Gas-Forming Chemical Reactions

Gas-forming reactions are chemical reactions in which one of the products is a gas that escapes from the mixture, and the most common ones are acids with carbonates (giving carbon dioxide), acids with active metals (giving hydrogen), ammonium salts with bases (giving ammonia) and acids with sulfites or sulfides (giving sulfur dioxide or hydrogen sulfide). In brief: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂; the bubbles show that a gas has formed, and a simple test, such as turning limewater cloudy, identifies it as carbon dioxide.

The release of a gas drives a reaction forward because the gas leaves the solution. Many everyday phenomena, such as the fizzing of an antacid tablet and the rising of a cake, are gas-forming reactions. This article explains the main types and their equations, gives tests for the common gases, shows how to calculate the volume of gas formed using molar volume and the ideal gas law, includes tables, works through examples, discusses safety, and gives the history of the discovery of gases.

What are the main types of gas-forming reactions?

1. Acid + carbonate or hydrogencarbonate → salt + water + carbon dioxide. Carbonic acid, H₂CO₃, is unstable and breaks down into water and CO₂. Example: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. Net ionic: CO₃²⁻ + 2H⁺ → H₂O + CO₂.
2. Acid + active metal → salt + hydrogen. Zn + 2HCl → ZnCl₂ + H₂.
3. Ammonium salt + strong base → salt + water + ammonia. NH₄Cl + NaOH → NaCl + H₂O + NH₃. Warming helps to release the gas.
4. Sulfite + acid → salt + water + sulfur dioxide. Na₂SO₃ + 2HCl → 2NaCl + H₂O + SO₂.
5. Sulfide + acid → salt + hydrogen sulfide. FeS + 2HCl → FeCl₂ + H₂S.
6. Decomposition reactions that release gas: 2H₂O₂ → 2H₂O + O₂, and the decomposition of carbonates by heat.
7. Reactions of metals with water: 2Na + 2H₂O → 2NaOH + H₂.
8. Oxidation of hydrochloric acid by strong oxidizers: MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂ (heat).

Summary of gas-forming reactions

How do you identify the gas?

How do you calculate the volume of gas?

First find the moles of gas from the balanced equation. Then convert moles to volume.

• Molar volume: at 0 °C and 1 atm (STP), 1 mol of an ideal gas occupies 22.4 L. At 25 °C and 1 atm, it occupies about 24.5 L.
• Ideal gas law: PV = nRT, with R = 8.314 J/(mol·K), or R = 0.08206 L·atm/(mol·K). For example, V = nRT ÷ P with T in kelvin.

Worked examples

1. Carbon dioxide from marble. 5.00 g of CaCO₃ (100.09 g/mol) reacts with excess HCl. n(CaCO₃) = 0.04996 mol. The ratio is 1 : 1, so 0.04996 mol of CO₂ forms. Mass: 0.04996 × 44.01 = 2.20 g. Volume at 25 °C and 1 atm: 0.04996 × 24.5 = 1.22 L.
2. Ideal gas law. The same CO₂ at 25 °C (298 K) and 101.3 kPa: V = nRT ÷ P = 0.04996 × 8.314 × 298 ÷ 101,300 = 1.22 × 10⁻³ m³ = 1.22 L. The two methods agree.
3. Hydrogen from zinc. 3.27 g of Zn (65.38 g/mol) is 0.0500 mol. Zn + 2HCl → ZnCl₂ + H₂ gives 0.0500 mol of H₂, which is 0.0500 × 22.4 = 1.12 L at STP, or 0.100 g.
4. Ammonia from ammonium chloride. 2.68 g of NH₄Cl (53.49 g/mol) with excess NaOH: n = 0.0501 mol. NH₄Cl + NaOH → NaCl + H₂O + NH₃ gives 0.0501 mol of NH₃, with a mass of 0.0501 × 17.03 = 0.853 g.
5. Antacid tablet. A tablet contains 1.68 g of NaHCO₃ (84.01 g/mol), which is 0.0200 mol. In water with citric acid, it releases 0.0200 mol of CO₂, or 0.0200 × 24.5 = 0.490 L at 25 °C. This volume is about 490 mL of gas.
6. Net ionic equation. Magnesium carbonate with nitric acid: MgCO₃(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂O(l) + CO₂(g). The nitrate ions are spectators. Since MgCO₃ is a solid, it is written as a formula unit.
7. Limiting reagent. 0.100 mol of Na₂CO₃ and 0.100 mol of HCl react: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. HCl is limiting: it can react with only 0.0500 mol of Na₂CO₃, and gives 0.0500 mol of CO₂ (1.23 L at 25 °C).

Where do you see gas-forming reactions?

• Baking: baking powder (sodium hydrogencarbonate and an acid) releases CO₂ in dough, which expands and makes bread and cakes rise.
• Effervescent tablets: citric acid and sodium hydrogencarbonate release CO₂ when dissolved in water.
• Fire extinguishers: the classic soda-acid extinguisher released CO₂ by this reaction.
• Cleaning: baking soda and vinegar release CO₂, which helps to loosen deposits in drains.
• Geology: acid rain dissolving limestone releases CO₂, and carbonated spring waters form from this reaction in nature.
• Airbags and rockets: decomposition reactions that release large volumes of gas in milliseconds.

What are the safety hazards?

• Do not seal a container in which a gas is being produced. The pressure can burst the container.
• Hydrogen is flammable and forms explosive mixtures with air.
• Hydrogen sulfide, sulfur dioxide, chlorine and ammonia are toxic or irritating. Work in a fume hood.
• Never mix bleach with acids or ammonia-containing cleaners. Bleach and acid release chlorine, and bleach with ammonia releases toxic chloramines.
• Carbon dioxide displaces air in enclosed spaces and can cause suffocation at high concentrations.

How were the gases discovered?

The Flemish chemist Jan Baptist van Helmont coined the word “gas” in the early 17th century after studying the vapors from burning charcoal and fermentation. Joseph Black identified carbon dioxide in 1754 as “fixed air” released from limestone and magnesium carbonate. Henry Cavendish isolated hydrogen in 1766 by reacting metals with acids. Joseph Priestley discovered ammonia, sulfur dioxide and oxygen in the 1770s, and Carl Wilhelm Scheele discovered chlorine in 1774.

Which mistakes are common?

Mistake 1: Writing H₂CO₃ as a product

Carbonic acid is unstable. Write H₂O and CO₂(g) instead.

Mistake 2: Using the molar volume at the wrong conditions

22.4 L/mol applies at 0 °C and 1 atm. At 25 °C, use 24.5 L/mol or the ideal gas law.

Mistake 3: Expecting all metals to give hydrogen with acids

Metals below hydrogen in the activity series, such as copper and silver, do not displace hydrogen from dilute acids.

Mistake 4: Smelling gases directly

Never inhale directly. Waft a small amount toward you only if instructed.