Download Energy - Chemistry R: 4(AE)

Energy
• Without energy, your body will not function,
technology will not exist, and there probably
would not be life on earth.
• We eat food to obtain energy, part of this
source is through the combustion of glucose.
• C6H12O6 + 6O2  6CO2 + 6H2O + energy
Energy
• Energy = the capacity to do some kind of work
– For example: moving an object, forming a new
compound, generating light
• SI unit = joule (J)
Remember changes in matter
• Physical change = only the physical properties of
matter are effected
– Ex: ice melting, water boiling
• Chemical change = a new substance is made
– Ex: chemical rxns
• Every change in matter involves a change in
energy!
– Sometimes energy is supplied (melting/evaporating)
– Sometimes energy is released (condensation/freezing)
Conservation of Energy
• During any physical or chemical change, the
total quantity of energy REMAINS CONSTANT!
• Energy can not be destroyed or created!
• Energy is being transferred between the
reaction system and its surroundings
The Law of the Conservation of Energy
• Energy is neither created nor destroyed
during a chemical reaction or a physical
change. It is just changed from one form to
another.
Forms of Energy
• Energy can be chemical (KE/PE), mechanical,
light, heat, electrical, sound, etc.
– Ex: photosynthesis – the chlorophyll in the plant’s
cells (system) absorb light energy from the sun
(surroundings) – endothermic
– Ex: light stick – chemicals react inside the stick
(system) to produce light energy (surroundings) –
exothermic
Chemical energy
• Chemical energy = total energy stored in
matter as kinetic energy or potential energy
• Kinetic energy = energy of motion
• Potential energy = stored energy
Kinetic Energy (KE)
• Kinetic Energy = the energy an object has due
to its motion
– For Ex: A hockey puck gains KE through a tap, or a
slam. The puck has energy that is why goalies
wear a mask!
• The amount of kinetic energy depends on the
velocity of the object and it’s mass.
Potential Energy (PE)
• Potential Energy = energy possessed by objects
due to their position (No apparent evidence of
energy is observed)
• One form of energy can be transformed into
other forms of energy.
– For example: a spring :
• Pushed down, all coiled up = _________
• Let go = _________
– For example: water dam
• Water behind a dam has PE because of its elevated position.
• As the H2O level increases, so does its PE.
• When the water is released from the dam, its PE is
converted to KE as it falls to a lower level.
• This KE increases with speed
KE vs. PE
Endothermic Reactions
-Energy is written before the arrow
2 H2 + O2 + 242kJ  2 H2O
Ex: Melting/Evaporating
Ex: Cold Pack
Ex: Baking
Exothermic Reactions -
-Energy is written after the arrow
2 H2 + O2  2 H2O + Heat
or
2 H2 + O2  2 H2O + 242 kJ
Ex: Condensation/Freezing
Ex: Heat Pack
Ex: 2 H2 + O2  2 H2O (and an explosion)
Energy as Heat
• Most chemical reactions, physical changes of
state, and dissolving processes involve energy
changes that can be measured.
Heat
• Heat = The transfer of energy between objects at
different temperatures
• Heat of reaction = heat energy gained or released
during a chemical reaction
• Heat energy can be measured as a result of
temperature change.
• Heat is different from temperature!
Temperature
Measure of the average kinetic energy of
the particles in a substance
What is the SI Unit for temperature?
What do we use to measure temperature?
In lab, our thermometers use a Celsius
scale, therefore we need to convert values
to Kelvin when performing calculations.
C =
K
-
273
K = C + 273
Try these conversions:
• Boiling Point of water is 100C, what is the
temperature in Kelvin degrees?
• Room Temperature is 25C, what is it in Kelvin
degrees?
Heat vs. Temperature
• Transfer of heat does not always change the
temperature
– Ex: As ice melts in a closed container, the
temperature of the ice-water mixture remains
0C, even though heat energy is being transferred.
– Ex: As water boils in a closed container and water
vaporizes, the gas-liquid mixture remains 100C,
even though heat energy is being transferred.
• The energy is being used to move molecules!
Water Phase Changes
Temperature remains __________
during a phase change.
http://www.kentchemistry.com/links/Matter/HeatingCurve.htm
Phase Changes
S
L
G
G
L
S
Endothermic or
Exothermic
Review: Phase Changes
Type of Change
Solid
Liquid
Liquid
Gas
Solid
Gas
Gas
Liquid
Liquid
Solid
Gas
Solid
Name
Heating Curve
Label the phases/changes:
25
Temperature (C)
e
15
d
c
5
a
b
-5
Energy
Heating Curve Energy Changes
• During a phase change
• The kinetic energy remains the same but the
potential energy increases.
• During a phase
• The kinetic energy increases and the potential
energy remains the same
http://zonalandeducation.com/mstm/physics/mechanics/energy/heatAndTem
perature/changesOfPhase/changeOfState.html
Heating Curve
Label the KE/PE:
Kinetic energy
f
d
b
c
a
Time
e
Cooling Curves Energy Changes
• During a phase change
•
the kinetic energy remains the same but the
potential energy decreases.
• During a phase
• The kinetic energy decreases and the potential
energy remains the same
Cooling Curve
Label the phases/changes:
60
A
B
40
KE
C
D
20
E
0
TIME
Do Now
• Which do you think requires more energy:
melting water or vaporizing water? Why?
Heat of Fusion
• The amount of heat needed to MELT (solid to
liquid) 1 gram of a given substance (J/g)
Temp.
Heat of fusion
Energy
Heat of Vaporization
• The amount of heat needed to VAPORIZE
(liquid to gas) 1 gram of a given substance
(J/g)
Heat of vaporization
Temp.
Energy
Specific Heat
• Different substances are effected by heat in different
ways, resulting in different temperature changes
• Specific heat = the amount of heat energy required
to raise the temperature of 1 g of a substance 1C or
1 K (J/gK)
• Specific Heat of water is high and found on your
reference tables (Table B)
– What is the heat of fusion? ____________
– What is the heat of vaporization? _________
– Which is larger heat of fusion or heat of vaporization?
___________
FORMULAS
Reference Table page 12 Table T*
q = heat
m = mass
C = specific heat
ΔT= change in temperature (final - initial)
• q = m x C x ΔT
• q = m x Heat of Fusion (Hf)
• q = m x Heat of Vaporization (Hv)
Q (heat) is __________ during
EXOTHERMIC REACTIONS (energy
released)
Q (heat) is ________ during
ENDOTHERMIC REACTIONS (energy
absorbed)
Formula Diagram
Using: q = m x C x ΔT
Temp
Heat Energy Added
Using: q = m x C x ΔT
Specific heat of water = 4.18 J/g∙K
• How many kilojoules of heat are needed to
raise the temperature of 500. grams of water
from 10.0˚C to 30.0˚C?
Using: q = m x C x ΔT
Specific heat of water = 4.18 J/g∙K
• If 4.0 grams of water at 1˚C absorbs 33 joules
of heat, what will be the change in
temperature of the water?
Using: q = m x C x ΔT
Specific heat of water = 4.18 J/g∙K
• When 84 joules of heat are added to 2.0
grams of water at 15˚C, what will be the final
temperature of the water?
Using: q = m x C x ΔT
Specific heat of water = 4.18 J/g∙K
• The temperature of 50. grams of water was
raised to 50.˚C by the addition of 4,180 joules
of heat energy. What was the initial
temperature of water?
Types of Change
Physical Change
A change in a
substance that does
not involve a change in
the identity of the
substance.
Solid Liquid Gas
Chemical Change
A change in which one
or more substances are
converted into
different substances.
Can be detected
through:
•Energy changes
•Change in color
•Emission of gases
States of Matter
Solid
Liquid
Heat of ReactionAmount of heat
released or
absorbed during a
chemical reaction
Exothermic
reactions that release
Energy to their
surroundings.
-result in a temperature
increase
-Energy is written after
the arrow
Endothermic Reactions
reactions that absorb
energy from their
surroundings
Usually results in a
temperature decrease
-Energy is written
before the arrow
Gas
Forms of Energy
Phase Changes
Heating Curve
Endothermic Reaction
Cooling Curve
Exothermic Reaction
Potential Energy
Kinetic Energy
Energy possessed
by objects through
their position
type of energy that only
moving objects have.
Energy in motion.
Review: Endothermic vs. Exothermic
• Endothermic Reactions - usually results in a
temperature decrease (absorb energy)
• Energy is written before the arrow
2NaHCO3 + Heat  Na2CO3 + H2O + CO2
• Exothermic Reactions - result in a temperature
increase (release energy)
• Energy is written after the arrow
2H2 + O2  2H2O + Heat
Phase Diagram
•Represents phases as a function of
temperature and pressure.
•Critical temperature: temperature above
which the vapor can not be liquefied.
•Critical pressure: pressure required to
liquefy AT the critical temperature.
•Critical point: critical temperature and
pressure (for water, Tc = 374°C and 218 atm).
There is no observable change in the
quantity of matter during a chemical
reaction or a physical change.
In other words, matter cannot be created
nor destroyed. It is just converted from
one form to another
Energy is neither created nor
destroyed instead it changes from
one form to another.
• http://zonalandeducation.com/mstm/physics/
mechanics/energy/heatAndTemperature/chan
gesOfPhase/changeOfState.html
• http://www.dlt.ncssm.edu/TIGER/Flash/phase
/HeatingCurve.html
• http://chemistry.bd.psu.edu/jircitano/heatcur
v.html