Chem 201A
Dr. Lara Baxley
Thermochemistry Worksheet
Answers given at the end
This is NOT the Thermochemistry Homework that is due for credit. Look on the course website
for “Thermochemistry Assignment Due”
1. Circle the correct choice to indicate weather energy is absorbed or released, and whether
each process is endo or exothermic.
•
Melting ice requires ice to (absorb OR release) energy; (exothermic OR endothermic)
•
Boiling water requires water to (absorb OR release) energy; (exothermic OR
endothermic)
•
Condensing steam to water (absorb OR release) energy; (exothermic OR endothermic)
•
Freezing water to ice (absorb OR release) energy; (exothermic OR endothermic)
2. There is an attractive force between a rock and the earth (gravity). Energy is released when:
a. the rock is lifted above the ground
b. the rock falls and hits the ground
c. whenever the rock is moved
3. There is an attractive force between a magnet and your fridge door. Energy is released when:
a. the magnet is pulled away from the fridge
b. the magnet is pulled out of your hand toward the fridge
c. whenever the magnet moves
4. There is an attractive force between atoms in a compound. Chemical energy is released
when:
a. chemical bonds are broken
b. chemical bonds are formed
c. during any chemical reaction
5. Is energy released or absorbed by the following processes?
a. Cl−Cl bond breaks, separating two atoms with an attractive force, __________________
b. C−C bond breaks, separating two atoms with an attractive force, ___________________
c. 2 C−Cl bonds form, joining two atoms with an attractive force,_____________________
Chem 201A
Dr. Lara Baxley
6. A 36.9 g sample of metal is heated to 100.0 °C, and then added to a calorimeter containing
141.5 g of water at 23.1 °C. The temperature of the water rises to a maximum of 25.2 °C
before cooling back down.
a. Did the water absorb heat or did it release heat?
b. How many joules of heat was exchanged between the water and the metal?
c. What is the identity of the metal?
7. 16.0 g of ice at 0.00ºC is placed in 525.0 mL of water at 80.2 ºC. What is the final
temperature? Cw = 4.184 J/gºC, ΔHfus = 6.01 kJ/mol.
Chem 201A
Dr. Lara Baxley
8. Butane, which is the fuel used in cigarette lighters, combusts according to the following
thermochemical equation:
2 C4H10(g) + 13 O2(g) → 8 CO2(g) + 10 H2O(g)
ΔHº = –5316.6 kJ
If 10.0 g of butane burns completely in oxygen and the heat is used to warm up 25.0 L of
water at 22.0 ºC, what will be the final temperature of the water (assuming that no heat is
lost to the surroundings and that d = 1.00 g/mL for water))?
9. Evaporating sweat cools the body because evaporation is an endothermic process, as shown
below. Estimate the mass of water that must evaporate from the skin to cool a 95 kg person
by 0.50 ºC. Assume that the specific heat capacity of the body is 4.0 J/gºC.
H2O(l) → H2O(g)
ΔH = +44.01 kJ
Chem 201A
Dr. Lara Baxley
10. Calculate the amount of heat energy released in burning 1 mole of liquid ethanol, C2H5OH in
O2, and for 1 mole of liquid methanol CH3OH in O2 using ∆Hf° values from your book (you
will have to look in Appendix B).
11. Use the information in the previous question to compare the energy density (kJ/g) of each
substance. Describe which substance would you choose as a fuel for your car.
Chem 201A
Dr. Lara Baxley
12. Write the equation that represents the ∆H°f for:
a. SO2(s)
b. HCl(g)
c. C2H5OH(l)
13. Care must be taken when diluting acids with water because the dilution process is very
exothermic. For the dilution of H2SO4, the process is shown below:
H2SO4(l) → H2SO4(aq)
a. Use Appendix B to find the ΔHº for dilution 1.00 mol of H2SO4(l) (d = 1.83 g/mL) to 1 L
of 1.00 M H2SO4(aq) (d = 1.060 g/mL).
b. Suppose you carry out the dilution in a calorimeter. The initial T is 25.0 ºC and the
specific heat capacity of the final solution is 3.50 J/gºC. What is the final T?
Chem 201A
Dr. Lara Baxley
Answers
1. Melting ice requires ice to absorb
energy; _endothermic______
Boiling water requires water to absorb
energy; ___ endothermic ____
Condensing steam to water release
energy; _ exothermic______
Freezing water to ice
release
energy; __ exothermic ____
2. b. the rock falls and hits the ground
3. b. the magnet is pulled out of your hand toward the fridge
4. b. chemical bonds are formed
5. a. absorbed b. absorbed
c. released
6. a. The water absorbed heat from the reaction
b. 1.2 X 103 J
c. the metal is iron
7. 75.5 ºC
8. 26.4 ºC
9. 78 g
10. C2H5OH(l): 1234.8 kJ; CH3OH(l): 638.5 kJ (ΔH is negative, but question doesn’t ask for ΔH, it
asks for the amount of heat energy released, which is positive).
11. C2H5OH(l): 1234.8 kJ/mol (1 mol / 46.07 g) = 26.80 kJ/g
CH3OH(l): 638.5 kJ/mol (1mol / 32.04 g) = 19.93 kJ/g
ethanol provides more energy per gram, so it’d probably be better in your car
12. a. S(s) + O2(g) → SO2(g) ∆H°f = –296.9 kJ/mol
b. ½ H2(g) + ½ Cl2(g) → HCl(g) ∆H°f = –92.5 kJ/mol
c. 2 C(graphite) + 3 H2(g) + ½ O2(g) → C2H5OH(l) ∆H°f = –278 kJ/mol
13. a. –93.52 kJ/mol b. 50.2 ºC