Part 1 A calorimeter contains 20.0 mL of water at 15.0 C . When 1.90 g of X (a s

Question

Part 1

A calorimeter contains 20.0 mL of water at 15.0 C . When 1.90 g of X (a substance with a molar mass of 78.0 g/mol ) is added, it dissolves via the reaction

X(s)+H2O(l)X(aq)

and the temperature of the solution increases to 30.0 C .

Calculate the enthalpy change, H, for this reaction per mole of X.

Assume that the specific heat of the resulting solution is equal to that of water [4.18 J/(gC)], that density of water is 1.00 g/mL, and that no heat is lost to the calorimeter itself, nor to the surroundings.

Express the change in enthalpy in kilojoules per mole to three significant figures.

Part 2

Consider the reaction

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

in which 10.0 g of sucrose, C12H22O11, was burned in a bomb calorimeter with a heat capacity of 7.50 kJ/C. The temperature increase inside the calorimeter was found to be 22.0 C. Calculate the change in internal energy, E, for this reaction per mole of sucrose.

Express the change in internal energy in kilojoules per mole to three significant figures.

Explanation / Answer

Part 1.

mass of water = volume x density

= 20 x 1 = 20 g

Total mass = 20 + 1.9 = 21.9 g

heat of reaction = mCp(Tf-Ti)

= 21.9 x 4.18 x (30 - 15)

= 1373.13 J

enthalpy = heat

dH = 1373.13 x 78/1.9

      = 56.37 kJ/mol

Part 2.

moles of sucrose = g/molar mass

= 10/342.30 = 0.029 mols

heat absorbed = 7.50 kJ/oC x 22 oC = 165 kJ

change in internal energy = n x dH = 165 x 0.029 = 4.785 kJ/mol

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