Chem 108 Lecture notes 10 PDF

Preview

Summary

Download Chem 108 Lecture notes 10 PDF

Description

Chem 108 Lecture notes 10: 2/16/15 Chapter 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+dD o −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 → products o Rate = k[A]n[B]m o Order: n and m  n is the partial reaction order with respect to A and m is the partial reaction order with respect to B o Overall order: n+m o Rate constant: k, unit of k is variable, it depends on the overall order of the reaction and k is affected by temperature, and does not change with concentration, since it is the rate constant.  Zero order: Rate = k[A]0 = k (concentration x 2 = same rate of reaction)  First order: Rate = k[A] (concentration. x 2 = rate of reaction x 2)  Second order Rate = k[A]2 (concentration x 2 = rate of reaction x 4; i.e Conc. 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]n Experiment

[NO] (M)

[O2] (M)

Initial reaction rate

1 2 3

0.1 0.1 0.05

0.1 0.05 0.1

1e-6 0.5e-6 2.5e-7

o o 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]2 o 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 equations that 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 + 1/[A]0...