The Chemical Stability of Copper
Copper (Cu) is a transition metal renowned for its excellent electrical conductivity and resistance to corrosion, which makes it an ideal material for electrical wiring and plumbing. This relative inertness, particularly under standard atmospheric conditions, is due to its moderate position in the electrochemical series. It is less reactive than common metals like iron and zinc but more reactive than truly noble metals like gold.
Why Copper Resists Non-Oxidizing Acids
One of the most notable examples of copper's resistance to chemical reactions is its behavior with non-oxidizing acids. According to the reactivity series, copper is positioned below hydrogen. This means that copper has a higher standard reduction potential than hydrogen and cannot displace hydrogen from a compound in a redox reaction. As a result, pure copper metal will not react with or dissolve in dilute hydrochloric acid (HCl) or dilute sulfuric acid ($$H_2SO_4$$). This property is why copper pipes can safely transport fresh water and acidic beverages, provided the acids are not highly concentrated or heavily oxygenated.
Non-Reactivity with Pure Water
Under normal conditions, copper does not react with water, a property that is fundamental to its use in plumbing and water transport systems. Over very long periods in moist air, however, it can undergo a slow oxidation process. The resulting green protective layer, known as patina, is a mixture of copper carbonate and other compounds, which actually protects the underlying metal from further corrosion. This differs significantly from iron, which rusts away completely when exposed to moisture.
Inertness to Certain Non-Metals and Compounds
At ambient temperatures and without strong oxidizing agents present, copper is stable and does not react with several common non-metals and compounds. For instance, in dry air, it does not corrode. It is also chemically resistant to carbon, dry gases, and many organic compounds like alcohols and phenol resins. This stability makes it useful in many chemical and manufacturing processes. However, this inertness changes dramatically under different conditions, such as high temperatures or the introduction of a stronger oxidizing agent.
The Limitations of Copper's Non-Reactivity
While copper is resistant to many common substances, its inertness has clear boundaries. Its reactions with powerful oxidizing agents, certain gases, and other metals demonstrate that its chemical stability is situational.
Reactivity with Strong Oxidizing Acids
Unlike with dilute non-oxidizing acids, copper readily reacts with strong oxidizing acids, such as nitric acid ($$HNO_3$$) and hot, concentrated sulfuric acid ($$H_2SO_4$$). In these cases, the acid itself acts as the oxidizing agent, not the hydrogen ions. The reaction with nitric acid, for example, produces copper nitrate, water, and toxic nitrogen oxide gases.
Reaction with Oxygen and Sulfur
When heated, copper reacts with oxygen to form copper(I) oxide ($$Cu_2O$$) and copper(II) oxide ($$CuO$$), which appear as black layers on the surface. Copper also tarnishes when exposed to sulfur compounds, forming various copper sulfides.
The Challenge of Saltwater
In contrast to its resistance to pure water, copper is susceptible to corrosion when exposed to saltwater, particularly in marine environments or aggressive soil conditions. The presence of chloride ions and other electrolytes accelerates the corrosion process over time.
Comparison of Copper's Reactivity
| Substance | Condition | Reaction? | Explanation |
|---|---|---|---|
| Dilute Hydrochloric Acid | Normal | No | Copper is less reactive than hydrogen. |
| Dilute Sulfuric Acid | Normal | No | Copper is less reactive than hydrogen. |
| Nitric Acid | Dilute or Concentrated | Yes | The acid is a strong oxidizing agent. |
| Pure Water | Ambient Temperature | No | Copper is stable under normal conditions. |
| Moist Air | Long-term Exposure | Slow Oxidation | Forms a protective green patina. |
| Hot, Concentrated $$H_2SO_4$$ | Heating Required | Yes | The acid becomes a powerful oxidizing agent. |
| Gold | Contact | No | Gold is less reactive than copper. |
Conclusion: The Selective Inertness of Copper
While the question "what can copper not react with" has a number of specific answers, the true takeaway is that copper's inertness is selective rather than absolute. It stands out for its resistance to non-oxidizing acids and pure water under normal conditions, properties that have made it invaluable for centuries. However, its behavior changes dramatically when exposed to stronger chemical reagents or elevated temperatures, as it can be easily oxidized or complexed. Understanding these chemical nuances is essential for both engineering applications and general material knowledge. You can find more information about copper's chemical properties and uses on resources like Britannica's entry on copper.
The Factors Behind Copper's Resistance
Position in the Reactivity Series: Copper sits below hydrogen, making it unable to displace hydrogen ions from non-oxidizing acids like dilute HCl.
Natural Passivation: Exposure to moist air leads to the formation of a protective green patina, a layer of copper carbonate that prevents further corrosion.
Weak Metallic Bonding: Unlike metals with incomplete d-electron shells, copper's filled d-shell results in relatively weak metallic bonds, contributing to its physical properties.
Resistance to Oxygen at Ambient Temperature: At room temperature, copper's reaction with oxygen is very slow, ensuring its stability.
Incompatibility with Certain Compounds: Copper has known incompatibilities with substances like alkali solutions, sodium azide, and acetylene.
FAQs
Q: Why doesn't copper react with dilute hydrochloric acid? A: Copper does not react with dilute hydrochloric acid because it is less reactive than hydrogen and cannot displace the hydrogen ions in the acid.
Q: Does copper rust like iron? A: No, copper does not rust like iron. Instead, it slowly oxidizes in moist air to form a protective green layer called patina, which prevents further corrosion.
Q: What about copper reacting with water? A: Pure copper does not react with neutral water. Its use in water pipes relies on this non-reactivity, though corrosion can occur in saltwater or with dissolved oxygen present over time.
Q: Why does copper react with nitric acid? A: Copper reacts with nitric acid because it is a strong oxidizing acid. The acid itself acts as the oxidizing agent, dissolving the copper, rather than the hydrogen ions.
Q: Can copper react with bases? A: Copper is generally resistant to alkalis (bases), but some compounds like copper(II) hydroxide can dissolve in concentrated hydroxide solutions.
Q: What happens when copper is heated in the presence of oxygen? A: When copper is heated in air or oxygen, it reacts to form a black coating of copper(II) oxide ($$CuO$$).
Q: Does salt or saltwater cause a reaction with copper? A: Yes, saltwater can cause corrosion over time. Copper has low resistance to salt-related corrosion, and exposure can cause it to turn bluish-green and eventually degrade.
