chapter 10: energy honors chemistry. what is energy? the ability to do work or produce heat
TRANSCRIPT
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Chapter 10: EnergyChapter 10: Energy
Honors ChemistryHonors Chemistry
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What is energy?What is energy?
The ability to The ability to do work or do work or
produce heatproduce heat
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But first…But first…• State functionState function: a property that : a property that
changes changes independently of its pathindependently of its path• Real life examples:Real life examples:
• DisplacementDisplacement• TanningTanning
• Chemistry examples:Chemistry examples:• Changes in Changes in energyenergy• Changes in Changes in temperaturetemperature
∆∆X = XX = Xfinalfinal – X – Xinitialinitial
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Two general classes of Two general classes of energy:energy:
• Potential energy: • Energy that has the potential of
being converted to other forms of energy (like kinetic energy) and do work in the process
• Think:• Water behind a dam• Raised object• Two charges separated in space
• Kinetic energy: • Energy due to motion
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Law of Conservation of Law of Conservation of EnergyEnergy• Energy can neither be created nor
destroyed• It can only be converted from one form to
another
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Thermodynamics!Thermodynamics!• The study of energy• Law of Conservation of Energy is AKA the First
Law of Thermodynamics:
The energy of the universe is The energy of the universe is constantconstant!!
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Types of energy…Types of energy…• Radiant energy: comes from the sun
and is earth’s primary energy source• Thermal energy: energy associated
with the random motion of atoms and molecules
• Chemical energy: energy stored in chemical bonds
• Nuclear energy: energy stored in the neutrons and protons of an atom
• Electrical energy: potential energy associated with the forces between charged particles
• Light energy
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Hot! Hot! Hot!Hot! Hot! Hot!
• Heat: energy transferred from a hotter object to a colder one• Heat flows due to
temperature differences• No temperature difference
= no heat flow
• Symbolized by q (or Q)
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More on heat…More on heat…
• What was heat again?!?• Your answer:
• Heat cannot be measured directly• Changes due to heat can be
measured• How do you think we do this?!?
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Temperature…Temperature…
100°C
Hi TE
25°C
Med TE
0°C
Lo TE
• Temperature is a measure of the thermal energy of something
• Thermal energy is how much random movement there is
• Temperature ≠ thermal energy• But the greater the
thermal energy, the higher the temperature
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Defining a system and its Defining a system and its surroundings…surroundings…• System: part of
the universe being studied• Usually reactants
and products
• Surroundings: everything else in the universe Surroundings:
Everything elseSystem:
Reactants and Products in flask
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Thermochemistry is the study of heat change in chemical reactions
openmass & energyExchange:
closedenergy
isolatednothing
SYSTEMSURROUNDINGS
System:
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Exothermic vs. Exothermic vs. Endothemric ProcessesEndothemric Processes
Exothermic• Process that gives of
energy• Energy flows out of
the system
• 2H2 (g) + O2 (g) 2H2O (l) + Energy
• H2O (g) H2O (l) + Energy
Endothermic• Process that requires
energy• Energy flows into the
system
• Energy + 2HgO (s) 2Hg (l) + O2 (g)
• Energy + H2O (s) H2O (l)
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Exothermic Reaction
• Reactants have more potential energy than products• Potential energy is stored
in chemical bonds
• Amount of energy released by system (c)
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Endothermic Reaction
• Reactants have less potential energy than products
• Amount of energy absorbed by the system (c)
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Exothermic Endothermic
Reactants
Reactants
2H2 (g) + O2 (g) 2H2O (l) + Energy
Products
Products
Energy + 2HgO (s) 2Hg (l) + O2 (g)
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• ∆E = Efinal – Einitial
• ∆E = Eproducts – Ereactants
• ∆E = (–) number
Exothermic Endothermic
Reactants
Reactants
2H2 (g) + O2 (g) 2H2O (l) + Energy
Products
Products
Energy + 2HgO (s) 2Hg (l) + O2 (g)
• ∆E = Efinal – Einitial
• ∆E = Eproducts – Ereactants
• ∆E = (+) number
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Measuring Energy Changes
• Calorie: amount of energy (heat) required to raise 1g of H2O by 1C • Unit = cal
• Kilocalorie: 1000 calories • Used to measure energy
content of food• Unit = kcal or Cal
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More units of energy…
• Joule (SI unit of energy)• Unit = J
• 1 cal = 4.184 joules
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Energy required to change temperature depends on…
1. Amount of substance being heated (grams)
2. Temperature change (°C or K)3. Material that is being heated
• Different materials respond differently to transfer of heat• Know: 1 cal, or 4.184 J, of energy raises the
temperature of 1 g of H2O by 1°C• But… 4.184 J of energy raises the temperature
of 1 g of silver metal by 17°C!!• Think: wooden vs. metal spoon!
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Specific Heat Capacity
• Amount of energy (heat) required to change the temperature of 1g of a substance by 1C
• Unit = J/g °C = Jg-1 °C-1
• Symbolized by “s” (“c” in other books?!?)
• See table on page 279…
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Heat requirements…
•Q = m x s x T •Q = amount of energy (heat)
required to heat up or cool down a sample
•m = mass of sample (g)•s = specific heat capacity of
sampleT = change in temperature
desired