Transcript Energy Changes

Warm-up
True or False
1) Energy is found only in living
things
2) Energy is created as the result of
activity
3) Energy is only associated with
movement
4) Photosynthesis converts light
energy to chemical energy
Answers
1) Energy is found only in living things
FALSE- everything has energy -heat,
light, sound, electricity, chemical
energy (living things convert energy)
2) Energy is created as the result of
activity
FALSE- activity transfers energy,
does NOT create.
Answers
3) Energy is only associated with
movement
FALSE- nonmoving objects have
potential energy (moving has kinetic)
4) Photosynthesis converts light
energy to chemical energy
TRUE- photosynthesis converts
energy from sunlight to chemical
forms
Energy Changes in
Chemical Reactions
ENERGY
ENERGY : capacity to do work
or produce heat
Two forms of energy:
Kinetic - energy due to movement
- energy of motion
Potential- energy due to position
Chemical Potential Energy –
energy due to chemical composition.
Law of conservation of energy
states that in any chemical
reaction or physical process, energy
can be changed from one form to
another, but it is neither created nor
destroyed
Example
• A macroscopic example of potential energy is
water stored behind a dam above the turbines
of a hydroelectric generating plant.
• When the dam gates are
opened, the water rushes
down and does work by
turning the turbines to
produce electrical energy.
Energy in Chemical Systems
• Chemical systems contain both kinetic
and potential energy.
• As temperature increases, the motion of
particles increases.
• The potential energy of a substance depends
upon its composition: the type of atoms in the
substance, the number and type of chemical
bonds joining the atoms, and the particular
way the atoms are arranged.
HEAT and TEMPERATURE
Heat
Heat Power point
Heat, which is represented by the symbol
q, is a form of energy that flows from a
warmer object to a cooler object.
• When the warmer object loses heat, its
temperature decreases
• When the cooler object absorbs heat, its
temperature rises.
HEAT
Measured with a calorimeter
Measured in calories (cal)
kilocalories(kcal)
joules (J)
kilojoules (kJ)
One calorie is the amount of heat
needed to raise the temperature of 1
g. of water by 1 deg. Celsius
Relationships Among
Energy Units
1J
= 0.2390 cal
1 cal
= 4.184 J
1 kJ
= 1000 J
1 Calorie = 1 kcal
1 kcal = 1000 cal
Conversion of Heat/Energy Units
Using Dimensional Analysis
Some Notes:
• Use the T-chart
• The number 1 (no unit) goes to the
bottom left part of the chart
• The given (write its unit) goes to
the top left of the chart
• The unit of the number at the
bottom right must match that of the
given
EXAMPLES
1. Convert 55 cal to joules
55 cal
4.184 J
1
1cal
Answer =
(55) (4.184) = 230.12 J
2. Convert 500 joules to calories
500 J
1 cal
1
4.184 J
Answer= 500 ÷ 4.184 = 119.5 cal
3. Convert 800 joules to kcal
800 J
1
1 cal
4.184 J
1 kcal
1000cal
Answer = 800 ÷ 4.184 ÷ 1000
= 0.19 kcal
Convert these heat measures:
1) 300 Joules to calories
300 J
1 cal
1
4.184 J
Answer = (300) (4.184) = 1255.2 cal
2) 4,590 Joules to kiloJoules
4590 J
1 kJ
1
1000 J
Answer = 4590 ÷ 1000 = 4.590 kJ
3. 0.85 kilojoules to Joules
0.85 kJ
1
1000 J
1 kJ
Answer = (0.85) ( 1000) = 850 J
4) 93 calories to Joules
93 cal
1
4.184 J
1 cal
Answer = (93 ) (4.184) = 389.11 J
5) 2.47 kcal to cal
2.47 kcal
1
1000cal
1 kcal
Answer = (2.47) (1000) = 2470 cal
6)
8.56 kJ to cal
8.56 kJ 1000 J
1cal
1
1kJ
4.184 J
Answer = (8.56) (1000) ÷ 4.184
= 2045.89 cal
In an experiment, Teddy and Kyle place four objects
( a wooden block, metal tray, woolen cap and glass
plate) on a table in the science classroom, and
connect each to a thermometer. They leave these
overnight and observe their temperatures the
following day at the same time.
Which do you think will occur?
a) none of the objects will have the same temps
b) two of the objects will have the same temps
c)three of the objects will have the same temps
d)all of the objects will have the same temps
Why?
TEMPERATURE
• Measured with a thermometer
• Measured in Celsius, Fahrenheit or
Kelvin
• Conversion of Units:
ºF = 1.8 (º C)
K
=ºC
+
+ 32
273
EXAMPLE 1
Changing Celsius to Kelvin
Convert 20° C to Kelvins (K)
K
= ºC
= 20
= 293
+
+
273
273
EXAMPLE 2
Changing Kelvin to Celsius
Convert 300 K to ° C
K = ºC
300 = º C
- 273
27 = ° C
+
+
273
273
-273
EXAMPLE 3
Changing Celsius to Fahrenheit
Convert 10 º C to °F
ºF
= 1.8 (º C)
= 1.8 (10)
= 18
+
= 50
+ 32
+ 32
32
EXAMPLE 4
Changing Fahrenheit to Celsius
Convert 77 °F to ° C
ºF = 1.8 (º C) + 32
77 = 1.8 (º C) + 32
-32
- 32
45 = 1.8 (º C)
Divide by 1.8
45
1.8
25
=
1.8 (º C)
1.8
= °C
Convert the following:
1.
383 Kelvins to Celsius
K = °C + 273
383 = °C + 273
-273
-273
110 = °C
2. 110°C to °F
ºF
= 1.8 (º C) + 32
= 1.8 ( 110) + 32
= 198
+ 32
= 230
Convert the following:
3.
-37 ° C to
K
K
K
K
= °C + 273
= -37 + 273
= 236
Convert the following:
212°F to ° C
ºF = 1.8 (º C) + 32
212 = 1.8 (º C) + 32
- 32
- 32
180 = 1.8 (º C)
(Divide both sides by 1.8)
180 = 1.8 (º C)
1.8
1.8
100 = º C
4.
Convert the following:
5. 212°F to K
(from last slide,212°F= 100°C)
K = °C + 273
= 100 + 273
= 373