride, Auric chloride

Laboratory Preparation

Gold(III) chloride can be prepared by reacting finely divided gold powder directly with excess chlorine gas at elevated temperatures. Alternatively, it can be obtained by careful evaporation and thermal decomposition of the tetrachloroauric(III) acid formed when gold dissolves in aqua regia.

Balanced Equations:

1. Direct Chlorination:

   2Au(s) + 3Cl₂(g) → 2AuCl₃(s)

2. From Aqua Regia: Gold first reacts with aqua regia to form tetrachloroauric(III) acid:

   Au(s) + 4HCl(aq) + HNO₃(aq) → H[AuCl₄](aq) + NO(g) + 2H₂O(l)

   Upon gentle heating and controlled decomposition, H[AuCl₄] can yield AuCl₃.

Properties

• Physical State: A reddish-brown crystalline solid.
• Solubility: Highly soluble in water, ethanol, and ether. In aqueous solution, it primarily exists as the tetrachloroaurate(III) ion, [AuCl₄]⁻.
• Hygroscopic: Readily absorbs moisture from the atmosphere.
• Thermal Stability: Decomposes upon heating above approximately 160°C.

Exam-Relevant Reactions

1. Hydrolysis/Complexation in Water:

   AuCl₃(s) + HCl(aq) → H[AuCl₄](aq)

   (This represents the formation of tetrachloroauric(III) acid in the presence of chloride ions.)
2. Reduction to Elemental Gold: Gold(III) chloride is a strong oxidizing agent and is readily reduced to elemental gold by various reducing agents.
   • With Tin(II) chloride (SnCl₂):

     2AuCl₃(aq) + 3SnCl₂(aq) → 2Au(s) + 3SnCl₄(aq)

     This reaction is famously used in the “purple of Cassius” test for gold, where colloidal gold particles impart a purple colour.
   • With Iron(II) sulfate (FeSO₄):

     2AuCl₃(aq) + 6FeSO₄(aq) → 2Au(s) + 2Fe₂(SO₄)₃(aq) + 2FeCl₃(aq)

Gold(I) Chloride (Aurous Chloride), AuCl

Chemical Formula and Common Name

• Chemical Formula: AuCl
• Common Name: Gold monochloride, Aurous chloride

Preparation

Gold(I) chloride is typically prepared by the controlled thermal decomposition of gold(III) chloride at a specific temperature range (e.g., around 160°C).

Balanced Equation:

AuCl₃(s) → AuCl(s) + Cl₂(g)  (at ~160°C)

Properties

• Physical State: A yellow crystalline solid.
• Stability: Less stable than AuCl₃. It is prone to disproportionation, especially upon further heating or in contact with water.
• Solubility: Insoluble in water.

Exam-Relevant Reactions

1. Disproportionation: Gold(I) chloride is unstable and disproportionates into more stable oxidation states (0 and +3) of gold.

   3AuCl(s) → 2Au(s) + AuCl₃(s)

Gold(III) Oxide (Auric Oxide), Au₂O₃

Chemical Formula and Common Name

• Chemical Formula: Au₂O₃
• Common Name: Gold(III) oxide, Auric oxide

Preparation

Gold(III) oxide is prepared indirectly. First, gold(III) hydroxide, Au(OH)₃, is precipitated from a gold(III) chloride solution by adding a strong base. The resulting hydroxide is then gently dehydrated to form the oxide.

Balanced Equations:

1. Formation of Gold(III) Hydroxide:

   AuCl₃(aq) + 3KOH(aq) → Au(OH)₃(s) + 3KCl(aq)

2. Dehydration to Gold(III) Oxide:

   2Au(OH)₃(s) → Au₂O₃(s) + 3H₂O(l)  (upon gentle heating)

Properties

• Physical State: A reddish-brown solid.
• Stability: Thermally unstable. It readily decomposes to elemental gold and oxygen upon moderate heating.
• Solubility: Insoluble in water.
• Amphoteric Nature: Exhibits both acidic and basic properties, dissolving in concentrated acids and strong bases.

Exam-Relevant Reactions

1. Thermal Decomposition:

   2Au₂O₃(s) → 4Au(s) + 3O₂(g)  (at ~160°C)

2. Reaction with Acid (Basic Oxide behavior):

   Au₂O₃(s) + 6HCl(aq) → 2AuCl₃(aq) + 3H₂O(l)

3. Reaction with Base (Acidic Oxide behavior):

   Au₂O₃(s) + 2NaOH(aq) + H₂O(l) → 2Na[Au(OH)₄](aq)

   (Alternatively, a simplified representation: Au₂O₃(s) + 2NaOH(aq) → 2NaAuO₂(aq) + H₂O(l))

Comparative Properties of Gold Compounds

Compound	Chemical Formula	Oxidation State	Colour	Key Property / Stability
Gold(III) Chloride	AuCl₃	+3	Reddish-brown	Hygroscopic, water-soluble (forms [AuCl₄]⁻), strong oxidizer.
Gold(I) Chloride	AuCl	+1	Yellow	Thermally unstable, water-insoluble, disproportionates.
Gold(III) Oxide	Au₂O₃	+3	Reddish-brown	Thermally unstable, water-insoluble, amphoteric.