5.4: Enthalpies of Reactions Louis Johnson, Zaineb Alattar

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5.4 Enthalpies of Reactions

• Louis Johnson, Zaineb Alattar, Marita Inglehart

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Definitions

• Energy the ability to do work or produce heat
• Work a force acting over distance
• Heat a transfer of energy due to temperature
  differences
• Temperature a property directly proportional to
  the random motions of particles in a substance
• Enthalpy of Reaction the change in heat in a
  chemical reaction, ?H or ?Hrxn.
• ?H Hproducts Hreactants

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Definitions (cont.)

• Because Enthalpy of Reaction is
• ?H Hfinal Hinitial, then a larger initial heat
  is an exothermic reaction, and a larger final
  heat is an endothermic reaction.
• Endothermic reaction ?H gt 0
• Exothermic reaction ?H lt 0
• The larger the magnitude for both of these
  values, the more highly exo/endothermic the
  reaction (the more powerful).

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Definitions (cont.)

• Enthalpy is a state function, meaning it depends
  on the initial and final states of a system. It
  does not depend on how the internal energy is
  used.
• A thermochemical equation is a balanced chemical
  equation that shows the associated enthalpy
  change.
• 2H2(g) O2(g) ? 2H2O(g) ?H -483.6kJ
• ?H depends on the number of moles of reactants
  and products associated with the ?H value.

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Important Points

• Enthalpy is an extensive property, therefore the
  amount of reactant consumed is directly
  proportional to the magnitude of the ?H.
• 2H2(g) O2(g) ? 2H2O(g) ?H -483.6kJ
• Vs
• 4H2(g) 2O2(g) ? 4H2O(g) ?H -967.2kJ
• (In Class Demonstration).
• Important
• This property means that it is possible to use
  Enthalpy of reaction as a conversion factor
  between heat and moles.

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• 2. The enthalpy change in the reaction is equal
  in magnitude, but opposite in sign to the ?H for
  the reverse reaction.
• For example
• CH4(g) 2 O2(g) ? CO2(g) 2H2O(l)
• ?H -890 kJ
• CO2(g) 2H2O(l) ? CH4(g) 2 O2(g)
• ?H 890 kJ

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• 3. The enthalpy change for the reaction depends
  on the state of the reactants and products. So it
  is important to specify states.
• 2H2(g) O2(g) ? 2H2O(g) ?H -483.6kJ
• Vs
• 2H2(g) O2(g) ? 2H2O(l) ?H -571.6kJ
• The enthalpy of H2O(g) is higher than that of
  H2O(l) and so 88kJ more is released.
• H2O(g) ? H2O(l) -88kJ

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Enthalpy Diagram

• Original Equation

Intermediate Change in Enthalpy
Total Change in Enthalpy
Intermediate Step
Final Equation
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Problem 5.33

• The complete combustion of acetic acid, HC2H3O2
  (l), to form H2O (l) and CO2 (g) at constant
  pressure releases 871.7 kJ of heat per mole of
  HC2H3O2
• a) Write a balanced thermo chemical equation for
  this reaction.
• HC2H3O2 (l) 2O2 (g) ? 2H2O(l) 2CO2(g)
• ?H -871.7 kJ
• b) Draw an enthalpy diagram for the reaction.

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Problem 5.35

• Consider the following reaction which occurs at
  room temperature and pressure
• 2Cl (g) ? Cl2 (g) ?H -243.4 kJ
• Which has the higher enthalpy under these
  conditions, 2Cl gas or Cl2 gas?
• 2Cl gas, the reactant has the higher enthalpy
  because it is an exothermic rxn.

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Problem 5.37

• Consider the following reaction
• 2Mg(s) O2(g) ? 2MgO (s) ?H -1204 kJ
• a) Is this reaction endothermic or exothermic?
• Exothermic, because the ?H is less than 0.
• b) Calculate the amount of heat transferred when
  2.4g of Mg(s) reacts at constant pressure.
• 2.4g Mg x (1 mol Mg/ 24.31g Mg) x (-1204 kJ/ 2
  mol Mg) -59 kJ heat transferred.

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Cont.

• c) How many grams of MgO are produced during an
  enthalpy change of -96.0 kJ?
• -96.0 kJ x (2 mol MgO/ -1204 kJ) x (40.31g MgO/ 1
  mol MgO) 6.43g MgO produced
• d) How many kJ of heat are absorbed when 7.50g
  of MgO(s) is decomposed into Mg (s) and O2(g) at
  constant pressure?
• 2MgO (s) ? 2Mg(s) O2(g)
• 7.50g MgO x (1 mol MgO/ 40.31g MgO) x (1204 kJ/ 2
  mol MgO) 112 kJ heat absorbed
• 1204 is positive here because it is the reverse
  reaction

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Problem 5.39

• When solutions containing silver ions and
  chloride ions are mixed, silver chloride
  precipitates
• Ag (aq) Cl- (aq) ? AgCl (s)
• ?H -65.5 kJ
• a) Calculate ?H for formation of 0.200 mol AgCl
  by this reaction.
• 0.200 mol AgCl x (-65.5 kJ/ 1 mol AgCl)
  -13.1 kJ
• b) Calculate ?H for the formation of 2.50 g of
  AgCl.
• 2.50g AgCl x (I mol AgCl/ 143.2g AgCl) x (-65.5
  kJ/ 1 mol AgCl) -1.14 kJ
• c) Calculate ?H when 0.150 mmol AgCl dissolved
  in water.
• 0.150 mmol AgCl x (I mol / 1000 mmol) x (65.5 kJ
  / 1 mol AgCl) x (1000 J/ 1 kJ) 9.83 J

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Problem 5.43

• A gas is confined to a cylinder under constant
  atmospheric pressure. When the gas undergoes a
  particular chemical reaction, it releases 79 kJ
  of heat to its surroundings and does 18 kJ of P-V
  work on its surroundings. What are the values of
  ?H and ?E for this process?
• ?H -79 kJ
• ?E -79 kJ -18 kJ -97 kJ

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Problem 5.45

• Consider the combustion of liquid methanol,
  CH3OH(l)
• CH3OH (l) 3/2 O2 (g) ? CO2 (g) 2H2O (l)
  ?H -726.5 kJ
• a) What is the enthalpy change for the reverse
  reaction?
• ?H 726.5 kJ
• b) Balance the forward reaction with whole
  number coefficients. What is ?H for the reaction
  represented by this equation?
• 2CH3OH (l) 3 O2 (g) ? 2CO2 (g) 4H2O (l)
• ?H -1453 kJ

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Cont.

• c) Which is more likely to be thermodynamically
  favored, the forward reaction or the reverse
  reaction?
• The exothermic, forward reaction is more
  thermodynamically favored because heat is given
  off and is not required for the reaction to take
  place which is the case for the endothermic,
  reverse one.
• d) If the reaction were written to produce H2O
  (g) instead of H2O (l), would you expect the
  magnitude of ?H to increase, decrease, or stay
  the same? Explain.
• The reaction would have a smaller negative value.
  It would be more endothermic due to the fact that
  enthalpy is ?H Hfinal Hinitial and that H2O
  (g) has more energy than H2O (l) Therefore it
  would have a larger final energy and would result
  in a smaller negative value, meaning a more
  endothermic reaction. Less energy is released, so
  H2O remains a gas and not a liquid.

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Works Cited

• Blaber, Micheal. "Energy Relations in Chemistry
  Thermochemistry." Mikeblaber.org. 1996. 7 Nov.
  2008 lthttp//www.mikeblaber.org/oldwine/chm1045/no
  tes/energy/hesslaw/energy04.htmgt.