AP Chemistry
Unit 8 Thermochemistry
Practice Problems
1.
4 Fe(s) + 3 O2(g)  2 Fe2O3(s)
H = -1640 kJ
a.
How much heat is released to produce 10.0 g Fe2O3?
b.
2.
CaSO4(s) + CO2(g)  CaCO3(s) + SO3(g) H = 224 kJ
a.
How much heat is absorbed when 10.0 g CaSO4 react.
b.
3.
4.
How many grams of iron are needed to generate 1.00 x 10 4 kJ of heat?
How much CaCO3 is produced when 500. kJ is absorbed?
When 1.51 g of NH4Cl are dissolved in 100.0 g of water the temperature drops 1.00oC. Determine
a.
qwater for the solution process.
b.
the number of moles of NH4Cl dissolved.
c.
H (in kJ) for the dissolving of one mole of NH4Cl.
12.8 g of MgSO4 is dissolved in 250.0 g of H2O in a coffee cup calorimeter. The temperature of the solution increases from
23.8oC to 33.1oC. Determine
a.
qwater for the solution process.
b.
the number of moles of MgSO4 dissolved.
c.
H (in kJ) for the dissolving of one mole of MgSO4.
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5.
H2(g) + F2(g)  2 HF(g)
Estimate H for the reaction using the bond energy values.
6.
C2H2(g) + 2 H2(g)  C2H6(g)
Estimate H for the reaction using the bond energy values.
7.
A bomb calorimeter with a constant of 921 J/oC contains 1,000 g of water. The combustion of 1.00 g of ethene (C 2H4)
increases the temperature 9.3oC. Determine
a.
qwater for the combustion process.
8.
b.
the number of moles of ethene reacted.
c.
H (in kJ) for the combustion of one mole of C2H4.
d.
Write the equation for the combustion of ethene (C2H4).
e.
Calculate H using bond energies.
Predict whether S > 0, S < 0 or S 0.
>0
 0
<0
Melting ice at 0oC
CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(l)
CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(g)
Distilling alcohol-water mixture
9.
2 Na2O2(s) + 4 HCl(g)  4 NaCl(s) + 2 H2O(l) + O2(g)
Determine H from the thermochemical reactions below.
2 Na2O2(s) + 2 H2O(l) 4 NaOH(s) + O2(g) H1 = -126 kJ
NaOH(s) + HCl(g) NaCl(s) + H2O(l)
H2 = -179 kJ
10. C2H2(g) + 5 N2O(g)  2 CO2(g) + H2O(g) + 5 N2(g)
Determine H from the thermochemical equations below.
2 C2H2(g) + 5 O2(g)  4 CO2(g) + 2 H2O(g) H1 = -2512 kJ
N2(g) + ½ O2(g)  N2O(g)
H2 = 104 kJ
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11. NO(g) + O(g)  NO2(g)
Determine H for the above reaction using the following thermochemical equations.
NO(g) + O3(g)  NO2(g) + O2(g)
H1 = -198.9 kJ
O3(g)  3/2 O2(g)
H2 = -142.3 kJ
O2(g)  2 O(g)
H3 = 495.0 kJ
12. Write the equation for the combustion of methanol, CH3OH(l). (other reactants and products are gaseous)
a.
Calculate H using Hfo values.
1.00 g of methanol is burned in a bomb calorimeter that contains 1200 g of water. The temperature increases 3.4 K.
b.
Calculate the heat generated by the combustion reaction.
c.
Calculate the calorimeter constant of the bomb.
13. Ca(s) + SO3(g) + 2 H2O(l)  CaSO3•2 H2O(s)
H = -795 kJ and S = -0.2535 kJ/K for the reaction.
a.
Calculate Hfo for CaSO3•2 H2O.
b.
Calculate So for CaSO3•2 H2O.
14. Cu(s) + 4 H+(aq) + 2 NO3-(aq)  Cu2+(aq) + 2 NO2(g) + 2 H2O(l).
a.
Calculate Ho using Hfo values.
b.
Calculate So using So values.
c.
Calculate Go at 25oC.
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15. NH4NO3(s)  NH4+(aq) + NO3-(aq)
Determine the following for the above reaction.
a. Is the reaction exothermic or endothermic?
b.
Is there an increase or decrease in entropy?
c.
Is the reaction spontaneous at 25oC?
16. 2 SO2(g) + O2(g)  2 SO3(g)
a.
Calculate Ho.
b.
Calculate So.
c.
Calculate G at 400 K.
d.
Determine the temperature range where the reaction is spontaneous.
17. C2H5OH(l) + 3 O2(g)  2 CO2(g) + 3 H2O(l)
a.
Calculate Ho.
b.
Calculate So.
c.
Calculate G at 20oC.
d.
What is the spontaneous temperature range?
18. When H2SO4(l) is dissolved in water, the temperature of the mixture increases. Predict the sign of H, S and G for this
process (justify your answer).
+/–
Justification
H
S
G
19. C2H5OH(l)  C2H5OH(g)
Calculate the boiling point (threshold temperature) given the information: H = 37.95 kJ and S = 0.1078 kJ/K.
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