CHAPTER 6 - Thermochemistry DR. PAHLAVAN 1. Calculate the total change in internal energy, E, of a system when 400 J heat is applied to expanding O2 (g) and the gas does 350 J of work on its surroundings. 2. Calculate the kinetic energy in J, calories, and kJ of a particle (mass = 9.11x10-28 g) at 6.00x105 m/s. 3. Given the following data: a) 1/2 Br2 (l) Br (g) H0 = 111.75 kJ b) 1/2 Cl2 (g) Cl (g) H0 = 121.38 kJ c) 1/2 Br2 (l) + 1/2 Cl2 (g) BrCl (g) H0 = 14.7 kJ determine H0rx for reaction; Br (g) + Cl (g) BrCl (g) 4. Calculate H0 in kJ for reaction; CuO(s)+ CO (g) Cu(s) + CO2 (g) given the following thermochemical data : 2CO(g) + O2 (g) 2CO2(g) 2Cu(s) + O2 (g) 2CuO(s) H0 = - 566.1 kJ H0 = - 310.5 kJ 5. Consider the following reactions: Calculate Hf0 for H2O2 (l) ; a) H2O2 (l) H2O (l) + 1/2 O2 (l) b) H2 (g) + 1/2 O2 (g) H2O (l) H2 (g) + O2 (g) H2O2 (l) H0 = -98.3 kJ H0 = -285.8 kJ 6. A 1.0 g sample of propane, C3H8, was burned in calorimeter. The temperature rose from 28.5 0C to 32.0 0C and heat of combustion 10.5 kJ/g. Calculate the heat capacity of the calorimeter apparatus in kJ/0C . 7. What is the resulting temperature when 35.0 g of water at 75 0C is mixed with 15.0 g of water at 10 0C? (Heat capacity of water = 4.184 J/g. 0C) 8. A 20.0 g piece of a metal with specific heat of 0.080 cal/g.0C at 68 0C dropped into 15.0 ml water in a calorimeter at 23 0C. Calculate the final equilibrium temperature of the mixture. 9. When 72 g of a metal at 97.00C is added to 100 g of water at 25.0 0C, the final temperature is 45.5 0C. What is the heat capacity per grams of the metal? (Heat capacity of H2O = 4.184 J/g. 0C) 10. The heat capacity of lead is 0.13 J/g.0C. How many joules of heat would be required to raise the temperature of 15.0 g of lead from 22 0C to 38 0C? 1 CHAPTER 6 – Thermochemistry (Answers) DR. PAHLAVAN 1. Calculate the total change in internal energy, E, of a system when 400 J heat is applied to expanding O2 (g) and the gas does 350 J of work on its surroundings. ΔE = q + w = 400J – 350J = 50J 2. Calculate the kinetic energy in J, calories, and kJ of a particle (mass = 9.11x10-28 g) at 6.00x105 m/s. E = (1/2)mv2 = (1/2)(9.11 x 10-31kg)(6.0 x 105 m/s)2 = 1.64 x 10-19J = (1.64 x 10-19J) ÷ 4.184J/cal = 3.93 x 10-20 calories = 1.64 x 10-22kJ 3. Given the following data: a) 1/2 Br2 (l) Br (g) ΔH0 = 111.75 kJ b) 1/2 Cl2 (g) Cl (g) ΔH0 = 121.38 kJ c) 1/2 Br2 (l) + 1/2 Cl2 (g) BrCl (g) ΔH0 = 14.7 kJ determine ΔH0rx for reaction; Br (g) + Cl (g) BrCl (g) Reverse a, ΔHº = -111.75 kJ Reverse b, ΔHº = -121.38 kJ Add to c. ΔHº = 14.7 kJ ΔH0rx = -218.4 kJ 4. Calculate H0 in kJ for reaction; CuO(s)+ CO (g) Cu(s) + CO2 (g) given the following thermochemical data : (a) 2CO(g) + O2 (g) 2CO2(g) ΔH0 = - 566.1 kJ (b) 2Cu(s) + O2 (g) 2CuO(s) ΔH0 = - 310.5 kJ Multiply (a) by ½ ΔHº = -566.1 kJ/2 = -283.05 kJ Reverse (b) & multiply by ½ ΔHº = 310.5 kJ/2 = 155.25 kJ ΔHº = -128 kJ 5. Consider the following reactions (a) & (b) : Calculate Hf0 for H2O2 (l) ; H2 (g) + O2 (g) H2O2 (l) a) H2O2 (l) H2O (l) + 1/2 O2 (l) ΔH0 = -98.3 kJ b) H2 (g) + 1/2 O2 (g) H2O (l) ΔH0 = -285.8 kJ Reverse (a) ΔHº = 98.3 kJ Add to (b) ΔHº = -285.8 kJ ΔHº = -187.5 kJ 2 . 6. A 1.0 g sample of propane, C3H8, was burned in calorimeter. The temperature rose from 28.5 0 C to 32.0 0C and heat of combustion 10.5 kJ/g. Calculate the heat capacity of the calorimeter apparatus in kJ/0C Equation for bomb calorimeter: ΔH = CbombΔT 10.5 kJ/g = Cbomb(32.0 ºC – 28.5 ºC) Cbomb = 3.00 kJ/ºC 7. What is the resulting temperature when 35.0 g of water at 75 0C is mixed with 15.0 g of water at 10 0C? (Heat capacity (Cp) of water = 4.184 J/g. 0C) Equation: m1CpΔT1 = m2CpΔT2 (35g)(Cp)(75-Tf) = (15g)(Cp)(Tf – 10) Tf = 55.5 ºC 8. A 20.0 g piece of a metal with specific heat of 0.080 cal/g.0C at 68 0C dropped into 15.0 ml water in a calorimeter at 23 0C. Calculate the final equilibrium temperature of the mixture. Equation: m1CmetalΔT1 = m2CpΔT2 (20g)(0.08cal/g)(68 -Tf) = (15g)(1.0 cal/gºC)(Tf – 23) Tf = 27.34 ºC 9. When 72 g of a metal at 97.00C is added to 100 g of water at 25.0 0C, the final temperature is 45.5 0C. What is the heat capacity per grams of the metal? (Heat capacity of H2O = 4.184 J/g. 0C) Equation: m1CmetalΔT1 = m2CpΔT2 (72g)(Cmetal)(97 – 45.5) = (100g)(4.184J/gºC)(45.5 – 25) Cmetal = 2.31 J/gºC 10. The heat capacity of lead is 0.13 J/g.0C. How many joules of heat would be required to raise the temperature of 15.0 g of lead from 22 0C to 38 0C? Equation : ΔH = mCmetalΔT = (15g)(0.13J/gºC)(38 – 22) = 31.2 J 3