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.

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Thermodynamics & Phase Change

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Thermodynamics phase change

Thermodynamics & Phase Change


Thermodynamics phase change

  • Heat can only do one thing at a time: either change the temperature OR change the state!


Thermodynamics phase change

  • Changes of state – seen in phase change diagram

temperature

heat added


Thermodynamics phase change

  • 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.


Thermodynamics phase change

  • 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 vaporization for water

Heat of Fusion & Vaporization for Water

  • 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


Thermodynamics phase change

  • Heat required to melt:

    Q = mHf

    *use for melting & freezing!

  • Heat required to vaporize:

    Q = mHv

    *use for evaporating & condensing!


Specific heat values

Specific Heat Values

  • 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

Example #1

  • 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

Example #1 Strategy:

  • 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

Example #1 Solution:

  • Q1 = 33400 J

  • Q2 = 8360 J

  • Q = 41,760 J


Example 2

Example 2

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


Example 21

Example 2

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


Thermodynamics phase change

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


Ex 2 cont d

Ex. 2 Cont’d

  • 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 st law of thermodynamics

1st 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 nd law of thermodynamics

2nd 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.


Thermodynamics phase change

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.


Thermal engine efficiency

Thermal Engine efficiency

  • 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.


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