CHEMISTRY 161
THERMO WORKSHEET
FALL 2015
1. A system absorbs 52.5 J of energy as it expands to a volume of 50.0 L. If the external pressure is 0.800
atm and the change in internal energy is -112 j, calculate the initial volume.
-112 j = 52.5 j – 0.0800 atm (50.0 – Vi)L 101.3 j
ΔE = q -P ΔV
L-atm
-112 j = 52.5 j – 0.0800 atm (Vf – Vi)
-112 j = 52.5 j – 81.04 j (50.0 – Vi)
Vi = 48.0 L
-112 j = 52.5 j – 0.0800 atm (50.0 – Vi)L
2. Use Hess’s law to find the heat of combustion of methane,
using the following thermochemical equations.
¼ (CH4 (g) + 2 O2 (g) → CO2 (g) +2 H2O (g)
ΔH = -802 kJ)
¼ (CH4 (g) + CO2 (g) → 2CO(g)
+ 2 H2 (g) ΔH = 247 kJ)
¼(CH4 (g) + H2O (g) → CO (g) + 3 H2 (g)
206 Kj)
CH4 (g) + ½ O2 (g) → CO (g) + 2 H2 (g),
ΔH = ¼(-802 +247 + 206) kj
ΔH = -36 kj
3. How much glucose must be metabolized to completely evaporate 1.00 g of water? The heat of
combustion of glucose is 2803 kj/mole.
2.257k j
g
180.1 g
1.00 g mole
= 0.145 g C6H12O6
2803 kj mole
kg
103g
4. The standard enthalpy of formation of NH3 is -46.1 kJ/mole. What is ΔH°rxn for the following reactions?
a. N2 (g) + 3 H2 (g) → 2NH3 (g)
b. NH3 (g) → N2 (g) + 3 H2 (g)
ΔH°rxn = 2(-46.1) = -92.2 kj/mole
ΔH°rxn = 2(-46.1) = +92.2 kj/mole
5. The heat of combustion of pentane (C5H12) is -3535 kj/mole
a. Find the heat given off when 1.00 kg of pentane is burned.
3535 kj
1.00 kg 1000 g mole
kg
72.14 g mole
= 4.90 X 104 kj
b. How many grams of pentane must be burned to increase the temperature of of water from20.0°C
to 90.0°C?
4.184 j 70.0 °C
g-°C
103 g
kg
mole
1.00 kg kj
3
10 j 3535 kj
72.12
mole
g
= 5.98 g C5H12
Brass has a density of 8.40 g/cm3 and a specific heat of 0.385 j/g°C. A 15.2 cm3 piece of brass at an initial
temperature of 163°C is dropped into an insulated container with 150.0 g H2O at 22.4°C. What will be the final
temperature of the brass-water mixture? g-°C
-q = q
4.184 j 150.0 g
-0.385 j 8.40 g 15.2 cm3 (Tf – 163)°C
=
3
g-°C
cm
g-°C
(Tf – 163)°C
Tf = 32.6 °C