Kinetics Chemistry Kinetics Study of reaction rates How

Kinetics Chemistry

Kinetics • Study of reaction rates • How fast does it happen? What variables influence the rate? What is the path the reaction takes to convert reactants to products? • Rate • Change in an amount over a period of time • Ex. Distance traveled, space travel

Reaction Rates • Change in the concentration of a chemical compound in the reaction over a period of time • Focus on one reactant or one product in the reaction • Want to know rate of disappearance for reactant/rate of appearance for a product • Rate = ∆X ∆t Units = M/time

Example 1: A + B → D + E • A) Rate of disappearance for A • Rate. A = -∆[A] ∆t • B) Rate of appearance for D • Rate. D = ∆[D] ∆t

Example 2: 2 AB → A 2 + B 2 Time = 0 seconds 15. 0 M 0 M 0 M Time= 60 seconds 5. 0 M 5. 00 M • Determine the rate of disappearance of AB • Determine the rate of appearance of A 2 and B 2

Instantaneous Rate of Reaction • Rate = • d(x) dt Enables us to get an accurate measurement of a reaction rate that is always changing.

Reaction Rate at ANY moment in time • Slope of line tangent to the reaction curve

Measuring Reaction Rates **Colorimeter/Spectrophotometer • p. H changes • Volume changes • Amount of precipitate formed

What can influence reaction rates? 1) 2) 3) 4) 5) 6) Temperature Concentration Catalyst Surface Area Volume/Pressure Reactant Properties

Rate Law • Must be experimentally determined • Describes the rate of the overall chemical reaction • Equation indicates how changes in reactant concentration affects the rate of the chemical reaction

Rate Law (cont. ) • A+B→ C+D • Rate = k [A]m[B]n • • • Rate = rate of disappearance of reactants k = rate constant, specific to reactions and temperature m = reaction order in terms of A n = reaction order in terms of B m + n = overall reaction order

Reaction Order • Indicates how concentration changes affect changes in the reaction rate • Orders: 0, 1, 2 • Overall order of reaction = Σ individual orders of each reactant • Order of a reaction in terms of a reactant ≠ reactant’s coefficient in chemical equation

Reaction Orders (cont. ) • Zero-order Reaction • Rate not dependent on reactant’s concentration • Constant reaction rate • First-order Reaction • Concentrate affects reaction rate • Example: Double concentration, double the rate. • Second-order Reaction • Concentration affects reaction rate • Example: double concentration, quadruple the rate

Rate Constant (k) Units Reaction Order Basic Formula Units 0 Rate = k Ms-1 1 Rate = k [A] s-1 2 Rate = k [A]2 M-1 s-1 3 Rate = k [A]3 M-2 s-1

Example 3: 2 NO(g) + O 2(g) 2 NO 2(g) Based on the reaction’s rate law of Rate = k(NO)2 (O 2) Classify this reaction’s order.

Example 4: • Determine the rate law, reaction order, and rate constant (k) for the following reaction at a specific temperature--- • 2 NO(g) + 2 H 2(g) N 2(g) + 2 H 2 O(g) Experiment [NO]initial [H 2]initial Rate initial 1 0. 20 M 0. 30 M 0. 0900 M/s 2 0. 10 M 0. 30 M 0. 0225 M/s 3 0. 10 M 0. 20 M 0. 0150 M/s

Example 5: • Determine the rate law for the following reaction--- • NH 4+(aq) + NO 2 -(aq) N 2(g) + 2 H 2 O(l) Experiment [NH 4+]initial [NO 2 -]initial Rate initial 1 5 x 10 -2 M 2. 70 x 10 -7 M/s 2 5 x 10 -2 M 4 x 10 -2 M 5. 40 x 10 -7 M/s 3 1 x 10 -1 M 2 x 10 -2 M 5. 40 x 10 -7 M/s

Homework • Kinetics Worksheet #1 -6