Textbook Notes (368,826)
United States (206,116)
Chemistry (297)
CHEM 001B (1)
Hill (1)
Chapter

# Chp 12.docx

9 Pages
116 Views

Department
Chemistry
Course
CHEM 001B
Professor
Hill
Semester
Summer

Description
Chapter 12: Chemical Kinetics  Focus on the question – How fast does it happen? o More closely we’ll look at the speeds, or rates at which chemical reactions occur  The area of chemistry concerned with reaction rates and sequence of steps by which reactions occur is called chemical kinetics o Subject of crucial environmental, biological, and economic importance 12.1 Reaction Rates  Chemical reactions differ greatly o Examples:  Combination of Na and Br, occur instantly  Rusting of Fe, imperceptibly slow  To describe a reaction rate quantitatively o Need to specify  How fast the concentration of a reactant or product changes per unit time o Rate =  The reaction rate can be defined either as the increase in the concentration of a product per unit time or as the decrease in the concentration of a reactant per unit time o Example:  2 N O254NO + O 2 2  If we want to find the rate formation of O 2n the decomposition of N O2 5 o 1. Use this equation:  Rate of formation of O =2 = o 2. Plug in the values necessary o 3. Solve o The most common units :  M/s  mol/(L*s) o Defined in terms of concentration because we want the rate to be independent of the scale of the reaction o Don’t forget a minus sign when calculating the rate of disappearance of reactant because reaction rate is defined as a positive quantity  Example:  Rate of decomposition of N O 2 5 = -5 = 1.9 x 10 M/s o Depend on the concentration of at some of the reactants and therefore decrease as the reaction mixture runs out of reactants, as indicated by the decreasing slopes of the curves  The slope of the tangent to a concentration-versus-time curve at a time t is called the instantaneous rate at that particular time  The instantaneous rate at the beginning of a reaction (t = 0) is called the initial rate 12.2 Rate Laws and Reaction Order Ddexxxxdxxedx9drrx9wzxz  The dependence of the reaction rate on the concentration of each reactant is given by an equation called the rate law o Rate = - = k[A] [B] n  k is the proportionality constant called the rate constant  exponents m and n in the rate law indicate how sensitive the rate is to changes in [A] and [B]  Small positive numbers  If m is negative, the rate decreases as [A] increases  If m is zero, the rate is independent of the concentration of A  If m = 1 and [A] is doubled, the rate doubles  If m = 2 and [A] is doubled, [A] quadruples and the rate increases by a factor of 4  The values of the exponents m and determine the reaction order with respect to A and B  The sum of the exponents (m + n) defines the overall reaction order o Example: 2  Rate = k[A] [B]  m = 2, n = 1 so m + n = 2 + 1 = 3 12.3 Experimental Determination of a Rate Law  Methods to determine the values of the exponents in a rate law o The method of initial rates  Carry out a series of experiments in which the initial rate of a reaction is measured as a function of different sets of initial concentrations  Ways to determine rate law: o Establish the reaction order o Evaluate the numerical value of the rate constant k  Rate = k  zeroth order  Rate = k[A]  first order  Rate = k[A][B]  second order 2  Rate = k[A][B]  third order  The rate depends on the concentrations, whereas the rate constant does not  The rate is usually expressed in units of M/s, units of the rate constant depend on the overall reaction order  Example: o Initial rate date for the decomposition of gaseous N O a2 55 degrees are as follows: Experiment Initial N2O 5 Initial rate of Decomposition -5 1 .020 3.4 x 10 2 .050 8.5 x 10 -5 a) What is the rate? Compare experiments 1 and 2 and when we do find that both the initial N O and initial rate of decomposition come out to be 2.5 2 5 and with this we can say the rate is proportional to the concentration of N O2a5d therefore the rate law is of first order Rate = - = k [N 2 ]5m b) What is the value of the rate constant? -3 -1 Solve by substituting k = = = 1.7 x 10 s c) What is the initial rate of decomposition of N O2at555 degrees when its initial concentration is .030M? Substitute initial concentration of .030 M and rate constant from part b -5 Rate = - = k [N 2 ]5= (.030 M) = 5.1 x 10 M/s 12.4 Integrated Rate Law  A first order reaction is one whose rate depends on the concentration of a single reactant raised to the first power o Rate = - = k[A]  With calculus it’s possible to convert the rate law to another form, integrated rate law o ln = - kt  ln = natural log  [A] = concentration of A at some initial time 0  [A] =tconcentration of A at any time t thereafter  = fraction of A that remains at time t o Concentration-time equation that lets us calculate the concentration of A or the fraction of A that remains at any time t o Used to calculate the time required for the initial concentration of A to drop to any particular value or to any
More Less

Related notes for CHEM 001B
Me

OR

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Join to view

OR

By registering, I agree to the Terms and Privacy Policies
Just a few more details

So we can recommend you notes for your school.