Name _______________________________________________ Date ___________________ Period ___
Class Work, Homework, and TEST GRADE RECOVERY
Chapter 5: Thermochemistry
Exercises: Sections 5.7: Enthalpies of Formation
1. Define each of the following:
(a) enthalpy of vaporization (ΔHvap)
(b) enthalpy of fusion (ΔHfus)
(c) enthalpy of formation(ΔHf)
(d) heat of formation
(e) standard state
(f) standard enthalpy change of reaction (ΔHo)
(g) standard enthalpy of formation (Δ𝐻𝑓𝑜 )
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2. List the standard state for each of the following :
(a) atmospheric pressure
(b) temperature
3.
(a) A standard enthalpy is given the following symbol, ΔHo. What does the superscript o indicate?
(b) An enthalpy of formation is given the following symbol, ΔHf. What does the subscript f indicate?
(c) An standard enthalpy of formation is given the following symbol, Δ𝐻𝑓𝑜 . What does this symbol
represent?
(d) What is the value of the standard enthalpy of formation of an element in its most stable form?
Explain. What are the most stable forms of carbon, hydrogen, and oxygen?
(e) Write the chemical equation for the reaction whose enthalpy change is the standard enthalpy of
formation of glucose, C6H12O6 (s), ΔHof [C6H12O6].
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4. Write balanced themochemical equations that describe the formation for each of the following
compounds from their elements in their standard states and use Appendix C of the textbook to obtain the
value of 𝛥𝐻𝑓𝑜 . (Remember: 𝛥𝐻𝑓𝑜 is tabulated for 1 mole of a substance!)
(a) HCl(g)
(b) AgNO3(s)
(c) Al2O3(s)
(d) CH3OH(l)
𝑜
5. Use Hess's law and 𝛥𝐻𝑓𝑜 to calculate the 𝛥𝐻𝑟𝑥𝑛
for the following. Values for ΔHfo can be found in
Table 5.3 on page 189.
C2H5OH(l) + 3 O2(g) → 2 CO2(g) + 3 H2O(l)
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6. Benzene, C6H6(l) is combusted.
(a) Write the balanced chemical equation for this reaction. Don't forget phases of matter!
(b) Calculate the standard enthalpy change for the complete combustion of 1 mole. Consult
Table 5.3 on page 189 for ΔHfo values.
𝑜
7. Write the mathematical equation which can be used instead of Hess's law when determining 𝛥𝐻𝑟𝑥𝑛
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8. Many cigarette lighters contain liquid butane.
(a) Write the balanced chemical equation for this reaction. Don't forget phases of matter!
𝑜
(b) Use the 𝛥𝐻𝑟𝑥𝑛
equation from question #7 to calculate the heat of reaction for the equation in 8(a).
Consult Appendix C or Table 5.3 for 𝛥𝐻𝑓𝑜 values.
(c) Calculate the standard enthalpy change for the complete combustion of 1 mole of liquid butane.
(d) Calculate the quantity of heat produced when 1.0 gram of liquid butane is combusted.
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𝑜
9. Using values from Appendix C or Table 5.3 in the textbook, calculate the value of 𝛥𝐻𝑟𝑥𝑛
for each of the
following reactions: Follow ALL math work rules!
a) the combination reaction between solid sodium hydroxide and carbon dioxide gas to form sodium
solid sodium bicarbonate
b) 4 NH3(g) + O2(g)
→ 2 N2H4(g) + 2H2O(l)
ΔHorxn = ?
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9. Con’t
c) the metathesis reaction between solid iron (III) oxide and gaseous hydrogen chloride to form solid
iron (III) chloride and steam
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10. The following exothermic reaction releases 127.2 kJ of heat. Use data from Appendix C of the textbook,
to calculate ΔHof for CaC2. Follow ALL math work rules!
CaC2(s) + 2H2O(l) → Ca(OH)2(s) + C2H2(g)
ΔHorxn = − 127.2 kJ
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11. Use Hess's law to calculate the standard enthalpy of formation of gaseous diborane, B2H6 The following
thermochemical information may be of help.
4 B(s) + 3 O2(g) → 2 B2O3(s)
ΔHorxn = − 2509.1 kJ
2 H2(g) + O2(g) → 2 H2O(l)
ΔHorxn = − 571.7 kJ
B2H6(g) + 3 O2(g) → B2O3(s) + 3 H2O(l)
ΔHorxn = − 2147.5 kJ
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