Law of Conservation of Energy

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.


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.


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.


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.


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)


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


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.


9

An Exothermic Reaction

Pot

enti

al e

nerg

y

Reactants

Products

Surroundings

reaction

Amount of energy released


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.


11

An Endothermic Reaction

Pot

enti

al e

nerg

y

Products

Reactants

Surroundings

reaction

Amount of energy absorbed


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


14

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.


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.


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


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.


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


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.

Documents

Law of Conservation of Energy