Synthesis (Combination) and Decomposition Reactions

A synthesis (combination) reaction joins two or more substances to form one product, A + B → AB, and a decomposition reaction breaks one compound into two or more simpler substances, AB → A + B. In brief: magnesium and oxygen combine to form magnesium oxide (2Mg + O₂ → 2MgO), while calcium carbonate decomposes on heating into calcium oxide and carbon dioxide (CaCO₃ → CaO + CO₂); recognizing the pattern lets you predict the products.

Synthesis and decomposition reactions are opposite in pattern. Many of them are industrially important, from the production of ammonia to the manufacture of lime. This article lists the common patterns for synthesis and for decomposition, including the typical products of heating carbonates, hydroxides, chlorates and nitrates, shows tables of examples, works through calculations with masses and gas volumes, and mentions the history of the main reactions.

What are the patterns of synthesis reactions?

• Two elements form a binary compound: metal + nonmetal → ionic compound (2Na + Cl₂ → 2NaCl), or nonmetal + nonmetal → molecular compound (N₂ + 3H₂ → 2NH₃).
• Metal + oxygen → metal oxide: 4Al + 3O₂ → 2Al₂O₃.
• Nonmetal + oxygen → nonmetal oxide: S + O₂ → SO₂.
• Metal oxide + water → metal hydroxide (base): CaO + H₂O → Ca(OH)₂.
• Nonmetal oxide + water → oxyacid: SO₃ + H₂O → H₂SO₄.
• Metal oxide + nonmetal oxide → salt: CaO + CO₂ → CaCO₃.
• Compound + compound → larger compound: NH₃ + HCl → NH₄Cl.

The charges of the ions show the formula of an ionic product. Transition metals can give more than one product: iron and chlorine give FeCl₃ (with excess chlorine), and iron with sulfur gives FeS.

Synthesis examples

What are the patterns of decomposition reactions?

• Binary compounds → elements: 2HgO → 2Hg + O₂ (heat), 2H₂O → 2H₂ + O₂ (electricity).
• Metal carbonates → metal oxide + CO₂: CaCO₃ → CaO + CO₂. (Group 1 carbonates, except lithium carbonate, are very stable.)
• Metal hydroxides → metal oxide + water: Cu(OH)₂ → CuO + H₂O.
• Metal chlorates → metal chloride + oxygen: 2KClO₃ → 2KCl + 3O₂ (with MnO₂ as a catalyst).
• Hydrogencarbonates → carbonate + water + CO₂: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂.
• Nitrates: group 1 nitrates give nitrites and oxygen (2KNO₃ → 2KNO₂ + O₂), and many others give oxides with NO₂ and O₂ (2Cu(NO₃)₂ → 2CuO + 4NO₂ + O₂).
• Hydrogen peroxide: 2H₂O₂ → 2H₂O + O₂.
• Unstable acids: H₂CO₃ → H₂O + CO₂.
• Hydrates: CuSO₄·5H₂O → CuSO₄ + 5H₂O.

Decomposition examples

Worked examples

1. Oxygen from potassium chlorate. How much oxygen forms from 10.0 g of KClO₃ (122.55 g/mol)? n(KClO₃) = 10.0 ÷ 122.55 = 0.0816 mol. From 2KClO₃ → 2KCl + 3O₂, n(O₂) = 0.0816 × 3 ÷ 2 = 0.1224 mol. The mass is 0.1224 × 32.00 = 3.92 g.
2. Carbon dioxide from baking soda. 5.00 g of NaHCO₃ (84.01 g/mol) is 0.0595 mol. From 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂, n(CO₂) = 0.0595 ÷ 2 = 0.02976 mol, which has a mass of 0.02976 × 44.01 = 1.31 g. At 25 °C and 1 atm (24.5 L/mol), the volume is 0.02976 × 24.5 = 0.729 L.
3. Mass of residue. The solid left after the baking soda decomposition is sodium carbonate: 0.02976 mol × 105.99 g/mol = 3.15 g. Check: 5.00 g = 3.15 g Na₂CO₃ + 0.536 g H₂O + 1.31 g CO₂ = 5.00 g. ✓
4. Airbag. An airbag needs 70.0 L of N₂ at 25 °C and 1 atm. n(N₂) = 70.0 ÷ 24.5 = 2.86 mol. From 2NaN₃ → 2Na + 3N₂, n(NaN₃) = 2.86 × 2 ÷ 3 = 1.90 mol, with a mass of 1.90 × 65.01 = 124 g.
5. Predict the products. Heating copper(II) carbonate, CuCO₃: a metal carbonate gives a metal oxide and CO₂. CuCO₃ → CuO + CO₂. The green powder turns black.
6. Synthesis prediction. Aluminum and bromine: Al³⁺ and Br⁻ give AlBr₃. The balanced equation is 2Al + 3Br₂ → 2AlBr₃.

Where are these reactions used?

• Lime and cement: limestone is decomposed in kilns, and the quicklime is slaked with water to give calcium hydroxide.
• Fertilizer: synthesis of ammonia by the Haber process supplies the nitrogen for most fertilizers.
• Metal extraction: electrolysis (a decomposition) gives aluminum from aluminum oxide.
• Oxygen generation: chlorate candles in aircraft and submarines release oxygen by decomposition.
• Cooking: baking powder and baking soda decompose when heated, releasing CO₂.

How were the reactions discovered?

Joseph Priestley heated mercury(II) oxide with sunlight in 1774, releasing oxygen. Fritz Haber demonstrated the laboratory synthesis of ammonia in 1909, and Carl Bosch scaled it up at BASF by 1913, which made artificial nitrogen fertilizer possible. Haber received the Nobel Prize in Chemistry in 1918 and Bosch in 1931. The electrolytic decomposition of molten aluminum oxide in cryolite was developed independently by Charles Martin Hall and Paul Héroult in 1886.

Which mistakes are common?

Mistake 1: Forgetting the diatomic elements

Oxygen, hydrogen, nitrogen and the halogens are written as O₂, H₂, N₂, Cl₂ and so on when they are the free elements.

Mistake 2: Writing the wrong formula for an ionic product

Use the ion charges to find the formula. Magnesium oxide is MgO, not MgO₂.

Mistake 3: Assuming that all carbonates decompose with gentle heat

Group 1 carbonates (except Li₂CO₃) do not decompose in a Bunsen flame, while copper(II) carbonate does.

Mistake 4: Writing a decomposition without the energy source

Decompositions need heat, light, electricity or a catalyst. Show the condition above the arrow.