Enthalpy and Phase Change

Phase changes and Latent Heat

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Equilibrium

• two opposing changes happen at an equal rate

• Pressure helps to characterize gas. P= F/A. unit=N/m²

• 1 atm, 101.325 kPa,101325 Pa,760 torr, 760 mm Hg- all pressure units

• Equilibirum vapor pressure- pressure of a gas in equilibrium with a liquid

• Boiling Point- vapor pressure= atmospheric pressure

Open Ended

Discussion: Does water boil at 100 degrees C at the top of a mountain? Why?

Open Ended

Why does evaporation keep you cool?

Latent heat

• thermal energy required to change 1 kilogram of a substance from one phase to another, L_f

• Latent heat of fusion- heat needed to freeze a liquid

• Latent heat of vaporization- heat needed to boil, L_v

• Latent heat of sublimation- heat needed to boil a solid into a gas, L_s

• heat needed to change phase= mass x latent heat or Q= mL

Open Ended

Does temperature change while something is inequilibrium between two states?

Open Ended

Does boiling water or steam produce a more severe burn?

Fill in the Blank

How much thermal energy must be removed from .72 kg of water at 0 degrees C to make ice cubes at 0 degrees C? Latent heat of fusion for water is 33.5 x10⁴ J/kg

Calculate the thermal energy required to change .55 kg of ice at -20 C to liquid water at 20 C

• specific heat of ice is= 2090 J( kgx C)

• latent heat of fusion for water= 33.5 x 10⁴ J/kg

• specific ehat of water= 4186 J(kgxC)

Open Ended

Why is placing an ice cream mixture in a bag in a bowl filled with ice and salt effective at freezing the mixture