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Synthesis reactions combine two or more simpler reactants into a single complex product through bond formation. Decomposition reactions are the opposite—a single complex reactant breaks down into simpler products like elements or compounds. Both are types of redox reactions involving electron transfer and observable changes in oxidation states.

Synthesis reactants may include only elements, elements combined with compounds, or only compounds. For example, elemental hydrogen and oxygen combine to form water, carbon monoxide and oxygen form carbon dioxide, and calcium oxide reacts with water to produce calcium hydroxide. All synthesis reactions result in a single, more complex product.

Decomposition reactions require energy to break the bonds holding a complex molecule together. This energy can come from heat, electricity, or light. For instance, water decomposes into hydrogen and oxygen under an electric field, hydrogen peroxide breaks down into oxygen and water in sunlight, and calcium hydroxide decomposes into calcium oxide and water when heated.

In these redox reactions, one reactant is oxidized (loses electrons and increases oxidation state) while another is reduced (gains electrons and decreases oxidation state). Synthesis reactions typically release energy as bonds form between reactants. For example, sodium and chlorine combine to form sodium chloride, releasing 787 kJ of heat energy during the process.

Decomposition reactions occur naturally in digestion, where proteins, fats, and carbohydrates break down into simpler molecules. Historically, Joseph Priestley discovered oxygen in 1774 by heating mercury oxide, decomposing it into mercury and oxygen gas. Sodium azide decomposes into sodium metal and nitrogen gas, demonstrating how complex compounds break into simpler substances.

Decomposition products may be all elements, a combination of elements and compounds, or only compounds. For example, aluminum oxide decomposes into aluminum metal and oxygen gas, potassium chlorate breaks down into potassium chloride and oxygen, and ammonium chloride produces ammonia gas and hydrogen chloride gas.

Synthesis reactions are generally accompanied by the release of energy. When reactants combine to form new bonds, energy is released as heat. The formation of sodium chloride from sodium and chlorine exemplifies this, releasing 787 kJ of heat energy. This energy release reflects the stability gained when simpler reactants bond into a more complex product.