Energy Part III: Calculation of H from a Thermochemical Equations b Heat of Formation Chapter 6 Sec

1
Energy Part IIICalculation of ?H froma)
Thermochemical Equationsb) Heat of
FormationChapter 6 Sec 6 Sec 8of Brady
Senese 5TH ed

• Dr. C. Yau
• Fall 2009

1
2
Standard Heat of Reaction
?H enthalpy of rxn or heat of reaction heat
transferred in a rxn (usu. in kJ, not
kJ/mol) ?Ho standard heat of reaction
?H at standard conditions (1 atm, 25oC,
1M if aq soln) Remember these conditions!
2
3
Thermochemical Equations
N2 (g) 3H2 (g) 2NH3 (g) ?Ho
-92.39 kJ This tells us that 1 mole of N2 would
produce 92.39 kJ of heat, that 3 moles of H2
would produce 92.39 kJ of heat, that production
of 2 moles of NH3 would be accompanied by a
release of 92.39 kJ of heat. NOTE that ?Ho is in
kJ and not kJ/mol.
3
4
Thermochemical Equations
N2 (g) 3H2 (g) 2NH3 (g) ?Ho
-92.39 kJ The value of ?Ho depends on the
coefficients in the equation. If coefficients are
doubled, ?Ho would be doubled 2N2 (g) 6H2
(g) 4NH3 (g) ?Ho -92.39x2 kJ Note
also that ?Ho is dependent on the physical states
as stated in the equation. CH4 (g) 2O2 (g)
CO2(g) 2H2O (l) ?Ho - 890.5kJ CH4 (g) 2O2
(g) CO2(g) 2H2O (g) ?Ho - 802.3 kJ
4
5
Thermochemical Equations

• If we reverse a reaction, the magnitude of ?Ho is
  the same but the sign is changed
• C (s) O2 (g) ?? CO2 (g) ?H ? 393.5 kJ
• CO2 (g) ?? C (s) O2 (g) ?H 393.5 kJ

6
Example 6.6 p.228

• The following thermochemical equation is for the
  exothermic reaction of hydrogen and oxygen that
  produces water.
• 2H2 (g) O2 (g) 2H2O (l) ?Ho
  -571.8kJ
• What is the thermochemical equation for this rxn
  when it is conducted to produce 1.000 mol H2O?
• Do Pract Exer 7 8 p.229

7
Determination of ?H

• Example 6.8 p. 233
• Fe2O3 (s) 3CO (g) ?? 2Fe(s) 3CO2(g) ?H ?
  26.7 kJ
• CO(g) ½ O2 (g) ?? CO2 (g)
  ?H ? 283.0 kJ
• Calculate the value of ?H for the following
  reaction
• 2 Fe (s) ?O2 (g) ?? Fe2O3 (s)

8
Determination of ?H

• Ethylene glycol, HOCH2CH2OH, is used as
  antifreeze. It is produced from ethylene oxide,
  C2H4O, by the reaction
• C2H4O (g) H2O (l) ?? HOCH2CH2OH (l)
• What is the heat of reaction of this reaction...
• Given
• 2C2H4O (g) 5O2 (g) ?? 4CO2 (g) 4H2O (l)
• ?H ?2612.2 kJ
• HOCH2CH2OH(l) 5/2 O2(g) ??2CO2(g) 3H2O(l)
• ?H ?1189.8 kJ
• Do Pract Exer 11, 12, 13 p.234

9
Hess' Law

• The value of ?H for any reaction that can be
  written in steps equals the sum of the values of
  ?H of each of the individual steps.
• This is based on the fact that enthalpy is a
  state function.
• The implication is that regardless of how many
  steps are taken the overall enthalpy change is
  the same.

10
Enthalpy as a State Function

• Enthalpy (H) depends only on its current state
  and not the path taken to get there.
• It is not affected by how many steps are used to
  get there.
• A ????? B
• C F
• D E

11
Hess' Law

• For example
• A B C
• D E

?H1
?H3
?H2
?H1 ?H2 ?H3 This is because
H is a state function. This is extremely useful
because if ?H1 cannot be measured, it can be
calculated from ?H2 and ?H3.
12
Hess' Law

• What is ?H1 in terms of ?H2 and ?H3 ?
• F G J
• K L

?H1
?H2
?H3
?H1 ?H2 - ?H3
13
Enthalpy Diagrams

• C (s) ½ O2 (g) ?? CO (g) ?H ?110.5 kJ
• Construct an enthalpy diagram for the reaction.
• Learn the terminology, "enthalpy diagram."
• Know what is asked for on an exam.

14
Enthalpy Diagrams

• N2 (g) O2 (g) ?? 2 NO (g) ?H 181 kJ
• Draw the enthalpy diagram for this reaction.
• We are skipping Example 6.7, Pract Exer 9 10.
  You do not need to know how to draw enthalpy
  diagrams of that sort. However, you should know
  how to add equations together to determine the
  enthlapy change for the overall reaction.

15

• Use the two equations below to determine the
  standard enthalpy change for the reaction
• H2O2 (l) ?? H2O (l) ½ O2 (g)
• H2 (g) O2 (g) ?? H2O2 (l) ?H ?188kJ
• H2 (g) ½ O2 (g) ?? H2O (l) ?H ?286kJ

• Solve the problem by both methods
• By sketching enthalpy diagram
• By manipulating the given eqns.

16

• 2Cu (s) O2 (g) ?? 2CuO (s) ?H ?310 kJ
• 2Cu (s) ½ O2 (g) ?? Cu2O (s) ?H ?169 kJ
• Use the two equations above to determine the ?H
  of this reaction
• Cu2O (s) ½ O2 (g) ?? 2CuO (s)
• Is this exothermic or endothermic?
• Solve the problem by both methods
• By sketching enthalpy diagram
• By manipulating the given eqns. ANS
  -141 kJ
• Use the equations given in Pract Exer 10 p.232
  find the ?H for the reaction
• ½ N2 (g) O2 (g) ?? NO2 (g)
• Is this exothermic or endothermic?
• You do not need to construct an enthalpy diagram
  to solve this problem.

17
Standard Heat of Combustion

• ?Hoc standard heat of combustion
• It is the amount of heat released when one mole
  of a fuel substance is completely burned in pure
  oxygen gas with all reactants and products
  brought to 25oC and 1 bar pressure (1 atm).
• Combustion reactions are always exothermic.
  Therefore, ?Hoc is always negative.

18

• Example 6.9
• How many moles of carbon dioxide gas are produced
  by a gas-fired power plant for every 1.00 MJ
  (megajoule) of energy it produces? the plant
  burns methane, CH4 (g), for which ?Hoc is -890
  kJ/mol
• Do Pract Exer 14, 15 p.235

19
Standard Enthalpy of Formation

• LEARN THIS DEFINITION!
• ?Hfo standard enthalpy of formation
• It is the amount of heat absorbed or evolved when
  specifically one mole of substance is formed at
  25oC at 1 atm from its elements in their standard
  states.

20

• ?Hfo for solid potassium sulfate is -1433.7 kJ.
  Write the thermochemical equation corresponding
  to this value.
• ?Hfo for solid ammonium chloride is 315.4 kJ.
  Write the thermochemical equation corresponding
  to this value.
• ?Hfo for solid calcium hydroxide is -986.6 kJ.
  Write the thermochemical equation corresponding
  to this value.
• Do Example 6.10 and Pract Exer 16 17 on p.237

21
?Hfo of Elements

• Write the thermochemical equation corresponding
  to the ?Hfo of chlorine gas.
• What do you think the value of ?Hfo would be for
  chlorine gas?
• What about the value of ?Hfo of solid silver? of
  liquid mercury?
• Remember this! You will not be provided ?Hfo
  elements in their standard states. They are
  always ZERO kJ.

22
Applying Hess' Law to Heats of Formation

• A B C
  D

All elements in their standard states.
What is heat of reaction of AB CD?
23
Hess' Law of Summation

• ?Hfo values are given in Table 6.2 p.237 and will
  be provided on exams.

24
Example 6.11 p.238

• Some chefs keep baking soda, NaHCO3, handy to put
  out grease fires. when thrown on the fire, baking
  soda partly smothers the fire and the heat
  decomposes it to give CO2, which further smothers
  the flame. The eqn is
• 2NaHCO3 (s) ?? Na2CO3 (s) H2O (l) CO2 (g)
• Use the data in Table 6.2 (p.236) to calc the ?Ho
  for this reaction in kilojoules.
• http//www.youtube.com/watch?vz7JzJlgNuU8

25
Calculating ?H For Reactions Using ?Hf

• 2Fe(s) 6H2O(l) ? 2Fe(OH)3(s)
  3H2(g)
• ?Hf -285.8 -696.5
  
• kJ mol-1
• CO2(g) 2H2O(l) ? 2O2(g) CH4(g)
• ?Hf -393.5 -285.8 -74.8
• kJ mol-1
• Do Pract Exer 18, 19, 20 p.239

26
SUMMARY

• What are the different ways you can determine the
  ?H of a reaction?
• Measure q from calorimetry experiment. In open
  containers, q ?H
• Calculate by manipulating given thermochemical
  equations.
• Calculate from ?Hfo
• Calculate from bond energies (handout)