Answers to Questions that involve a calculation
ALE 28. Hess’s Law and Standard Enthalpies of Formation
(Reference: Chapter 6 - Silberberg)
Important!! For answers that involve a calculation you must show your work neatly using dimensional analysis with correct
significant figures and units to receive full credit. No work, no credit. Report numerical answers to the correct number of
significant figures. CIRCLE ALL NUMERICAL RESPONSES.
Exercises
A. Use the table of thermodynamic data in Appendix B of your textbook to calculate the standard heat of
reaction for the following reactions at 25 C. Show your work and circle your answer.
5
 2 NH3(g) + 3 NO(g)
/2 N2(g) + 3 H2O(g)
o
Hrxn = -904.0 kJ/mol
 NaHCO3(s)
Na2CO3(s) + H2O(l) + CO2(g) (UNBALANCED!)
o
Hrxn = +85.2 kJ/mol
 MgCl2(aq) + 2 NaOH(aq)
Mg(OH)2(s) + 2 NaCl(aq)
Hints: Write the net ionic equation, paying attention to the phase of the reactants and products. Use the
net ionic equation to calculate Hrxn. The
of Mg(OH)2(s) is -924.66 kJ/mol.
o
Hrxn = -2.82 kJ/mol
B. An “instant ice pack” when purchased has one compartment filled with solid ammonium nitrate and
another compartment filled with water. When a seal between the two compartments is broken, the
water and ammonium nitrate mix, dissolving the salt:
NH4NO3(s)
o
H2O
NH4+(aq) + NO3-(aq)
o
 Calculate Hrxn (in kJ/mol) for the solvation of ammonium nitrate. Hints: Hf of solid
o
ammonium nitrate at 25 C is -365.56 kJ/mol, the H f of aqueous nitrate at 25 C is -205.0
kJ/mol and the
of aqueous ammonium at 25 C is -133.8 kJ/mol. “H2O” is written
above the reaction arrow to let you know it is necessary for the solvation to occur, but you
should not consider water to be a “reactant”. Show your work and circle your answer.
o
Hrxn = +27.8 kJ/mol
 Suppose an instant ice pack has within it 36.2 g of NH4NO3 (80.05 g/mol) and 254.6 mL of water.
(The density of the water is 0.9978 g/mL and its molar mass is 18.02 g/mol.) When the two
components mix, how much heat is exchanged? Show your work and circle your answer.
q = 12.6 kJ absorbed
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 Assume that no heat is exchanged between the solution in the instant ice pack and its surroundings.
(i.e. The reaction takes place in an insulated container.) Suppose that the initial temperature of the
instant ice pack was 22.0 C. If the resultant aqueous solution of ammonium nitrate has a specific heat
of 4.184 J/g C, what is the final temperature of the solution? Hints: Use the density of water to
convert the volume of water into the mass of water. What is the mass of the ammonium nitrate
solution? From the q calculated in  and the known values of m and C, calculate T. Is heat added
to or taken from the water? Does the temperature of the water increase or decrease? Show your work
and circle your answer.
Tf = Ti – T = 11.7 C.
D. Physicians and nutritional biochemists recommend eating vegetable oils rather than animal fats to
lower risks of heart disease. In olive oil, one of he healthier choices, the main fatty acid is oleic acid:
C18H34O2; Hocomb= -1.11 x 104kJ/mol. Use Appendix B and the Hocomb of oleic acid to calculate the
Hfo of oleic acid. Start by writing the balanced chemical equation for the combustion of oleic acid.
Show your work and circle your answer.
842 kJ/mol = -800 kJ/mole =
Hfo, oleic acid
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