# How do you find heat of formation of a solution?

Mar 17, 2014

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.

Step 1. Measure the mass of solute in grams using a balance. I am dissolving solute Sodium Hydroxide. The mass I have taken is 4 g or 0.1 moles.

Step 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.

Step 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.

Step 4. Add 4 g of Sodium Hydroxide to the water. Dissolve the sodium Hydroxide using a stirrer and also record the temperature of solution. Let us assume that the final temperature of the solution is ${48}^{o}$ C

Step 5. In this dissolution experiment Sodium hydroxide dissolves in water and gives off heat energy to the surrounding water , which causes the temperature of water to go up from ${27}^{o}$ C to ${48}^{o}$ C. The change in temperature is ${48}^{o}$ C - ${27}^{o}$ C = ${21}^{o}$ C

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

Step 7 Q = 100 g . 4.18 J / ${g}^{o} C$. ${21}^{o}$ C

Q = 8778 J or 8.778 kJ

Water has gained 8.778 kJ of heat energy from the salt, or salt has lost 8.778 kJ of heat energy to water. So energy lost by salt is -8.778 kJ.

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

= -8.778 kJ / 0.1 mol = -87.88 kj /mol