Chemistry 122 [Tyvoll]
Quiz 3 Key
September 15, 2008
Part I. Multiple Choice (2 points each)
1. In the reaction, A products, the initial concentration of A is 1.60 M, and 45 s later it is 1.20
M. What is the initial rate of this reaction in M • s−1?
Answer 3. 8.9 × 10−3 M • s−1
(rate = (1.60 M – 1.20 M)/45 s = 0.40 M/45 s = 8.9 × 10−3 M • s−1
2. Which of the following statements is correct regarding 2 H2 (g) + O2 (g) → 2 H2O (g) ?
Answer 2. The rate of H2 disappearance is twice the rate of disappearance of O2.
−(1/1)Δ[O2]/Δt = −(1/2)Δ[H2]/Δt @ Δ[H2]/Δt = −(2/1)[ −(1/1)Δ[O2]/Δt] = 2Δ[O2]/Δt]
3. In the reaction 2A + B 2C + D, −Δ[A]/Δt is 5.0 M · min−1. What is the rate of reaction of B?
Answer 1. 2.5 M · min−1
(−(1/2)Δ[A]/Δt = −(1/1)Δ[B]/Δt @ Δ[B]/Δt = −(1/1)·(−(1/2)Δ[A]/Δt = 1/2 x Δ[A]/Δt
so Δ[B]/Δt = 1/2 x 5.0 M · min−1 = 2.5 M · min−1
4. Which of the following is not a possible initial rate law for 2 H2 + C2H2 C2H6 ?
Answer 4. All of these rate laws are possible.
(all rate laws for this reaction would be of the general form, rate = k [H2]x[C2H2]y … in the
case of Answer #3, y =0)
5. For the reaction, 2 A (g) + B (g) → C (g), rate = k[A]1[B]2 where k = 5.70 M-2 s-1. What
would be the initial rate if [A] = 0.0510 M and [B] = 0.0125 M?
Answer 4. 4.5 × 10−5 (Ms−1)
(rate = k[A]1[B]2 = (5.70 M-2 s-1)(0.0510 M)(0.0125 M)2 = 4.5 × 10−5 Ms−1
Part II. Problem (10 points) Show all work for credit!
C2H4 (g) + O3 (g) 2 CH2O (g) + ½ O2 (g)
At 298 K,
Three experiments were performed in which the concentrations of C2H4 (g) and O3 (g) were
systematically varied, as shown in the table below:
[C2H4]
[O3]
Rate, M/s
1
0.50 x 10-7
1.0 x 10-8
1.0 x 10-12
2
1.50 x 10-7
1.0 x 10-8
3.0 x 10-12
3
0.50 x 10-7
2.0 x 10-8
4.0 x 10-12
Experiment
(a) Determine and write the correct algebraic rate equation, including numerical exponents, for
this reaction, using the data from the table above. (7 points)
Here, rate = k [C2H4]X[O3]Y
Exp. 1 and Exp. 2 for x
Exp. 1 and Exp. 3 for y
[O3] = constant
[C2H4] = constant
3X = 3
2Y = 2
x=1
y=2
Rate = k [C2H4]1[O3]2
(b) Calculate the value for the rate constant, k, including correct units, for the reaction above.
(3 points)
Arbitrarily choose data from Exp. #1 and substitute for rate, [C2H4] and [O3].
Rate = k [C2H4]1[O3]2 = k(0.50 x 10-7)1(1.0 x 10-8)2 = 1.0 x 10−12 Ms−1
k = (1.0 x 10−12 Ms−1)/{(0.50 x 10-7)1(1.0 x 10-8)2} = 2.0 x 1011 M-2s−1