Download Thermochemistry Questions

CHEM 100
Thermochemistry
Questions
Question 11
• The complete combustion of ethanol,
C2H5OH(l), to form H2O(g) and CO2(g) at
constant pressure releases 1235 kJ of heat per
mole of C2H5OH. Write a balanced equation
for this reaction. Draw the enthalpy diagram
for the reaction.
Question 12
• Consider the following reaction:
2Mg(s)+O2(g)→2MgO(s) ΔH=−1204kJ
a) Is this reaction exothermic or endothermic?
b) Calculate the amount of heat transferred when
3.51g of Mg(s) reacts at constant pressure.
Question 13
• When solutions containing silver ions and
chloride ions are mixed, silver chloride
precipitates:
Ag+(aq)+Cl−(aq)→AgCl(s)ΔH=−65.5kJ
a) Calculate ΔH for formation of 0.450mol of AgCl
by this reaction.
b) Calculate ΔH for the formation of 9.50g of AgCl.
c) Calculate ΔH when 9.29×10−4mol of AgCl
dissolves in water.
Question 14
• The specific heat of octane, C8H18(l), is 2.22
J/g⋅K.
a) How many J of heat are needed to raise the
temperature of 70.0g of octane from 11.2∘C to
26.0∘C?
b) Which will require more heat, increasing the
temperature of 1 mol of C8H18(l) by a certain
amount or increasing the temperature of 1 mol
of H2O(l) by the same amount?
Question 15
• When a 6.50-g sample of solid
sodium hydroxide dissolves in
100.0 g of water in a coffee-cup
calorimeter (Figure), the
temperature rises from 21.6 ∘C
to 37.8 ∘C. Calculate ΔH (in
kJ/mol NaOH) for the solution
process
• NaOH(s)→Na+(aq)+OH−(aq)
• Assume that the specific heat
of the solution is the same as
that of pure water.
Question 16
• Calculate the enthalpy change for the reaction
P4O6(s)+2O2(g)→P4O10(s)
• given the following enthalpies of reaction:
• P4(s)+3O2(g)→P4O6(s)ΔH=−1640.1kJ
• P4(s)+5O2(g)→P4O10(s)ΔH=−2940.1kJ
Question 17
•
•
•
•
•
From the enthalpies of reaction:
H2(g)+F2(g)→2HF(g)ΔH=−537kJ
C(s)+2F2(g)→CF4(g)ΔH=−680kJ
2C(s)+2H2(g)→C2H4(g)ΔH=+52.3kJ
calculate ΔH for the reaction of ethylene with
F2: C2H4(g)+6F2(g)→2CF4(g)+4HF(g)
Question 18
• For each of the following compunds, write a
balanced thermochemical equation depicting the
formation of one mole of the compound from its
elements in their standart states and use the
table given in in this sheet to obtain the value of
ΔHf
a)
b)
c)
d)
NO2(g)
SO3(g)
NaBr(s)
Pb(NO3)2(s)
Question 19
• Using values from Table is given in this sheet;
calculate the standard enthalpy change for
each of the following reactions.
a)
b)
c)
d)
2SO2(g)+O2(g)→2SO3(g)
Mg(OH)2(s)→MgO(s)+H2O(l)
N2O4(g)+4H2(g)→N2(g)+4H2O(g)
SiCl4(l)+2H2O(l)→SiO2(s)+4HCl(g)
Question 20
• Complete combustion of 1 mol of acetone
(C3H6O) liberates 1790 kJ:
• C3H6O(l)+4O2(g)→3CO2(g)+3H2O(l)ΔH∘=−1790kJ
• Using this information together with data
from Table in this sheet, calculate the enthalpy
of formation of acetone.
Question 21
• The thermite reaction is highly exothermic:
• 2Al(s) + Fe2O3  2 Fe(s) + Al2O3(s)
H: - 849 kJ
• Howmuch heat is liberated when 36.0 g of Al
reacts with excess F2O3 ?
Question 22
• Magnesium metal burns quite vigorously in solid
carbon dioxide, forming magnesium oxide and
carbon. The balanced reaction looks like this:
• 2Mg (s) + CO2(s)  2 MgO (s) + C(S)
• Using the enthalpy data provided on the sheet,
calculate the enthalpy change for this reaction
(H  rxn ) in kJ.
Question 23
• Calculate the standard enthalpy of formation
(ΔH  f) of solid Mg(OH)2,
• given the following information:
• Rxn 1: 2Mg (s) + O2 (g) → 2MgO (s) ΔH  = -1203.6 kJ
• Rxn 2: Mg(OH)2 (s) → MgO (s) + H2O (l) ΔH  = +37.1 kJ
• Rxn 3: 2H2O (l) → 2H2 (g) + O2 (g) ΔH  = +571.7 kJ