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: 2AB → A 2 + B 2 Time = 0 seconds 15.0 M0M Time= 60 seconds 5.0 M5.00M 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 pH changes Volume changes Amount of precipitate formed

What can influence reaction rates? 1) Temperature 2) Concentration 3) Catalyst 4) Surface Area 5) Volume/Pressure 6) 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 OrderBasic FormulaUnits 0Rate = kMs -1 1Rate = k [A]s -1 2Rate = k [A] 2 M -1 s -1 3Rate = k [A] 3 M -2 s -1

Example 3: 2NO (g) + O 2(g)  2NO 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---- 2NO (g) + 2H 2(g)  N 2(g) + 2H 2 O (g) Experiment[NO] initial [H 2 ] initial Rate initial 10.20M0.30M M/s 20.10M0.30M M/s 30.10M0.20M M/s

Example 5: Determine the rate law for the following reaction---- NH 4 + (aq) + NO 2 - (aq)  N 2(g) + 2H 2 O (l) Experiment[NH 4 + ] initial [NO 2 - ] initial Rate initial 15 x M2 x M2.70 x M/s 25 x M4 x M5.40 x M/s 31 x M2 x M5.40 x M/s

Homework Kinetics Worksheet #1-6