(1) Sucrose (C 12 H 22 O 11 ) is found to have a standard enthalpy of combustion

Question

(1) Sucrose (C12H22O11) is found to have a standard enthalpy of combustion of -5,640.9 kJ/mole. The standard enthalpies of formation of CO2 (g) and H2O (l) are -393.51 and --285.83 kJ/mole, respectively.

Compute deltaH_f^0 for sucrose.

(2) A student determined the enthalpy of a solution of NaOH by dissolving 4.00 g of NaOH (s) in 180 mL of pure water (density 0.999 g/mL) at an initial temperature of 19.5 degrees Celsius. Temperature-time data collected after mixing was extrapolated back to the time of mixing to obtain a temperature change, deltaT = +5.5 degrees. The density of the final solution was 1.013 mL^-1 at 25 degrees Celsius and the solution heat capacity was 4.08 J g^-1 deg^-1. In a separate experiment, the calorimeter constant was found to be 21 J deg^-1.

(a) What was the molar concentration of the final NaOH solution at 25 degrees Celsius?

(b) What was the molar enthalpy of dissolution of NaOH in water?

Explanation / Answer

(1) C12H22O11(s) + 12 O2(g) => 12 CO2(g) + 11 H2O(l)

DH(combustion) = DHf(products) - DGf(reactants)

= 12 x DHf(CO2) + 11 x DHf(H2O) - DHf(C12H22O11) - 12 x DHf(O2)

-5640.9 = 12 x (-393.51) + 11 x (-285.83) - DHf(C12H22O11) - 12 x 0

DHf(C12H22O11) = -2225.35 kJ/mol

(2) (a) Moles of NaOH = mass/molar mass of NaOH

= 4.00/40.00 = 0.1 mol

Mass of water = volume x density of water

= 180 x 0.999 = 179.82 g

Mass of solution = mass of NaOH + mass of water

= 4.00 + 179.82 = 183.82 g

Volume of solution = mass/density of solution

= 183.82/1.013 = 181.46 mL = 0.18146 L

Concentration of NaOH = moles of NaOH/volume of solution

= 0.1/0.18146 = 0.551 M

(b) Heat absorbed by solution = mass x specific heat x temperature change of solution

= 183.82 x 4.08 x 5.5 = 4124.92 J

Heat absorbed by calorimeter = calorimeter constant x temperature change

= 21 x 5.5 = 115.5 J

Enthalpy of dissolution = -total heat absorbed/moles of NaOH

= -(4124.92 + 115.5)/0.1

= -42404 J/mol = -42.4 kJ/mol

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