AP chemistry Kinetics

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AP chemistry Kinetics

• Things to know

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Things to remember.

• Rate depends on temperature
• Temp is the avg. KE
• Order depends on rxn mechanism
• Rate is determined by the slow step
• Temp affects k
• Increase temp 10oC, rate doubles
• Rxns occur when collisions have sufficient Ea and
  correct orientation
• Factors that can affect rate
• Nature of reactants, surface area, concentration,
  temp, catalyst, pressure

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Outline Kinetics
Reaction Rates How we measure rates.
Rate Laws How the rate depends on amounts of reactants.
Integrated Rate Laws How to calc amount left or time to reach a given amount.
Half-life How long it takes to react 50 of reactants.
Arrhenius Equation How rate constant changes with T.
Mechanisms Link between rate and molecular scale processes.
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Reaction Rates
C4H9Cl(aq) H2O(l) ??? C4H9OH(aq) HCl(aq)

• A plot of concentration vs. time for this
  reaction yields a curve like this.
• The slope of a line tangent to the curve at any
  point is the instantaneous rate at that time.

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Reaction Rates
C4H9Cl(aq) H2O(l) ??? C4H9OH(aq) HCl(aq)

• The reaction slows down with time because the
  concentration of the reactants decreases.

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Reaction Rates and Stoichiometry

• What if the ratio is not 11?

H2(g) I2(g) ??? 2 HI(g)

• Only 1/2 HI is made for each H2 used.

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• 3. 2 A(g) B(g) ltgt 2 C(g)
• When the concentration of substance B in the
  reaction above is doubled, all other factors
  being held constant, it is found that the rate of
  the reaction remains unchanged. The most probable
  explanation for this observation is that
• (A) the order of the reaction with respect to
  substance B is 1(B) substance B is not involved
  in any of the steps in the mechanism of the
  reaction(C) substance B is not involved in the
  rate-determining step of the mechanism, but is
  involved in subsequent steps(D) substance B is
  probably a catalyst, and as such, its effect on
  the rate of the reaction does not depend on its
  concentration(E) the reactant with the smallest
  coefficient in the balanced equation generally
  has little or no effect on the rate of the
  reaction

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• Step 1) N2H2O2 ltgt N2HO2 H fast equilibrium
• Step 2) N2HO2 ---gt N2O OH (slow)
• Step 3) H OH ---gt H2O (fast)
• 4. Nitramide, N2H2O2, decomposes slowly in
  aqueous solution. This decomposition is believed
  to occur according to the reaction mechanism
  above. The rate law for the decomposition of
  nitramide that is consistent with this mechanism
  is given by which of the following?
• (A) Rate k N2H2O2(B) Rate k N2H2O2
  H(C) Rate (k N2H2O2) / H(D) Rate (k
  N2H2O2) / N2HO2(E) Rate k N2H2O2 OH

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Rate Laws

• A rate law shows the relationship between the
  reaction rate and the concentrations of
  reactants.
• For gas-phase reactants use PA instead of A.
• k is a constant that has a specific value for
  each reaction.
• The value of k is determined experimentally.
• Constant is relative here-
• k is unique for each rxn
• k changes with T (section 14.5)

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First-Order Processes

• When ln P is plotted as a function of time, a
  straight line results.
• The process is first-order.
• k is the negative slope 5.1 ? 10-5 s-1.

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Determining rxn order
Graphing ln NO2 vs. t yields

• The plot is not a straight line, so the process
  is not first-order in A.

Time (s) NO2, M ln NO2
0.0 0.01000 -4.610
50.0 0.00787 -4.845
100.0 0.00649 -5.038
200.0 0.00481 -5.337
300.0 0.00380 -5.573
Does not fit
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Second-Order Processes
A graph of 1/NO2 vs. t gives this plot.

• This is a straight line. Therefore, the process
  is second-order in NO2.

Time (s) NO2, M 1/NO2
0.0 0.01000 100
50.0 0.00787 127
100.0 0.00649 154
200.0 0.00481 208
300.0 0.00380 263
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Second-Order Processes
A graph of 1/NO2 vs. t gives this plot.

• This is a straight line. Therefore, the process
  is second-order in NO2.

Time (s) NO2, M 1/NO2
0.0 0.01000 100
50.0 0.00787 127
100.0 0.00649 154
200.0 0.00481 208
300.0 0.00380 263
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10. The graph above shows the results of a study
of the reaction of X with a large excess of Y to
yield Z. The concentrations of X and Y were
measured over a period of time. According to the
results, which of the following can be concluded
about the rate of law for the reaction under the
conditions studied? A) It is zero order in
X.B) It is first order in X.C) It is second
order in X.D) It is the first order in Y.E)
The overall order of the reaction is 2.
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 Experiment Initial NO(mol L1 Initial O2(mol L1 Initial Rate ofFormation of NO2(mol L1 s1)
1 0.10 0.10 2.5 x 104
2 0.20 0.10 5.0 x 104
3 0.20 0.40 8.0 x 103
The initial-rate data in the table above were
obtained for the reaction represented below. What
is the experimental rate la for the reaction?
(A) rate kNO O2(B) rate kNO O22(C)
rate kNO2 O2(D) rate kNO2 O22(E)
rate kNO / O2
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Half-Life

• Half-life is defined as the time required for
  one-half of a reactant to react.
• Because A at t1/2 is one-half of the original
  A,
• At 0.5 A0.

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Half-Life

• For a first-order process, set At0.5 A0 in
  integrated rate equation

NOTE For a first-order process, the half-life
does not depend on A0.
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Half-Life- 2nd order

• For a second-order process, set
• At0.5 A0 in 2nd order equation.

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Outline Kinetics
First order Second order Second order
Rate Laws
Integrated Rate Laws complicated
Half-life complicated
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What is the order of the rxn?
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Potential energy diagram
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Arrhenius equation

• KE converts to PE during collisions to break
  bonds.
• The transition state (aka activated complex) is
  the unstable intermediate that forms at the peak
  of the PE diagram.
• Increase Ae, k decreases, and therefore rate
  decreases.
• When temp doubles, all particles speed up (way
  more than double) therefore, the relationship is
  not linear, but rather exponential.

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Arrhenius equation

• When to use it? When given k and time or asked to
  solve for Ea.

• Plot ln k vs 1/T linear graph
• Slope of the line -Ea/R\
• Therefore, Ea -R x slope
• R 8.31 J/K mol

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