Energy

Unit 6

What is Energy?

A burning match releases energy.

Source: ElektraVision/PictureQuest

A Hindu firewalking ceremony in the Figi Islands. Could you do this?

Source: Jack Fields/Photo Researchers, Inc.

Energy Notes

Energy – Capacity to do work or produce heat– Capacity to move or change matter

Energy Notes

Work

Examples

• Moving an object over a distance against a resisting force– Energy stored in the organized motion of

molecules

• A car traveling down the street• A train moving down the tracks• Pushing a book across the desk

Types of Energy• Stored

– Energy of position (water wheel, book on desk)

– Chemical energy (gas, food, batteries)

• Motion– Mechanical energy (moving parts of

machines)

• Thermal energy (Sun’s heat) -Light

• Vibration of molecules

Potential

Kinetic

Radiant

Sound

Energy Notes

Law of Conservation of Energy

• In any chemical or physical process, energy is neither created nor destroyed.– Different forms of energy are equivalent.

• A particular amount of one type of energy can be converted into an exactly amount of another type of energy.

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?

– STOP Period 6

Brain Teaser

• Explain why the rug on your floor feels warmer than the tile in the same room

• Convert 200 Calories to Joules– 4.184 Joules = I calorie– 1 Calorie = 1000 calories

Agenda

• Brain Teaser• Collect Energy Lab Packet• Energy Notes:

– Heat and Temperature– Specific Heat

• Homework– Temperature Conversion Worksheet

Heat and Temperature

Continue on this slide (P5)

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.

Heat and Temperature

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

Heat and TemperatureExothermic

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

Endo vs. Exo

• Why are these types of reactions useful?• What are some examples in your daily life?

Temperature

Temperature

3 scales

• Measure of the average kinetic energy of the particles in a sample of matter.

• Fahrenheit (ºF)• Celsius (ºC)• Kelvin (K)

Temperature

Converting between ºC and K

Example

Practice

• ºC = K – 273• K = ºC + 273

• Normal human body temperature is 37 ºC. What is your temperature in K?

• Surgical instruments must be sterilized at 170 ºC. What is this in K?

Temperature

Converting between ºC and ºF

Example

Practice

• ºC = 5/9(ºF – 32)• ºF = 9/5 ºC + 32

• It is 100 ºF outside. What temperature is this in ºC?

• The boiling point of methanol is -164 ºC. What is the boiling point is ºF?

Heat versus Temperature(continue on this slide P2,4)

• 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

Refer to Temperature Worksheet

Brain Teaser (1/17)

• Convert 200 Calories to Joules

Agenda

• Brain Teaser• Notes: Specific Heat and Calorimetry• Post Lab Activity: Energy Lab• Homework

– Practice Problems 1-6

Specific Heat and Calorimetry

Specific Heat and Calorimetry

• Specific Heat

• What this means

• amount of heat that can be absorbed by 1 gram of material when its temperature increases by 1oC; unit = J/g oC

• What bench would you rather sit on, wood or iron, in the summer in AZ? Why?

Specific Heat• Which holds more heat when

exposed to 120oF sunlight -- a wood or an iron bench (both at the same temperature)?

• Compare the specific heat capacities.

Iron 0.448 J/goC, wood 1.76 J/goC• The wood bench holds onto

more heat so it does not transfer as much to you.

Specific Heat and Calorimetry

• Calorimetry

• Calorimeter

• Measurement of the amount of energy released or absorbed during a chemical reaction

• Device used to measure the transfer of energy to water

Complex Calorimeter

Simple Calorimeter

Test Tube

Stirring RodThermometer

Reaction

Water

Sealed Container

Endo vs. Exo

• Endothermic- reaction absorbs energy temperature of the surrounding water goes down

• Exothermic - reaction gives off energy temperature of the surrounding water goes up

Energy

How does the Heat Flow and How do you Know?

Three factors determine the amount of energy (Q) transferred to or from the water.

Q= mCpT 1. Mass of the water in the calorimeter (units = grams)

2. Specific Heat of water

3. Change in the water’s temperature. T = Tfinal- Tinitial

(units = ºC)

Cg

Jor

Cg

cal

184.41

Q= mCpT• Example 1

• Practice 1

• How many calories must be added to 5000 g of water to change it temperature from 20 to 30ºC?

• How many calories are needed to increase the temperature of 150.0 grams of water from 40.0 to 65.0ºC?

Cg

Jor

Cg

cal

184.41specific heat

for water

Q= mCpT• Example 2

• Practice 2

• A 2.0 x104 grams sample of water has its temperature raised by 3ºC. How many Joules of energy was produced?

• What is the number of Joules needed to increase the temperature of 50.0 grams of water 15.0ºC?

Cg

Jor

Cg

cal

184.41specific heat

for water

Q= mCpT• Example 3

• Practice 3

• If 500.0 g of water at 25ºC loses 2500 calories, what will be the final temperature?

• What is the final temperature after 80.0 calories is absorbed by 10.0 grams of water at 25ºC?

Cg

Jor

Cg

cal

184.41specific heat

for water

Q= mCpT• Example 4

• Practice 4

• What temperature change,in Celsius, is produced when 6.0 x102

grams of water gives off 9.60 kilocalories?

• What is the change in temperature when 640 calories is given off by 40 grams of water?

Cg

Jor

Cg

cal

184.41specific heat

for water

Q= mCpT• Example 5

• Practice 5

• What is the specific heat of lead if a 30.0 g piece of lead undergoes a 250ºC change while absorbing 229.5 calories?

• What is the specific heat of an unknown substance if the addition of 950 J of heat energy caused a 20 gram sample to warm from 18ºC to 42ºC?

Q= mCpT

• Example 6

• Practice 6

• A quantity of water is heated from 25.0ºC to 36.4ºC by absorbing 325 calories. What is the mass of the water?

• What is the mass of a piece of copper that undergoes a 25.0ºC temperature change when it absorbs 755 J of energy? Copper has a specific of

Cg

J

387.0

PreLab: Energy Content in Foods

Time to get your lab books ready for Lab!

Use the lab handout to help you complete all of the following according to the grading rubric.

1. Title2. Purpose3. Background information4. Materials5. Procedure and safety6. Set up data table

Class ResultsGroup Peanut

J/g

Potato Chip

J/g

Popcorn

J/g

Marshmallow

J/g

1

2

3

4

5

6

7

8

Ave.

Class Results 6th PeriodGroup Peanut

J/g

Cashew

J/g

Popcorn

J/g

Marshmallow

J/g

1

2

3

4

5

6

7

8

Ave. 16,500 11,300 687 2830

Review Lab- Energy Content in Foods

5. Food energy is expressed in Calories. 4.184 kJ in 1.000 Calories. Based on the class average for peanuts, calculate the number of calories in a 50.0 g package.

6. Product labels. Determine a serving size and use the class average to calculate the number of calories present in a single serving. Compare your results to the nutritional information.

Review Lab- Energy Content in Foods

1. What are 2 possible sources of error? What could be done to reduce these errors?

2. How does calorimetry make use of the law of conservation of energy?

3. What generalization can you make about the relative energy content of fats and carbohydrates?

Product Labels

16 oz = 453.59 gPeanuts – 1 oz, 180 CaloriesCashews – 1 oz, 160 CaloriesPopcorn – 1 cup popped, 25 Calories (7g~1 cup)Marshmallow – 30 g, 100 Calories