Challenge: To determine the melting point of water. Your write up should include the following components : I. TITLE II. PURPOSE III.MATERIALS- IV. PROCEDURE.
Post on 26-Dec-2015
215 Views
Preview:
Transcript
• Challenge: To determine the melting point of water.• Your write up should include the following
components:• I. TITLE• II. PURPOSEIII.MATERIALS- • IV. PROCEDURE• V. DATA• VI. CALCULATIONS/GRAPH• VII.CONCLUSION (including a discussion of sources of
error.)•
• Types of energy
• POTENTIAL ENERGY :• STORED ENERGY. The energy inside the
substance.• KINETIC ENERGY : Associated with motion.• Average KE = TEMPERATURE
HEAT OF FUSION FOR WATER (TABLE B)
• Amount of heat needed to completely melt 1g of water (ice!).
• 334 J/g• 334 Joules of heat are necessary to
completely melt 1 g of water.• HOW MUCH HEAT IS NEEDED TO MELT 10 g
OF WATER?
HEAT OF VAPORIZATION FOR WATER (TABLE T)
• The amount of heat needed to completely vaporize one g of water at its boiling point.
• 2260 J/g
• Water needs 2260 J of heat per gram to convert to gas!
Summarizing
When there is a change in temperature useQ = m x C x T While the substance is melting/freezing use Q = m x H f While the substance is boiling/condensing use Q = m x H vap
Do now!
• How much heat is needed to completely melt 10 g of ice at 0 0 C ?
• How much heat is needed to vaporize 10 g of water at 100 0 C ?
Energy Changes Associated with Changes of State
• The heat added to the system at the melting and boiling points goes into pulling the molecules farther apart from each other.
• The temperature of the substance does not change during a phase change !!!
Objectives• Describe what happens with PE and KE as
phase changes• Identify MP and BP from heating curve• MP=FP and BP=condensation point• Calculate the Hfus and Hvap from the heating
curves• Cooling curves
Objective: How to calculate heat from heating curves
Do now: Draw a heating curve for the heating of 2 grams of substance that are being heated at a rate of 50 J/min.Melting Point 20 CBoiling Point 80 C Substance begins to melt at minute 2 and melts for 1 minute.It begins to boil at minute 8 and it takes 3 minutes to completely boil.
Endothermic Phase Changes
• If the substance is melting or boiling, heat is being absorbed, and is being used to change the state of matter.
• DURING THE PHASE CHANGE THE AVERAGE KINETIC ENERGY DOES NOT CHANGE!!! THE POTENTIAL ENERGY INCREASES.
Melting Point and Freezing Point
• The temperature at which a substance melts. Is the same temperature at which the substance freeze.
• Boiling Point and Condensation Point are the same temperature.
Normal boiling point is the temperature at which a liquid boils at normal pressure.
Exothermic changes
• If the substance is undergoing condensation or freezing then heat energy is being released. The potential energy is decreasing and the TEMPERATURE REMAINS CONSTANT!!!
Objective: How to represent heat changes
Do now: Draw a cooling curve for the heating of 50 grams of substance that are being heated at a rate of 200 J/min. At minute 0 the substance is a gas. Substance begins to condense at minute 2 and it takes 7 minutes to turn completely to liquid at 70C.At minute 16 it begins to freeze and it takes 3 minutes till it solidifies completely at 20 C.Indicate Boiling Point, Melting Point and calculate heat of fusion and heat of vaporization.
VAPORIZATION IS ENDOTHERMIC
• In hot climates drinking water is cooled by evaporating water from the surfaces of porous clay pots. As water evaporates it ABSORBS heat from the water inside the container which is maintained cool.
• Like cooling yourself off on a hot day by pouring water over your body. As water evaporates it absorbs heat
FREEZING IS EXOTHERMIC
• In freezing weather, citrus crops are sprayed with water to protect the fruit from frost damage. As the water freezes (around the fruit-outside the fruit!) it releases heat, which helps to prevent the fruit from freezing.
How to calculate the amount of heat that a substance absorbs while it is being heated up.
• DO NOW• 1 .-Calculate the amount of heat needed to increase the
temperature of 200g of water from 0 0C to 100 0C. The heating rate is 100J/min.
• 2.-How much heat is needed to completely vaporize 200 g of water at 100 0C
Heating curve Homework answers
1) 20 C2) b and d 3) KE (temperature) remains
constant PE increases 3) S and L 4) 20 C 5) 60 C 6) Gas 7) Increases (is T!)
8) E
TOPICS FOR TEST
• TEMPERATURE• SCALES/CONVERSIONS• HEAT TRANSFER• CALORIMETRY
PROBLEMS• SPECIFIC HEAT
• PHASES OF MATTER• HEAT OF VAPORIZATION
AND HEAT OF FUSION• ENDO AND EXO
CHANGES• HEATING AND COOLING
CURVES• BRING PENCIL AND
REFERENCE TABLES• AND CALCULATOR!!!!!
HEATING/COOLING CURVE WORKSHEET ANSWERS
1. 40 C2. BC AND DE3. AB, CD AND EF4. 9min x 60 kJ/min= 540kJ5. 7minx 60kJ/min= 420 kJ6. 24minx60kJ/min = 140 kJ7. 8minx60kJ/min=480kJ Hvap= 480kJ/10kg Hvap=48J/g
8. AB9. CD10. EF11. DE
THE ROLE OF ENERGY IN CHANGES OF PHASE ANSWERS (Fill in)1. PHASE2. ABSORPTION
(ADDITION)3. MELTING4. BOILING5. TEMPERATURE6. KINETIC7. MOLECULES8. POTENTIAL
9. HEAT10. KINETIC11. POTENTIAL12. COOLING13. DECREASE14. DECREASING
How to calculate heat from heating curves
9) Use information in the heading of the problem
Mass of substance=Heating rate=100 Joule/minute
20minx100J/min=2000J
2000J/20 g = 100J/g10)1000J/20 g = 50 J/g11)60 C
Twenty grams of a solid substance is heated starting from a temperature below its melting point at a constant
rate of a 100 joule/minute.
Heat Answers• SET 11. 42. 43. 44. 45. 16. 27. 28. 39. 3
• SET 2• 13 3• 14 4• 15 4• 16 3• 17 3• 18 2• 19 2• 20 3• 21 122J/g x 7.5 g= 915 J
• q (amount of heat ) table T
• q = m C T while T is changing (one phase)
• q = m x Hfus during melting (2 phases- T constant )
• q = m x Hvap during boiling (2 phases- T constant )
Heat of Fusion Hfus
Amount of heat needed to completely melt 1 gram of substance at its melting point.
At the melting point the heat is calculatedHeat = m x Hfus
top related