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Thermodynamics & Phase Change

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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

- Diagonal regions: 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 – 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 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:
Q = mHf

*use for melting & freezing!

- Heat required to vaporize:
Q = mHv

*use for evaporating & condensing!

- Cice = 2060 J/kg·˚C
- Cwater = 4180 J/kg·˚C
- Csteam = 2020 J/kg·˚C
- Use these values for temps in Celsius or Kelvin!

- 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?

- 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

- Q1 = 33400 J
- Q2 = 8360 J
- Q = 41,760 J

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

- 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

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

- 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)

- 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.

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

2nd 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.

- 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.