Assignment #2: Thermochemistry
Comprehension:
1. Explain the difference between heat capacity, thermal energy, specific heat capacity,
enthalpy and molar enthalpy. (5 marks)
2. a) Explain the three types of molecular motion as a result of kinetic energy (3marks)
b) Why does neon have a lower molar heat capacity than nitrogen ? (2 mark)
C) When a fire extinguisher is filled the container becomes very warm and when it is
discharged to put out a fire, it becomes very cold? Explain this phenomenon. (4 marks)
3. Why is the standard enthalpy of formation of O 2(g) zero when the enthalpy of
formation of O(g) is +249.2 kJ (2 marks)
4. Using methane gas as an example, explain why one can not assume the heat of
combustion is the reverse of the heat of formation (3 marks)
5. Determine the complete combustion of ethanoic acid using bond energies and heats
of formation (assume reaction occurs at SATP). Compare your results for each
enthalpy and explain any differences? (10 marks)
6. Explain why the fusion of ice is spontaneous using the theory of Gibbs free energy,
entropy and enthalpy. (No calculations are necessary) (5 marks)
7. The reaction of magnesium metal with oxygen gas to produce magnesium oxide has
both a negative delta H value and a negative delta S value. Explain how the free
energy of this reaction varies with temperature. (4 marks)
8. Why do different states of water have different molar enthalpies? (2 marks)
9. Predict whether the entropy will increase or decrease or remain unchanged based on
the evidence provided for each of the following.
A) Frost forms on the roof of a house
C) The melting of glaciers.
B) A raft is inflated
D) The sublimation of dry ice.
E) The formation of a precipitate
F) 2NO2(g) -----> N2O4 (g)
G) CaCO3(s) -----> CO2(g) + CaO(s)
H) N2 (g) + O2(g) -----> 2NO (g)
Inquiry: Calculation
1. Ammonium nitrate can decompose explosively by the following equation:
NH4NO3 (s)------> NO2(g) + 2H2O(g)  H= -37.0 kJ
Calculate the quantity of heat produced when 2.50 g of ammonium nitrate decomposes
at a constant temperature. (2 marks)
2. A typical sausage (without the bun) has an energy value of 26 kJ/g. If a student ate
one of Mr. Mihoren’s sausages (a five sausage pack weighs 375 g) and if none of the
energy was stored in the student’s body, how much water must the student sweat off in
perspiration so that his/her body temperature remains constant? (3 marks)
3. How much heat in kJ is required to increase the temperature of 250.0 g of water from
25 oC to 42 oC? (2 marks)
4. Write the thermochemical equation for the neutralization of 120.0 mL of 4.00 mol/L
hydrochloric acid with 10.0 g of magnesium hydroxide when the reaction was carried
out in a 80.0 g copper calorimeter having obtained the following values.
The initial temperature of the acid was 19 oC. The maximum temperature reached by
the acid was 30 oC.
5. Calculate the heat of complete combustion of 50.0 g of methane and write a
thermochemical equation for the reaction.
Given:
C(s) + 2 H2(g) ------> CH4(g)
 H= -74.86 kJ
C(s) + O2(g) ------> CO2(g)
 H=-393.5 kJ
H2(g)+ 1/2 O2(g) ------> H2O(l)
 H= -241.9 kJ
(5 marks)
6. Calculate the change in enthalpy for
NO(g) + O (g) ------> NO2(g)
Given:
NO(g) + O3(g) ------> NO2(g)+ O2(g)
 H= -198.9 kJ
O3(g) ------> 3/2 O2(g)
 H= -142.3 kJ
O2(g)------> 2O (g)
 H= 495.0 kJ
(5 marks)
7. When 50.0 mL of 0.100 mol/L silver nitrate is allowed to sit in a copper solution
calorimeter with a mass of 80.0 g for 10 minutes the temperature reaches 23.11oC.
When 50.0 mL of 0.100 mol/L hydrochloric acid are mixed with the silver nitrate the
temperature increases from 23.11 oC to 22.0 oC.
AgNO3(aq) + HCl (aq) ------> AgCl(aq)+ HNO3(aq)
Create a potential energy diagram for the above reaction. List 5 assumptions you used
to complete this question. (11 marks)
8. The following three reactions are carried out in a solution calorimeter. (12 marks)
Use this information to demonstrate Hess’s Law.
A) 200 mL of 1.0 mol/L sulfuric acid is added to a solution calorimeter. The initial
temperature is 18 oC. When 5.6 g of potassium hydroxide is added to the acid
the final temperature reaches 34.7 o C.
B) 100 mL of 2.0 mol/L sulfuric acid is added to a solution calorimeter. The initial
temperature is 20 oC. When 100 mL of 1.0 mol/L potassium hydroxide is added
the temperature of the mixture reaches 30.7 oC.
C) 100 mL of water is added to a solution calorimeter. The initial temperature is 19
o
C. When 5.6 g of potassium hydroxide is added to the solution the mixture
reaches a temperature of 25 oC.
9. Calculate the enthalpy of formation for: (7 marks)
10.5 g of COCl2(g) and express the answer as a thermochemical equation, a delta H, a
potential energy diagram, and as a molar enthalpy.
10. Calculate the standard entropy change for each of the following:
A) Calcium oxide + hydrogen chloride gas produces calcium chloride solid and water.
B) Ethyne + hydrogen produces ethane
11. Using heats of formation, determine the heat of reactions for the following: (6
marks)
a) 2F2(g)+ 2H2O(l) ------> O2(g)+ 4 HF(g)
b) CS2(s)+ 2H2O(l) ------> CO2(g)+ 2H2S(g)
c) 2CH3OH(l) + 3O2(g) ------> 2CO2(g) + 4H2O(g)
12. Using heats of formation values, calculate the amount of heat released if 150.0 g of
brandy (containing 50% ethanol by weight) is poured over plum pudding and burned?
(5 marks)
13. Determine the temperature at which steam undergoes thermal decomposition. (hint:
gibbs) [5 marks]
14. The delta H of the gaseous reaction
XeF2(g) + H2(g) ------> 2HF(g) + Xe(g)
Is found to be -430 kJ. Use this value and average bond energies to determine the
average bond energy of the Xe-F bond. (4 marks)
15. Determine if the following reactions will be spontaneous at room temperature
(23 oC). (8 marks)
a) 2 NO(g) + O2(g) ------> 2 NO2 (g)
b) 2 SO2 (g) ------> 2 SO3 (g)