A calorimeter is an insulated device in which a chemical reaction is contained.

Question

A calorimeter is an insulated device in which a chemical reaction is contained. By measuring the temperature change, Delta T, we can calculate the heat released or absorbed during the reaction using the following equation: q = specific heat Times mass Times Delta T Or, if the calorimeter has a predetermined heat capacity, C, the equation becomes q = C Times Delta T At constant pressure, the enthalpy change for the reaction, Delta H, is equal to the heat, q_p, that is, Delta H = q_p but it is usually expressed per mole of reactant and with a sign opposite to that of q for the surroundings. The total internal energy change, Delta E, is the sum of heat, q, and work done, w: Delta E = q + w However, at constant volume (as with a bomb calorimeter) w = 0 and so Delta E = q_v. A calorimeter contains 20.0 mL of water at 14.0 degree C. When 1.80 g of X (a substance with a molar mass of 62.0 g/mol) is added, it dissolves via the reaction X(s) + H_2O(l) rightarrow X(aq) and the temperature of the solution increases to 25.0 degree C. Calculate the enthalpy change, Delta H, for this reaction per mole of X. Assume that the specific heat and density of the resulting solution are equal to those of water [4.18 J/(g degree C) and 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. Consider the reaction C_12H_22O_11 (s) + 12O_2(g) rightarrow 12CO_2(g) + 11H_2O(l) in which 10.0 g of sucrose, C_12H_22O_11, was burned in a bomb calorimeter with a heat capacity of 7.50 kJ/degree C. The temperature increase inside the calorimeter was found to be 22.0 degree C. Calculate the change in internal energy, Delta E, for this reaction per mole of sucrose. Express the change in internal energy in kilojoules per mole to four significant figures.

Explanation / Answer

Part A

     X(s) + H2O(l) ---> X(aq)

Volume of water = 20 ml

mass of water = 20*1 = 20 grams

totalmass of mixer = 20+1.8 = 21.8 grams

specific heat of solution = 4.18 j/g.c

q = m*s*DT

   = 21.8*4.18*(25-14) = 1002.364 joule.

No of moles of X = 1.8/62 = 0.029 mole

0.029 mole of X = 1002.364 joule.

1mole of X = 1002.364/0.029 = 34564.276 joule. = 34.56 kj

part B

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

No of moles of sucrose = 10/342.2965 = 0.029 mole

Molar mass of sucrose = 342.2965 g/mol

DT = 22 c

heat capacity = 7.50 kJ/C

heat reased (q) = Cp*DT = 7.5*22 = 165 kj

so that,

0.029 mole of sucrose = 165 kj

1 mole of sucrose = 165/0.029 = 5689.65 kj/mol

DH = 5689.65 kj/mol

as Dn = 0 FOR the reaction.

DH = DE = 5689.65 kj/mol

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