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# Thermodynamics & Phase Change - PowerPoint PPT Presentation

Thermodynamics & Phase Change. Heat can only do one thing at a time: either change the temperature OR change the state!. Changes of state – seen in phase change diagram. temperature. heat added.

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### Thermodynamics & Phase Change

temperature

• Diagonal regions: temperature where thermal energy is added and particles increase in motion (temperature changes)

• Flat regions: where changes in state occur. All added thermal energy goes to overcome the forces holding the particles together.

• Heat of fusion temperature – The amount of energy required to melt one kilogram of a substance

• Heat of vaporization – the amount of energy required to vaporize one kilogram of a substance

Heat of Fusion & Vaporization for Water temperature

• Heat of fusion for water:

• Hf = 3.34x105 J/kg

• positive when melting, negative when freezing

• Heat of vaporization for water:

• Hv = 2.26x106 J/kg

• positive when evaporating, negative when condensing

• Heat required to melt: temperature

Q = mHf

*use for melting & freezing!

• Heat required to vaporize:

Q = mHv

*use for evaporating & condensing!

Specific Heat Values temperature

• Cice = 2060 J/kg·˚C

• Cwater = 4180 J/kg·˚C

• Csteam = 2020 J/kg·˚C

• Use these values for temps in Celsius or Kelvin!

Example #1 temperature

• You are asked to melt 0.100 kg of ice at its melting point and warm the resulting water to 20.0 ºC. How much heat is needed?

Example #1 Strategy: temperature

• Calculate heat needed to melt the ice.

Q1 = mHf

• Calculate heat needed to raise water temperature.

Q2 = mCΔT

• Add all heats together to get the total heat needed.

Q = Q1 + Q2

Example #1 Solution: temperature

• Q1 = 33400 J

• Q2 = 8360 J

• Q = 41,760 J

Example 2 temperature

• How much energy will be used to convert a 3.5 kg block of ice at 240 K into steam at 390K?

Example 2 temperature

• First, look at your phase change diagram and locate where you are starting in the process and where you are ending.

⁰C = K - 273.15 temperature

Q=mCΔt

Δt=tf-ti

mCΔta = -mCΔtb

Water Phase Change Diagram

Water Vapor

Steam

Q=m(2020)Δt

Q=m Hv

Hf = 3.34 x 105

Hv = 2.26 x 106

100⁰ C 373K

Temperature

Liquid Water

Q=m(4180)Δt

0⁰ C 273K

Q=m Hf

Solid Water/Ice

Q=m(2060)ΔT

Heat

Ex. 2 Cont’d temperature

• Then write down all the formulas you will need

Q=3.5(2060)(33)

Q=3.5(3.34x10^5)

Q=3.5(4180)(100)

Q=3.5(2.26x10^6)

Q=3.5(2020)(17)

1 temperature st Law of Thermodynamics

• The total increase in the thermal energy of a system is the sum of the heat added to it and the work done on it.

• Heat engines are devices which convert thermal energy to mechanical energy continuously along with some waste heat.

2 temperature nd Law of Thermodynamics

• Entropy of the universe is always maintained or increased. All things will become more and more disordered unless some action is taken.

2 temperature nd Law of Thermodynamics

• Entropy is a measure of the disorder in a system.

• Entropy increases when heat is added and decreases when heat is removed.

Thermal Engine efficiency temperature

• Called Carnot efficiency, or the ideal efficiency of a heat engine

• (Thot-Tcold)/Thot = Ideal efficiency

• Temperatures must be in Kelvin.

• Result is a percentage.