Unit 5: Topic 2 - Introduction to Rate Law Flashcards
What experimental methods are used to determine the rate of a reaction?
The rate of a reaction changes according to the concentration of certain reactants. The rate law for a reaction can be determined experimentally by using initial rates, where the instantaneous reaction rate is measured immediately when the reactants are mixed. The process is repeated for several trials, as the concentration of one reactant is changed as the other is held constant. These trials are then compared to determine how changing the concentration of each reactant affects the initial rate.
What is the rate law?
For a single-step reaction mA + nB -> products, the rate law is defined as R = k[A] ͫ [B]ⁿ, where R is the rate of the reaction, k is the reaction-specific rate constant, [A] and [B] are the concentrations of each reactant, and m and n are reaction orders for each reactant (found by the coefficients from the reaction equation). It is rare for a reaction to have more than two reactants since the probability of all the reactants colliding in the right way for the reaction to occur is exceedingly low. Rate laws can only be determined experimentally.
What is the order of a reaction?
Give the rate law, m and n define the order of the reaction with respect to the corresponding reactant. The reaction order describes the factor by which the reaction rate increases when the concentration of a reactant is increased by a specific amount.
For example, given R = k[A]²[B], if the concentration of A doubles as the concentration of B is held constant, the reaction rate will quadruple. Similarly, if the concentration of B doubles as the concentration of A is held constant, the reaction rate will double. We say that the reaction is second-order with respect to A, first-order with respect to B, and third-order overall by adding up the rate law exponents.
What is the rate constant?
The rate constant, k, is a proportionality constant that quantifies the rate of a reaction and is temperature dependent. Its unit reflects the overall reaction order as follows:
Zeroth order: M/s (mol/(Ls))
First order: 1/s
Second order: 1/Ms (1/(mol/(Ls)))
…
The unit for reaction rate is always M/s or mol/(L*s), so the units of the rate constant can always be found by plugging the reaction rate and concentration units into the rate law.
Find the reaction order with respect to each reactant of the reaction 2NO + Cl₂ -> 2NOCl by comparing the initial rates.
R = 0.152[NO]²[Cl₂]
The concentration of NO doubles from Experiment 1 to Experiment 2, and to find the factor by which the initial rate increases, divide 7.6 x 10⁻² by 1.9 x 10⁻² to get 4. This means that doubling the concentration of NO quadruples the reaction rate. Therefore, the reaction order with respect to NO is 2, as the rate increases 2² times. The concentration of Cl₂ doubles from Experiment 2 to Experiment 3, and the initial rate doubles as well. This means that doubling the concentration of Cl₂ doubles the initial rate, and therefore, the reaction order with respect to Cl₂ is 1, as the rate increases 2¹ times. To find the value of k, plug in any of the data points for the rate and concentration of reactants, and solve. Using Experiment 1, we get 1.9 x 10⁻² M/s = k(0.50M)²(0.50M), and k = 1.52 x 10⁻¹ 1/M²s.
