Unit 5 Thermodynamics

5.2 Exercises

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Section 5.2 Exercises

  1. Would the amount of heat measured for an exothermic reaction be greater, lesser, or remain the same if we used a calorimeter that was a poorer insulator than a coffee cup calorimeter? Explain your answer.
  2. Would the amount of heat absorbed appear greater, lesser, or remain the same if the heat capacity of the calorimeter were taken into account? Explain your answer.
  3. How much will the temperature of a cup (180 g) of coffee at 95 °C be reduced when a 45 g silver spoon (specific heat 0.24 J/(g·°C)) at 25 °C is placed in the coffee and the two are allowed to reach the same temperature? Assume that the coffee has the same density and specific heat as water.
  4. The temperature of the cooling water as it leaves the hot engine of an automobile is 115 °C. After it passes through the radiator it has a temperature of 80 °C. Calculate the amount of heat transferred from the engine to the surroundings by 4.00 L of water with a specific heat of 4.184 J/(g·°C).
  5. If a reaction produces 1.506 kJ of heat, which is trapped in 30.0 g of water initially at 26.5 °C in a calorimeter, what is the resulting temperature of the water?
  6. Dissolving 3.0 g of CaCl2(s) in 150.0 g of water in a calorimeter at 22.4 °C causes the temperature to rise to 25.8 °C. What is the approximate amount of heat involved in the dissolution, assuming the heat capacity of the resulting solution is 4.18 J/(g·°C)? Is the reaction exothermic or endothermic?
  7. The addition of 3.15 g of Ba(OH)2·8H2O to a solution of 1.52 g of NH4SCN in 100 g of water in a calorimeter caused the temperature to fall by 3.1 °C. Assuming the specific heat of the solution and products is 4.20 J/(g·°C), calculate the approximate amount of heat absorbed by the reaction, which can be represented by the following equation:

    Ba(OH)2·8H2O(s) + 2NH4SCN(aq) ⟶ Ba(SCN)2(aq) + 2NH3(aq) + 10H2O(l)

  8. If the 3.21 g of NH4NO3 were dissolved in 100.0 g of water, how much would the temperature change? Explain your answer.  This question is based on Section 5.2 Example 7.
  9. When a 0.740-g sample of trinitrotoluene (TNT), C7H5N2O6, is burned in a bomb calorimeter, the temperature increases from 23.4 °C to 26.9 °C. The heat capacity of the calorimeter without water is 534 J/°C.  The calorimeter contains 675 mL of water. How much heat was produced by the combustion of the TNT sample?

Solutions

  1. lesser; more heat would be lost to the coffee cup and the environment and so ΔT for the water would be lesser and the calculated q would be lesser
  2. greater, since taking the calorimeter’s heat capacity into account will compensate for the thermal energy transferred to the solution from the calorimeter; this approach includes the calorimeter itself, along with the solution, as “surroundings”: qrxn = −(qsolution + qcalorimeter); since both qsolution and qcalorimeter are negative, including the latter term (qrxn) will yield a greater value for the heat of the dissolution
  3. The temperature of the coffee will drop 1 degree.
  4. 5.9 × 102 kJ
  5. 38.5 °C
  6. 2.2 kJ; The heat produced shows that the reaction is exothermic.
  7. 1.4 kJ
  8. The solution temperature decreases by 2.4 °C. Since the mass and the heat capacity of the solution is approximately equal to that of the water, the nearly two-fold increase in the amount of water, compared to Example 7, leads to slightly more than two-fold decrease of the temperature change.
  9. 11.7 kJ

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