heat, energy and phases of matter energy – ability to do work work - force x distance two types...
TRANSCRIPT
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Heat, Energy and Phases of Matter
Energy – ability to do work Work - force x distance
Two types of energy Potential Energy – stored energy
ex: stretched bowKinetic Energy – energy in motion
ex: swinging a bat
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Energy
Joule - SI unit for measuring energy (J)
Law of Conservation of Energy- Energy can not be created nor destroyed- total energy before and after a reaction must be the same
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Energy and Chemical Reactions
Endothermic - absorption of heat energy in a chemical reaction- increase in potential energy
Exothermic – release of heat energy in a chemical reaction- decrease in potential energy
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Energy and Chemical Reactions
Activation Energy - initial input of energy needed to get a reaction startedex: striking a match – provides frictional heat
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Heat Energy and Temperature
Thermometers are based on the principle of expanding liquid
Celsius Scale – is what our thermometers measure in
0 oC = freezing point of water
100 oC = boiling point of water
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Heat Energy and Temperature
Fahrenheit Scale - not part of the SI system
- Is what our weather is measured in.
32 oF = freezing point of water
212 oF = Boiling point of water
oF = (oC x 1.8) + 32
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Heat Energy and Temperature
Kelvin Scale – absolute zero represents the theoretical lowest possible temperature
- absolute zero has yet to be reached- Unit = K- Absolute zero = -273.15 oC- Conversion:
- K = oC + 273- oC = K - 273
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Heat and It’s Measurement
Calorie – non-SI unit to measure quantity of heat- quantity of heat that will increase the temperature of 1 gram of water by 1oC
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Calorimetry
Measurement of the amount of heat released or absorbed during a chemical reaction
Instrument used to measure this heat is called a Calorimeter
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Calorimetry
Reaction occurs inside a reaction chamber surrounded by an unknown mass of water
- heat released – enters the water and raises its temperature
- Heat absorbed – lowers the temperature of the water
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Calorimetry
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Calculating Heat
Q = mC∆TWhere: Q = heat (either lost or absorbed)
unit = Joule or caloriem = mass of substance
unit = gramC = specific heat of the substance
unit = J/g.oC or cal/g.oC∆T = change in temperature
unit = oC
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Calculating Heat
Specific Heat – amount of heat energy required to raise the temperature of 1gram of a substance by 1 degree Celsius
C for water = 1 cal/g.oC
or 4.184 J/g.oC
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Q = mC∆T
Ex: How much heat (J) is needed to raise 138.0g of water at 18.0 oC to 75.0 oC?
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Q = mC∆T
Ex: How much heat was lost as 458.0g of water was cooled from 50.0 oC to 15.0oC?
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Q = mC∆T
Ex: What is the final temperature of 15.0g of water if 238J of heat were added when it was 8.0 oC?
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Q = mC∆T
Ex: A piece of unknown metal with a mass of 23.8g is heated to 100.0 oC and dropped into 50.0 ml of water at 24.0 oC. The final temperature of the system is 32.5 oC. What is the specific heat of the metal?
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Phase Changes
During a phase change, we use this formula:
Q = mass X heat of fusion (of heat of vaporization)
Heat of fusion for water = 333 j/gHeat of vaporization = 2260 J/g
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Phases of Matter
Pressure - force exerted on one unit area
- SI unit = Newton (N)- other units – atm, torr,
mm Hg, psi, and kPa equalities
1 atm = 760 mm Hg = 101.3 kPa = 760 torr
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Pressure
Liquid pressure – exerted equally in all directions- swimmers feel an increase in pressure as they go deeper down into the ocean
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Pressure
Atmospheric Pressure – (air pressure, barometric pressure)- at sea level, air pressure = weight of a kg mass on every square centimeter of surface exposed to it** we are not conscious of air pressure because it is exerted in all directions
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Measuring Pressure
Hg Barometer – measures AIR PRESSURE
- Pressure varies with altitude - Decrease in air pressure as you
increase altitude- Drop in air pressure before a storm- Normal Atmospheric Pressure - 760
mmHg or 1atm
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Measuring Pressure
Manometer – measures pressure exerted by a gas in a closed container- 2 types – closed and open
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Measuring Pressure
Closed Tube Manometer
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Phase Changes
As a substance is heated, the particles gain energy, vibrate farther and farther apart, until they have enough energy to break out from the fixed pattern and enter a new phase
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Boiling and Melting
Water boils at 100 oC –NOT NECESSARILY TRUE
- Depends on the atmospheric pressure
- Less pressure – boils faster- Ex: baking recipes in high
altitudes
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Boiling and Melting
Boiling Point - Temperature at which a substance changes from a liquid to a gas
Melting/Freezing Point - temperature at which a substance changes from a solid to a liquid (melting) or from a liquid to a solid (freezing)
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Other phase changes
Evaporation – process by which liquid water enters the gas phase
Condensation – (reverse of evaporation) process by which a gas is cooled from above the boiling point
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Other Phase Changes
Sublimation – process by which a solid changes directly to a gas without passing through the liquid phaseie: moth balls, dry ice, iodine
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Heating Curve Diagram
Temperature – Energy graph that describes the way a substance absorbs heat as it moves from the solid to liquid to gas states
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Generic Heating Curve
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Heating Curve Diagram
http://www.kentchemistry.com/links/Matter/HeatingCurve.htm
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Heating Curves
Heat of Vaporization – amount of energy needed to vaporize a mass of liquid at a given temperature
Q = mHvwhere: m = mass (g)
Hv = enthalpy of heat = constant value= Hv for water = 2260 J/g
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Heating Curves
Heat of Fusion – heat needed to change a mass of solid to a liquid at a given temperature
Q = mHfwhere: m = mass (g)
Hf = enthalpy of fusion = constant value = Hf for water = 334 J/g
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How much heat is needed to raise 15g of water from 35.8 oC to 118 oC?
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How much heat is needed to raise 25g of water from -24.5 oC to 434 oC?
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http://www.dlt.ncssm.edu/TIGER/Flash/phase/HeatingCurve.html
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Phase Diagram
http://www.youtube.com/watch?v=zSwG59d8OCc
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Phase Diagram
Triple Point – point at which all three phases can coexist
Critical Point – Point at which a substance can no longer exist as a liquid regardless of the temp. or pressure
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Phase Diagram
Normal Melting/Freezing Point – melting/freezing point of a substance at standard pressure (ex: 1 atm or 760 mmHg)
Water’s Normal MP = 0 0C
Normal Boiling Point – boiling point of a substance at standard pressure (ex: 1 atm or 760 mmHg)
Water’s Normal BP = 100 oC
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Vapor Pressure
Pressure of a vapor in equilibrium with its liquid- in a container equilibrium is reached when the space above the liquid holds as much vapor as it can (saturated)- as temperature of a liquid increases, the liquids vapor pressure increases- when the vapor pressure = atmospheric pressure, the liquid begins to boil****Heating of a boiling liquid DOES NOT raise its temperature
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Vapor Pressure Curve