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, T, we can calculate the heat released or absorbed during the reaction using the following equation:

q=specific heat×mass×T

Or, if the calorimeter has a predetermined heat capacity, C, the equation becomes

q=C×T

At constant pressure, the enthalpy change for the reaction, H, is equal to the heat, qp; that is,

H=qp

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, E (sometimes referred to as U), is the sum of heat, q, and work done, w:

E=q+w

However, at constant volume (as with a bomb calorimeter) w=0 and so E=qv.

Part A

A calorimeter contains 18.0 mL of water at 14.0 C . When 1.80 g of X (a substance with a molar mass of 43.0 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

Q = m c T
Q = quantity of heat in joules (J)
m = mass of the substance acting as the environment in
grams (g)
c = specific heat capacity (4.19 for H2O) in J/(g oC)
T = change in temperature = Tfinal - Tinitial in oC

PART A

The heat genrated by compound is equal to the heat absorbed by water

(18 +1.8) x 4.18 x (25-14) = Q

Q= 910.4 J

molar mass of substance = 43.0 g/mol

weight of substance dissolved in water = 1.8

Moles of substance dissolved = 1.8 / 43 = 0.04186 Mole

Hence change in enthalpy per mole = 910.4 x 1/ 0.04186 = 21748 J or 21.748 kJ

PART B

m =  10.0 g of sucrose

c= heat capacity is 7.50 kJ/C

T = 22

Q = 10 x 7.5 x 22 = 1650 j of heat will be released

10.0 g of sucrose (Molar mass: 342.2965 g/mol) = 10 / 342.29 = 0.02921 Mole

change in internal energy per mole = 1650 x 1 / 0.02921 = 56487 J or 56.487 kJ

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