CHEMISTRY
September 21, 2010
Warm Up
• What is the difference
between heat and
temperature?
• Which bench will you sit on
when exposed to 120oF
sunlight -- a wood or an iron
bench? Why?
Agenda
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Warm Up
Grade Unit 3 Worksheet 1 and 2
Answers to Post Test Questions
Unit 3 Lecture
Homework
– Unit 3 Worksheet 3
Grade Homework
• Unit 3 Worksheet 1: Forms of Energy
• Unit 3 Worksheet 2: Energy Calculations
Post-Test Questions (Video)
• Refer to Packet
Chemistry
September 22, 2010
Warm Up
• Why is it cold when you hold on to a piece
of ice cube?
– How is heat transferred?
Agenda
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Warm Up
Unit 3: Energy
Practice Problems
Homework:
– Unit 3 Worksheet
– Unit 3 Quiz
Unit 3 Recap: Energy
• Part I: Forms of Energy
• Part II: Measurements of Energy and SI Unit
Measured by amount of work it can do
- ORamount of heat it can be changed into
SI unit of measure = Joule (J)
• Part III: Energy Conversions and Energy Efficiency
A. Energy is changed, or converted, from one form to another
B. Law of Conservation of Energy
– The total amount of energy remains the same during all energy changes.
– Energy cannot be created or destroyed
Thermochemistry
Chemical Energy and the Universe
• Thermochemistry is the study of heat
changes that accompany chemical
reactions and phase changes.
• In thermochemistry, the system is the
specific part of the universe that contains
the reaction or process you wish to study.
Chemical Energy and the Universe
• Everything in the universe other than the
system is considered the surroundings.
• Therefore, the universe is defined as the
system plus the surroundings.
universe = system + surroundings
Enthalpy and enthalpy changes
• For many reactions, the amount of energy
lost or gained can be measured
conveniently in a calorimeter at constant
pressure.
• To more easily measure and study the
energy changes that accompany such
reactions, chemists have defined a property
called enthalpy.
• Enthalpy (H) is the heat content of a
system at constant pressure.
Complex Calorimeter
Simple Calorimeter
Test Tube
Thermometer
Stirring Rod
Water
Reaction
Sealed
Container
Enthalpy and enthalpy changes
• You can measure the change in enthalpy, which
is the heat absorbed or released in a chemical
reaction.
– What is the direction of heat flow?
• The change in enthalpy for a reaction is called
the enthalpy (heat) of reaction (∆Hrxn).
Enthalpy and enthalpy changes
IV. Energy and Chemical Reactions
A. Exothermic Reaction
– Release heat energy
B. Endothermic Reaction
– Absorb heat energy
C. Activation energy
– Minimum amount of energy needed to get a
reaction started
V. Heat and Temperature
A. Celsius Scale
– Freezing point of water = 0 oC
– Boiling point of water = 100 oCelsius
– Interval between them is divided into 100 parts
B. Kelvin Scale
– Absolute zero
• Lowest temperature theoretically possible
– Absolute zero = 0 K = -273 oC
– Size of degree is same as Celsius
• Fahrenheit Scale
C. Converting
– Kelvin = Celsius + 273 (K = oC + 273)
– Celsius = Kelvin – 273 (oC = K -273)
– Fahrenheit = (oC × 9/5) + 32
D. Heat
– Measured by temperature change it produces in
substance
– Calorie
• Quantity of heat that will increase the temperature of 1
gram of water by 1oC
– 1 calorie (cal) = 4.19 Joules (J)
E. Heat vs Temperature
– Refer to the board
Heat versus Temperature
• Heat
– Energy that transfers from one object to
another because of a temperature difference
between them.
• Temperature
– A measure of the average kinetic energy of
the particles in a sample of matter
VI. Changes in Temperature
• When heated, different substances change
temperature at different rates.
• Specific Heat
– The amount of heat it takes to raise the
temperature of 1 g of substance by 1oC.
• Specific heat of water = 1 cal/g.oC
• Specific heat of Iron = 0.11 cal/g.oC
VII. Calorimetry
• Measurement of the amount of heat released
during a reaction
• Heat measured using a calorimeter (refer to
diagram)
– Calorimeter = device to measure the transfer of
heat to water
– To calculate the calories of heat transferred during a
chemical reaction, multiply:
• Mass of substance in calorimeter (g)  m
• Change in temperature (oC)  T
• Specific heat (calorie/g.oC)  c
– Formula
• Refer to the board
Simple Calorimeter
Test Tube
Thermometer
Stirring Rod
Water
Reaction
Sealed
Container
Examples
• 2000 grams of water has its temperature
raised by 3.0 oC. How much heat was
produced?
• How many calories must be added to 5000
g of water to change its temperature from
20oC to 30 oC?
• If 500 g of water at 25oC loses 2500
calories, what will be the final
temperature?
CHEMISTRY
September 23, 2010
Warm Up
• No Warm Up
• Grade Worksheet
Agenda
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Warm Up
Grade Worksheet 1
Complete Unit 3 Notes
Unit 3 Quiz: Conversions and Calorimetry
Unit 3: Research Project
Homework
– Research Project
– Organize Binder
Homework
• Calorimetry Worksheet 1
Energy Measurement
Joule
Energy Notes
Unit:
• Metric = joule (J)
• US = calorie (c)
– 1 calorie is the amount of heat needed
to raise 1 g of H2O by 1 °C
– 1000cal = 1Cal = 1 kilocalorie = food
Calories
Conversion
• 4.184 J = 1 cal
(c to J)
Energy Notes
Example
– A student uses 30 J of energy putting
books on the shelf in the classroom.
How many calories of energy did the
student exert?
– A student eats an entire bag of plan
popcorn which contains 60.1 Calories.
How many Joules of energy is this?
Group Quiz
Calories and Joules
1. Convert 3.26 calories to joules.
2. How many joules of energy are in a
snickers candy bar (320 Calories)?
3. How many calories are in 83.2 kJ?
4. If you ran 6 miles you would burn
approximately 3000. kJ, how many
Calories is this?
VIII. Kinetic Theory
A. Matter has small particles in continuous
motion
B. The faster a particle moves, the greater the
kinetic energy
C. Temperature
– Measure of the average kinetic energy of
particles in the sample.
• At absolute zero, the average kinetic energy is zero
• Higher temperatures have a greater average kinetic
energy
• Samples at the same temperature have the same
average kinetic energy
Figure 3.12: Equal masses of hot
water and cold water separated by a
thin metal wall in an insulated box.
Is anything
going to
happen?
Figure 3.13: The H2O molecules in
hot water have much greater random
motions than the H2O molecules in
cold water.
Figure 3.14: The water samples now
have the same temperature (50°C)
and have the same random motions.
Energy Notes
Heat
• Energy transferred from one body to
another due to a temperature
difference.
– Energy stored in the random motion of
molecules
Examples
• Sunlight heating the earth
• Hot plate with beaker of water
D. Transfer of heat into or out of a sample
– Heat transferred into a sample can be used to
increased the average kinetic energy of the
particles
• This causes an increase in temperature
• When a sample cools, the particles lose
kinetic energy. Heat is given off.
– Heat can also enter or leave a sample
without causing a change in temperature.
• When ice melts, the heat absorbed is used to
rearrange the particles, not to increase the
kinetic energy of the particles
• No change in temperature occurs
Energy Notes
Exothermic
Endothermic
• System that releases energy into its
surroundings
• Release energy because a change
has occurred
• Combustion reactions
• System that takes energy in as heat
from the surroundings
• Need energy from outside source in
order for a change to occur
• Phase change – boil water
Reaction Spontaneity
• Entropy (S) is a measure of the disorder or
randomness of the particles that make up a
system.
• Spontaneous processes always result in an
increase in the entropy of the universe.
• Several factors affect the change in entropy of a
system.
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Changes of state
Dissolving of a gas in a solvent
Change in the number of gaseous particles
Dissolving of a solid or liquid to form a solution
Change in temperature
Unit 3 Quiz
• Refer to Handout
• Make sure you show your work!
– No work = No credit!
Practice Problems
• What amount of heat would be given off by
3.0 x 103 g of water in order to lower its
temperature from 95oC to 12oC?
Practice Problem
• If 5603 joules of heat is added to 5.6 g of
water at a temperature of 15oC, what will
the final temperature be?
• What amount of heat would be given off by
7.0 x 103 g of water in order to lower its
temperature by 5oC?
Practice Problem
• What is the specific heat of lead that has a
mass of 30 g and undergoes a 250oC
change while absorbing 229.5 calories?
• Copper has a specific heat of 0.387 J/g.oC.
What is the mass of a piece of copper that
undergoes a 25oC temperature change
when it absorbs 755 J of energy?