PART A: A calorimeter contains 25.0 mL of water at 13.0 C . When 1.20 g of X (a

Question

PART A: A calorimeter contains 25.0 mL of water at 13.0 C . When 1.20 g of X (a substance with a molar mass of 67.0 g/mol ) is added, it dissolves via the reaction X(s)+H2O(l)X(aq) and the temperature of the solution increases to 25.0 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 B: 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 A)

mass of water = 25 g

mass of solution = 25 + 1.20 = 26.20 g

temperature rise = 25 - 23 = 12 oC

Q = m Cp dT

    = 26.20 x 4.18 x 12

    = 1314.2 J

moles of X = 1.20 / 67 = 0.0179 mol

H = - Q / n = - 1.314 / 0.0179

enthalpy H = - 73.4 kJ/mol

Part B)

Q = Cp x dT

    = 7.50 x 22

   = 165 kJ

moles of sucrose = 10 / 342 = 0.0292

E = -165 / 0.0292 = 5.64 x 10^3 kJ/mol

E = - 5.64 x 10^3 kJ/mol

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