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

• 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

• 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 - OR-

amount of heat it can be changed intoSI unit of measure = Joule (J)

• Part III: Energy Conversions and Energy EfficiencyA. Energy is changed, or converted, from one form to anotherB. 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

Stirring RodThermometer

Reaction

Water

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 TemperatureA. 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

Stirring RodThermometer

Reaction

Water

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

• 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:

Conversion (c to J)

• 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

• 4.184 J = 1 cal

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 TheoryA. 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

Examples

• Energy transferred from one body to another due to a temperature difference.– Energy stored in the random motion of

molecules

• 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. – 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?