Rate of Reaction

First balance the reaction. Next decide whether the species is a reactant or product. Use a negative sign for disappearance and a positive sign for formation. Finally divide by the stoichiometric coefficient and include mol L^-1 s^-1 unless another time unit is given.

A concentration-time graph is useful. Reactant concentration curves slope downward and product concentration curves slope upward. Instantaneous rate is read from the tangent slope at the required time.

Writing 0.004 mol L^-1 s^-1 as the reaction rate for A → 2B when it is only the rate of formation of B.

Rate of reaction is the change in concentration of a reactant or product per unit time. For a reactant it is written with a negative sign because its concentration decreases, while for a product it is written with a positive sign because its concentration increases.

Rate of reaction is the change in concentration of a reactant or product per unit time. For a reactant it is written with a negative sign because its concentration decreases, while for a product it is written with a positive sign because its concentration increases. Average rate for reactant = -Delta[R]/Delta t; average rate for product = +Delta[P]/Delta t. For aA + bB → cC + dD, rate = -(1/a)d[A]/dt = -(1/b)d[B]/dt = (1/c)d[C]/dt = (1/d)d[D]/dt. Usual unit: mol L^-1 s^-1. For N₂ + 3H₂ → 2NH₃, rate = -d[N2]/dt = -(1/3)d[H2]/dt = (1/2)d[NH3]/dt. If ammonia concentration increases by 0.020 mol L^-1 in 10 s, the rate of reaction based on NH₃ is (1/2)(0.020/10) = 0.0010 mol L^-1 s^-1.

Reaction rate measures how quickly a chemical reaction proceeds. Average rate is calculated over a time interval, while instantaneous rate is the rate at a particular moment, usually obtained from the slope of a concentration-time curve. For a reaction aA + bB → cC + dD, the rate must be divided by stoichiometric coefficients so that the same reaction rate is obtained from any species. Average rate for reactant = -Delta[R]/Delta t; average rate for product = +Delta[P]/Delta t. For aA + bB → cC + dD, rate = -(1/a)d[A]/dt = -(1/b)d[B]/dt = (1/c)d[C]/dt = (1/d)d[D]/dt. Usual unit: mol L^-1 s^-1. For A → 2B, [B] rises from 0.10 mol L^-1 to 0.22 mol L^-1 in 30 s. Rate of formation of B = 0.12/30 = 0.004 mol L^-1 s^-1. Reaction rate = (1/2)(0.004) = 0.002 mol L^-1 s^-1. Questions ask students to calculate average rate from concentration data, compare rates of disappearance and formation, or identify the correct expression using a balanced chemical equation. Writing 0.004 mol L^-1 s^-1 as the reaction rate for A → 2B when it is only the rate of formation of B.