Thermodynamics Practice 3 - AnneReganWiki

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Oct 27, 2013 (3 years and 5 months ago)

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AP Chemistry


Unit 11: Thermodynamics Practice
3


1.





2 NO
(g)

+ O
2
(g)



2 NO
2
(g)



H
°=
-
114.1 kJ,

S
°=
-
146.5 J K
-
1



For the reaction at 25

C, the value of the standard free
-
energy change,

G

, is
-
70.4 kJ.


(a)

Calculate
the value of the equilibrium co
n
stant,
K
eq
, for the reaction at 25

C.


(b)

Indicate whether the value of

G


would b
e
come more negative, less negative, or remain unchanged
as the temperature is increased. Justify your answer.




2.
AgNO
3
(s)



Ag
+
(aq)

+ NO
3

(aq)




The dissolving of AgNO
3
(s)

in pure water is rep
resented by the equation above.


(a)

Is ∆
G

for the dissolving of AgNO
3
(s)

positive, negative, or zero? Justify your answer.


(b)

Is ∆
S

for the dissolving of AgNO
3
(s)

positive, negativ
e, or zero? Justify your answer.


(c)

The solubility of AgNO
3
(s)

increases with increas
ing temperature.



(i)

What is the sign of ∆
H

for the dissolving pro
cess? Justify your answer.



(ii)

Is the answer you gave in part (a) consistent with your answers t
o parts (b) and (c) (i)? Ex
plain.


The compound NaI dissolves in pure water according to the equation NaI
(s)



Na
+
(aq)

+ I

(aq)
. Some of the

information in the table of standard reduction potentials given below may be useful in answering the

que
s
tions t
hat follow.


Half
-
reaction



EV)

O
2
(g)

+ 4 H
+

+ 4
e
-



㈠2
2
O
(l)

1.23

I
2
(s)

+ 2
e
-



㈠2

=
〮㔳
=
㈠2
2
O
(l)

+ 2
e
-



H
2
(g)

+ 2 OH

=
J
〮㠳
=

+

+
e
-




(s)

-
2.71

(d)

An electric current is applied to a 1.0
M

NaI solu
tion.



(i)

Write the balanced oxidation

half reaction for the reaction that takes place.



(ii)

Write the balanced reduction half
-
reaction for the reaction that takes place.



(iii)

Which reaction takes place at the anode, the ox
i
da
tion reaction or the reduction reaction?



(iv)

All electrolys
is reactions have the sa
me sign for ∆

. Is the sign positive or negative? Justify



your answer.


3
.

Answer the following questions that relate to the che
m
istry of nitrogen.



(a)

Two nitrogen atoms combine to form a nitrogen molecule, as represented by the following equ
a
tion.


2 N
(g)


N
2
(g)



Using the table of average bond energies below, determine the enthalpy change, ∆
H
, for the rea
c
tion.

Bond

Average Bond Energy
(kJ mol

1
)

N

k
=
ㄶ1
=
k㵎
=
㐲4
=
k

N

㤵9


(b)

The reaction between nitrogen and hydrogen to form ammonia is repres
ented below.


N
2
(g)

+ 3 H
2
(g)



2 NH
3
(g)


H
˚ =

92.2 kJ




Predict the sign of the standard entropy change, ∆
S
˚, for the reaction. Justify your answer.


(c)

The value of ∆
G
˚ for the reaction represented in part (b) is negative at low temperatures but pos
i
t
ive at

high temperatures. Explain.


(d)

When N
2
(g)

and H
2
(g)

are placed in a sealed co
n
tainer at a low temperature, no measurable amount of

NH
3
(g)

is produced. Explain.

FULL

ANSWERS
below



Answer
s
:

1.

(a
)
K
eq

=

2.22

10
12



(b
) less negative
;

G


=

H




T

S

; as temperature increases,

T

S








becomes a larger positive value causing an increase in

G


(less neg
a
tive).


2.



(a)

sign of ∆
G

is (

)
;

Because AgNO
3
(s) is known to be soluble in water, the solution process must





be spontaneous.



(b)

sign of ∆
S

is (+)
; an increase in entropy occurs when a solid becomes aqueous and the products
contain more particles than the reactants.



(c)

(i) sign of ∆
H

= (+
)
; an endothermic process will be favored when the temperature is increased.




(ii) yes
;

G

= ∆
H



T∆
S
, as the temperature in
creases the

T∆S term will increase, keeping ∆
G






negative.



(d)

(i) 2 I



I
2
(s)

+ 2
e
-





(ii) 2 H
2
O
(l)

+ 2
e
-



H
2
(g)

+ 2 OH





(iii) anode = oxidation




(iv) sign ∆


= (+); by definition, an electrolysis is
a non
-
spontaneous process and requires the




input of energy to get it to proceed.

3
.


(a)

a triple bond is formed, an exothermic process



H

=

950 kJ mol

1

(b)

(

);

the 4 molecules of gas are reduced
to 2

(c)


G
˚ = ∆
H
˚


T∆
S
˚;
a negative change in entr
opy means that the
T∆
S

factor is positive. When this factor
exceeds the negative value

of the change in enthalpy,
such as at high temperatures, the value of

G
becomes positive (nonspontaneous).


(d)

at low temperatures, the kinetic energy of the mol
e
cule
s is low and very few molecules have enough
activ
a
tion energy