Transcript energy - Pleasantville High School

Matter and Energy
Matter and energy interact and cause
changes in matter.
Energy
Definition: the ability to do work
Is NOT matter
Is measured in the unit Joules (J)
Can be POTENTIAL or KINETIC
Potential Energy
Energy that is stored (i.e. in a chemical
bond).
Something has the “potential” to do
some kind of work
Example: the child at the top of the
slide has potential energy
REGENTS QUESTION:
Given the balanced equation:
F2 + energy → F + F
Which statement describes what occurs during
this reaction?
(1) Energy is absorbed as a bond is formed.
(2) Energy is absorbed as a bond is broken.
(3) Energy is released as a bond is formed.
(4) Energy is released as a bond is broken.
Kinetic Energy
Energy of motion
Example: the child going down the slide
now has kinetic energy
REGENTS QUESTION:
The graph below represents the relationship between time and
temperature as heat is added at a constant rate to a sample of a
substance. During interval AB, which energy change occurs for the
particles in this sample?
(1)The potential energy of the particles increases.
(2) The potential energy of the particles decreases.
(3) The average kinetic energy of the particles increases.
(4) The average kinetic energy of the particles decreases.
So why do we want to study
energy?
Energy = the ability to
do work!
Having something do
work is great!
Examples: our bodies
need energy to do
work, our cars rely on
energy to do work, our
homes need to be
heated via energy,
etc…
Different types of energy…
Light  is waves, visible or invisible
Electrical involves moving electrons
Heat movement of molecules
Chemical is contained in foods
Nuclear responsible for the sun
Sound waves/vibrations of molecules
Mechanical involves moving objects
Magnetic opposing poles
REGENTS QUESTION:
Three forms of energy are
(1) chemical, exothermic, and temperature
(2) chemical, thermal, and electromagnetic
(3) electrical, nuclear, and temperature
(4) electrical, mechanical, and endothermic
Light
Responsible for colors
Responsible for sight
We have found ways to use light to
improve how much we can see and
what we see (example: TV)
Chemical and Electrical Energy
Energizes everything from
remote controls to cars.
Chemical Energy
Chemical energy from
crude oil (natural,
non-renewable
resources) is used to
heat our homes and
run factories that
produce consumer
goods.
Law of Conservation of Energy
Energy, like matter, is neither created
nor destroyed, rather it is converted.
Examples of Conservation of Energy
When you watch TV, it starts as electrical
energy and converts to radiant and sound
energy.
The radiant energy (or light energy) goes into
your eye and converts to electrical energy in
your nerves and then to the brain.
The sound energy (vibrations) go to your ear
drum where is vibrates sending electrical
impulses to the brain.
REGENTS QUESTION:
Which quantities must be conserved in
all chemical reactions?
(1) mass, charge, density
(2) mass, charge, energy
(3) charge, volume, density
(4) charge, volume, energy
Summary:
What will we study in this unit?
What is heat? How is it different from
temperature?
How does energy relate to chemical
reactions?
How energy relates to phase changes?
REGENTS QUESTION:
Which temperature change indicates an
increase in the average kinetic energy
of the molecules in a sample?
(1) 15°C to 298 K
(2) 305 K to 0°C
(3) 37°C to 273 K
(4) 355 K to 25°C
Understanding Heat Flow
Heat (q) is defined as the energy that
transfers from one object to another.
Heat flows from warm  cool.
What will happen if the two objects are
touching? (example)
REGENTS QUESTION
A student made a copper bracelet by hammering a
small copper bar into the desired shape. The bracelet
has a mass of 30.1 grams and was at a temperature of
21°C in the classroom. After the student wore the
bracelet, the bracelet reached a temperature of 33°C.
Later, the student removed the bracelet and placed it
on a desk at home, where it cooled from 33°C to 19°C.
Explain, in terms of heat flow, the change in the
temperature of the bracelet when the student wore
the bracelet. [1]
Heat Energy vs. Temperature
Temperature is measure of the heat flow.
Temperature is a measure of the average
kinetic energy of the particles in matter.
REGENTS QUESTION:
The temperature of a sample of matter is
a measure of the
(1) total kinetic energy of the particles in the
sample
(2) total potential energy of the particles in
the sample
(3) average potential energy of the particles
in the sample
(4) average kinetic energy of the particles in
the sample
Heat Energy and Changes in Matter
In virtually all changes in matter,
energy is released or absorbed.
System vs. Surroundings (together they
make the universe).
Examples
REGENTS QUESTION:
A few pieces of dry ice, CO2(s), at 78°C are placed in a
flask that contains air at 21°C. The flask is sealed by
placing an uninflated balloon over the mouth of the
flask. As the balloon inflates, the dry ice disappears and
no liquid is observed in the flask.
State the direction of heat flow that occurs between
the dry ice and the air in the flask. [1]
Write the name of the process that occurs as the dry
ice undergoes a phase change in the flask. [1]
Exothermic Reactions (Changes)
Exothermic reactions
RELEASE ENERGY (i.e.
explosions).
A good way to
remember this is to
associate “EXO” with
“OUT”.
Has a –q value because
heat is leaving the
system.
Heat is a product.
Endothermic Reactions (Changes)
Endothermic reactions ABSORB ENERGY
A good way to remember this is to
associate “ENDO” with “INSIDE”.
Has a +q value because heating is
entering the system.
Heat is a reactant.
REGENTS QUESTION:
What occurs when two fluorine atoms
react to produce a fluorine molecule?
(1) Energy is absorbed as a bond is broken.
(2) Energy is absorbed as a bond is formed.
(3) Energy is released as a bond is broken.
(4) Energy is released as a bond is formed.
Activation Energy
Sometimes reactions can’t occur on their
own (they can be exothermic).
They need a little input of energy to get it
started.
This energy is called ACTIVATION ENERGY.
Can you think of a common example of a
reaction that requires activation energy?
Energy and Phase Changes
Energy of particles of matter relates to
the phase or state of matter (solid,
liquid, or gas)
Therefore, changes in energy result in
changes of matter.
Let’s review what we already know…
Phase Changes
Other
terms
Change in
phase
Endo or
Exo
Sign of
ΔH or q
Endo
+
Melting
Liquefying S  L
Freezing
Solidifying
L S
Exo
-
Vaporization
Boiling
LG
Endo
+
Condensation
_______
GL
Exo
-
Sublimation
_______
SG
Endo
+
Monitoring Energy in Phase
Changes
Have you ever sat and
watched a pot or kettle of
water boil?
Did you ever wonder how
energy from the stove
causes the water to
change phase?
Have you ever thought
about PE and KE changes
in the process of heating
water?
Heating Curve of Water
F
D
B
A
C
E
What changes in phase are
occurring…
AB- solid, ice
BC- melting
CD- liquid
DE- boiling
EF- gas
REGENTS QUESTION:
The graph below represents the uniform
heating of a substance from the solid to the
gas phase. Which line segment of the graph
represents boiling?
(1) AB—
(2) CD—
(3) BC—
(4) DE—
What changes in energy are
occurring…
AB- increase in KE (as evident by
increase in temperature)
BC- no change in KE, but continually
adding heat, so increase in PE
CD- increase in KE
DE- increase in PE
EF- increase in KE
REGENTS QUESTION diagram
REGENTS QUESTION:
A sample of a substance is a liquid at 65°C. The
sample is heated uniformly to 125°C. The
heating curve for the sample at standard
pressure is shown below.
Determine the boiling point of the sample at
standard pressure. [1]
State what happens to the potential energy of
the particles of the sample during time
interval BC. [1]
What is specific heat capacity?
The amount of heat energy required to
raise 1 unit of mass of a substance by 1
unit temperature.
C=4.18 J/g°C (specific heat capacity of
water). Table B
Heat of Fusion
Heat of fusion (Hf) = amount of heat
energy absorbed or released when
melting or freezing.
See Reference Table B for values
Ex: Hf H2O = 334 J/g
Heat of Vaporization
Heat of vaporization = heat absorbed or
released when vaporizing or
condensing.
See Reference Tables for values.
Ex: HVH2O = 2260 J/g
Kinetic Theory of Heat
Molecules and atoms are constantly in
motion, even in the SOLID phase.
They are said to have kinetic energy or
the energy of motion.
As the energy of the particles increases,
temperature increases.
Heat Calculations and
Phase Changes
One can calculate how much heat is absorbed
or released.
Potential energy
If the problem says…
Melt/freeze
Vaporize condense
At 0°C (melting/freezing point)
At 100°C (boiling/condensing point)
Use
Q = mHf (melting/freezing) or
Q = mHv (vaporization/condensation)
Kinetic energy
If the problem says…
Temperature changes
Increase in temp from __ to __
Decrease in temp from __ to __
Heat a liquid/solid
Cool a liquid/solid
Use
Q = mCΔT
Examples
How much heat is needed to melt 10.5
grams of ice at 0°C?
Use Q = mHf
Q = (10.5 g) (334 J/g)
Q= 3507 J
Examples
What mass of liquid water can be
vaporized if 680 J of heat energy is added
at 100°C?
Use Q = mHv
680 J = (m) (2260 J/g)
m= 0.30 g
Examples
How much energy is needed to increase
the temperature of 5.0 grams of water
from 0°C to 10°C?
Use Q = mCΔT
Q = (5.0 g) (4.18 J/g°C)(10-0)
Q= 209 J
Examples
What is the mass of water that can be
increased in temperature by 15°C by the
addition of 800 J?
Use Q = mCΔT
800 J = (m) (4.18 J/g°C)(15°C)
m= 12.8 g
Calorimetry
Used to measure amount of heat
released or absorbed during a
chemical/physical change that occurs in
water solution.
“calorimeter” is used to measure the
change in temperature of water
surrounding a reaction.
Cheap Calorimeter- insulation
REGENTS QUESTION diagram
REGENTS QUESTION:
Base your answers to questions1 through 3 on the information
below and on your knowledge of chemistry.
Starting as a solid at 25°C, a sample of H2O is heated at a constant
rate until the sample is at 125°C. This heating occurs at standard
pressure. The graph below represents the relationship between
temperature and heat added to the sample.
1. Describe what happens to both the potential energy and the
average kinetic energy of the molecules in the H2O sample
during interval AB. [1]
2.Using the graph, determine the total amount of heat added to
the sample during interval CD. [1]
3. Explain, in terms of heat of fusion and heat of vaporization,
why the heat added during interval DE is greater than the heat
added during interval BC for this sample of water. [1]