Sulfuric acid, a powerful and highly corrosive mineral acid, dissolves a wide range of substances, primarily through its strong acidic, oxidizing, and dehydrating properties.
Sulfuric acid is an exceptionally reactive and concentrated acid. It interacts with various materials by oxidizing, dehydrating, or sulfonating them, often leading to significant chemical changes, including charring in organic compounds. Its formidable reactivity means it can dissolve or chemically alter most metals and a vast array of organic and inorganic substances.
How Sulfuric Acid Dissolves Substances
The dissolving power of sulfuric acid stems from several key chemical properties:
- Strong Acidity: As a diprotic acid, it readily donates protons (H⁺), initiating reactions with bases, metals, and certain salts.
- Oxidizing Agent: Especially in its concentrated form and when heated, it acts as a strong oxidizing agent, accepting electrons from other substances and causing them to dissolve.
- Dehydrating Agent: It has a very strong affinity for water molecules, effectively removing them from compounds, which often leads to decomposition or charring in organic materials.
- Sulfonating Agent: It can introduce a sulfonyl group (–SO₃H) into organic molecules, a process known as sulfonation.
What Dissolves in Sulfuric Acid?
1. Metals
Sulfuric acid dissolves most metals, although the specific reaction depends on the acid's concentration, temperature, and the metal's reactivity.
- Active Metals: Highly reactive metals like magnesium (Mg), zinc (Zn), iron (Fe), and aluminum (Al) readily dissolve in dilute sulfuric acid, producing hydrogen gas and a metal sulfate.
- Example: $\text{Zn (s)} + \text{H}_2\text{SO}_4 \text{ (aq)} \rightarrow \text{ZnSO}_4 \text{ (aq)} + \text{H}_2 \text{ (g)}$
- Less Reactive Metals: Metals like copper (Cu), silver (Ag), and mercury (Hg) do not react with dilute sulfuric acid but will dissolve in hot, concentrated sulfuric acid. In these cases, the acid acts as an oxidizing agent, producing sulfur dioxide gas instead of hydrogen.
- Example: $\text{Cu (s)} + 2\text{H}_2\text{SO}_4 \text{ (conc)} \rightarrow \text{CuSO}_4 \text{ (aq)} + \text{SO}_2 \text{ (g)} + 2\text{H}_2\text{O (l)}$
- Passivation: Some metals, like aluminum (Al) and chromium (Cr), can become passivated (form a protective oxide layer) when exposed to concentrated sulfuric acid, preventing further dissolution.
- Lead (Pb): While lead initially reacts with sulfuric acid, it quickly forms an insoluble layer of lead(II) sulfate ($\text{PbSO}_4$) on its surface, which then prevents further reaction, making lead resistant to dissolution. This property makes lead useful in handling sulfuric acid.
2. Organic Compounds
Sulfuric acid reacts with most organic compounds through oxidation, dehydration, or sulfonation, often leading to their breakdown or transformation. Its strong dehydrating action means it commonly causes charring of organic materials.
- Dehydration: Organic compounds containing hydroxyl groups (–OH) are highly susceptible to dehydration.
- Examples: Sugars (like sucrose) are rapidly dehydrated, turning into black carbon (charring). Alcohols can be dehydrated to form alkenes or ethers.
- $\text{C}{12}\text{H}{22}\text{O}_{11} \text{ (s)} \xrightarrow{\text{H}_2\text{SO}_4} 12\text{C (s)} + 11\text{H}_2\text{O (l)}$
- Sulfonation: Aromatic compounds readily undergo sulfonation, where a sulfonic acid group is introduced. This is a crucial reaction in the synthesis of dyes and detergents.
- Example: Benzene reacts with fuming sulfuric acid to produce benzenesulfonic acid.
- Oxidation: Many organic compounds, particularly those with easily oxidizable functional groups, can be oxidized by concentrated sulfuric acid, leading to their decomposition.
3. Inorganic Compounds
Sulfuric acid dissolves many inorganic compounds, especially bases, carbonates, and some oxides.
- Bases/Alkalis: It neutralizes metal hydroxides (e.g., sodium hydroxide, NaOH) and metal oxides (e.g., copper(II) oxide, CuO) to form sulfates and water.
- Example: $\text{2NaOH (aq)} + \text{H}_2\text{SO}_4 \text{ (aq)} \rightarrow \text{Na}_2\text{SO}_4 \text{ (aq)} + 2\text{H}_2\text{O (l)}$
- Carbonates: It reacts vigorously with metal carbonates (e.g., calcium carbonate, $\text{CaCO}_3$) to produce carbon dioxide gas, water, and a metal sulfate.
- Example: $\text{CaCO}_3 \text{ (s)} + \text{H}_2\text{SO}_4 \text{ (aq)} \rightarrow \text{CaSO}_4 \text{ (aq)} + \text{H}_2\text{O (l)} + \text{CO}_2 \text{ (g)}$
- Other Salts: Some salts, particularly those of weaker acids, can react with sulfuric acid to release the weaker acid (e.g., chlorides to release HCl, although this requires high temperatures).
4. Water and Alcohols
While not "dissolving" in the conventional sense, sulfuric acid reacts violently with alcohol and water to release a significant amount of heat. This exothermic reaction, particularly when diluting concentrated sulfuric acid, can be dangerous, causing boiling and splashing. Therefore, it is crucial to add acid slowly to water, not the other way around, to dissipate the heat safely. Learn more about the safe handling of acids from reputable sources like the Royal Society of Chemistry.
What Does NOT Dissolve or Is Resistant?
Certain materials exhibit resistance to sulfuric acid:
- Noble Metals: Gold (Au) and platinum (Pt) are highly unreactive and do not dissolve in sulfuric acid, even when hot and concentrated.
- Insoluble Sulfates: As mentioned, lead (Pb) forms an insoluble layer of lead(II) sulfate, preventing further reaction. Barium sulfate ($\text{BaSO}_4$) is also highly insoluble in sulfuric acid.
- Certain Non-Metals/Ceramics: Glass (primarily silicon dioxide) is generally resistant to sulfuric acid, especially at room temperature. Some specialized plastics, such as PTFE (Teflon), are also highly resistant.
- Specific Alloys: Certain alloys are designed for acid resistance, often containing high proportions of metals like nickel or chromium.
Summary Table: What Reacts with Sulfuric Acid
| Category | Dissolves/Reacts | Resistant/Doesn't Dissolve | Conditions/Notes |
|---|---|---|---|
| Metals | Magnesium, Zinc, Iron, Copper, Silver | Gold, Platinum, Lead (due to passivation) | Depends on acid concentration and temperature; some metals passivate. |
| Organic | Sugars, Alcohols, Aromatic compounds, most other organics | Highly resistant polymers (e.g., PTFE) | Undergoes dehydration (charring), oxidation, or sulfonation. |
| Inorganic | Metal Hydroxides, Metal Oxides, Carbonates | Barium Sulfate, Glass | Neutralization reactions, gas evolution. |
| Solvents | Water, Alcohols | (N/A – reacts exothermically) | Reacts violently with heat release; requires careful handling (acid to water). |
Sulfuric acid's broad reactivity makes it an essential chemical in numerous industrial processes, from fertilizer production and petroleum refining to metal treatment and chemical synthesis. Understanding what dissolves in it is crucial for safe handling and effective application.
