Introduction to Chromium
Chromium, identified by the symbol Cr and atomic number 24, is a hard, silvery, lustrous, and brittle transition metal. It is well-known for its high polish, resistance to tarnishing, and high melting point. The element was discovered in 1797 by Louis Nicolas Vauquelin.
Chemical Reactivity of Chromium
Chromium exhibits diverse chemical behaviors, primarily due to its ability to exist in multiple oxidation states, most commonly +2, +3, and +6.
Reactivity with Air
Under normal atmospheric conditions, solid chromium metal reacts with oxygen in the air to form a very thin, passive layer of chromium(III) oxide (Cr₂O₃) on its surface. This oxide layer is extremely stable, dense, and adherent, effectively preventing further oxidation of the underlying metal. This phenomenon, known as passivation, is responsible for chromium’s corrosion resistance and is a key reason for its use in stainless steel. Stainless steel, an alloy of iron, carbon, and chromium (typically 10.5% or more chromium), is widely used globally in everything from kitchen utensils to architectural structures due to this inherent corrosion resistance.
When finely powdered, chromium can react more readily with oxygen at elevated temperatures, leading to combustion.
Reactivity with Water
Chromium metal shows very limited reactivity with water. At room temperature, it does not react with water, including steam, due to the protective passive oxide layer formed in the presence of air. This inertness towards water makes chromium an ideal material for plating, providing a durable and corrosion-resistant surface to other metals.
Reactivity with Acids and Bases
Chromium metal reacts with non-oxidizing acids like hydrochloric acid (HCl) or sulfuric acid (H₂SO₄) to produce hydrogen gas (H₂) and chromium(II) ions (Cr²⁺), which are typically blue in solution. However, strong oxidizing acids, such as nitric acid (HNO₃), tend to passivate the chromium surface, similar to its reaction with air, making it resistant to further attack. Chromium does not react with bases.
Specific Properties and Hazards
Toxicity
The toxicity of chromium is highly dependent on its oxidation state.
- Chromium(III) [Cr(III)] compounds are generally considered essential trace nutrients for humans, involved in sugar and fat metabolism. They exhibit low toxicity and are found naturally in many foods.
- Chromium(VI) [Cr(VI)] compounds, also known as hexavalent chromium, are significantly more toxic and are classified as human carcinogens. Exposure to Cr(VI) can occur through inhalation, ingestion, or skin contact. It is associated with respiratory cancers, skin ulcers, and kidney damage. Industrial activities, such as electroplating, leather tanning, and pigment production, historically led to environmental contamination by Cr(VI), which has necessitated stringent regulations globally. For instance, in some parts of the United States, groundwater contamination by Cr(VI) has led to significant public health concerns and remediation efforts.
Radioactivity
Naturally occurring chromium consists of four stable isotopes: chromium-50, chromium-52, chromium-53, and chromium-54. There are no naturally occurring radioactive isotopes of chromium. While several artificial radioactive isotopes have been synthesized in laboratories, they are not naturally found in the environment and do not pose a common radioactive hazard.
Flammability
Solid, bulk chromium metal is not considered flammable under normal conditions. It has a high melting point (1907 °C) and requires very high temperatures to ignite. However, finely divided chromium powder can be pyrophoric, meaning it can ignite spontaneously in air, particularly when exposed to friction or heat. This is a common hazard associated with many finely powdered metals.
Illustrative Chemical Reaction: Ammonium Dichromate “Volcano”
A classic and visually striking chemical reaction involving chromium is the thermal decomposition of ammonium dichromate ((NH₄)₂Cr₂O₇). This reaction, often demonstrated as a “chemical volcano,” illustrates a redox process and the formation of chromium(III) oxide.
The reaction is: (NH₄)₂Cr₂O₇(s) → Cr₂O₃(s) + N₂(g) + 4H₂O(g)
When heated, the orange ammonium dichromate crystals decompose vigorously, producing voluminous green chromium(III) oxide, nitrogen gas, and water vapor. The solid chromium(III) oxide resembles ash from a volcano, hence the demonstration’s name. This reaction highlights the stability of the Cr(III) oxidation state.