Question

TC01H02 Solid calcium hydroxide weighing 1.9984 g (molar mass = 74.09 g mor') is dissolved in 100.0 mL of 1.000 mol/L HCl in an open calorimeter which has a heat capacity of 15.87 JK! The initial temperature of the Ca(OH)2 and HCl is 19.14 °C and and the temperature of the solution is 24.20 °C at the completion of the reaction. Density of 1.000 mol/L HCl = 1.015 g/mL. Heat capacity of the final solution = 3.950 JK'g? Use the experimental measurements above to calculate the molar enthalpy change for the reaction: Ca(OH)2(s) + 2HCl(aq) - CaCl2(aq) + H200) Select one: O a. -79.7 kJ/mol b. -76.7 kJ/mol O C.-78.2 kJ/mol d. -78.6 kJ/mol e. -21.5 kJ/mol

Answer 1

First part,

mass of the solution (after reaction) = mass of HCl + mass of Ca(OH)₂ = 100 mL x 1.015 g.mL^-1 + 1.9984 g= 103.4984 g

heat capacity = 3.950 J.K^-1.g^-1

Initial temperature = 19.14 ^oC = 292.14 K, Final temperature = 24.20 ^oC = 297.20 K, So, change in temperature = 5.06 K

So, heat absorbed by solution = mass of the solution x heat capacity x change in temperature

= 103.4984 g x 3.950 J.K^-1.g^-1 x 5.06 K = 2068.62 J

again,

heat capacity of calorimeter = 15.87 J.K^-1

temperature change = 5.06 K

So, heat absorbed by the calorimeter = 15.87 J.K^-1 x 5.06 K = 80.30 J

So, total heat released = total heat absorbed = 2068.62 J + 80.30 J = 2148.92 J

Now, since Ca(OH)₂ is in lower amount and gets completely neutralized. So, we have to calculate heat of neutralization based on moles of Ca(OH)₂.

moles of Ca(OH)₂ = (mass / molar mass) = 1.9984 / 74.09 mol = 0.02697 mol

Thus,

$enthalpy\, change=- \frac{2148.92}{0.02697}J.mol^{-1} =- 79678J.mol^{-1}=-79.7kJ.mol^{-1}$

it is negative because it is exothermic. So answer is a.

Second part,

Moles of Cu = (mass / molar mass) = 29.5 / 63.5 mol = 0.4646 mol

heat capacity = 24.44 J/K/mol

initial temperature = 21.4 ^oC = 294.4 K, final temperature = 98.7 ^oC = 371.7 K, temperature change = 77.3 K

Thus, heat = Moles of Cu x heat capacity x temperature change = 0.4646 mol x 24.44 J/K/mol x 77.3 K = 877.7 J = 878 J (approx)

In this case heat is absorbed thus endothermic process. So, the sign is positive. Correct answer is a.