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What is the melting point of this substance? - PowerPoint PPT Presentation


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Just to review before we start…. What is the melting point of this substance?. 50˚C. The boiling point?. 100˚C. Topic: Calculating Energy Changes at Phase Changes (H v and H f ). It takes energy to heat stuff up!.

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What is the melting point of this substance?

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Just to review before we start…

What is the melting point of this substance?

50˚C

The boiling point?

100˚C


Topic: Calculating Energy Changes at Phase Changes (Hv and Hf)


It takes energy to heat stuff up!

  • for pure substance in single phase - can calculate how much E needed using:

    Q = mCT

    • Q = energy in Joules

    • m = mass in grams

    • C = specific heat capacity

    • T = change in temperature = Tf - Ti

  • on other hand, when something cools down, energy is released!


Temperature

Time

Q = mCliquidT

Q = mCgasT

Q = mCsolidT

IV

I

II

III

V

  • C = specific heat capacity

    (amount heat required to raise temp of 1g of pure substance by 1C)

  • C is a physical constant

    • unique for every pure substance

    • CAN YOU FIND THE SPECIFIC HEAT OF WATER ON YOUR REFERENCE TABLE?


Temperature

Time

Why can’t I use Q = mCT for II and IV??

IV

I

II

III

V

Because T = 0, temp isn’t changing!!!!


So, how do we calculate the amount of energy required during a phase change?

  • HF = Heat of Fusion (Q = mHF)

  • HV = Heat of Vaporization (Q=mHV)

  • We use one of these two constants instead of specific heat and delta T

  • Q = mCT


Hf =Heat of Fusion

is amount energy required to change 1 gram

of pure substance from solid to liquid at its MP (meaning you aren’t changing the temperature)

Is a physical constant

Check out Reference Table B, what is the heat of fusion for water?

The Equation Q = mHf


How much heat is absorbed when 10 grams of ice melts at 0oC?

Heat absorbed =

mass of substance x heat of fusion of substance

Q = mHf = (10 g)(334 J/g) = 3340 J

Where does this energy go?

Particles must overcome forces of attraction to move farther apart during phase change (s → l)


HV = Heat of Vaporization

is the amount energy required to convert 1 gram

of pure substance from liquid to gas at its BP (meaning you aren’t changing the temperature)

Is a physical constant

Check out Reference Table B, what is the heat of vaporization for water?

The Equation Q = mHv


How much energy does it take to vaporize 10 g of water?

Q = mHv

Q = (10 g)(2260 J/g) = 22600 J


It takes a lot more energy to go from liquid to gas than from solid to liquid. Why?

H2O changing from liquid to gas requires 22,600J/g

H2O changing from solid to liquid requires 3,340J/g

* greater energy required to change from liquid to gas because particles are spreading farther apart!


Temperature

Time

Q = mCsolidT

Q = mCLIQUIDT

Q = mCgasT

Q = mHV

Q = mHF

IV

I

II

III

V


Heating curve of H2O


3 equations for Q

1. Q = mCT

2. Q = mHf

3. Q = mHv

figure out which to use

depends on section of heating curve

look for hints in word problem


Q = mCT

Temperature changed

T ↑

T ↓

Initial temperature

Start temperature

Final temperature

Ending temperature

From __ ˚C to __ ˚C

Water


Q = mHf

Ice

Freezing

Melting

Occurs at 0C (for H2O)

At constant temperature


Q = mHv

Steam

Boiling

Condensation

Occurs at 100C (for H2O)

At constant temperature


heating rate = 150 J/min

If the substance takes 4 minutes to melt, how much heat energy was used to melt it?

150J/min x 4min = 600J


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