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 25.0 mL of water at 15.0 C . When 1.70 g of X (a substance with a molar mass of 75.0 g/mol ) is added, it dissolves via the reaction

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

and the temperature of the solution increases to 27.0 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.

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Explanation / Answer

Mass of water = 25ml = 25g

Mass of substance dissolved = 1.70g

Total mass,m = 26.70g

Specific heat of solution = 4.184(J/g)

Temperature increase , T = 27-15= 12

Heat absorbed by solution ,q = 26.7g × 12 × 4.184(J/g)

= 1340.6J

Therefore, H = -1340.6J

This value is for 1.70 g of X

For 1mol of X i.e for 75gm ,H =(-1340.6 J/1.70g) × 75g = -59144.1 J = - 59.14kJ/mol

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