Reactant
A reactant (also called a reagent) is any substance that enters into and is altered in the course of a chemical reaction, written on the left-hand side of a chemical equation. Reactants supply the atoms that are rearranged to form products, and their amounts determine how much product can be formed. The concept of a limiting reactant is especially important in industrial chemistry, where maximising product yield while minimising waste requires precise control of reactant quantities.
Worked Example
Problem
In the reaction N₂ + 3 H₂ → 2 NH₃, if you start with 14 g of N₂ and 6 g of H₂, which is the limiting reactant?
Solution
Step 1: Find moles of each reactant. Molar mass of N₂ = 28 g/mol → moles of N₂ = 14/28 = 0.50 mol Molar mass of H₂ = 2 g/mol → moles of H₂ = 6/2 = 3.00 mol Step 2: Determine moles needed. The reaction requires 3 mol H₂ per 1 mol N₂. For 0.50 mol N₂, we need 0.50 × 3 = 1.50 mol H₂. Step 3: Compare available H₂ (3.00 mol) with required H₂ (1.50 mol). We have more H₂ than needed, so N₂ is fully consumed first.
Answer
N₂ is the limiting reactant; H₂ is in excess.
Reactants vs Products in Common Reactions
| Reaction | Reactants | Products | Type |
|---|---|---|---|
| Combustion of methane | CH₄ + 2 O₂ | CO₂ + 2 H₂O | Combustion |
| Rusting of iron | 4 Fe + 3 O₂ | 2 Fe₂O₃ | Oxidation |
| Neutralisation | HCl + NaOH | NaCl + H₂O | Acid-Base |
| Photosynthesis | 6 CO₂ + 6 H₂O | C₆H₁₂O₆ + 6 O₂ | Synthesis |
| Decomposition of water | 2 H₂O | 2 H₂ + O₂ | Decomposition |
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Related Terms
Chemical Reaction
A chemical reaction is a process in which one or more substances (reactants) are transformed into one or more different substances (products) through the breaking and forming of chemical bonds. Chemical reactions involve changes in the arrangement of atoms, resulting in new materials with different properties from the original substances. They are fundamental to all biological, industrial, and environmental processes, from cellular respiration to the manufacture of medicines.
Chemical Product
A chemical product is a substance formed as a result of a chemical reaction, written on the right-hand side of a chemical equation after the arrow. Products have different chemical properties and structures compared to the reactants from which they originate, because chemical bonds have been broken and reformed in new arrangements. Understanding products is essential for predicting reaction outcomes, designing syntheses of useful materials, and assessing environmental impacts of chemical processes.
Balancing Chemical Equations
Balancing chemical equations is the process of adjusting the stoichiometric coefficients in front of each formula in a chemical equation so that the number of atoms of each element is equal on both the reactant and product sides, satisfying the Law of Conservation of Mass. Only coefficients (not subscripts) may be changed during balancing, because altering subscripts would change the identity of the compounds. Balanced equations are essential for calculating the exact quantities of reactants and products in stoichiometric calculations used in laboratories and industry.
From Latin "reagere", meaning "to react or act back", with the suffix "-ant" indicating a thing that performs the action. The distinction between reactants and products became formalised in chemical notation by Antoine Lavoisier in the late 18th century.