Sulfuric acid (\(\text{H}_2\text{SO}_4\)) is a highly corrosive mineral acid, while copper (\(\text{Cu}\)) is a durable metal widely used in wiring and plumbing. Whether copper dissolves in sulfuric acid depends entirely on the acid’s concentration and temperature. Under ordinary conditions, no reaction occurs, but changing the environment causes the sulfuric acid to behave in a chemically different manner. This shift allows for the dissolution of the metal, transforming the copper into a soluble salt.
The Non-Reaction Under Standard Conditions
Copper metal does not react with dilute sulfuric acid at room temperature. The lack of reaction is explained by the metal activity series, which ranks metals based on their tendency to lose electrons. For a metal to dissolve in a non-oxidizing acid, it must be reactive enough to displace hydrogen ions (\(\text{H}^+\)) from the solution.
Copper is positioned below hydrogen on this reactivity scale, indicating it has a lower tendency to be oxidized. Because copper cannot donate electrons to the hydrogen ions, the metal remains unchanged. When copper is placed in dilute \(\text{H}_2\text{SO}_4\), no bubbles of hydrogen gas are produced, and the metal maintains its solid form.
When Sulfuric Acid Acts as an Oxidizer
The reaction changes completely when concentrated sulfuric acid is used, especially when it is heated. Under these conditions, the acid stops acting merely as a source of hydrogen ions and begins to function as a powerful oxidizing agent. This oxidizing power enables the dissolution of the copper metal.
The concentrated sulfuric acid molecules are reduced during the reaction, allowing the copper to be oxidized despite its low position on the activity series. The copper atoms lose electrons, changing their oxidation state from 0 to \(\text{+2}\). Simultaneously, the sulfur atom within the sulfuric acid is reduced, changing its oxidation state from \(\text{+6}\) to \(\text{+4}\).
This chemical exchange, known as a redox reaction, is facilitated by the high concentration of the acid and the energy supplied by heating. The process converts the solid copper into an ionic compound dissolved in the solution.
Identifying the Reaction Products and Byproducts
The interaction between hot, concentrated sulfuric acid and copper metal yields three distinct products: copper(II) sulfate, sulfur dioxide, and water. Copper(II) sulfate (\(\text{CuSO}_4\)) is the primary product and appears as a blue salt in the resulting solution. This compound is the dissolved form of the copper.
The most notable byproduct is sulfur dioxide (\(\text{SO}_2\)), a colorless gas with a strong, pungent odor often described as similar to a burnt match. This gas is toxic and highly irritating to the eyes and respiratory system, requiring proper ventilation.
Water (\(\text{H}_2\text{O}\)) is also generated from the chemical rearrangement of the hydrogen and oxygen atoms. The balanced chemical equation shows that one copper atom reacts with two sulfuric acid molecules to form one molecule each of copper(II) sulfate and sulfur dioxide, along with two molecules of water.
Essential Safety Procedures for Handling Concentrated Acids
Working with concentrated sulfuric acid, particularly when heated, requires strict adherence to safety protocols due to the extreme hazards involved. Personal protective equipment (PPE) must be worn at all times, including a lab coat, chemical splash goggles, and gloves made of a chemical-resistant material like neoprene or nitrile.
Because the reaction produces toxic sulfur dioxide gas, the procedure must be performed inside a fume hood or a workspace with exceptional ventilation. When preparing solutions, always add the acid slowly to cold water, rather than adding water to the acid. This procedure manages the large amount of heat released during dilution, preventing the mixture from boiling violently and splattering the corrosive liquid.