Chapter 6 Study Questions
1. Draw an energy diagram for an exothermic reaction where DH = - 20 kJ.
2. Classify each of the following processes as endothermic or exothermic. In each case, indicate which has greater heat content (enthalpy), reactants or products.
a) combustion of natural gas
b) condensation of water vapor
c) splitting of carbon dioxide into carbon and oxygen
d) solidification of melted wax
e) formation of sodium chloride (NaCl) from it elements
f) evaporation of alcohol
3. Propane gas, C3H8, is a common fuel for camp stoves.
a) What is DHf° for propane? (Use DHf° Table.)
b) Write a balanced thermochemical equation for the formation of propane from its elements.
c) Is this reaction endothermic or exothermic?
d) How many moles of H2 are needed to produce 1090 kJ according to the equation in b)?
e) How much heat is produced during the formation of 30.1 g of propane?
f) Write a balanced chemical equation for the complete combustion of propane.
g) Find DH for the reaction in f) using DHf° Table and Hess’ Law.
4. For each of the reactions below, calculate DH and indicate whether the reaction is endothermic or exothermic. (Use DHf° Table as needed.)
a) 2 NO(g) + O2(g) ® 2 NO2(g)
b) 6 PbO(s) + O2(g) ® 2 Pb3O4(s)
5. A coffee cup calorimeter is used to calculate the heat change when NH4Cl is dissolved in water. Use the data table below to calculate the heat change when 1.00 mole of NH4Cl is dissolved in water. (Assume that the heat change for the solution is the same as that of water alone and that you can ignore the mass of solid in the water, so use only the mass of water and the specific heat of water, 4.18 J/g °C, in calculating the heat change.)
Mass of NH4Cl = 5.03 g
Mass of water in the coffee cup = 60.0 g
Initial temperature of the water = 24.78 °C
Final temperature of the water = 19.23 °C
Is this process endothermic or exothermic?
6. To change the temperature of a particular calorimeter and the water it contains by one degree Celsius requires 6485 Joules. The complete combustion of 1.40 g of ethylene gas, C2H4, in the calorimeter causes a temperature rise of 10.7 degrees. Find the heat of combustion per mole of ethylene.
7. Calculate DH for the reaction: N2H4(l) + O2(g) ® N2(g) + 2 H2O(l)
given the following data:
2 NH3(g) + 3 N2O(g) ® 4 N2(g) + 3 H2O(l) DH = -1010 kJ
N2O(g) + 3 H2(g) ® N2H4(l) + H2O(l) DH = -317 kJ
2 NH3(g) + ½ O2(g) ® N2H4(l) + H2O(l) DH = -143 kJ
H2(g) + ½ O2(g) ® H2O(l) DH = -286 kJ
Summary of Chapter 6: Thermochemistry
heat content = enthalpy
change in heat content (DH)
heat content diagrams
change in heat content associated with physical and chemical changes
calorimetry: calculation of heat change from temperature change
DHfus & DHvap
DHf°, definition & use of table to find heat changes of chemical reactions