Finding reaction orders by comparing initial rates across experiments with different starting concentrations.
Practise Kinetics & Equilibrium All Kinetics & Equilibrium topics
The method of initial rates determines the order of a reaction with respect to each reactant by running several experiments in which one initial concentration is varied while the others are held fixed. The initial rate is measured in each run, and the ratio of two rates is compared with the ratio of the corresponding concentrations. If doubling a concentration doubles the rate, the order is one; if the rate quadruples, the order is two; if the rate does not change, the order is zero. Once every order is found, the rate constant follows from any experiment by substituting the measured rate and concentrations into the rate law.
Key ideas: Set up a table of runs, pick two runs where only one concentration changes, and solve rate2/rate1 = (c2/c1)^order. Repeat for each reactant, then compute k from all runs and average it. Use a short time so that less than about 5 percent of reactant is consumed and the concentrations are still near initial values.
Common mistakes and connections: A frequent mistake is comparing two runs where two concentrations changed at once, which mixes the orders. Another is guessing an order such as 1 from a ratio of about 1.4 that really means a half order. Students also forget to include temperature control. The method gives orders but not mechanisms, so it pairs with integrated plots for confirmation.
Doubling [A] each run doubles the initial rate, so the order in A is one. Comparing rate ratios with concentration ratios gives each order. The interactive lesson shows this as a live animation or chart.
If doubling [A] doubles the rate while doubling [B] causes no change, the rate law is rate = k[A].
Finding reaction orders by comparing initial rates across experiments with different starting concentrations.
Method of Initial Rates is a intermediate topic in Kinetics & Equilibrium. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Kinetics & Equilibrium first: Second Order Reaction, Half-life of Reactions, Pseudo First Order Reaction.
The next topics in Kinetics & Equilibrium are Integrated Rate Laws and Molecularity.
Yes. The interactive lesson includes a self-check question on Method of Initial Rates, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Method of Initial Rates is topic 12 of 91 in Kinetics & Equilibrium.