Part A A calorimeter contains 18.0mL of water at 13.0?C . When 2.30g of X (a sub

Question

Part A

A calorimeter contains 18.0mL of water at 13.0?C . When 2.30g of X (a substance with a molar mass of 70.0g/mol ) is added, it dissolves via the reaction

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

and the temperature of the solution increases to 29.0?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/(g??C)], 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

The number of moles of the substance dissolved is 2.3/70=0.03285 mole of X

The energy produced by dissolution is

E= m*c*(tf-ti) tf and ti are final and initial temperature mass m=18g

So E = - 18*4.18*(29-13) = -1203.84 Joules (I put sign - since the system gives heat)

this for 0.03285 moles

For 1 mole -1203.84/0.03285 = -36646.57J = -36.646kJ/mole

Part B

The heat produced in a bomb calorimeter when combusting a given amount of known substance is the internal energy of the substance combusted per the number of moles of the substance that was burned.

Here 10.0 g of sucrose is (10.0 g of sucrose) / (342.3 g/mol) = 0.02921 moles of sucrose

You are given that the heat capacity of the calorimeter is 7.50 kJ/ deg C. This means that the temperature of calorimeter increases 1 deg C when 7.50 kJ of heat is absorbed by the calorimeter from the combustion process. Since the temperature increase was 22.0 deg C when combusting the 0.02921 moles of sucrose, then the combustion process must have given off (22.0 deg C)(7.50 kJ/deg C) = 165.0 kJ..

Therefore, the change in internal energy per mole of sucrose combusted would be = (165.0 kJ) / (0.02921 moles of sucrose combusted) = 5649 kJ/mol of sucrose combusted

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