Transcript Chapter 10 Section 2
Energy Changes in Chemical Reactions Chemistry TEKS 11
Learning Objective • TLW understand energy and its forms – kinetic, potential, chemical, and thermal (TEKS 11.A) • TLW understand the law of conservation of energy and processes of heat transfer (TEKS 11.B) • TLW use thermochemical equations to calculate energy changes in chemical reactions (enthalpy and entropy) and classify reactions as exothermic and endothermic (TEKS 11.C) • TLW perform calculations involving heat, mass, temperature change, and specific heat (TEKS 11.D) • TLW perform calorimetry to calculate heat of a chemical process (TEKS 11.E)
Forms of Energy
• Potential – stored energy – Mechanical - energy of position, such as effect of gravity or a spring – Chemical potential - energy is a form of potential energy related to the structural arrangement of atoms or molecules – Also electrical, electrostatic, nuclear, and magnetic • Kinetic – energy of motion • Chemical – energy from chemical reactions where substances are transformed into new substances as bonds break and reform • Thermal – portion of the thermodynamic or internal energy of a system that is responsible for the temperature of the system
How is Energy Transferred???
• Conduction • Convection • Radiation
I. Methods Of Energy Transfer A. Three ways energy can
transfer
1.
Conduction
– transfer of energy as heat between particles that
collide
a.
two
objects that are in
contact
with each other at
unequal
temperatures b. particles
within
an object
c. objects must be
direct
contact d. Ex. In case of a wire in a campfire, the rapidly moving
air
molecules close to the flame
collide
with the atoms at the end of the wire. The energy
transferred
to the atoms in the wire causes them to
vibrate
rapidly .
NO HEAT HEATED
•
As a group think of 5 other examples of conduction
2.
Convection
energy – transfer of by the
movement
of
fluids
a. fluids are either a
liquid gas
; Ex. water & air or a these particles are
free
move around to b. particles in a
solid
are
not
free to
move
, so convection can’t take place
c. heated substances follow a convection
current
1)
heated
air
rises
, then
cools
and
falls
back down 2) when something heats up – it
expands
3) when something cools down – it
contracts
d. This
cycle
of a
warm
fluid that rises and then cools and falls is called a
convection current
e. Example While roasting marshmallows, you may notice that tiny glowing embers from the fire
rise
and begin to swirl. They are following the movement of air
away
from the fire. The air
close
to the fire becomes hot and
expands
so that there is more
space
between the air
particles
As a result the air becomes less
dense
and moves upward, carrying its
energy
with it The rising warm air is
replaced
by cooler,
denser
air. Eventually, the rising hot air cools and
contracts
becomes denser, and sinks.
•
As a group think of 5 other examples of convection
3.
Radiation
– the transfer of energy by
electromagnetic
waves a. does
not
involve the movement of
matter
b.
only
method of energy transfer that can take place in a
vacuum
c. includes
infrared
radiation, visible
light
and ultraviolet rays
d. a
hot
object
radiates
more energy than a cooler object e. much of the energy we receive from the
sun
is transferred by
radiation
f. objects
do not
have to
touch
transfer heat by radiation to g. Ex. Warmth from the fire without standing
IN
the fire
As the
molecules
in your skin
absorb
the energy from the fire, the average
kinetic
energy of these molecules and the
temperature
of your skin –
increases
.
•
As a group think of 5 other examples of radiation
Which energy transfer is indicated by each arrow?????
Convection Conduction Radiation
convection YouTube Video conduction conductor insulator radiation
Science in the News • Find 3 articles each that describe the three types of energy transfer (conduction, convection, radiation) • Example – Moon” space – that will be a total of 9 articles • Write down the title/headline of each article • Under this briefly describe which type of energy transfer it is and why – “Scientists Find Evidence of Global Warming on the – Radiation - Satellite data shows the moon’s surface temperature has increased by 0.1 o C over the past 20 years. The moon receives the sun’s radiant heat as this type of energy can travel through the vacuum of
TOMORROW….
Specific Heat
I. Measuring Heat A.
Heat
is
measured
called
calories
in units (cal) 1. a calorie is the
amount
of heat needed to raise
1 gram
of water
1 o C
2. Note the two important factors: a) It's
1 gram
of a substance b) and it moves
1 °C
3. Ex. To raise the temperature of 1 gram of water from 4 o C to 5 o C, 1
calorie
of heat is needed 4. The amount of
heat change
the T o needed to depends on the
mass
B. Specific Heat 1. The ability of a substance to
absorb
heat
energy
called
specific heat
is 2. Different substances absorb different amounts of heat 3.
Water
has a
high
heat specific
4.
Water
has a specific heat of
1
this is one of the
highest
specific heats of any substance
Wood
is
0.42
Aluminum is 0.22
Mercury is 0.03
5. Specific heat can be used to
calculate
the amount of heat
gained
or
lost
II. Calculating Heat Energy A.
Formula: Heat gained or lost (Q = ∆H ) =
Mass X ∆T o X Specific heat
1 .
∆T o
=
change
in Temperature (T o final – T o initial) 2.
Raise in temperature would be positive
∆T o
, lower temperature is negative (-
∆T o )
3. Heat gained would be positive, heat lost is a negative 4.
Specific heat is an absolute number
B. Ex. How much heat is needed to raise the T o of 4 grams of aluminum 5 o C?
Heat gained =
4 g X 5 o C X 0.22 cal/g o C
Heat gained =
4.4 cal
C. Ex. Calculate the heat lost by 10 g of copper if it is cooled from 35 o C to 21 o C.
Heat lost =
10 g X (21 o - 35 o ) X .09 cal/g o C = - 12.6
Heat lost
12.6 calories
1. A 500 g piece of iron changes 7 ° C when heat is added. How much heat energy produced this change in temperature? Mass X ∆T o X Specific heat
500 g X 7 o C X 0.11 cal/g o C
Heat gained =
385 cal
2. When 300 cal of energy is lost from a 125 g object, the temperature decreases from 45 °C to 40 °C. What is the specific heat of this object? Specific heat = cal/ (mass X ΔT)
300 cal 125 g X (40 o C – 45 o C)
specific heat =
.48 cal/g o C
3. A piece of food is burned in a calorimeter that contains 200 g of water. If the temperature of the water rose from 20 °C to 45 °C, how much heat energy was contained in the food?
Heat gained = Mass X ∆T X Specific Heat
200g X (45 o C – 20 o C) X 1 cal/g o C 5000 cal
• Independent practice set – calculating specific heat
Enthalpy & Entropy • Enthalpy – • Entropy -
Calculating Heat of Reaction
Chemical Reactions & Energy Changes
•
Classified as:
–
Exothermic – chemical reactions releasing energy . Indicated by temperature increase .
•
List examples
–
Endothermic -- chemical reactions using energy . Indicated by temperature decrease
•
List examples
What’s Next • Lab – Calorimetry (and/or specific heat) – In Periodic Groups read all procedures – Determine potential hazards, precautions to take, any PPE needed – Set up lab according to scientific method – Complete any pre-work