Chapter13 week 4

# Chapter13 week 4 - 6C Chapter 13 Week 2 and 3 Bill Trogler...

This preview shows pages 1–11. Sign up to view the full content.

6C Chapter 13 Week 2 and 3 Bill Trogler

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
Kinetics The study of how fast reactions occur. A topic distinct from thermodynamics, which only tells whether a reaction is energetically favorable overall. Overhead 1. The speed of a chemical reaction is called the rate and is defined for the reaction: a A + b B + … … + x X + y Y + z Z Rate of rxn = (1/x) d[X]/dt = (1/z) d[Z]/dt = - (1/a) d[A]/dt = - (1/b) d[B]/dt It has units of conc/t or M/s
Consider the Simple Combustion of Hydrogen Reaction time

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
Rate of Diappearance of a Reactant at times of 5 and 10 Weeks Estimated from Tangent to Conc vs. t curve at these Times
Rate from Tangents: At 5 weeks 0 M - 0.076 M = - 0.00633 M/wk for rate of disappearance 12 wk 0 wk Rate of rxn = 0.00633 M/wk At 10 weeks 0 M - 0.045 M = - 0.00264 M/wk for rate of disappearance 17 wk 0 wk Rate of rxn = 0.00264 M/wk

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
In general: Rate(fn of T, P, Conc, Specific rxn, Phase, Solvent, Catalyst,…) For homogeneous phase reactions(e.g. all species in soln or all in gas phase), the concentration dependence of the rate is expressed in terms of the Rate Law = product of concentrations of reactants (and rarely products), each raised to an experimentally determined power. e.g. for the rxn 2 A + 3 B P Rate Law : Rate = k[A] x [B] y [P] z x, y, z are determined by expt, often z = 0 and x = 0, 1, or 2 and y = 0, 1, or 2; however, any value (e.g. ½, 0.2, 1.3) may be found. k is a constant characteristic of a rxn at a specific temperature, which is called the rate constant.
Reaction Order Reaction Order = sum of the powers of reactants in the Rate Law For the previous example = x + y + z The order in a reactant, A, is simply its power in the Rate Law For the previous example the reaction is x order in A Most common rate laws: Zero order reaction : Rate of rxn = k First order reactions : Rate = k[A] or Rate = k[B] or rarely Rate = k[A] 0.5 [B] 0.5 Second order reactions : Rate = k[A] 2 or Rate = k[B] 2 or Rate = k[A][B]

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
Units of the Rate Constant Note that k has units, which depend on the rate law, since the rate must always have units of conc/time (e.g. M/s for soln or atm/s for gas phase reactions). Zero order rate constant units are M/s First order rate constant units are 1/s or s -1 Second order rate constant units are M -1 s -1
How do you experimentally measure the rate law and rate constant? Two methods are commonly used: 1) Method of initial rates. Measure the initial values of the rate of reaction at various concentrations of reactants and observe how the rate varies as one concentration changes while the others held constant. 2) Integrated rate law. Assume a first or second order dependence in the rate law and see if the concentration vs. time curve of a reactant obeys a first or second order behavior. Often one reactant is the limiting reactant, while the others are present in such a large excess, such that their concentrations remain approximately constant.

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
Method of Initial Rates Initial Rate obsd(M/s) [A], M [B], M 0.01 0.2 0.1 0.03 0.6 0.1 0.05 1.0 0.1 0.04 0.2 0.2 0.09 0.2 0.3 Observation, the initial rate varies like [B] 2 and [A] 1 so Rate law: Rate = k[A][B] 2 A third order reaction, which is first order in A and second order in B.
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

### What students are saying

• As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

Kiran Temple University Fox School of Business ‘17, Course Hero Intern

• I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

Dana University of Pennsylvania ‘17, Course Hero Intern

• The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

Jill Tulane University ‘16, Course Hero Intern