Chemistry Problems Thermodynamics

1
Chemistry ProblemsThermodynamics

• Blase Ferraris (did these problems for Gangluff)
• Final Project
• 1996B and 1997D

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1996 B

• C2H2(g) 2H2(g) ? C2H6(g)
• Information about the substances involved in the
  reaction represented above is summarized in the
  following tables.

Substance ?S (J/molK) ?Hf (kJ/mol)
C2H2(g) H2(g) C2H6(g) 200.9 130.7 ----- 226.7 0 -84.7
Bond Bond Energy (kJ/mol
C-C CC C-H H-H 347 611 414 436
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• A. If the value of the standard entropy change
  ?S, for the reaction is -232.7 joules per mole
  Kelvin, calculate the standard molar entropy, ?S,
  of C2H6 gas.
• B. Calculate the value of the standard
  free-energy change, ?G, for the reaction. What
  does the sign of ?G indicate about the reaction
  above?
• C. Calculate the value of the equilibrium
  constant, K, for the reaction at 298 K.
• D. Calculate the value of the C C bond energy in
  C2H2 in kilojuoles per mole.

4

• If the value of the standard entropy change ?S,
  for the reaction is -232.7 joules per mole
  Kelvin, calculate the standard molar entropy, ?S,
  of C2H6 gas.

• Use this equation to solve for Entropy
• ?Srxn ?Sproducts-?Sreactants
• ?Srxn?S(C2H6)- ?S(C2H2)2?S(H2)
• Just plug and chug
• -232.7 ?S(C2H6)- 200.92130.7
• Solve for variable
• ?S(C2H6) 229.6J/K

5
Calculate the value of the standard free-energy
change, ?G, for the reaction. What does the sign
of ?G indicate about the reaction above?

• ?G?H-T?S
• ?H ?Hproducts-?Hreactants
• ?H(- 84.7 kJ) - (226.7 kJ) -311.4kJ
• kJ units are needed for ?S
• -232.7J/K(from part A) 1kJ/1000J - 0.2327kJ/K
• ?G -311.4kJ-T(- 0.2327kJ/K)
• note(use standard temperature (25 C))
• ?G -311.4kJ-(298K)(- 0.2327kJ/K) -242.1 kJ
• Negative ?G therefore reaction is spontaneous

6
Calculate the value of the equilibrium constant,
K, for the reaction at 298 K.

• Use the following equation
• ?G -RTlnK
• Just divide by RT
• -lnk ?G/(RT)
• Plug and Chug
• -ln K -242.1 (8.31 x 10-3) (298) 97.7
• e97.7k
• K3 x 1042

7
Calculate the value of the CC bond energy in
C2H2 in kilojuoles per mole.

• ?H bonds broken- bonds formed
• Plug and chug all over again
• - 311.4 kJ (2) (436) ?H (CC) (2) (414) -
  (347) (6) (414)
• Solve for variable
• 820 kJ ?H (CC)

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ITS DONE

• wait no, one more