CHEM 108 1st Edition Lecture 10Chapter 14: Chemical Kinetics:Chemical Kinetics: Study of rates at which reactants/products change during a reaction. - Includes study of factors that affect rate of reactions, and steps involved as a reaction proceeds from reactants to products. - Reaction Rate: Change in concentration of reactant or product over time.- N2 (g) + O2 (g) → 2 NO(g)o Rate of NO formation: (ΔNO)/(Δt)[Units: M/s]o Rate of N2 consumption: -(ΔN2)/(Δt)[Units: M/s]o Rate of O2 consumption: -(ΔO2)/(Δt)[Units: M/s]- Relative reaction rates: Can compare relative rates of consumption of reactants and formation of products based on stoichiometry.o -(ΔO2)/(Δt) = -(ΔN2)/(Δt) = 0.5(ΔNO)/(Δt)- Average rate: change in concentration of reactant or product over a specific time interval.o (ΔNO)/(Δt)- General Example: aA+bB→cC+dDo −1/a(ΔA/Δt) =−1/b(ΔB/Δt)=1/c(ΔC/Δt)=1/d(ΔD/Δt)- Instantaneous Rate: Reaction rate at a particular instant. It is determined graphically as tangential slope of concentration vs time plot.Factors that affect rates of reaction:- Concentration on the rate of reactions: Increasing the concentration of a substance increases the rate of reaction because there are going to be more collisions, and therefore more reactions happening.- Physical state of the reactants, reactions in the solid state tend to react slower than those reactions in the liquid and gas state.- Temperature, as temperature increases the rates of chemical reaction increases, because the likely hood of particle collisions increase- The presence of catalysts. Catalysts are materials that accelerate reactions.Order of Reaction: - How the concentration of the reaction affects the rate of the reaction. o Ex: if the concentration of a reaction doubles then the rate of that reaction will double too (first order).o Overall order of reaction: sum of exponents of concentration terms in the rate law.- Rate Law: Equation defining relationship between reactant concentrations and reaction rate. o aA + bB → productso Rate = k[A]n[B]mo Order: n and m n is the partial reaction order with respect to A and m is the partial reactionorder with respect to Bo Overall order: n+mo Rate constant: k, unit of k is variable, it depends on the overall order of the reaction and k is affected by temperature Zero order: Rate = k[A]0 = k (concentration x 2 = samerate of reaction) First order: Rate = k[A] (concentration. x 2 = rate of reaction x 2) Second order Rate = k[A]2 (concentrationx 2 = rate of reaction x 4; i.eConc. x n = n2 )Determining Rate law:- Two methods used: o Method of initial rate: In a reaction you will keep one reactant at a constant rate while changing the other reactant (usually halving or doubling it) and see how the overall reaction changed due to the change in that reactant. o Ex:Rate = k[O2 ]m [NO]nExperiment [NO] (M) [O2] (M) Initial reaction rate1 0.1 0.1 1e-62 0.1 0.05 0.5e-63 0.05 0.1 2.5e-7oo To find m: Compare experiments #1 and #2. [NO] is constant; [O2] is halved, and rate is halved. Rate (proportional to)o [O2]m; m = 1. After finding n through a similar fashion you can find k can be calculated from the rate law using data for any experiment. Rate = k[O2 ]*[NO]2o Graphical method – Reactions that involve a single reactant – Plots of concentration-time data are used All of the concentration-time reactions can be rearranged to give equationsthat form a straight line:Where slope of the line is the k constant. Zero Order: [A]t = -kt + [A]0 First Order: ln[A]t = -kt + ln[A]0 Second Order: 1/[A]t = kt +
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