08/11115
CHEM lB
Quiz 5 (30 points)
1. ( lf points)
(a) What factors determine whether a collision between two molecules will lead to a chemical
.
?
(f)
reaction.
.s uf-h· c,'e nl-
k; h£ -h·c
ene"(!Y o/ -1-k.
e-0//r · <Yt· (f ru...0/e cu lt7 s ,;,
(;))
frafe- r
or/-enla/,'on of /-h_e_rn,
(b) According to the collision model, why does temperature affect the value of the rate constant?
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le J/tAf-er-cz_lus-e /ne-re~; -I~ fracl-101'7 o~
Y'«J le cu(e <:.
iv/IL h t-c.e-lc. ene.IJt exeeecft~· fke_
Nr'nt'muY11 €YfJ(_Jft1 Y~CZ,tL-t'rec{_ /Sr 'fh_ri_ cOll/t::r,'
~le cu le s
ar
2. (Spoints) The gas-phase reaction Cl(g) + HBr(g) ~ HCl(g) + Br( has an overall enthalpy
change of-66 kJ/mol. The activation energy for the reaction is 7 kJ/mol. ./o un~~ c4-
(a) Sketch the energy profile for the reaction, and label Ea and m. C ~ ~ c a. I c 'h
I= ·
i11creeures ex?oruzf'th'all
Yle.ray
- - -·
E
--1---_J
,
a._
t::.H
r~a.cHOrJ pro
0 r.ess
(b) What is the activation energy for the reverse reaction? Show calculations.
Ee-
-
6.
)
f/ /On.Jf/, +- Ea.) forw,
66 +t-
=
==
7-3 kJ/Yi<.O/
3. (6points) Ammonia decomposes on the surface of a hot tungsten wire. Following are the halflives that were obtained at 1100°C for different initial concentrations ofNH3: [NH3]0 = 0.0031
M, t112 = 7.6 min; 0.0015 M, 3.7 min; 0.00068 M, 1.7 min. For this decomposition reaction,
what is (a) the order of the reaction; (b) the rate constant, k?
(c..-)
-tf(:,
TJ11s 'd.
(b)
tz
k -==
he dtr-<!!cf)' pro;_:;orh'oHa.l lo [lv'f67o,
corresponds -lo
a... zero- oroUz.s- reac I-ion,
a.ppea..r.s fa
-
[ /Jll3] 0
cJk
[ Nfl3] o
ci
-t ~
0.00311'1
;;_x 7. G x 600
f1
( ()-r OL.
0 X/0
-
'I
== 3 .'f><-lo-G H, 5
AA
,
-1
-r) PLEASE TURN OVER!!!
rr 'rvru1 J
4. Cf-points) The cloth shroud from around a mummy is found to have a 14C activity of 9.7
disintegrations per minute per gram of carbon as compared with living organisms that undergo
16.3 disintegrations per minute per gram of carbon. From the half-life for 14C decay, 5715 yr,
calculate the age of the shroud. (Radioactive decay follows the first-order kinetics.) Show
calculations.
[A Jo=- / 6. 3 d;;r>I
9, 7 dfM
{ A ]t ==
t/la
5t!S
t ::::
(J
(J
[AJo _
lb. 3
[ A].c
<f. ?-
-t:
-
t~
yr
= ;) -t /tJ&_
/07 ! 08
-=
Er
l!!J d
:::=
a.
aCJ-s- ::::
o. 001
=== o. 7'-17-
7
r == S?!Sy
x
o.
l'it- == L/()1-0 j
5. lSpoints) Urea, (NH2)2CO, is the end product in protein metabolism in many animals. The
decomposition of urea in 0.1 M HCl occurs according to the following chemical equation
(NH2)2CO(aq) + H+(aq) + 2 H20(l) ~ 2 NH/(aq) + HC03-(aq)
The reaction is first order in urea and first order overall. When [(NH2)2CO] = 0.200 M, the rate
at 61.05°C is 8.56x 10-5 Mis.
(a) What is the rate constant? Show calculations.
r~/e_ ==
k [ (/JflJ-)cJ- CD]
ra_le
[(/J~)d- CO]
(b) What is the concentration of urea in the solution after 4.00x 103 s if the starting concentration
of urea is 0.500 M? Show calculations.
C (Nr-/;)_)d- c.-o J = C
-kt
C =
Coe
==- 0.070 M
(c) What is the concentration of ammonium ion in the solution after 4.00xl03 s ifthe starting
concentration of urea is 0.500 M? Show calculations.
Y
Jx
-==
===
de cr€a4{ in con c, o{ ur-ea_
/11 c reei,J,R_
[ NH/J =
( ; r1
J
x
==
0. SDO - O. 090 ==
1 VJ
o. l/10/V/
:f
t'-h c;_ I c,o 11 c, of AJfl
O. 8:JO M
w~
0
H)
o, L/IOH