Topic: Calculating Energy and H › Uploads › 382 › Q Equations Notes.pdf · 2017-08-14 · •for pure substance in single phase can calculate how much E needed using: Q = mC∆T

Q equations.notebook

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October 01, 2015

Sep 23­9:51 AM

What is the melting point of this substance?

The boiling point?

Just to review before we start…

Sep 23­9:51 AM

Topic: Calculating Energy Changes at Phase Changes

(Hv and Hf)

Sep 23­9:51 AM

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!

Sep 23­9:54 AM

Sep 23­9:51 AM

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 

Sep 23­9:51 AM

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

Q equations.notebook

2

October 01, 2015

Sep 23­9:55 AM Sep 23­9:51 AM

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

• 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)

Sep 23­9:51 AM

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

Sep 23­9:51 AM

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

• Q = mHv

• Q = (10 g)(2260 J/g) = 22600 J

Sep 23­9:51 AM

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

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

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

Sep 23­9:51 AM

Heating curve of H 2O

Q equations.notebook

3

October 01, 2015

Sep 23­9:51 AM

3 equations for Q1. 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

Sep 23­9:51 AM

Q = mC∆T• Temperature changed

• T ↑

• T ↓

• Initial temperature

• Start temperature

• Final temperature

• Ending temperature

• From __ ˚C to __ ˚C

• Water

Sep 23­9:51 AM

Q = mHf• Ice

• Freezing

• Melting

• Occurs at 0°C (for H2O)

• At constant temperature

Sep 23­9:51 AM

Q = mHv• Steam

• Boiling

• Condensation

• Occurs at 100°C (for H2O)

• At constant temperature

Sep 23­9:51 AM

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