Calculating Enthalpy Change

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Calculating Enthalpy Change

• 15.4 Pgs. 534 - 541

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Main Idea

• The enthalpy change for a reaction is the
  enthalpy of the products minus the enthalpy of
  the reactants

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Hesss Law

• Most reactions occur in steps
• What if you need to know the heat change of a
  step, but you cant measure it?
• Hesss law allows you to gather this information
  indirectly
• Elemental carbon can be found as graphite and
  diamond at 25oC
• C(diamond) ? C(graphite)
• Takes millions of years to go from diamond to
  graphite
• Reaction is too slow to measure the heat change

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Hesss Law

• Hesss Law of Heat Summation
• If you add two or more thermochemical equations
  to give a final equation, then you can also add
  the heats of reaction to give the final heat of
  reaction
• Example Conversion of diamond to graphite
• C(diamond) ? C(graphite)
• C(s, diamond) O2 (g) ? CO2 (g) ?H
  -395.4kJ
• C(s, graphite) O2 (g) ? CO2 (g) ?H
  -393.5kJ
• REVERSE the above equation, so that we can show
  diamond being converted into graphite
• CO2 (g) ? C(s, graphite) O2 (g) ?H 393.5kJ

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• Add the equations together!
• C(s, diamond) ? C(s, graphite) ?H -1.9 kJ
• The overall ?H is negative, so you can see this
  is an exothermic process!

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Now you try some!

• Try 32 and 33 on pg. 537
• 32 -385.4 kJ
• 33 -521 kJ cuz direction of b is changed to get
  desired equation!

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Standard Enthalpy (Heat) of Formation
- standard heat of formation the change in
enthalpy that accompanies the formation of one
mole of a compound from its elements with all
substances in their standard states at 298 K
(25oC)

• Free elements at 298 K 1 atm have an enthalpy
  of zero.
• Examples, HONClBrIF
• Other ?Hºf values have been calculated
• ?Hºreaction can be calculated as follows

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Lets do an example!

• What is the standard heat of reaction (?Hº) for
  the following equation
• O2 (g) 2CO (g) ? 2CO2
• From your text
• ?Hºf O2 (g) 0 (free element)
• ?Hºf CO (g) -110.5 kJ/mol
• ?Hºf CO2 (g) -393.5 kJ/mol
• Calculate the ?Hºf of the reactants
• 2 mol CO, 1 mol O
• 2 mol of CO ? 2 mol x -110.5 kJ/mol -221.0 kJ
• 1 mol O2 0 KJ (free element)
• TOTAL 0 kJ -221.0 kJ -221 kJ (reactants)
• Calculate the ?Hºf of the products
• 2 mol CO2
• 2 mol CO2 ? 2 x -393.5 kJ/mol -787.0 kJ
• Use
  to find the total!

?Hº -787.0 kJ (-221.0) kJ ?Hº -566.0 kJ
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Try a Practice Problem

• 2NO (g) O2 (g) ? 2NO2 (g)
• ?Hºf NO 90.37
• ?Hºf O2 0.0
• ?Hºf NO2 33.85
• 2(33.85) 2(90.37) 0 -113.04 kJ