Energy And Enthalpy Worksheet With Answers Page 2
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1
2
3
4
5-1
-1
ΔU = ΔH – Δn
RT = -110.5 kJ – (½ mol)(8.315 J K
mol
)(298 K) = -111.7 kJ
g
5. Calculate ΔH°
(in kilojoules per mole) for benzene, C
H
, from the following data:
f
6
6
(g) ➝ 12 CO
2 C
H
(l) + 15 O
(g) + 6 H
O(l)
ΔH = -6534 kJ
6
6
2
2
2
ΔH°
(CO
) = -393.5 kJ/mol
f
2
ΔH°
(H
O) = -285.8 kJ/mol
f
2
ΔH°
= [12 ΔH°
(CO
) + 6 ΔH°
(H
O)] – [2 ΔH°
(C
H
)]
rxn
f
2
f
2
f
6
6
-6534 kJ = [(12 mol)(-393.5 kJ/mol) + (6 mol)(-285.8 kJ/mol)] – [(2 mol)( ΔH°
f
(C
H
))]
6
6
Solve for ΔH°
(C
H
)
f
6
6
-6534 kJ = -6436.8 kJ – [(2 mol)( ΔH°
(C
H
))]
f
6
6
97.2 kJ = (2 mol)( ΔH°
(C
H
))
f
6
6
ΔH°
(C
H
) = +48.6 kJ/mol
f
6
6
6. Use the bond dissociation energies found in the eBook under Data and Tables to
calculate an approximate ΔH° (in kilojoules) for the reaction of ethylene with hydrogen
to yield ethane.
(g) ➝ CH
H
C=CH
(g) + H
CH
(g)
2
2
2
3
3
ΔH°
= D(Reactant bonds) – D(Product bonds)
rxn
ΔH°
= (D
+ 4 D
+ D
) – (6 D
+ D
)
rxn
C=C
C-H
H-H
C-H
C-C
ΔH°
= [(1 mol)(602 kJ/mol) + (4 mol)(413kJ/mol) + (1 mol)(436 kJ/mol)] – [(6
rxn
mol)(413 kJ/mol) + (1 mol)(346 kJ/mol)] = -134 kJ
7. In your own words describe what entropy, spontaneity, microstates are in relation to
thermodynamics.
Among the many ways entropy can be defined one way to describe it is the amount
of molecular randomness in a system. Entropy is a thermodynamic state function of
a system, determined by the number of microstates available to the molecules of the
system; changes in entropy, and therefore in the number of available microstates,
determine the direction of spontaneous processes. A spontaneous process is one that
occurs by itself, given enough time and without any continuous external influence.
Microstates are a microscopic state of a system, characterized by a particular
distribution of molecules among the positions and momenta accessible to them. In
other words, a microstate specifies all molecular details about the system including
the position and velocity of every molecule. The more such states available to the
system, among which energy can be shared, with appreciable probability, the
greater the entropy.
8. Predict whether ΔS° is likely to be positive or negative for each of the following
reactions:
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