Question

How do you find heat of solvation of `NH_4Cl`?

Answer 1

Enthalpy change for an aqueous solution can be determined experimentally. Utilizing a thermometer to measure the temperature change of the solution , (along with the mass of the solute) to determine the enthalpy change for an aqueous solution, as long as the reaction is carried out in a calorimeter or similar apparatus . You can use a coffee cup calorimeter.

1. Measure the mass of solute in grams using a balance. I am dissolving solute Ammonium chloride The mass I have taken is 5.4 g or 0.1 moles.

2. Measure the volume of water. I am going to use 100 ml of water. Record the Density of water. Using Density and volume of water I can calculate the mass of water using formula. Mass = Volume x Density (let us assume the density of water to be 1g / ml, the mass of 100 ml of water is 100g).

3. Pour the 100 ml of water in a coffee cup calorimeter, record the temperature of water , the initial temperature of water is `27^o` C.

4. Add 5.4 g of Ammonium chloride to the water. Dissolve the Ammonium chloride using a stirrer and also record the temperature of solution. Let us assume that the final temperature of the solution is `19^o` C

5. In this dissolution experiment Ammonium chloride dissolves in water and absorbs heat energy from the surrounding water , which causes the temperature of water to go down from `27^o` C to `19^o` C. The change in temperature is `19^o` C - `27^o` C = -`8^o` C

6. Use the formula Q = mass of water . specific heat of water . change in Temperature to calculate amount of heat gained by water.

7. Q = 100 g . 4.18 J / `g^oC`. -`8^o` C

Q = -3344 J or -3.34 kJ

Water has lost -3.34 kJ of heat energy to the salt, or salt has gained + 3.34kJ of heat energy from water. So energy gained by salt is + 3.34 kJ.

Change in enthalpy = Q gained by salt / # of moles of salt

= +3.34 kJ / 0.1 mol = +33.4 kj /mol