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Law of Conservation of Energy. “ Energy can neither be created nor destroyed.” The total amount of energy in the universe is constant. There is no process that can increase or decrease that amount. - PowerPoint PPT Presentation
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Tro's "Introductory Chemistry", Chapter 3
1
Law of Conservation of Energy
• “Energy can neither be created nor destroyed.” • The total amount of energy in the universe is
constant. There is no process that can increase or decrease that amount.
• However, we can transfer energy from one place in the universe to another, and we can change its form.
Tro's "Introductory Chemistry", Chapter 3
2
Matter Possesses Energy• When a piece of matter
possesses energy, it can give some or all of it to another object.It can do work on the other
object.
• All chemical and physical changes result in the matter changing energy.
Tro's "Introductory Chemistry", Chapter 3
3
Kinds of EnergyKinetic and Potential
• Potential energy is energy that is stored. Water flows because gravity pulls it
downstream. However, the dam won’t allow it to
move, so it has to store that energy.• Kinetic energy is energy of motion,
or energy that is being transferred from one object to another. When the water flows over the dam,
some of its potential energy is converted to kinetic energy of motion.
Tro's "Introductory Chemistry", Chapter 3
4
Some Forms of Energy• Electrical
Kinetic energy associated with the flow of electrical charge.
• Heat or Thermal EnergyKinetic energy associated with molecular motion.
• Light or Radiant EnergyKinetic energy associated with energy transitions in an
atom.• Nuclear
Potential energy in the nucleus of atoms. • Chemical
Potential energy in the attachment of atoms or because of their position.
Tro's "Introductory Chemistry", Chapter 3
5
Units of Energy• Calorie (cal) is the amount of energy needed to
raise one gram of water by 1 °C.kcal = energy needed to raise 1000 g of water 1 °C.food calories = kcals.
Energy Conversion Factors
1 calorie (cal) = 4.184 joules (J)
1 Calorie (Cal) = 1000 calories (cal)
1 kilowatt-hour (kWh) = 3.60 x 106 joules (J)
Tro's "Introductory Chemistry", Chapter 3
6
Energy Use
Unit
Energy Required to Raise Temperature of 1 g of Water by 1°C
Energy Required to Light 100-W Bulb for 1 Hour
Energy Used by Average U.S. Citizen in 1 Day
joule (J) 4.18 3.6 x 105 9.0 x 108
calorie (cal) 1.00 8.60 x 104 2.2 x 108
Calorie (Cal) 1.00 x 10-3 86.0 2.2 x 105
kWh 1.1 x 10-6 0.100 2.50 x 102
Tro's "Introductory Chemistry", Chapter 3
8
Exothermic Processes
• When a change results in the release of energy it is called an exothermic process.
• An exothermic chemical reaction occurs when the reactants have more chemical potential energy than the products.
• The excess energy is released into the surrounding materials, adding energy to them.Often the surrounding materials get hotter from the
energy released by the reaction.
Tro's "Introductory Chemistry", Chapter 3
9
An Exothermic Reaction
Pot
enti
al e
nerg
y
Reactants
Products
Surroundings
reaction
Amount of energy released
Tro's "Introductory Chemistry", Chapter 3
10
Endothermic Processes
• When a change requires the absorption of energy it is called an endothermic process.
• An endothermic chemical reaction occurs when the products have more chemical potential energy than the reactants.
• The required energy is absorbed from the surrounding materials, taking energy from them.Often the surrounding materials get colder due to the
energy being removed by the reaction.
Tro's "Introductory Chemistry", Chapter 3
11
An Endothermic Reaction
Pot
enti
al e
nerg
y
Products
Reactants
Surroundings
reaction
Amount of energy absorbed
Tro's "Introductory Chemistry", Chapter 3
12
Temperature Scales• Fahrenheit scale, °F.
Used in the U.S.
• Celsius scale, °C.Used in all other countries.A Celsius degree is 1.8
times larger than a Fahrenheit degree.
• Kelvin scale, K.Absolute scale.
Temperature Scales
Celsius Kelvin Fahrenheit-273°C-269°C
-183°C
-38.9°C
0°C
100°C
0 K4 K
90 K
234.1 K
273 K
373 K
-459 °F-452°F
-297°F
-38°F
32°F
212°F
Absolute zero
BP helium
Boiling point oxygen
Boiling point mercury
Melting point ice
Boiling point water
0 R7 R
162 R
421 R
459 R
671 R
Rankine
Room temp25°C 298 K 75°F 534 R
Tro's "Introductory Chemistry", Chapter 3
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Temperature Scales
• The Fahrenheit temperature scale used as its two reference points the freezing point of concentrated saltwater (0 °F) and average body temperature (96 °F).More accurate measure now sets average body
temperature at 98.6 °F.
• Room temperature is about 72 °F.
Tro's "Introductory Chemistry", Chapter 3
15
Temperature Scales, Continued
• The Celsius temperature scale used as its two reference points the freezing point of distilled water (0 °C) and boiling point of distilled water (100 °C).More reproducible standards.Most commonly used in science.
• Room temperature is about 22 °C.
Tro's "Introductory Chemistry", Chapter 3
16
Fahrenheit vs. Celsius• A Celsius degree is 1.8 times larger than a
Fahrenheit degree.• The standard used for 0° on the Fahrenheit
scale is a lower temperature than the standard used for 0° on the Celsius scale.
1.8
32-F C
Tro's "Introductory Chemistry", Chapter 3
17
The Kelvin Temperature Scale• Both the Celsius and Fahrenheit scales have
negative numbers.Yet, real physical things are always positive amounts!
• The Kelvin scale is an absolute scale, meaning it measures the actual temperature of an object.
• 0 K is called absolute zero. It is too cold for matter to exist because all molecular motion would stop.0 K = -273 °C = -459 °F.
Tro's "Introductory Chemistry", Chapter 3
18
Kelvin vs. Celsius• The size of a “degree” on the Kelvin scale is the
same as on the Celsius scale.
• The 0 standard on the Kelvin scale is a much lower temperature than on the Celsius scale.
• When converting between kelvins and °C, remember that the kelvin temperature is always the larger number and always positive!
273C K
Tro's "Introductory Chemistry", Chapter 3
19
Energy and the Temperature of Matter• The amount the temperature of an object
increases depends on the amount of heat energy added (q).If you double the added heat energy the
temperature will increase twice as much.
• The amount the temperature of an object increases depending on its mass.If you double the mass, it will take twice as
much heat energy to raise the temperature the same amount.
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