Chapter 13: Kinetics Flashcards
in general, how does increasing the concentration of reactants change the rate of a reaction?
Most rate laws are of the form rate = k[reactant]^n. increasing the reactant concentration normally increases the reaction rate. Additionally, keep in mind that a higher concentration of reactants means more collisions occur, which also means a higher observed reaction rate.
Define reaction rate. How does the rate of a reaction change as the reaction proceeds?
a reaction rate is the change in reactant or product concentration per unit time. a reaction slows down as it proceeds because the concentrations of reactant molecules decreases. there are thus fever collisions between reactant molecules, fewer chemical transformations, and a slower rate.
Write four rate equations for the following reaction
4NH3(g) + 5O2(g) -> 4NO(g) + 6H2O(g)
rate = -1/4 delta[NH3]/delta T
rate = -1/5 delta[O2]/delta T
rate = 1/4 delta[NO]/delta T
rate = 1/6 delta[H2O]/delta T
the following reaction: 2MnO-4 + 5H2C2O4 + 6H+ -> 2Mn+2 + 10CO2 + 8H2O had the rate law: k [MnO-4] [H2C2O4][H+]
a) what is the order with respect to each reactant?
b) what is the overall order of the reaction?
c) how would the rate change if the MnO-4 concentration was doubled?
d) how would the rate change of the H+ concentration was increased by a factor of four?
e) how would the rate change if all reactant concentrations were doubled?
a) the reaction is first order with respect to MnO-4, first order with respect to H2C2O4, and first order respect to H+
b) overall the reaction is third order (1+1+1=3)
c) the rate would double
d) the rate would increase by a factor of four
e) the rate would increase by eight (2 x 2 x 2)
The A concentration in the reaction A -> Products was monitored as a function of time. In the first 10 seconds, the reactant concentration decreased from 1.5 M to 0.75 M. It took another 10 seconds for the reactant concentration to drop from 0.75 M to 0.375 M. Is the reaction first or second order? Explain?
The reaction is first order because the half-life is constant. In a second order reaction, the half-life changes as the reaction proceeds.
Calculate the average rate of reaction in the first 10 seconds of the reaction in the previous problem
rate = 0.75-1.50/10 = -0.075 m/s. This is the average rate of disappearance of A. The rate of the reaction (not a specific reactant or product) is always a positive number, so in this case should be reported as 0.075 m/s
The reaction: 2NO + H2 -> N2O + H2O has the following rate law: Rate = k[NO]2 the initial concentration of NO is 0.729 M and the rate constant is 3.2x10^3 1/(ms). How long will it take for the NO concentration to reach 0.0729 M?
which order is the reaction? look at the units on the rate constant. its 1/(ms), which is a second order rate constant. that means we need to use the second order integrated rate law:
1/[A] = kt + 1/[A]0
1/0.0729 M = (3.2 X 10^3 1/ms)t + 1/0.729 M
t = 0.00386 s
Consider a first order reaction with k equal to 0.0175 s-1. what is the half-life of the reaction? if the initial concentration of the reactant is 0.380 M, what is the reactant concentration when t=120 seconds?
first order half-life: t1/2 = in2/k
t1/2 = in2/0.0175 s-1 = 39.6 s
then use the first order integrated rate law for the second part: [a] = [a]oe^-kt
[a] = [0.380 M]e^-0.0175s-1 x 120s
[a] = 0.0465 M
the kinetics of following reaction were studied by the initial rates method
NO(g) + O3(g) -> NO2(g) + O2(g)
Initial NO, Initial O3, Initial rate of reaction
Trail 1: 2.1, 2.1, 1.6
Trial 2: 4.2, 2.1, 3.2
Trial 3: 6.3, 2.1, 4.8
Trial 4: 6.3, 4.2, 9.6
Trial 5: 6.3, 6.3, 14.4
a) what is the order of the reaction with respect to NO?
b) what is the order of the reaction with respect to O3?
c) write the reaction’s rate law
d) calculate the reaction’s rate law
a) How much did they change NO? Trial 2/Trial 1: 4.2/2.1 = 2
How much did the rate change? Trial 2/Trial 1: 3.2/1.6 = 2
what does that mean for the rate law with respect to NO rate = [NO]^x, 2 = [2]^x, x=1
b) How much did they change O3? Trial 4/Trial 3: 4.2/2.1 = 2
How much did the rate change? Trial 4/Trial 3: 9.6/4.8 = 2
What does that mean for the rate law with respect to O3? rate = [O3]^x, 2 = [2]^x, x = 1
c) so the rate law is: rate=k[NO][O3]
d) what is the rate constant?
rate = k[NO][O3]
1.6 = k[2.1][2.1]
k = 3.63x10^6 1(ms)
If a plot of 1/[A] versus time produces a straight line, which of the following is true?
a) this reaction is first order in A
b) the reaction is second order in A (see 2nd order integrated rate law)
c) the reaction is first order in two reactants
d) the rate of reaction does not depend on the concentration of A
b) the reaction is second order in A (see 2nd order integrated rate law)
which of the following statements about the half life of a reaction is true?
a) the half life doesn’t depend on the order of the reaction
b) the half life of a first order reaction increases with time
c) the half life of a second order reaction is independent of concentration
d) none of the above are true
d) none of the above are true
to clarify: a) half life does depend on the order of reaction - that’s why there are separate equations for 1st and 2nd order; b) the half life of a first order rection in constant while the half life of a second order reaction increases with time; c) the half life of a second order reaction depends on concentration (see reason for b as well as the equation for the half life of a second order reaction)
consider the following reaction coordinate
a) label the reactants, products and transition state
b) calculate the activation energies of the forward and reverse reactions
the reactants are labeled (A+B). the products are labeled (C+D). the transition state is the highest point of the large peak. the activation energy is 100-40=60 kj
consider the following mechanism
1) H2 + ICI -> HI + HCI (slow)
2) HI + ICI -> I2 + HCI (fast)
a) what is the overall balanced reaction?
b) does this mechanism have an intermediate?
c) identify the rate-determining step
d) write the rate law for each step and the rate law for the overall reaction
a) H2 + 2 ICI -> I2 + 2 HCI
b) HI is the intermediate because it is produced in step 1 and used up in step 2
c) step 1 is slow and is the rate determining step
d) the reactions rate law is the rate law of the rate determining step:
rate = k[H2][ICI]
