Unit Eight: Heat
https://sites.google.com/site/hoyathermochemistry/
What is temperature?
• A measure of the average kinetic energy
of the particles of a sample; how fast the
particles are moving
• Common temperature units are
Fahrenheit, Celsius, and Kelvin.
• We’ll use Celsius.
Heat is NOT Temperature.
• Heat energy flows from areas of high energy to
areas of low energy.
• Heat will continue to flow until all areas have
reached an EQUAL temperature.
Heat is ENERGY that FLOWS.
What units measure heat?
•
•
•
•
•
calories
Calories (nutritional calories)
Joules
4.184 J = 1 cal
1000 cal = 1 Cal
The amount of heat that
flows can be measured.
Exothermic Reactions:
• Release heat energy to the
surroundings
• Cause test tubes to feel
hot
• Products have less heat
than the reactants
• Show an overall loss of
energy (negative heat
change)
Endothermic Reactions:
• Absorb heat energy from
the surroundings
• Cause test tubes to feel
cold
• Products have more heat
than the reactants
• Show an overall gain of
energy (positive heat
change)
Review: Exothermic &
Endothermic Reactions
Review: Exothermic &
Endothermic Reactions
• Look at the reaction described below:
2S + 3O2 --> 2SO3
∆H = -791.4 kJ
• Analyze the reaction:
1. Is heat absorbed or released?
2. What conversion factors could be written to include the
heat?
• Here is a synonymous reaction:
2S + 3O2 --> 2SO3 + 791.4 kJ
Enthalpy values in the product = EXOTHERMIC
Enthalpy values in the reactant = ENDOTHERMIC
Heat Stoichiometry
2S + 3O2 --> 2SO3
∆H = -791.4 kJ
2S + 3O2 --> 2SO3 + 791.4 kJ
• ∆H can be used as a conversion factor
with the coefficients from the equation.
• 2 mole S = 791.4 kJ lost
• 3 mole O2 = 791.4 kJ lost
• 2 mole SO3 = 791.4 kJ lost
Understanding the Equation
• How much heat will be released when 6.44 g of
sulfur reacts with excess O2 according to the
equation above?
2S + 3O2 --> 2SO3
∆H = -791.4 kJ
• Since you have a chemical reaction, you have to use
stoichiometry.
• Label the equation according to the question.
• Write the given and draw the chart.
• Change grams to moles LIKE ALWAYS.
• Use a ratio from the equation to convert moles to
energy.
Calculations Using ∆H As A
Conversion Factor
• Solve the 12-2 Practice Problems
in your packet.
Stoichiometry Practice
Heat flow in PHYSICAL CHANGES
• Phase changes involve a flow of heat that
can be measured.
• Solid to liquid to gas
• Melting & Evaporation
• Gas to liquid to solid
• Freezing & Condensation
Learning How to
Calculate Heat Flow
Heating Curve of Water
Endothermic graph
•
•
•
•
•
The graph shows heat
continually flowing into this
sample of water.
Solid = heat in causes particles
to increase in speed (ΔTemp)
Melting = heat in causes
intermolecular forces to weaken
Liquid = heat in causes
particles to increase in speed
(ΔTemp)
Boiling = heat in causes
intermolecular forces to break
Gas = heat in causes particles
to increase in speed (ΔTemp)
Conceptual Understanding
Two Equations Used
to Calculate
Heat Flow
q = mcΔT
q = molΔH
You must be able to
recognize a change in temp
vs. a change in heat content.
Quantitative Understanding
• Diagonal Lines:
• show heat causing
changes in
temperature
• q = mcΔT
• Plateaus:
• Show heat causing
changes in heat
content that
weaken/break
intermolecular forces
without affecting
temperature
• q = molΔH
Putting It All Together
• A 5.0 gram sample
of water at -40C is
heated to 140C.
How much heat is
required?
Let’s use the constants listed on the
front of your practice packet.
The Calculation
• Calculate ONE
segment at a time.
• Watch the units
carefully.
• Add the five segments
together in the end.
• What if the question had said, “A 5.0
gram sample of water at 140C is
cooled to -40C. How much heat is
released?”
• What would the graph look like?
• How would the calculation be different
for each segment?
• Would the graph be endothermic or
exothermic?
What if…?
• What if the question had said, “A 5.0 gram
sample of water at 140C is cooled to -40C. How
much heat is released?”
• What would the graph look like?
• The graph would start at 140C and have a negative
slope to 40C.
• How would the calculation be different for each
segment?
• The numbers would NOT change at all. However,
the changes in temperature and heat content
should be negative.
• Would the graph be endothermic or exothermic?
• The graph shows heat being lost or released out of
the sample so it is exothermic.
What if…? Answers
• A 3.6 gram sample of water is cooled from
75C to -5C. How much heat is released?
• Draw the x-axis (time) and y-axis (temp).
• Mark the highest and lowest temperatures on the
y-axis.
• Mark the phase change temperatures that will
occur within the temperature range on the y-axis.
• Draw the line segments from starting to ending
temperature. Be sure to show plateaus at the
phase change temps.
What if you had to draw
your own graph?
Temperature
75
A 3.6 gram sample of water is cooled
from 75C to -5C. How much heat is
released?
0
-5
Time
Check your work.
• Solve the Heating Curve of Water
calculations on the first page of your
practice packet. Give each answer to
4 sig figs.
• NOTE: The questions will take you
step-by-step through the process.
Suggested Homework
• Question 1:
segments.
• Question 2:
• Question 3:
• Question 4:
• Question 5:
• Question 6:
158.1J
• Question 7:
We’ll answer as we go through the
(3.1g)(2.1J/gC)(0C- -20C) = 130.2J
(0.17mol)(6.01kJ/mol) = 1.022kJ
(3.1g)(4.18J/gC)(100C-0C)= 1296J
(0.17mol)(40.7kJ/mol) = 6.919kJ
(3.1g)(1.7J/gC)(130C-100C) =
9525J or 9.525kJ
Check your HW: Heating Curve of Water
• Goal 1: With a small group, draw a heating or
cooling curve to represent a sample. Calculate
the heat required to make the
temperature/phase changes on the curve.
Everyone is responsible for the graph,
calculations, and explaining the process.
• Goal 2: In a new small group, explain your
graph and calculations. Learn about the graphs
and calculations for five more samples.
• Goal 3: Return to your desk to draw and
calculate a curve independently. Select
answers in I-Respond.
Heating Curve Jig-Saw
1)
2)
3)
4)
Assign each group member a number (1-4).
Draw the heating or cooling curve to reflect your sample.
Calculate the heat flow described in your question and curve.
Answer the following questions:
a) Is the sample undergoing endothermic or exothermic heat
flow?
b) Will the q value be positive or negative?
c) Will the ΔH value be positive or negative?
5) Discuss the curve and calculations as a group until ALL
members are comfortable explaining them to another student.
10 minute time limit
Goal 1 Group Instructions
1) Open your Goal 2 folder and pull out the six heating
or cooling curve examples.
2) Draw and discuss calculations of Curve 1. (5 minute)
3) Draw and discuss calculations of Curve 2. (5 minute)
4) Draw and discuss calculations of Curve 3. (5 minute)
5) Draw and discuss calculations of Curve 4. (5 minute)
6) Draw and discuss calculations of Curve 5. (5 minute)
30 minute time limit
Goal 2 Group Instructions
• A sample of 4.9 grams of water is
cooled from 114°C to -8°C. Give your
answers to four sig figs in Joules.
•
•
•
•
•
∆H fusion = 6.02 kJ/mol
∆H vap = 40.6 kJ/mol
C solid = 2.03 J/g°C
C liquid = 4.184 J/g°C
C vapor = 1.7 J/g°C
Independent Heating Curve
Question (10 minute time limit)
Is the sample undergoing
endothermic or exothermic heat
flow?
A.) endothermic
B.) exothermic
Will the q value be positive or
negative?
A.) positive
B.) negative
Will the ΔH value be positive or
negative?
A.) positive
B.) negative
How much heat is lost in the gas
segment of the graph?
A.) 949.6 J
B.) 287.0 J
C.) -139.3 J
D.) -116.6 J
How much heat is lost in the
condensation segment of the graph?
A.) -11,040 J
B.) 11,040 J
C.) 1.640 J
D.) -1.640 J
How much heat is lost in the liquid
segment of the graph?
A.) 833.0 J
B.) -2050. J
C.) -994.7 J
D.) 245.3 J
How much heat is lost in the freezing
segment of the graph?
A.) 29.50 J
B.) -11.04 J
C.) -1,637 J
D.) 198.9 J
How much heat is lost in the solid
segment of the graph?
A.) -79.58 J
B.) 66.64 J
C.) 164.0 J
D.) -4.420 J
Calculate the total heat required to
make the temperature change from
114C to -8C.
A.) -3894 J
B.) -2259 J
C.) -12,250 J
D.) -14,920 J
• A phase diagram gives the conditions of
temperature and pressure at which a substance
exists as solid, liquid, and gas.
• Each of the three regions represents a pure phase
(not a mix).
• Each line represents the temp & pressure
conditions where the phases exist in equilibrium and
phase changes occur.
• Triple point: set of conditions in which all phases
exist in equilibrium
Phase Diagram
• If you had a bottle of
X in your closet, what
state of matter would
it be in?
• At what temperature
and pressure will all
three phases exist
together?
• If I have a bottle of X
at 45 atm and 100C,
what will happen if I
raise the temperature
to 400C?
• Why can’t the
substance be boiled
at 200C?
Don’t forget to practice the phase diagram questions in your packet!
q = m c ΔT
Concept Video
Specific Heat Capacity
q = m c ΔT
• c= Amount of heat required to raise 1 g of the substance
by 1 degree Celsius. (J/gC)
• Specific to a substance; can be used to identify
substances as a result
• Example 10.4 A 1.6g sample of a metal that has the
appearance of gold requires 5.8 J of energy to change its
temperature from 23°C to 41°C. Is the metal pure gold?
c of Au = 0.129 J/gC.
• Specific Heat WS (Practice Packet) 1. A 15.75-g piece
of iron absorbs 1086.75 J of heat energy, and its
temperature changes from 25°C to 175°C. Calculate the
heat capacity of iron.
Specific Heat Capacity
Water
c = 4.184 J/g°C
∆Hfusion = 6.02 kJ/mol
∆Hvaporization = 40.6 kJ/mol
• Example 14.1 Calculate the energy required to
melt 8.5 g of ice at 0°C.
• Example 14.2 Calculate the energy (in kJ) required
to heat 25 g of liquid water from 25°C to 100°C and
change it to steam at 100°C.
• Section Review Question 7 Calculate the energy
required to change 1.00 mol of ice at -10°C to water
at 15°C.
Specific Heat Capacity in Other
Calculations
• So far, I’ve just told you that heat is added or
released from a substance. I haven’t included
where the heat is coming from or going.
• Example: A 25.0 g sample of pure iron at 85°C is
dropped into 75 g of water at 20°C. What is the final
temperature of the water-iron mixture?
• What direction will heat flow in this example? Fe to
H2O or H2O to Fe?
• When will the heat stop flowing?
• When we know about BOTH parties involved in heat
flow, we can calculate many variables.
Heat Exchange Between
Two Substances
• Heat will ALWAYS flow from hot to cold.
• Heat will ALWAYS stop flowing when the
same final temperature is reached.
• If the system is insulated, the amount of heat
lost by the hot substance will equal the
amount of heat gained by the cold
substance.
• qlost + qgained = 0
Understanding Heat Flow
Between Two Substances
• In analytical chemistry labs,
a calorimeter is used to
insulate heat exchange
situations.
• We’ll assume that any
exchanges calculated in here
are insulated in a
calorimeter.
• Therefore, qlost by the hot
substance will equal the
qgained by the cold substance.
qlost + qgained = 0
Insulated Heat Exchange
• A 25.0 g sample of pure iron at 85°C is
dropped into 75 g of water at 20°C. What is
the final temperature of the water-iron
mixture? (cFe = 0.45 J/gC; cH2O = 4.18J/gC)
• qlost + qgained = 0
• qFe + qH2O = 0
• (mcΔT)Fe + (mcΔT)H2O = 0
Calculating Heat Flow
Between Two Substances
Chemistry Thermo WS of Practice Problems
16. The specific heat capacities of Hf and ethanol
are 0.146J/gC and 2.45J/gC, respectively. A
piece of hot Hf weighing 15.6 g at a temperature
of 160.0C is dropped into 125 g of ethanol that
has an initial temperature of 20.0C. What is the
final temperature that is reached, assuming no
heat loss to the surroundings?
Another Example
• A sample of silver with a mass of 63.3 g is
heated to a temperature of 111.4ºC and placed
in a container of water at 17ºC. The final
temperature of the silver and the water is
19.4°C. Assuming no heat loss, what mass of
water was in the container? The specific heat of
water is 4.184 J/gºC, and the specific heat of
silver is 0.24 J/gºC.
• Mass = 139.3 grams
A Third Example
• An unknown substance at 152C is
dropped into H2O at 25C. The mass of
the unknown is 12g, and the water has
a mass of 100g. If the final
temperature of the mixture is 32C,
what is the specific heat capacity of the
unknown substance?
• C = 2.03 J/gC
Final Example
A rectangular aquarium, 37.4 cm by 30.7 cm by
67.7 cm, is filled with water at 13.5C. How much
energy is required to raise the temperature of the
water to 22.3C?
(1cm3 = 1 mL = 1 gram; cH2O = 4.18J/gC)
A.) 1,375 J
B.) 324,918 J
C.) 2, 859, 278 J
D.) Not enough information to calculate
How much heat does a 23.0 g ice cube
absorb as its temperature increases from
-17.4°C to 0.0 °C ? The specific heat of ice
is 2.1 J/gC.
A.) 840.42 J
B.) 1,673 J
C.) 84.0 J
D.) Not enough information
A sample of an unknown metal has a mass of
120.7 g. As the sample cools from 90.5 °C to
25.7 °C , it releases 7020J of energy. What is
the specific heat of the sample?
A.) -0.8975 J/gC
B.) 0.8975 J/gC
C.) 1.114 J/gC
D.) -1.114 J/gC
True or false. Temperature increases
as a sample of silver melts.
A.) True
B.) False
Calculate the heat required to vaporize 6.5g of
gold. The specific heat capacity of gold is 0.129
J/gC. The ΔHfus is 12.5 kJ/mol, and the ΔHvap is
334.4 kJ/mol.
A.) 0.4124 kJ
B.) 0.8385 kJ
C.) 11.03 kJ
D.) 0.0043 kJ
What are the temperature and pressure
coordinates of the triple point?
A.) 50 atm & 350C
B.) 90 atm & 750C
C.) 40 atm & 400 C
D.) 22 atm & -10C
• DO NOT SIT DOWN!
• DO NOT EAT CHIPS IN THE LAB AREA!
• EVAPORATING DISH WILL BE HOT!
AFTER THREE TRIALS,
• CLEAN UP YOUR STATION AND RETURN TO
THE CLASSROOM AREA.
• WORK WITH YOUR GROUP TO ANSWER THE
LAB QUESTIONS.
• ALSO, SOLVE THE “INTERPRETING GRAPHICS”
HANDOUT ON THE BACK OF YOUR PRACTICE
PACKET.
Lab Details
• So far, we’ve been analyzing temperature changes and
calculating the heat involved in these PHYSICAL
changes.
• Now, we are going to transition back to chemical
changes...chemical reactions. Look at the reaction
described below:
2S + 3O2 --> 2SO3
∆H = -791.4 kJ
• Analyze the reaction:
1. Is heat absorbed or released?
2. What conversion factors could be written to include the
heat?
Heat Stoichiometry
2S + 3O2 --> 2SO3
∆H = -791.4 kJ
2S + 3O2 --> 2SO3 + 791.4 kJ
• ∆H tells if the reaction is endothermic or
exothermic. + = endo; - = exo
• ∆H can be used as a conversion factor with the
coefficients from the equation.
• 2 mole S = 791.4 kJ lost
• 3 mole O2 = 791.4 kJ lost
• 2 mole SO3 = 791.4 kJ lost
• ∆H can also be written in the equation.
• - = exo = product
• + = endo = reactant
Understanding the Equation
• How much heat will be released when 6.44 g of
sulfur reacts with excess O2 according to the
equation above?
2S + 3O2 --> 2SO3
∆H = -791.4 kJ
• Since you have a chemical reaction, you have to use
stoichiometry.
• Label the equation according to the question.
• Write the given and draw the chart.
• Change grams to moles LIKE ALWAYS.
• Use a ratio from the equation to convert moles to
energy.
Calculations Using ∆H As A
Conversion Factor
• Solve the 12-2 Practice Problems
in your packet.
Stoichiometry Practice
• Use the specific heat capacity to identify your
unknown metal sample.
• What equation will you use to find the specific heat
capacity?
• So, what measurements should you make in the lab to
plug into the equation?
• You’ll have to use a second substance (water) to
calculate a q value to use in your equation.
• q lost by the metal + q gained by the water = 0
• A calorimeter is an instrument used to insulate the
heat exchange. We’ll build one for your lab.
Which is your metal?
Specific Heat Capacity Lab
• Hot Water Bath to Initially Heat the Metal
• Hot plate turned up to 10
• 500mL beaker of water
• Test tube containing metal with test tube
tongs
• Calorimeter to Measure Heat Exchange
• Two foam cups stacked
• 75mL of water inside
• Lid with a hole for thermometer and/or stirring
rod
• Thermometer to measure initial temp of water
Set Up
• Follow steps 1-3 to set up your calorimeter.
• Replace steps 4-7 with the following:
4. Add 300 mL of water to a 500 mL beaker, and place the
beaker on a hot plate.
5. While water is heating, put your sample into a test tube.
Use test tube tongs to hold the tube.
6. Place the test tube holding the metal sample in the hot
water bath. Allow the metal to remain in the water for three
minutes while the water boils. Record the temperature of
the boiling water.
7. After three minutes, quickly pour the metal sample from the
test tube into the 75 mL of water in the calorimeter.
• Follow steps 8-10 to complete the lab.
Procedure
• Answer the “Analysis and Conclusions”
questions.
• If the question requires a calculation, please
show all of your work. Be organized so that
you can receive full credit. Do not leave off
units in your answer!
• Each group member will turn in his/her own
lab report.
• Questions will be posted on the blog for those
who do not finish in class.