# Worksheet 5d answers - Iowa State University ```Leader: Sara
Course: Chem177
Instructor: Kingston
Supplemental Instruction
Date: October 20, 2014
Iowa State University
1) How do you relate heat to enthalpy? How do you relate heat to energy?
When there is constant P qsys = ΔH
When there is constant V qsys = ΔE
Worksheet 5D:
Heat
2) You mix 150. mL of 1.0 M nitric acid with .150 moles of solid sodium hydroxide
(neglect mass of solid). You know the process H+ + OH-  H2O has a heat
ΔHneutralization= -56.2 kJ/mol. Find the change in temperature of the water
HNO3 + NaOH  H2O + NaNO3
H+ + OH-  H2O
ΔHneutralization= -56.2 kJ/mol
Qrxn =.150 mol* -56.2 kJ/mol = -8.43 kJ = -qsurr
qsurr = mCs ΔT = 150 g * 4.184 J/mol K * ΔT= 8430 J
ΔT = 13.4
3) You drop a block of Al into water. The water is 5.000 times as massive as the
metal. You know the water starts at 25.00 degrees C and after a long time its
temperature is 27.00 degree C. The specific heat of Al is .9000 J/gK. What is the
initial temperature of the Al?
qwater = -qAl
mwater Cswater ΔTwater = - mAl CsAl ΔTAl
5*4.184*(27-25) = (1) * .90 * (27 – T)
T = 73.5
4) Write the formation reactions for the following compounds?
a. CH4 (g)
C(s) + 2H2(g)  CH4(g)
b. HCl (g)
&frac12; H2 (g) + &frac12; Cl2 (g)  HCl (g)
c. C6H12O6 (s)
6C(s) + 6 H2 (g) + 3 O2 (g)  C6H12O6 (s)
d. Na2CO3 (s)
2Na (s) + C (s) + 3/2 O2 (g)  Na2CO3 (s)
𝑜
5) What is the standard enthalpy change for the reactions? ∆𝐻𝑟𝑥𝑛
=
𝑜
𝑜
∑ 𝑚∆𝐻𝑓 (𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠) − ∑ 𝑛∆𝐻𝑓 (𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠)
a. Mg(OH)2 (s)  MgO (s) + H2O (l)
ΔH = ΔHfMgO + ΔHfH2O (l) - ΔHfMg(OH)2
=-601.8 + -285.8 - -924.7
=37.1
b. 2C3H7OH (l) + 9 O2 (g)  6CO2 (g) + 8H2O (g)
ΔH = 6ΔHfCO2 + 8ΔHfH2O (g) - 9ΔHfO2 - 2ΔHf C3H7OH
= 6 * -393.5 + 8 * -241.8 – 9 * 0 – 2 * -304.4
=-3686.6 kJ
c. H2 (g) + F2 (g)  2HF (g)
ΔH = 2ΔHfHF - ΔHfH2 - ΔHfF2
= 2*-268.6 – 0 – 0
=-537.2 kJ
1060 Hixson-Lied Student Success Center  515-294-6624  [email protected]  http://www.si.iastate.edu
Formula
Mg(OH)2 (s)
MgO (s)
C3H7OH (l)
∆Hf kJ/mol
-924.7
-601.8
-304.4
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