Test your skills on the following problems.
Example #1: An unknown gas expands in a container increasing the volume from 8.7 L to 18.9 L at a constant pressure of 1380 mmHg.
(a) Calculate the work done (in J) by the gas as it expands. (1 L· atm = 101.3 J).
(b) Calculate the internal energy of the system if the system absorbs 235.5 J of energy.
(c) Calculate the internal energy of the system if work was done against a vacuum. (1 L · atm = 101.3 J).
Example #2: Calculate the amount of heat absorbed when 12.0 g of water is heated from 20oC to 100oC. (c = 4.184 J/g· oC).
Example #3: 101.3 g of an unknown metal has an initial temperature of 25oC. If it absorbs 639.1 J of energy to obtain a final temperature of 32.01oC identify the unknown metal.
Example #4: Which substance has the highest molar heat capacity?
a) Copper (specific heat Cu (s): 0.39 J/g · oC)
b) Silver (specific heat Ag (s): 0.23 J/g · oC)
c) Iron (specific heat Fe(s): 0.46 J/g · oC)
d) Lead (specific heat Pb (s): 0.13 J/g · oC)
Example #5: 25.00 g of heated metal ore is placed into an insulated beaker containing 615.5 g of water at 42.18oC. If the metal gains 19.11 kJ of energy, what is the final temperature of the water? (cwater = 4.184 J/g · oC).
Example #6: If 53.2 g Al at 25.0 oC is placed in 110.0 g H2O at 90 oC, what is the final temperature of the mixture? The specific heat capacities of water and aluminum are 4.184 J/g · oC and 0.897 J/g · oC, respectively.
Example #7: A 20.0 g sample of iron, specific heat Fe (s) = 0.46 J/g⋅oC, has an initial temperature of 30.2 oC. If 0.310 kJ are applied to the iron sample, calculate its final temperature.
Example #8: A sample of H2O (l) containing 2.50 moles has a final temperature of 45.0 oC. If the sample absorbs 3.00 kJ of heat, what is the initial temperature of the H2O (l)? The specific heat of H2O (l) is 4.184 J/g⋅ oC .
Example #9: Determine the heat released when 80.0 g H2O (l) at 90 oC is cooled to ice at – 10.0 oC. Specific Heat of H2O (l) = 4.184 J / g⋅oC. Specific Heat of H2O (s) = 2.09 J / g⋅oC. Heat of Fusion of water = 333 J/g .
Example #10: If 1050 g of aluminum metal with a specific heat capacity of 0.902 J/g⋅oC at – 20 oC is placed in liquid water at 0.00 oC, how many grams of liquid water are frozen by the time that the aluminum metal has warmed to – 10 oC? Heat of Fusion of water = 333 J/g.
Practice: How much heat (in kilojoules) is evolved when 255.0 g of aluminum (MW: 26.98 g/mol) reacts with excess Fe2O3?
2 Al (s) + Fe2O3 (s) → 2 Fe (s) + Al2O3 (s) ∆Hrxn = - 852.0 kJ
Practice: Solve the following question based on the given chemical equation:
NO2Cl (g) + NO (g) → NOCl (g) + 3 NO2 (g)
Calculate the ∆Hrxn if the standard enthalpies of NO2Cl, NO, NOCl and NO2 are – 724.2 kJ/mol , – 83.5 kJ/mol , 119.3 kJ/mol and – 393.5 kJ/mol.
Practice: Choose the thermochemical equation that illustrates ΔHf°for Li2SO4.
a) 2 Li+ (aq) + SO42- (aq) → Li2SO4 (aq)
b) 2 Li (s) + 1/8 S8 (s, rhombic) + 2 O2 (g) → Li2SO4 (s)
c) Li2SO4 (aq) → 2 Li+ (aq) + SO42- (aq)
d) 8 Li2SO4 (s) → 16 Li (s) + S8 (s, rhombic) + 16 O2 (g)
e) 16 Li (s) + S8(s, rhombic) + 16 O2 (g) → 8 Li2SO4 (s)
Practice: Calculate the enthalpy, ∆Hrxn, for
CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (l)
Given the following reactions:
CH2O (g) + H2O (g) → CH4 (g) + O2 (g) ∆Ho = 284 kJ
CH2O (g) + O2 (g) → CO2 (g) + H2O (g) ∆Ho = -518 kJ
H2O (l) → H2O (g) ∆Ho = 44.0 kJ