# Thermochemistry of Phase Changes

Thermochemistry | Hess's Law, Enthalpy and Heating/Cooling Curve.

Tip: This isn't the place to ask a question because the teacher can't reply.

## Key Questions

Thermochemistry is used to calculate the amount of energy required to change from state to state.

#### Explanation:

For example, if you need to change from solid water to liquid, there is a constant known as heat of sublimation (specific heat), which has been experimentally determined. However, there is a constant for all liquids and that can be derived with simple algebra. Without making things complicated, here is a table to help guide you:

• When a compound is changing phase, i.e. it is in a phase transition, it is in equilibrium. That means $\Delta {G}_{\text{trs}} = 0$. So:

cancel(DeltaG_"trs") = DeltaH_"trs" - T_"trs"DeltaS_"trs"

$\textcolor{b l u e}{\Delta {H}_{\text{trs" = T_"trs"DeltaS_"trs}}}$

If there was more you were looking for to your answer, please clarify.

• 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 Sodium Hydroxide. The mass I have taken is 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 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

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

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 / ${g}^{o} C$. ${21}^{o}$ 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

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