reactants

Meaning: Reactants are the substances written left of the arrow in a chemical equation. Their bonds break and atoms rearrange into products, so reactant amounts fall and product amounts rise as the reaction proceeds. In a reversible reaction, the left side is called the reactants by convention; at equilibrium both sides are present.

Equation: aA + bB → cC + dD, rate = −(1/a)(Δ[A]/Δt), where A and B are reactants, a to d are dimensionless coefficients, [A] is molarity (mol/L), t is time (s), and rate is in mol/(L·s).

On the MCAT: Limiting reactant problems: convert masses to moles, divide each by its coefficient, and the smallest value marks the limiting reactant, which caps the theoretical yield. Rate passages give concentrations and initial rates so you can find each reactant's order. Le Chatelier questions: adding reactant shifts equilibrium toward products. On energy diagrams, reactants sitting higher in energy than products means the reaction is exothermic.

Common confusion: Reaction orders come from experimental data, not from the coefficients, unless the step is elementary. Catalysts are not reactants because they are regenerated; intermediates are formed and consumed, so they are absent from the overall equation.