Kinetics
t
t984
98B tt
'ifr"-O**Posrtion of compound X is an elementary process thai Proceeds as follows: kg
&e)FK. A1g1+ Bg1
AFI"- +15 kilocalories
but The lorward reaction is slow at rmm. temPemture added' is catalyst 6o-"t rapid when a reacdon co' {a) Draw a diagram of potential energy vs Lln trus otaordinate foi the uncatal-vzed reacnon' grarn label: ( the ares the energres of the r%ctalts andrhe products the enerly of the activated complex dt tlgtii.-tt energy differences indicate tbe change. or (b) \*/ On the sarne dragram . .t *g., that result irom the addition of the catal,v-st. E.W6; the role of the catalyst in changlng the rale of the reactlon'
l) iZi i:l i;i
krp is increase4 will the tauo increase, remarn the same, or decrease? Justify vour ans\{'er wrth a one or two sentence explanaii"". ffi *O x, are the specific ftIte .constants for the for-t"*ard and the revers€ reactlons' resPectrvell'.]
(c) Il the temperature
lE8 \ gET-E Natl + 2 Braat' 3 C(qt + Dtoc) or the retctlon above, camed out in solution of 30'C' re iollorl rng krnenc data were obtained:
ErperrmeDl I
)
PeactmlS
Reaction
liter-I)
(mole'liter- l'hr-
I
0 240 0.2-10
0.480 0.120
E.00
o.2N
9.m 0.500
5
0.2{o
0.120 0.0600
0.0140
135
7.0xlO4 1.4'
(a)
10-3
2.8' lO3 4.2'10'3 Give the rate law for this reacuon from
the data
above.
(b) calculate the specific
rat€ constant for this reaction
and specifY its uruts. (c) How long must the reagtioq plmeed tq-nrodyce. 1 concentriti on at Zequal !o 0'20 molar' if the irutigl reactlon concentratioirs are [Xlo = 0'8O molar' [Y]o 0 molar? = 0.6O molar and [Zlo = (d) Select from the mectranisms below the one most your **itr.nt with the obsewed data' and explain reaction are N and M .f,or... ln these mechanisms intermediates.
\-/
(l) X+Y -M X +M'Z
(slow)
(?) X+Xt=M
(fast)
+M-Z (3) Y*M M+X-N N+X*Z Y
1985
(fasO
(slow) (slow) (fasO
(fas0
PCl5tr) = above, the forward reaction is first orc'
1.00 ',
Wnte the rate-law' expression for tlus reacuon' C-alculate the Ialue of the specific rate constant k 30"C and specif.v- its uruts.
D
PCl3re) +Cl2(e)
2.00
at
Caiculate the vaiue of the irutral rate o[ this reacuon at -iO'C for the rruual conc€nl,rations shown in expenment 6. d) Assume that the rea.cnon goes to completion. Under the condrtrons specrfied for erpenment f , w'hat rr ouici be the llnal moiar concenLlatlon of C?
c)
mol r-'-l'sec-l)
)
AOIB' 0 360 0.120
6
b)
Inrtial Rate of
{
J
r)
Inrtrai Cooc. of (mole
B
por a hvpotheucal chemical reaction that has the stoittriornrfi 2 X + Y - Z, the following irutial rate data were obrarned. All measurements were made at the
In the equation in both PCl3 and Cl2 and the reverse reaction is lirst o der in PCl5. (a) Suppce that 2 moles of PCl3 and I mole of C are mixed in a closed contatner at constant temper ture. Draw a gaph that shows how the concentr tions of PCI3,-C12, and PCl5 change with time un after equlibrium has been firmly established (b) Give the rnilral rale law for the forward reaction' (c) hovide a molecular explanation for the dependen' of the rate of the fonvard reaction on the concentr tions o[ the reactants. (d) Provide a molecular explanation for the dependen o[ the rate of the forward reacuon on temperzlture.
,
lqgb
ne overail order
> o[ a reaction may not be
predictable
t987
om the stoichiometry of the reactton' true' r) Erpla.rn hou' this statement can be
r)
2 HgCl1w) + C2aa2- - 2 Cl- + 2 CO2rs) + Hg2Cloroa The equarion for the reaction beween mercuric chlonde and oxalate ion in hot aqueous solution is shown above. The reaction rat€ may be determined by measuring the irutial rate of formation of chloride ion, at constant temperature, for various initial concentrations o[ mercunc lhlonde and oxalat€ as shown in the following table
2Xy -\1+Y2
For the h1'potheucal reaction above' give a rate iu*' ttt^t ittot*'t that the reaction is first order in the reactant XY' Give the unirs for the specific rate constant for this rate law. with Propose a mechanism that is consistent both the rate la*'and the stoichiometry'
l.
1. 3.
ment
0
(d) The presence of powdered nickel causes a larger of
lqg8
0 ConriO.t the following general equation ibi'a cherriictl reaction. ' --
+ Drs t All" reaction = - l0 kJ Descnbe the frvo facnrs that determine whether a coilision benx'een molecules of A and B results in a reaction. Horv would a decrease in temperarure affect the rate of the reactlon shown above? Erplzun your an-
i,
(a)
(b)
,+
s
(c)
816;
-
Cr8
)
U.'f,UZM
0.52x
(2) (3) (4)
M 0.0418 M 0.0316 M
0.4&1M 0.404M
2.08'
0.B6
,l
shown, what is the rate la According for the reaction above?
J""i",.
(d) Calculate the initial oxalate Erpenment 4.
a
t99t
ion concentration for
B
2 CrO1 s t + F 2( il * 2 ClOzFrgt The following rezults were obtained when the reaction represented above was studied aL25'CInitial Rate ol Increase of Initiat Irutial Exp.riment
I
) J
(a) Write
lcrot,
lF,
fmol'L-l) 0. o I 0 0.010 0.020
rmol'L-l) 0 Io 0.40 0.20
lcro2F], (mol'L-l'ro-l)
2.4rl0-i 9.6' l0-3 9.6' l0-3
for the reactton
the rate law expression
above.
(fast)
(slow)
catall'st increases.the EIte
(b) Calculate the numerical value of the rate constant and specify the unig. (c) In expenment 2, what is the irutial rate of decrease
of a reac-
tion but does not .hunge the value of the equrlibnur conslant ior thar reactlon'
(d)
JUtorrF
lF lF 1.06* lF l.27,lF
0.rc6 M
(?)' (b) On the basis of the rate law determined in Pgt ne specihc tzlt€ constant' Spectfy the units. rate of (c) What is the numerical value fbr the initiat ? disappearance of C2O42- for Experiment I
Wnre lhe rate lau' erpres$on that would result i[ the reaction proceeded bv the mechantsm shown be lotr'.
(d) E.tplarn whl'a
l'Ein-l)
l)
ffi
(a)
\\'e r.
A+B=[AB] tABl+B-C+D
IniualR4!e of - lmtralof ClFormation lczo+2-l (mol'L_
C.tH4(s) + H2(s) - C2H1st AF{o = -137 kJ Account for the following observations regarding the exothennic reaction repr€s€nted by the equatilon ab6ve. (a) An increase in the pressure of the reactants causes an increase in the reaction rate. (b) A small increase in rempemfure causes a large incrals€ in the rarhon mte. (c) The presence of metallic nickel ciruses an increase in re..r-tion rate. mcreas€ in reactjon rate than does the presence sin.gle piece of njckei of the same mass.
IruualtHeclzl
ExPen-
(
t989
B
->
ci
clr)
\_
of [F2]? Which of the following reactton mechanisms is consistent w'ith the rate law developed in (a). Justifv vour choice. (fast) L CIOr + F.r d AOtFz
J.r
ClOlFz- CIOzF + F ClO2+F-CIOzF F2* 2F
(slow) fast) (slorr')
(
r995
1992 0 (mandatorg) H2(e)+12(s)-2tfl(st For the exothermic reaction represented above, carried out at 29,t3K, the rate law is as follows. Rate = ktHzl[z] Predict the effect of each of the following changes on the irurial rate of the reaction and explain your prediction. (a) Addition of hydrogen ga-s at constant temperature and volume
(b) Increase in volume of the reaction stant lemFraiurc
(c)
vessel
at
B
(l) A2+ 82 (lI) X2+Y2--
2 AB 2 XY Two reactions are represented above. The potential-energy diagram for reaction I is shown below. The potenual energy of the reactants in reaction II is also indicatec on the diagram. Reaction II is endothermic, and the activuion energy of reaction I is greater than that of reac-
tion II.
con_
I
Addition of catalvst. In your explanation, include a diagram ol potential energy versus reaction coordi_
ao
!
nate.
trl
(d) Increase in temperature. In your explanation, in_ clude a diagram showing rhe numbef of molecules as a funcuon ol-energv.
2AB Rcaetioo Pathway
r994
(a)
I
2 NOrer +2Hzt:-t-' N2(e) + 2 HzQs) the rale of the reLrpenmens were conOlicted to stud.Y' b-v the equation above lnitial concenin the table beLrauons and rates of reaction are Blven lorv.
;,i;;il;i.niJ
lnrnal Concen-
rauon (moliL)
Formauon of N2
mol'L'mrn)
0.m60 0.m10 0 0060 0.m20 0"0010 0.m60
3.6 x
0.0060
1.2 x
0.0020
(a)
1.8 x
l0-+ l0-a
0.30 x lO-J
ld of the reactans'
(r) Derrmine_the order for each shou I'our NO and H2, lrom ttre dara given and reasoni ng.
(ii) Wnte the overall rate la$'for the reactlon' k' for the (b) Calculate the value of the rale constant' reaction. lnclude unis' o[ (c) For expenment 2' calculare the concentrauon ;;t;ng ohen exactll'one-half of the onpnal (d)
ilo of Hi had been consumed' "-o*t is a The foilo*'ing sequence of elementar;' step proposed mechanism for the reactlon'
l. NO+NO:NzQ II ' NzQ + H2' H2o + N2o lll' N2o + H2 - N2 + H2o the above is
Based on the d?tr pres€nted' u hch.of tir. o"-,i.terrnlnlng step? Shou ttnt the mechanlsm ls conslst€nt \\'lth (ri rhe obsened nte larr lor the reactlon' and reactton' i ii) tie 'rr erail stolcilometry of the
Complete the potential+nergy diagram for reaction II on the graph above..
(b) For reaction I, predict how each of the following i: affected as the temperaftre is increased by 20'C Explarn the basis for each prediction. (i) Rate of reaction (ii) Heat of reaction (c) For reaction II, the form of the rale law is tvle = t[Xz].[YzJn. Briefly describe an experiment tt]ar can be conducted in order to determine the value of m and n in the rate law for the reaction. (d) From the information given, determine which reacnon irunally proce€ds at the laster rate under the same conditions of concentration and tremperarure. Justifl' )'our answer.
'Acoc
ft
Fo rn^
o:(s) + No(s) -+ o2k) + N o2k)
6.
Consider the reaction represented above'
(a) Refening to the data in the table below, calculate the standard enthalpy change, AIl", for the reaction
at
25'C. Be sure to show Your work.
o:(s)
No(s)
Noz(s)
143
90.
33
Standard enthalpy of
formation, LH] , at25"C
ftJ mol-l) (b) Make a qualitative prediction about the magnitude of the standard entropy change, ASo, for the reaction 25'C. Justify your answer
at
(c) On the
basis of your answers to parts (a) and (b), predict the sign of the standard free-energy change, AGo, for the reaction al25oC. Explain your reasoning.
(d) Use the information in the table below to write the rate-law expression for the reaction, and explain how you obtained your answer.
Experiment Number
Initial [O3l (mol L-r)
Initial [NO]
I
0.0010 0.0010 0.0020 0.0020
0.0010 0.0020 0.0010 0.0020
2 3
4
(mol
L-r)
Initial Rate of of NO,
Formation (mol
L-l 5-t; x 2x 2x 4x
(e) The following three-step mechanism is proposed for the reaction. Identify the step that must be the slowest in order for this mechanism to be consistent with the rate-law expression derived in part (d). Explain. step
I:
II: Step III: Step
o. + No -+ o + No, O+
O, -+
NO, + NO
2 O,
-+
2 NO2
-1 1-
:",. .Ansu,er tlre
)Oc{ (orn ft
followjne questions related to the kinetics of chemical reactions.
l-(dq) + C)O-(oq) oH- , IO-toq) + Cl-(aqt Iodide ion, I -. is oxidized to hvpoiodite ion. lO-. b1' hvpochlorite. CIO-. in basic solurion accorcting to rhe equation above. Three initial-rate experiments were conducted; the results are showrr in the follou'ilre rable.
Initial Rate of
tr-l (mol L-r)
tcro-l (mol L-l;
0.0r 7
0.0r 5
0.r56
2
0.052
0.015
0.416
3
0.016
0.061
0.596
Experiment
Formation of IO(mol L-l's'l;
(a) Determine the order of the reactjon with respect lo each reactant listed below. Sbou' your work.
,
(i) I-(aq) (ii) CIO -(na)
(b) For
the reaction
ii)
rvrite the rate law that is consistent with the calculatjons in part (a):
(ii)
calculate the value of the specific rate conslant- k, and specify units.
The catalyzed decomposition of hydrogen peroxide, HrO2@d. is represented by the following equation. catalyst
2H,O2@Q)
2HzO(l) + O2(s)
The kia-etics of the d,ecomposition reaction were srudied and the analysis of the results strow that it first-order reaction. Some of the experimental data are shown in tbe table below. ]
.-
:]
lHzozl:' (mol L-r)
(minutes)
r00
0.0
0.78
5.0
0.6r
10.0
!ime
-B-
is
a
(c; During
the analysis of the data. the graph below u,as produced.
Time (minutes)
(i) (ii)
(iii)
Label the vertical axis of the graph. What are the units of the rate constant,
ft
,
for the decomposition
of HrO2Gfi
?
On the graph, draw the line that represents the plot of the uncatalyzed first-order decomposition
of
1.00
M HrOr(ad
.
STOP ll you finish belore time is called, you may check your work on this part only. Do not turn to the other part ol the test until you are told to do so.
-9-
A
co g
(on
,u-
X-+2Y+Z to produce gases Y and Z is represented by the equation above. In a certarn experiment, the reaction tookpiace in a 5.00 L flask at 428 K. Data from this experiment were used to produce the i,nformation in the table below, which is plotted in the graphs that follow.
3. The decomposition of gas
X
txl
Time
0.00633 0.00520 0.04421
0 10.
20. 50.
0.00349 0.00236
70.
0.00160
30.
lxl-t
txl
(L moi-l ) 158
-5.062 -5.259 -5.456 -5.658
0.000900
100.
r
ln
(mol L- r)
(minutes)
r92 234 287
*6.M9
424
-6.438 -7.013
1,1r0
625
il.007
*5.0
0,fi)6
----5.rt
iJ.txj5
*5.8
X
o-{xr,{.
t).r-Hx
-fr.2 -{r.fi
i).0{}}
1.$
i),ml
---.-1.4
O-TXXJ
{}. i(}. {}.
-'1,0.
lO. ;t{i. 5{i.
1i}. ?(i. ..!0. J{i- 5{}. fi{). 7{}, ti{i. 1x}. l{X}.
trt}.
"line {rxin)
Tirrre {ruin} 1.ztxl 1-0{x}
s{i{i
X
600 4ix)
2ff) t)
ti. li). 10. lt]. 4{i. 5{}. {1i). ?tl. 3iJ. 9{}. l{I-}. Tim* (nrin)
(a) How many moles of
X
were initially in the flask?
(b) How many molecules of Y
were produced in the first 20. minutes of the reaction?
(c) What is the order of ttris reaction with (d) Write
respect
to X
?
Justify your answer.
the rate law for ttris reaction.
(e) Calculate
the specific rate constant for this reaction. Specify unis.
(f) Calcuiate
the concentration
of X
in the flask after a total of 150. minutes of reaction.
It().
9t1.
iti{}.
2OO8
AP' CHEMISTRY FREE.RESPONSE OUESTIONS A(c) + B(g)
2.
-r
(D.OTM B)
C(3) + D(6)
Forthe gas-phase reaction represented above, the following experimental data were obtained.
Experiment
Initial [B] (mol L-t1
Initial [A] (mol L-t1
I
0.033
0.034
2
0.034
0.13'1
J
0.t36
0.1
4
0.202
0.233
Initial Reaction Rate
(mol.L-t 5t1
x l0-4 1.08 x 1O-2
6.67
1.07
36
x l0-2 ,|
(a) Deiermine the order of the reaction with respect to reactant A . Justify your answer. (b) Determine the order of the reaction with respect to reactant B . Justify your answer.
(c) Write
the rate law for the overall reaction. :
(d) Determine the value of
the rate constant,
k, for the reaction. Include units with your answer.
(e) Calculate the initial reaction rate for experiment
(f)
4.
The following mechanism has been proposed for the reaction'
Stepl: B+B-+E+D Step2: E+A I B+C
slow
fastequilibrium
Provide two reasons why the mechanjsm is acceptable.
(g) In the mechanism in part (f), is species E
@
a catalyst, or is
it an intermediate? Justify your answer.
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2OO9 AP@
CHEMISTRY FREE-RESPONSE OUESTIONS (FOTM B)
SrOrz-(ad
-Ji1
SOr2-(ag) + S(s)
an experiment to investigate the decomposition of sodium thiosulfate, NarSrOr, in acidic solution, as represented by the equation above. In each trial the student mixed a different concentration of sodium thiosulfate with hydrochloric acid at constant temperature and determined the rate of disappearance of
2. A student peibrmed
SrOr2-(aO). Dara from five trials are given below in the table on the left and are plotted in the graph on the right.
0.060
Trial
Initial Rate of
Iqit:ial Concentration of 52C32- (aq)
Disappearance of SrO32- (ae)
tlul)
(M s-r)
0.0s0
0.020
2
0.075
0.030
3
0.088
0.034
4
o.) l2
0.045
5
0.
t25
0.051
q)
o
c
! cJo.
0.050 I
a,+ l-.1 -N
0.040
r
oO qJN
eu)
0.030
Eo
0.020
0.010 0.05
0.07 0.09
0.1l
Initial Concenlration of SrOr2-
(i4
(a) Identify the independent variable in the experiment.
(b) Determine the order of the reaction with respect to SrOr2-. Justify your answer by using the information above.
lc) Determine the value of the rate constant, ft, for the reaction. Include units in your answer. Show how you arrived al your answer.
M NarSrO3 with hydrochloric acid. Calculate the amount of time it would take for the concentration of SrOrt- to drop to0.020 M .
(d) In another trial the student mixed 0.10
(e) On the graph above, sketch the line that shows the results that would be expected five trials at a temperature lower than that during the lust set of trials.
if
the student repeated the
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'
-7-
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2O1O AP@
CHEMISTRY F'REE-RESPONSE OUESTIONS
8 H*(aq) + 4 C|-(aq) +MnOa-@Q)
:.
-t
2 Cl2(g) + Mn3*(ag) + 4
HrO(f
Cl2(S) can be generated in the laboratory by reacting potassium permanganate with an acidified solution of sodium chloride. The net-ionic equation for the reaction is given above.
(a) A 25.00 mL sample of 0.250 M NaCl reacts completely with excess KMnOo(cg). the Cl2(S) produced is dried and stored in a sealed container. AI22"C the pressure of the Cl2k) in the container is 0.950 atm.
(i) Calculate (ii) Calculate
the number of moles
of
Cl-(aq) present before any reaction occurs.
the volume, in L, of the Clt (g) in the sealed container.
An initial-rate study was performed on the reaction system. Data for the experiment are given in the table below.
Trial
of MnOo- in M s-l
Rate of Disappearance
tcl-l
[MnOo-]
IH*]
0.0104
0.00400
3.00
2.25
x l0'8
2
0.0312
0.00400
3.00
2.03
x
3
0.0312
0.00200
3.00
1.02
x l0-7
1A-1
(b) Using the information in the table, determine the order of the reaction with respect to each of the following. Justify your answers.
(i) cl(ii) MnOo(c) The reaction is known to be third order with respect to H+. Using this information
and your answers to
part (b) above, complete both of the following:
(i) (ii)
Write the rate Iaw for the reaction. Calculate the value of the rate constant, ft, for the reaction, including appropriate units.
(d) Is it likely that the reaction occurs in a single elementary
step? Justify your answer.
STOP lf you finish belore time is called, you may check your work on this part only. Do not turn to the other part of the test untilyou are told to do so.
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I\,-
-8',r\'t,/
CHEMISTRY FREE.RESPONSE OUESTIONS (FOTM B)
2O1O AP@
H2G)+ Cl2k)
6.
The table below gives data for
a
Initial IH2)
Initid
[Cl2]
(mol L-r)
(mol
L-r)
I
0.00100
0.000500
1.82
x l0-12
2
0.00200
0.000500
3.64
x
10-12
J
0.00200
0.000250
1.82
x
10-12
(a) Determine the order of the reaction with
(c)
2HCl(s)
reaction rate study of the reaction represented above.
Experiment
(b) Determine
+
respect
the order of the reaction with respect
to Ht
to
Cl
r
Initial Rate of Formation
of HCI
and
(mol
L-l
s-1)
justify your answer.
and
justify your hnswer.
l Write the overall rate law for the reaction.
(d) Write the units of the rate
constant.
(e) Predict the initial rate of the reaction if the initial concentration of H, is 0.00300 mol L-l and the initial concentration of Cl, is 0.000500 mol L-I. The gas-phase decomposition of nitrous oxide has the following two-step mechanism.
(f
Step
l:
Step
2:
NrO -+ N, + O O+
NrO -+ N, + O,
Write the balanced equation for the overall reaction.
(g) Is the oxygen atom, O, a catalyst for
the reaction or is
it
an intermediate? Explain.
(h) Identify
the slower step in the mechanism if tbe rate law for the reaction was determined to be rate = k [NzO]. Justify your answer.
STOP END OF EXAM
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-13-
2011 AP@ CHEMISTRY FREE-RESPONSE OUESTIONS 2.0 L flask is pumped out. Then some liquid ethanol is injected into the amount of liquid ethanol initially decreases, but after five minutes the The sealed flask, which is held at 35'C. amount of liquid ethanol in the flask remains constant. Ethanol has a boiling point of 78.5'C and an equilibrium
6. In an experiment, all the air in a rigid vapor pressure of 100 torr at 35oC.
(a) When the amount of liquid ethanol in the flask is constant, is the pressure in the flask greater than, less
than,
or equal to 100 ton? Justify your answer. heated to 45"C, and the pressure in the flask increases. In terms of kinetic molecular theory, provide TWO reasons that the pressure in the flask is greater at 45'C than at 35'C.
(b) The flask is then
ln a second experiment, which is performed at a much higher temperature, a sample of ethanol gas and a copper catalyst are placed in a rigid, empty 1.0 L flask. The temperature of the flask is held constant, and the initial concentration of the ethanol gas is 0.0100 M.The ethanol begins to decompose according to the chemjcal reaction represented below. CH3CH2OH(S)
-g-,
CH3CHO(g) + H2(s)
The concentration of ethanol gas over time is used to create tbe three graphs below. 500
0.0100 0.0080
.=
€
400
o -5.00
0.0040
F 300 ts zoo
g, -5.s0
0.0020
100
-5.00
0.0060 !.1
-4.50
0
0
0
500 1000 1500 2000
d
0
-6.50
s00 1000 rs002000
0
500 1000 1s002000 Time (s)
Time (s)
Time (s)
(c) Given that the reaction order is zero, one, or two, use the information in the graphs to respond to the
following.
(i) (ii) (iii)
Determine the order of the reaction with respect to ethanol. Justify your answer.
Write the rate law for the reaction. Determine the rate constant for the reaction, including units.
(d) The pressure in the flask at the beginning of the experiment is 0.40 atm. decomposes, what is the final pressure in the flask?
If
STOP END OF EXAM
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the ethanol completely
