Aim: ENERGY

AP CHEMISTRY

AIM: ENERGY

DO NOW: USING YOUR TEXTBOOKSAND PRIOR KNOWLEDGE OF CHEMISTRYTO ANSWER THE FOLLOWING QUESTIONS:

1.WHAT IS ENERGY?

2.WHAT ARE EXOTHERMIC REACTIONS?

3.WHAT ARE ENDOTHERMIC REACTIONS?

ENERGY

•Law of Conservation of Energy - Energy can

neither be created or destroyed in a chemical

reaction but may be converted from one form to

another.

•Energy can be assigned to one of the two

classes: potential or kinetic energy.

POTENTIAL ENERGY

•Potential energy is stored energy or the energy

of position. That is, a book 8 feet off the floor

has a greater potential energy than a book 2 feet

off the floor. Potential energy is important in

chemistry because the energy stored in chemical

bonds is potential energy.

•The energy liberated or absorbed in a chemical

reaction is similar in many ways to the energy

released or needed when a book is raised or

lower.

KINETIC ENERGY

•Kinetic energy is the energy of motion.

Particles with high kinetic energy are moving

rapidly and those with low kinetic energy of

moving slowly. The topic of kinetic energy is

often discussed during a lesson on kinetics

(rates of reaction).

•Some forms of energy are heat, light, sound,

electrical, chemical, and mechanical energy.

EXOTHERMIC

Energy is released in an

exothermic process. Any process in which energy isreleased is an exothermic process.

•The burning of paper, an explosion, or the

condensation of steam are all exothermic

process.

ENDOTHERMIC

•Endothermic - Energy is absorbed in an

endothermic process. Any process which

requires energy in classified as an endothermic

process.

•The baking of a cake, boiling water, or

the of melting ice are all endothermic processes.

UNITS OF ENERGY

•The unit of energy used in chemistry is the

“calorie” or “kilocalorie” (1000 calories).

•A calorie is the amount of energy needed to

change the temperature of 1.0 gram of water,

1.0°C.

HOW DO YOU FIND THE NUMBER OFCALORIES?

Since only water (specific heat = 1.00 calorie/gram°C) will be used for energy calculations the equationsimplifies to:

PROBLEMS?

How much heat is needed to change thetemperature of 100. grams of water from 20.°Cto 35°C?

# 𝑐𝑎𝑙𝑜𝑟𝑖𝑒𝑠=𝑚𝑎𝑠𝑠 𝑥 1.00 𝑐𝑎𝑙 𝑔𝑟𝑎𝑚 𝐶 𝑥 𝑡
# 𝑐𝑎𝑙𝑜𝑟𝑖𝑒𝑠=100𝑔 𝑥 1.00 𝑐𝑎𝑙 𝑔 𝐶 𝑥 15𝐶

amount of energy = 1500 calories

PROBLEMS????

HEAT OF FUSION

•Hfus - energy necessary to change one gram of solidinto one gram of liquid at its melting point

•The heat of fusion for ice is 79.72 calories/gram

•# calories = mass of solid x Hfus

HEAT OF VAPORIZATION

•Hvap - the energy necessary to change one gram ofliquid into one gram of vapor at its boiling point.

•The heat of vaporization for ice is 539.4calories/gram

•# calories = mass of solid x Hvap

PROBLEMS????

1. Calculate the amount of energy needed to melt200. grams of ice.

•# calories = mass ice x 80 cal/gram

•# calories = 200. grams x 80 cal/g

•# calories = 16,000 calories

PROBLEMS????

2. How much steam can be prepared by theaddition of 2160 calories of heat a sample of water at100°C?

•# calories = mass H2O x 540 cal/g

•# 2160 calories = mass H2O x 540 cal/g

•mass of H2O = 4 grams H2O

3. How many calories of heat energy are absorbed inraising the temperature of 10. grams of water from5.0°C to 20°C

A.2.5 x 102

B.2.0 x 102

C.1.5 x 102

D.5.0 x 101

# calories = mass x specific heat x ∆ T

# calories = 10. g x 1.0 cal/ g ∙ °C (20°C - 5°C)

# calories = 150 calories = 1.5 x102 (C)

4. What is the maximum number of grams of water

at 10.°C that can be heated to 30.°C by the

addition of 40.0 calories of heat?

A.1.0 gram

B.2.0 grams

C.20 grams

D.30 grams

# calories = mass x specific heat x ∆ T

40.0 calories = mass x 1.0 cal/ g ∙ °C (30°C - 10°C)

40.0 = 20 x mass

Mass = 2.0 grams

5. How many grams of ice can be melted at 0°C by

the addition of 4800 calories of energy?

A.60 grams

B.166 grams

C.2.1 grams

D.600 grams

# calories = mass x heat of fusion

4800 calories = mass ice x 80 cal/ g

Mass ice = 4800 calories / 80 cal/g

Mass = 60 grams

MEASURE OF THE AVERAGE KINETIC ENERGY OFTHOSE MOLECULES. SOME OF THE MOLECULESIN THE SYSTEM ARE MOVING VERY FAST ANDSOME ARE MOVING SLOWING OR NOT MOVINGAT ALL, BUT THE TEMPERATUREMEASURES THE AVERAGE VELOCITY.

TEMPERATURE

TEMPERATURE

•That is, the temperature measures the speed atwhich the molecules move. As the temperatureincreases, the average velocity increases. If thetemperature drops, the average velocity of themolecule slows down.

HEAT

•Heat flows spontaneously from a body at a high

temperature to a body at a lower temperature.

•Heat flows spontaneously from a body at a high

temperature to a body at a lower temperature.

Heat is transferred when faster moving, high

temperature molecules collide with slower moving,

lower temperature molecules. As a result of the

collision, the slower moving molecules speed up andthus heat up.

FAHRENHEIT

•Temperatures are measured by thermometers.

The first thermometer was created by Gabriel

Fahrenheit (1686-1736). Fahrenheit noticed that

the level of some mercury in a tube rose and fell

according to how hot or cold it was in his

laboratory.

FAHRENHEIT

•Fahrenheit used hisbody temperature(100°) and the lowesttemperature he couldachieve by mixing iceand salt (0°) as thefixed points on histhermometer. Thedistance betweenthese two points wasthen divided into 100equal parts.

CELSIUS

•Anders Celsius (1701-1744) studied Fahrenheit’s

work and devised a thermometer using the boiling

point and freezing point of water as his fixed points.

The freezing point and boiling points of water are

more reproducible than Fahrenheit’s body

temperature and freezing point of an ice salt mixture.

CELSIUS

•Celsius divided the distancebetween the 0° mark andthe 100° mark into 100 equaldivisions. Each divisionequals 1°C.

•The equation to convertfrom degrees Celsius to

degrees Fahrenheit is

°F = (°C X 9/5) + 32

KELVIN

•William Thomson (1824-1907), known as Lord

Kelvin, first suggested a temperature scale which

was based on the Celsius scale but does not have

negative numbers. He achieved this by setting the

lowest possible temperature as “0."

KELVIN

•The Kelvin scale uses the same fixed points, the

boiling point and freezing point of water, and the

same divisions as the Celsius scale.

ABSOLUTE ZERO

•Lord Kelvin’s zero point became known as

“Absolute Zero.” At absolute zero, all molecular

motion ceases.

•Absolute zero is defined as the lowest possible

temperature and is believed to be -273.15°C.

STANDARD TEMPERATURE

•Standard temperature is defined as 0°C or 273 K.

•0°C is chosen as standard temperature because it

is easily obtained and maintained. The sample

need only to immersed in a container of melting

ice and the temperature is maintained at a constant0°C.

CONVERTING BETWEEN °C AND °K

•The equation used to convert from degrees Celsius

to degrees Kelvin is:

°K = °C + 273.

Convert 100°C to degrees Kelvin.

°K = 100°C + 273 = 373 K

PROBLEMS????

•Convert from 200 K to °C.

•°C = 200 K - 273 = -73 °C

•Perform the following conversions:

•300 K = ? °C

•25°C = ? K

•200 °C = ? K

•600 K = ? °C

PRESSURE

•The Pressure is the force exerted on a given area

of an object.

•Gases exert a force on their container by their

repeated collision with its sides. This force is

dependent upon three factors; the mass of the

particles, the number of particles in the container,

and their velocity which is related to their

temperature.

PRESSURE

•The most common pressure measuredis the atmospheric pressure.Atmospheric pressure is created bythe weight of a column of airmolecules

from the outer limits of our atmosphereto the earth’s

surface being pulled towards thesurface of the earth by gravity.

PRESSURE

•A balloon reaches acertain size becausethe force of themolecules pushingagainst the insidethe balloon equalsthe atmosphericpressure pressing onthe outside surfaceof the balloon. Ifeither of these forceschange, the size ofthe balloonchanges.