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CHOPPED-UP CHAPTER
We’ll cut through this one. :)
ENERGY
CHAPTER 10
WHAT DO YOU KNOW?
What first comes to your mind when you hear the term energy?
What do calories measure?
List three sources of energy.
Have you heard of the term entropy? What does it mean?
HERE WE WILL...
understand the general properties of energy
understand the concepts of temperature and heat
understand the direction of energy flow as heat
10.1 THE NATURE OF ENERGY
Energy is the ability to do work or produce heat
can be classified as kinetic or
potential
kinetic = energy due to motion
potential = energy due to position
one of the fundamental characteristics of energy is that it is conserved
it is changed in form but not created or destroyed in a typical rxn
this the law of conservation of energy
S
10.2 TEMPERATURE
AND HEAT
the temp of something is a
reflection of the vigor of the
random motions of the
components of that substance
heat is a flow of E between
two objects due to a temp
difference in the two objects
notice here that matter has a
boundary; energy does not
on the next slide, notice that there was a heat flow b/t the two sides
that were different T’s…
which way did it flow?...
the random motions of the components of an object constitute the
thermal energy of that object
the flow of energy called heat is the way in which thermal energy is
transferred from a hot object to a colder object
10.3 EXO- AND ENDOTHERMIC
what is this????
in an exothermic rxn,
E flows from system ➛ surroundings
in an endothermic rxn,
E flows from surroundings ➛ system
exo often feel warmer (like fire);
endo often feel cooler
FRINKING FRAP TIME
Exo or Endo process? (from point of view of bold things)
1. Your hand gets cold when you touch ice.
2. The ice melts when you touch it. (think!)
3. Ice cream melts.
4. Propane burns in a propane torch.
5. After swimming, water drops on your skin evaporate.
6. Two chemicals mixing in a beaker give off heat.
WALK AWAY WITH...1A
energy is conserved
the law of conservation of energy says
that energy is neither created nor destroyed in
a chemical process
energy can be changed from one form to
another but the total amt of E remains the
same
WALK AWAY WITH...1B
energy is classified as either kinetic or potential
kinetic due to motion, potential due to position
WALK AWAY WITH...2
temperature indicates the vigor of the random
motions of the components of that substance
heat is the flow of energy b/t two objects b/c
of a temp diff
exothermic is where heat flows from
system to surroundings
endothermic is opposite
HERE WE WILL...
understand how heat is measured
understand energy as a driving force
for natural processes
understand the basic concept of
entropy
10.4 THERMODYNAMICS
thermodynamics is the study of E and its changes
from Gk therme ϑέρµη (heat) + dynamikós
δυναµικός (force or power)
internal E of a system is the sum of the kinetic (due
to motion) and the potential (due to position) E’s of
a substance
10.5 MEASURING ENERGY CHANGES
a calorimeter [L. calor = heat] is used to
measure the heat of chemical reactions
units used are joules (but also calories)
(low joule products)
to measure E changes you need to know 1) the T change (∆˚C)
of your sample...
(which takes more energy: to heat this pool by 10˚C or by 20 ˚C?)
and 2) the amount of substance being heated (g)...
(Which takes more energy? to heat this cup of water 10˚C or the pool?)
and 3) the specific heat capacity of a substance
i.e. E required to change the temp of 1 gram by 1 degree C
water has an enormous heat capacity (4.18 J/g˚C),
sand not so much (0.8 J/g˚C)...
that’s why wet sand is so much cooler and easier to walk through on
a hot day than dry sand - same amount of sunlight, but the water
absorbs a lot of heat as its temp rises slooooowly
which is why we can do this
all three (mass, change in T, sp/heat/cap) can be used to calculate
the changes in energy that occur in reactions or special
circumstances… ready?
Q = sm∆T
Q = heat (J)
s = specific heat capacity (a constant, measured in J/g˚C)
m = mass (g)
∆T = change in T (˚C)
EXAMPLE
1.6 g of a mystery gold-like metal
requires 5.8 J of energy to change its T
from 23˚C to 41˚C. Is it gold?
(sAu = 0.13 J/g˚C)
solve for s, so:
s = Q/m∆T
s = 5.8/(1.6 x 18)
s = 0.20 J/g˚C, so not gold!
QUICK QUIZ
You want to heat a mug of tea (250 g) from
room temp, 25˚C to 100˚C.
How much E would you need?
(sH2O for water = 4.184 J/g•˚C)
Q = sm∆T
Q = (4.184)(250)(75)
Q = 78,000 J
Q = 78 kJ
10.10 ENERGY AS A
DRIVING FORCE
Why does a log burn to ash, but ash never gathers to make a log?
Why does a toot spread throughout a room, but not gather in one place
again?
two important things we’ve figured out in science:
nature likes to spread energy around
nature likes to spread matter around
If both energy spreading and matter spreading take place, your reaction will
go. (like an explosion)
If neither, then it won’t happen by itself. (like CO2 and H2O getting together to
make sugar)
If both are competing, like hydrogen and oxygen getting together (bad) but
giving off energy when it does (good!) it depends on other things like
temperature. (too complicated to figure out now)
but what is this “matter spreading” out business?...
ENTROPY
entropy (S) is a way of keeping track of the “randomness” of a process
the lower the entropy the more “ordered” it is (like ice or a Lego
creation)
the higher the entropy the more disordered (like steam or your room)
since the tendency is for E and matter to spread, the entropy of the
universe is increasing.
= second law of thermodynamics
thus the universe is headed toward total randomness. Woo oho!
:)
WALK AWAY WITH...1
thermodynamics is the study of energy and
its changes
the common units for heat are calories and
joules
the amount of heat (Q) involved can be found
Q = sm∆T
WALK AWAY WITH...2
natural processes occur in the direction that
leads to an increase in the disorder (increase in
entropy) of the universe
the principal driving forces for natural forces can
be described in terms of “matter spread” or
“energy spread”
10EOCS
14, 24, 26, 52
quiz time...
Chapter 10 Chop Quiz
1. Energy of position, that is, energy which depends on where
something is, is also called....
A. kinetic energy
B. potential energy
C. heat energy
D. enthalpy
Chapter 10 Chop Quiz
2. Temperature is
A. a measure of the potential energy stored in a substance.
B. the same as heat.
C. a measure of the random motion of the particles of a system.
Chapter 10 Chop Quiz
3. Which of the following processes is exothermic?
A. candle wax melting
B. a puddle evaporating
C. dry ice subliming (evaporating) to form gaseous carbon dioxide
D. propane burning in a propane torch
Chapter 10 Chop Quiz
4. Water has a relatively _____ specific heat capacity, meaning it
responds rather _____ to cooling or heating.
A. low, quickly
B. low, slowly
C. high, slowly
D. high, quickly
Chapter 10 Chop Quiz
5. What two events drive nature?
A. matter spreading, energy spreading
B. matter coming together, energy spreading
C. matter spreading, energy coming together
D. matter coming together, energy coming together
Chapter 10 Chop Quiz
6. What is the study of energy and its changes?
A. calorimetry
B. thermodynamics
C. the law of conservation of energy
D. calorodynamicsmittywerbermannjensen
Chapter 10 Chop Quiz
2 points each...
Q = sm∆T
7. Calculate the heat given off when 45.0 g of water cools from 45˚C to
22˚C. The specific heat of water is 4.184 J/g˚C
8. A sample of water requires 200 J to be heated 1.7˚C. What mass of
water is present?
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Turn in
answers...
Chapter 10 Chop Quiz
1. Energy of position, that is, energy which depends on where
something is, is also called....
A. kinetic energy
B. potential energy
C. heat energy
D. enthalpy
Chapter 10 Chop Quiz
2. Temperature is
A. a measure of the potential energy stored in a substance.
B. the same as heat.
C. a measure of the random motion of the particles of a
system.
Chapter 10 Chop Quiz
3. Which of the following processes is exothermic?
A. candle wax melting
B. a puddle evaporating
C. dry ice subliming (evaporating) to form gaseous carbon dioxide
D. propane burning in a propane torch
Chapter 10 Chop Quiz
4. Water has a relatively _____ specific heat capacity, meaning it
responds rather _____ to cooling or heating.
A. low, quickly
B. low, slowly
C. high, slowly
D. high, quickly
Chapter 10 Chop Quiz
5. What two events drive nature?
A. matter spreading, energy spreading
B. matter coming together, energy spreading
C. matter spreading, energy coming together
D. matter coming together, energy coming together
Chapter 10 Chop Quiz
6. What is the study of energy and its changes?
A. calorimetry
B. thermodynamics
C. the law of conservation of energy
D. calorodynamicsmittywerbermannjensen
Chapter 10 Chop Quiz
2 points each...
Q = sm∆T
7. Calculate the heat given off when 45.0 g of water cools from 45˚C to
22˚C. The specific heat of water is 4.184 J/g˚C
4300 J
8. A sample of water requires 200 J to be heated 1.7˚C. What mass of
water is present?
28 g
EOC ANSWERS