10-3 changes in enthalpy during chemical...
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10-3 Changes in Enthalpy During Chemical Reactions
• 1) Explain the principles of calorimetry
• 2) Define and use Hess’s law and standard enthalpies of formation to calculate ΔH
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Application of Enthalpy
• ΔH used to determine calories in food
• Food burned in calorimeter
• Heat captured (enthalpy change) calories in food
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Standard Enthalpy of Formation, ΔH0
f
= ΔH to synthesize 1 mol of a compound from elements in standard states
Example:
• C + 2H2 → CH4 ΔH0f = -74.9 kJ/mol
Table 2 (pg. 355) and Appendix A
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Hess’s Law
Total energy change ΔH = sum of each of the steps in the overall reaction
Example:
You run the 5K in cross country. Your 1K splits add up to your final time for the race.
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Hess’s Law and Formation of NaCl
Sum of all steps = total enthalpy change (ΔH)
Na(s) + ½Cl2(g) → NaCl(s) ΔH = -411.3 kJ/mol
Vaporize Na: Na(s) -> Na(g) 107.3 kJ/mol
Ionize Na: Na(g) -> Na+(g) + e- 495.8 kJ/mol
Dissociate C2 bonds: ½ Cl2(g) -> Cl(g) 121.7 kJ/mol
Electron Affinity Cl: Cl(g) + e- -> Cl-(g) -348.8 kJ/mol
Lattice Energy NaCl: Na+(g) + Cl-
(g) -787.3 kJ/mol
-411.3 kJ/mol
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Why Use Hess’s Law?
• Some energy changes are difficult to measure directly
• 2 C + O2 2 CO ΔH = ? Hard to do in lab!
• Can measure ΔH for: • C + CO CO2
• CO + O2 CO2
• Use Hess’s Law to solve ΔH for formation of CO
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Hess’s Law and Algebra
Chemistry and algebra follow many of same rules!
Reverse direction of reaction:
2C + O2 → 2CO ΔH = -221 kJ
2CO → 2C + O2 ΔH = 221 kJ (note sign change!)
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Hess’s Law and Algebra
Addition
2C + O2 → 2CO ΔH = -221 kJ
CO2 → C + O2 ΔH = 393 kJ
2C + O2 + CO2 → 2CO + C + O2 ΔH = 172 kJ
Cancellation
2C + O2 + CO2 → 2CO + C + O2 ΔH = 172 kJ
C + CO2 → 2CO ΔH = 172 kJ
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Sample Problem – Using Hess’s Law to Calculate ΔH
The enthalpy of reaction for the combustion of C to CO2 is -393.5 kJ, and the enthalpy for the combustion of CO to CO2 is -566.0 kJ:
(1) C(s) + O2(g) CO2(g) ΔH1 = -393.5 kJ
(2) 2 CO(g) + O2(g) 2 CO2(g) ΔH2 = -566.0 kJ
Using these date, calculate the enthalpy for the combustion of C to CO:
(3) 2 C(s) + O2(g) 2 CO(g) ΔH3 = ?
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Example
Given:
SO2(g) + NO2(g) → SO3(g) + NO(g) ΔH = ?
How do we solve this?
• Find standard enthalpy of formation for reactants and products
ΔHtotal = ΔHproducts - ΔHreactants
-41.8 kJ
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Example
Given:
C + CO2 → 2CO ΔH = ?
ΔHtotal = ΔHproducts - ΔHreactants
(Elements have ΔH0f = 0)
ΔH = 172.5 kJ
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Practice
Calculate enthalpy change for
NO(g) + ½O2(g) → NO2(g)
ΔH = -57.2 kJ
Calculate ΔH for
C2H6(g) + 3.5 O2(g) → 2CO2(g) + 3H2O(g)
Is it exothermic or endothermic?
-1428.6 kJ; exothermic
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Guided Practice
Calculate the change of enthalpy, ΔH, for the formation of pentane, C5H12, using the info below:
C(s) + O2(g) CO2(g) ΔH = -393.5 kJ
H2(g) + 1/2O2(g) H2O(l) ΔH =-285.8 kJ
C5H12(g) + 8O2(g) 5CO2(g) +6H2O(l) ΔH = -3535.6 kJ
Ans:
5C(s) + 6H2(g) C5H12(g) ΔH = -145.7 kJ