Molar Enthalpy Questions

1. In a chemistry experiment to investigate the properties of a fertilizer, 10.0g of urea, NH2CONH2(s), is issolved in 150mL of water in a simple calorimeter. A temperature change from 20.4°C to 16.7°C is measured. Calculate the molar enthalpy of solution for the fertilizer urea.

T1 = 20.4°C ΔT = T2-T1 Q = mΔTc ΔH = Q/n T2 = 16.7°C = 16.7 – 20.4 = (150)(-3.7)(4.184) = 2322.12/.167 m = 150g = -3.7°C = -2322.12 J = 13904.9 J/mol c = 4.184 J/g°C Qrx = +2322.12 J = 13.9 kJ/mol M = 60.0g/mol n = m/M = 10/60 = .167 mol

2. In a 10.0g sample of liquid gallium metal, at its melting point, is added to 50.0g of water in a polystyrene calorimeter. The temperature of the water changes from 24.0°C to 27.8°C as the gallium solidifies. Calculate the molar enthalpy of solidification for gallium.

T1 = 24.0°C ΔT = T2-T1 Q = mΔTc ΔH = Q/n T2 = 27.8°C = 27.8-24.0 = (50)(3.8)(4.184) = -794.96/.143 m = 50g = 3.8°C = 794.96 J = -5559.16 J/mol c = 4.184 J/g°C Qrx = -794.96 J = -5.6 kJ/mol M = 69.7g/mol n = m/M = 10/69.7 = 0.143 mol

3. The energy involved in the process H2O(g) H2O(l) could be described as a molar enthalpy of condensation. Describe the type of molar enthalpy that would be associated with each of the following reactions:

a. Br2(l) Br2(g) Molar enthalpy of vapourization

b. CO2(g) CO2(s) Molar enthalpy of sublimation

c. LiBr(s) Li+1

(aq) + Br-1

(aq) Molar enthalpy of dissolving (solution)

d. C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g) Molar enthalpy of combustion

e. NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l) Molar enthalpy of neutralization

4. In a laboratory investigation into the reaction Ba(NO3)2(s) + K2SO4(aq) BaSO4(s) + 2KNO3(aq), a

researcher adds 261g sample of barium nitrate to 2.0L of potassium sulfate solution in a polystyrene calorimeter. As the barium dissolves, a precipitate is immediately formed.

T1 = 26.0°C and T2 = 29.1°C. Calculate the molar enthalpy of the reaction of barium nitrate. T1 = 26.0°C ΔT = T2-T1 Q = mΔTc ΔH = Q/n T2 = 29.1°C = 29.1-26.0 = (2)(3.1)(4.184) = -25.94/1 m = 2.0 kg = 3.1°C = 25.94 kJ = -25.94 kJ/mol c = 4.184 kJ/kg°C Qrx = -25.94 kJ M = 261.3 g/mol n = m/M = 261/261.3 = 1.0 mol

5. If the molar enthalpy of combustion of ethane is –1.56MJ/mol, how much heat is produced in the burning of :

a. 5.0 mol of ethane? ΔH = -1.56 MJ/mol Q = ΔHxn = (-1.56)(5) = -7.8 MJ or 7.8 MJ of energy RELEASED

b. 40.0g of ethane? ΔH = -1.56 MJ/mol n = m/M Q = ΔHxn M = 30.0 g/mol = 40/30 = (-1.56)(1.33) m = 40 g = 1.33 mol = -2.075 MJ or 2.075 MJ released

6. The molar enthalpy of solution of ammonium chloride is +14.8 kJ/mol. What would be the final

temperature of a solution in which 40.0g of ammonium chloride is added to 200.0mL of water, initially at 25°C?

ΔH = +14.8 kJ/mol n = m/M Q = mΔTc m = 40 g = 40/53.5 -11.07 = (.2)(ΔT)(4.184) M = 53.5 g/mol = 0.748 mol ΔT = -11.07/.8368 T1 = 25°C ΔT = -13.23°C c = 4.184 J/g°C Q = ΔHxn = (14.8)(.748) T2 = T1 + ΔT = 11.07 kJ T2 = 25 + -13.23 Qcal = -11.07 kJ T2 = 11.77°C

7. The molar enthalpy of combustion of decane (C10H22) is –6.78 MJ/mol. What mass of decane would have to be burned in order to raise the temperature of 500.0mL of water from 20.0°C to 55.0°C?

ΔH = -6.78 MJ/mol ΔT = T2-T1 Q = mΔTc m = ? g = 55-20 Q = (.500)(35)(4.184) M = 142 g/mol = 35°C = 73.22 kJ T1 = 20°C Qrx = -73.33 kJ c = 4.184 J/g°C n = Q/ΔH T2 = 55°C = -73.33/-6780 m = nxM = 0.0108 mol = (0.0108)(142) = 1.54 g

5. In a laboratory investigation into the neutralization reaction, HNO3(aq) + KOH(s) KNO3(aq)

+ H2O(l), a researcher adds solid potassium hydroxide to nitric acid solution in a polystyrene calorimeter. Calculate the molar enthalpy of neutralization of potassium hydroxide if 5.2g of the substance was dissolved in 200 mL of nitric acid and its temperature changed from 21.0°C to 28.1°C? T1 = 21.0°C ΔT = T2-T1 Q = mΔTc ΔH = Q/n T2 = 28.1°C = 28.1-21.0 = (.200)(7.1)(4.184) = -5.94/.093 m = 5.2 g = 7.1°C = 5.94 kJ = -63.88 kJ/mol c = 4.184 kJ/kg°C Qrx = -5.94 kJ M = 56.1 g/mol n = m/M = 5.2/56.1 = 0.093 mol