Ch.9 - Thermochemistry: Chemical Energy
Back
- Which of the following are state functions, and which are not? (a) The temperature of an ice cube (b) The volume of an aerosol can (c) The amount of time required for Paula Radcliffe to run her world-record marathon: 2:15:25
Problem 48
- Calculate the work done in joules by a chemical reaction if the volume increases from 3.2 L to 3.4 L against a constant external pressure of 3.6 atm. What is the sign of the energy change?
Problem 50
- Assume that the nutritional content of an apple-say, 50 Cal (1 Cal = 1000 cal) - could be used to light a light bulb. For how many minutes would there be light from each of the following? (a) A 100-watt incandescent bulb (1 W = 1 J/s)
Problem 52
Problem 54a
A reaction inside a cylindrical container with a movable RAN piston causes the volume to change from 12.0 L to 18.0 L while the pressure outside the container remains constant at 0.975 atm. (The volume of a cylinder is V = pr2h, where h is the height; 1 L # atm = 101.325 J.) (a) What is the value in joules of the work w done during the reaction?
Problem 54b
A reaction inside a cylindrical container with a movable RAN piston causes the volume to change from 12.0 L to 18.0 L while the pressure outside the container remains constant at 0.975 atm. (The volume of a cylinder is V = pr2h, where h is the height; 1 L # atm = 101.325 J.) (b) The diameter of the piston is 17.0 cm. How far does the piston move?
Problem 55a
At a constant pressure of 0.905 atm, a chemical reaction takes place in a cylindrical container with a movable piston having a diameter of 40.0 cm. During the reaction, the height of the piston drops by 65.0 cm. (The volume of a cylinder is V=pr2h,wherehistheheight;1Latm=101.3J.) (a) What is the change in volume in liters during the reaction?
Problem 55b
At a constant pressure of 0.905 atm, a chemical reaction takes place in a cylindrical container with a movable piston having a diameter of 40.0 cm. During the reaction, the height of the piston drops by 65.0 cm. (The volume of a cylinder is V=pr2h,wherehistheheight;1Latm=101.3J.) (b) What is the value in joules of the work w done during the reaction?
- When a sample of a hydrocarbon fuel is ignited and burned in oxygen, the internal energy decreases by 7.20 kJ. If 5670 J of heat were transferred to the surroundings, what is the sign and magnitude of work? If the reaction took place in an environ- ment with a pressure of 1 atm, what was the volume change?
Problem 56
- What is the difference between the internal energy change ∆E and the enthalpy change ∆H? Which of the two is mea- sured at constant pressure and which at constant volume?
Problem 58
- Under what circumstances are ΔE and ΔH essentially equal?
Problem 59
- The enthalpy change for the reaction of 50.0 mL of ethylene with 50.0 mL of H2 at 1.5 atm pressure (Problem 9.51) is ∆H = -0.31 kJ. What is the value of ∆E?
Problem 62
- Assume that a particular reaction evolves 244 kJ of heat and that 35 kJ of PV work is gained by the system. What are the values of ∆E and ∆H for the system? For the surroundings?
Problem 63
- What is the enthalpy change (ΔH) for a reaction at a constant pressure of 1.00 atm fi the internal energy chagne (ΔE) is 44.0 kJ and the volume increase is 14.0 L? (1 L-atm = 101.325 J)
Problem 64
- A reactiont akes place at a constant pressure of 1.10 atm with an internal energy change (ΔE) of 71.5 kJ and a volume decrease of 13.6 L. What is the enthalpy change (ΔH) for the reaction? (1 L-atm = 101.325 J)
Problem 65
- What is the difference between heat capacity and specific heat?
Problem 76
- Does a measurement carried out in a bomb calorimeter give a value for ∆H or ∆E? Explain.
Problem 77
- Sodium metal is sometimes used as a cooling agent in heat-exchange units because of its releatively high molar heat capacity fo 28.2 J/(mol·°C). What is the specific heat and molar heat capacity of sodium in J/g·°C?
Problem 78
- Titanium metal is used as a structural material in many high-tech applications, such as in jet engines. what is the specific heat of titanium in J/(g·°C) if it takes 89.7 J to raise the temeprature of a 33.0 g block of 5.20 °C? What is the molar heat capacity of titanium J/(mol·°C)?
Problem 79
- Assuming that Coca-Cola has the saem specific heat as water [4.18 J/(g C)], calculate the amount of heat in kilojoules transferred when one can (about 350 g) is cooled from 25 C to 3 C.
Problem 80
- Calculate the amount of heat required to raise the tempera- ture of 250.0 g (approximately 1 cup) of hot chocolate from 25.0 °C to 80.0 °C. Assume hot chocolate has the same spe-cific heat as water 34.18 J>1g °C24.
Problem 81
- Instant cold packs used to treat athletic injuries contain solid NH4NO3 and a pouch of water. When the pack is squeezed, the pouch breaks and the solid dissolves, lowering the tem-perature because of the endothermic reaction NH4NO31s2 ¡ NH4NO31aq2 ∆H = +25.7 kJ What is the final temperature in a squeezed cold pack that contains 50.0 g of NH4NO3 dissolved in 125 mL of water? Assume a specific heat of 4.18 J/(g C) for the solution, an initial temperature of 25.0 °C, and no heat transfer between the cold pack and the environment.
Problem 82
- Instant hot packs contain a solid and a pouch of water. When the pack is squeezed, the pouch breaks and the solid dis- solves, increasing the temperature because of the exothermic reaciton. The following reaction is used to make a hot pack: H2O LiCl1s2 ¡ Li 1aq2 + Cl 1aq2 ∆H = -36.9 kJ. What is the final temperature in a squeezed hot pack that contains 25.0 g of LiCl dissolved# in 125 mL of water? Assume a specific heat of 4.18 J>1g °C2 for the solution, an initial temperature of 25.0 °C, and no heat transfer between the hot pack and the environment.
Problem 83
- When 0.187 g of benzene, C6H6, is burned in a bomb calorimeter the temperature rises by 3.45 °C. If the heat capacity of the calorimeter is 2.46 kJ>°C, calculate the combustion energy 1∆E2 for benzene in units of kJ/g and kJ/mol.
Problem 86
- When 1.50 g of magnesium metal is allowed to react with 200 mL of 6.00 M aqueous HCl, the temperature rises from 25.0 °C to 42.9 °C. Calculate ΔH in kilojoules for the reaction, assumign that the heat capacity of the calorimeter is 776 J/°C, that the specific heat of the final soltuion is the same as that of water [4.18 J(g·°C)] and that the density of the solution is 1.00 g/mL
Problem 88
- A 110.0 g piece of molybdenum metal is heated to 100.0 °C and placed in a calorimeter that contains 150.0 g of water at 24.6 °C. The system reaches equilibirum at a final temeprature of 28.0 °C. Calcualte the specific heat of molybdenum metal in J/g·°C. The specific heat of water is 4.18 J/g·°C
Problem 89
- Citric acid has three dissociable hydrogens. When 5.00 mL of 0.64 M citric acid and 45.00 mL of 0.77 M NaOH are mixed at an initial temperature of 26.0 °C, the temperature rises to 27.9 °C as the citric acid is neutralized. The combined mixture ahs a mass of 51.6 g and a specific heat of 4.0 J/(g·°C). Assuming that no heat is transferred to the surroundings, cal- culate the enthalpy change for the reaction of 1.00 mol of cit- ric acid in kJ. Is the reaction exothermic or endothermic?
Problem 90
- What is Hess's law, and why does it 'work'?
Problem 92
- The following steps occur in the reaction of ethyl alcohol (CH3CH2OH) wiht oxygen to yield acetic acid (CH3CO2H). Show that equations 1 and 2 sum to give the net equation and calculate ΔH° for the net equation. (1) CH3CH2OH(l) + 1/2 O2(g) → CH3CHO (g) + H2O(l) ΔH° = -174.2 kJ (2) CH3CHO(g) + 1/2 O2(g) → CH3CO2H(l) ΔH° = -318.4 kJ (Net) CH3CH2OH(l) + O2(g) → CH3CO2H(l) + H2O(l) ΔH° = ?
Problem 93
- The industrial degreasing solvent methylene chloride, CH2Cl2, is prepared from methane by reaction with chlorine: CH4(g) + 2 Cl2(g) → CH2Cl2(g) + 2 HCl(g) Use the following data to calcualte ΔH° in kilojoules for the reaction: CH4(g) + Cl2(g) → CH3Cl(g) + HCl(g) ΔH° = -98.3 kJ CH3Cl(g) + Cl2(g) → CH2Cl2(g) + HCl(g) ΔH° = -104 kJ
Problem 94
- Hess's law can be used to calculate reaction enthalpies for hypothetical processes that can't be carried out in the labo- ratory. Set up a Hess's law cycle that will let you calculate ∆H° for the conversion of methane to ethylene: 2 CH4(g) → C2H4(g) + 2 H2(g) You can use the following information: 2 C2H6(g) + 7 O2(g) → 4 CO2(g) + 6 H2O(l) ∆H° = -3120.8 kJ CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l) ∆H° = -890.3 kJ C2H4(g) + H2(g) → C2H6(g) ∆H° = -136.3 kJ H2O(l) ∆H°f = -285.8 kJ/mol
Problem 95