Download U3 S1 L2 q=mct

U3 S1 L2
Calculating heat change!
Textbook Readings MHR
•pages 628-629: Temperature Change and Heat
•pages 629-630: Transfer of Kinetic Energy
•page 632: Mass and Energy Transfer
•page 632: Type of Substance and Energy Transfer
•pages 633-634: Calculating Heat Transfer
•pages 635-636: Heat Capacity
Textbook Practice Items MHR
•page 631: Thought Lab
•page 634: items 1, 2, 3 and 4
•page 636: items 6, 7, 8, 9 and 10
•page 638: items 4, 5, 6 and 7
•page 706: item 2
•page 707: items 15 and 20
Upon completion of this lesson, you should be able to:
• identify and explain that energy can be exchanged between a
system and its surroundings through heat gained or lost by a
system.
(qsystem = -qsurroundings)
• state and apply the first law of thermodynamics
• define joule, heat capacity and specific heat capacity
• identify that the amount of heat an object gains from or loses to its
surroundings is a function of the nature of its matter, its temperature,
and its mass
• perform calculations involving heat capacity and specific heat
capacity
1st Law of thermodynamics
• The total energy of the universe is constant.
• Energy can neither be created nor destroyed
∆Euniverse = 0
• Energy can not be lost or gained but simply transferred from
one body to another.
• (Energy can also be converted into other forms)
∆Euniverse = ∆Esystem + ∆Esurroundings
∆Esystem = - ∆Esurroundings
• That is;
– Energy is transferred from the system to the surroundings (or vice
versa)
• In this course we will be focusing on heat
energy. (q)
qsystem = - qsurroundings
qsys = - qsurr
• Units of Heat energy:
• The SI unit of heat is the Joule (J)
• Calorie = the energy it takes to heat 1.0 g of water 1oC
1 Calorie = 4.184 Joule
1 Cal = 4.184 J
Factors Affecting Heat Transfer
1.
Mass (the amount of material).
•
2.
When all other variables are equal, an object with higher mass can absorb or
release more energy than an object of lower mass.
Nature of material (Depends on what it is)
•
Heat capacity, C, is the amount of heat needed to cause a 1°C change in
temperature.
–
•
•
3.
It is measured in J/°C.
Specific heat capacity, c, is the energy needed to change the temperature of
1 g of a substance by 1°C
•
For example, the specific heat capacity of water is 4.184 J/g°C, whereas the
specific heat capacity of ethanol is 2.01 J/g°C.
•
See page 632, table 16.1 of your MHR textbook.
When all other variables are equal, a substance with higher specific heat
capacity can absorb or release more energy than a substance with lower
specific heat
Temperature change.
∆T = Tfinal - Tinitial
Different masses of water
Equal masses of canola oil and water
Comparing specific heat capacities
•
Given 1.0 g of each of the following which substance absorbs the most energy per
gram
• Cu
•
•
.
Fe
Al
H20(l)
Based on this data of the three metals, which one would be best to use to
make an air cooled snowmobile engine?
What is the advantage of having a liquid cooled engine?
Putting it all together.
q = mc∆T
where;
•
•
•
•
q = heat energy measured in Joules (J)
m = mass measured in grams (g)
c = specific heat capacity mesured in (J/g•oC)
∆T = change in temperature ∆T = Tfinal - Tinitial
Sign Convention
• a positive q value indicates that a substance (system) has absorbed
heat from its surroundings (an endothermic change)
• a negative q value indicates that a substance (system) has released
heat to its surroundings (an endothermic change)
Density
• the density of pure water @ 25oC is 1.0 g/mL
mass
density 
volume
m
d
v
Sample Exercise 1
• A student is required to use 225 mL of hot water in a lab
procedure. Calculate the heat change involved in raising the
temperature of 225 mL of water from 20.0°C to 100.0°C
Sample Exercise 2
• 175 g pieces of iron and aluminum are placed in a hot water bath so that
they are warmed to 99.7°C. The metal samples are removed and cooled to
21.5°C. Which sample undergoes the greater heat change?
• Heat capacity values are determined for specific devices or pieces of
equipment that undergo heat changes.
– For example a certain frying pan may be known to have a heat capacity of
1.20 kJ/°C.
– You may notice that there is no mass in this value!
• The heat change that it undergoes as it warms up is determined by
multiplying the heat capacity by its temperature change, q = C∆T
Sample Exercise 3
• A frying pan with a heat capacity of 1.20 kJ/°C is heated from 22°C to
198°C. Calculate the heat change.
Sample Exercise 4
• Calculate the specific heat capacity of an alloy if a 15.4 g
sample undergoes a heat change of 393 J when it is
heated from 0.0°C to 37.6°C.
1. Calculate the heat change involved when 2.00 L of water is heated from
20.0°C to 99.7°C in an electric kettle.
2. Calculate the heat change associated with cooling a 350.0 g aluminum bar
from 70.0°C to 25.0°C. Is the change endothermic or exothermic? Why?
(Hint: what is the sign of your answer?)
3. Calculate the heat change that occurs when 950.0 mL of water is heated
from 3.0°C to 95.0°C on a propane camp stove.
4. A 63.5 g sample of an unidentified metal absorbs 355 J of heat when its
temperature changes by 4.56°C.
a. Calculate the specific heat capacity of the metal.
b. Use a table of specific heat capacity values to identify the metal.
5. Calculate the specific heat capacity of titanium if a 43.56 g sample absorbs
0.476 kJ as its temperature changes from 20.13°C to 41.06°C.