Gold (Au): Chemical Properties and Reactions Study Guide
Chemical Properties Overview
Gold (Au) is a transition metal renowned for its inertness and nobility.
Reactivity Series Position
Gold is positioned at the very bottom of the reactivity series, below hydrogen and most other metals. This indicates its extremely low chemical reactivity.
Electronegativity
Gold has a relatively high Pauling electronegativity of 2.54. Despite this, its high ionization energy and stable electron configuration contribute to its chemical inertness.
General Reactivity
Gold is classified as a noble metal due to its exceptional resistance to corrosion and oxidation in most environments. It does not react with common acids, alkalis, or oxygen under normal conditions. Its most common oxidation states are +1 (aurous) and +3 (auric).
Action of Air and Oxygen
Gold is remarkably unreactive towards air and oxygen.
- At Room Temperature: Gold does not tarnish or oxidize in the presence of air.
- Upon Heating: Gold does not react with oxygen even at elevated temperatures. It retains its metallic luster indefinitely.
No significant chemical equations for reaction with air/oxygen.
Action of Water and Steam
Gold exhibits complete inertness towards water and steam.
- At Room Temperature: Gold does not react with water.
- Upon Heating (Steam): Gold remains unreactive even with superheated steam.
No significant chemical equations for reaction with water/steam.
Action of Acids and Bases
Gold is resistant to most individual acids and bases, making it unique among metals.
Action of Acids
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Non-oxidizing Acids (e.g., HCl, H₂SO₄ dilute): Gold does not react.
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Oxidizing Acids (e.g., HNO₃ concentrated): Gold does not react with nitric acid alone.
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Aqua Regia: This is the only common reagent that dissolves gold. Aqua regia is a mixture of concentrated nitric acid and concentrated hydrochloric acid, typically in a 1:3 molar ratio. The nitric acid acts as an oxidizing agent, forming Au³⁺ ions, while the hydrochloric acid acts as a complexing agent, forming the tetrachloroaurate(III) ion ([AuCl₄]⁻), which drives the reaction to completion by removing Au³⁺ ions from solution.
Au(s) + HNO₃(aq) + 4HCl(aq) → H[AuCl₄](aq) + NO(g) + 2H₂O(l)
Action of Bases
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Common Alkalis (e.g., NaOH, KOH solutions or fused): Gold is generally unreactive with most common bases.
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Cyanide Solutions (in the presence of oxygen): This reaction is crucial for the extraction of gold (cyanidation process). Gold dissolves in dilute solutions of alkali metal cyanides in the presence of oxygen and water to form soluble dicyanoaurate(I) complexes.
4Au(s) + 8NaCN(aq) + O₂(g) + 2H₂O(l) → 4Na[Au(CN)₂](aq) + 4NaOH(aq)
Key Laboratory Test/Identification Reactions
Gold in its ionic form, typically as Au³⁺ or [AuCl₄]⁻, can be identified by its reduction to elemental gold using various reducing agents.
Reduction with Tin(II) Chloride (SnCl₂)
This is a highly sensitive and characteristic test for gold ions. Gold(III) ions are reduced by tin(II) chloride to elemental gold, forming a purple precipitate known as “Purple of Cassius” (a colloidal dispersion of gold and tin oxides).
2AuCl₃(aq) + 3SnCl₂(aq) → 2Au(s) + 3SnCl₄(aq)
(Alternative ionic representation)
2Au³⁺(aq) + 3Sn²⁺(aq) → 2Au(s) + 3Sn⁴⁺(aq)
Reduction with Iron(II) Sulfate (FeSO₄)
Gold(III) ions can be reduced to metallic gold by iron(II) sulfate solution, especially in acidic conditions. A brown/black precipitate of gold metal is formed.
AuCl₃(aq) + 3FeSO₄(aq) → Au(s) + Fe₂(SO₄)₃(aq) + FeCl₃(aq)
(Alternative ionic representation)
Au³⁺(aq) + 3Fe²⁺(aq) → Au(s) + 3Fe³⁺(aq)
Other Reducing Agents
Gold ions can also be reduced by other reducing agents such as oxalic acid (H₂C₂O₄), hydrogen sulfide (H₂S), sulfur dioxide (SO₂), and even ethanol (C₂H₅OH) under specific conditions, forming a dark precipitate of metallic gold.