Thermodynamics NOTES SECTION: 10.4

10.4 Phase Change and Latent Heat

Particles move from one phase to another. When the particles “stop” moving, the system is in equilibrium.

Changing from one phase to another requires heat.

When a system is in a phase change, heat can be added and the temperature does not increase.

Example: You have an ice cube, when you add heat it starts to melt. The energy is goes into making the ice cube liquid (not increasing the temperature).

◦ Breaking bonds ◦ Increases potential energy of the liquid, not kinetic (why temperature doesn’t change)

10.4 Phase Change and Latent Heat

Latent heat is the heat (thermal energy) required to change 1 kilogram of a substance from one phase to another.

During this phase, the temperature of the substance does not change.

Heat to change phase= mass X latent heat

Q=mL

There are different values for the latent heat

Example:

Heat of fusion: liquid to a solid

Heat of vaporization: liquid to a gas

Heat of sublimation: solid directly to gas (ice sublimates at Mars)

Heating Curves The temperature of a substance in response to heating

Periods of no change in temperature (changing from one form to another) and rises in temperature.

To calculate total heat, add up all the energies required to get to a certain step.

Calculating Total Heat Follow the diagram and solve each step of the process.

Question: How much heat is required to change 0.55 kg of ice at -20 degrees Celsius to 20 degrees Celsius?

1. Find the thermal energy needed to warm ice from -20 degrees to 0 degrees. ◦ Use Q=cmΔT (ice)

2. Energy to melt the ice at O degrees◦ Use Q=mL

3 Energy to heat the water to 20 degrees Celsius◦ Use Q=cmΔT (water)

4. Add these values together.

Lesson Check 10.4 Page 374

Due Thursday:

Problems: 49, 53-56