Thermodynamics

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Thermodynamics

• The study of heat and energy

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So far

• Weve shown that matter (solids, liquids, etc)
  can be changed and rearranged in chemical
  reaction.
• However, we must also consider that energy is
  exchanged during all chemical and physical
  changes.

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Energy

• Energy The capacity (ability) to do work and/or
  supply heat. Exists in many different forms.
• Chemical potential energy is the type of energy
  stored in chemicals and released/absorbed in
  reactions.
• Gasoline, nitroglycerine, etc all have high
  chemical PEs.

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Movement of energy

• When describing the flow of energy, it is useful
  to set a few boundaries
• First, we will be examining what is known as a
  system, which will almost always be described by
  a chemical equation.

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System vs Surroundings

• A system is the part of the universe that we are
  studying while EVERYTHING else is known as the
  surroundings.
• In your notebooks, give me at least 2 examples of
  a system and its surroundings.
• For example a medical study would take a part
  of your body as its system while the rest of
  your body and universe would be the surroundings.

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Types of Systems.

• There are three different systems
• Open open to the surroundings and able to trade
  both matter and energy.
• Closed trades energy but not matter.
• Isolated trades absolutely nothing.
• In your notebooks, give me one example of each
  type. Closed and open systems are easy. For
  isolated, think of ways you keep food warm or hot.

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Energy flow

• Energy, like most things, likes to spread out to
  its surroundings. This is why hot objects cool
  off and vice versa.
• Keep in mind, energy is not destroyed nor is it
  created- just moved around.
• (E q w)
• Energy flow comes in two types endothermic and
  exothermic.

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Endo vs. Exo

• Endothermic processes (systems) absorb heat from
  the surroundings. Think cold.
• Reactions that build molecules, such as
  combination and synthesis.
• Ice absorbing heat in order to melt.

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Endo vs. Exo

• Exothermic reactions/systems release heat into
  the surroundings. Think hot.
• Destruction of chemicals by combustion and
  decomposition are good examples.
• Fire, explosions, digesting food, etc.

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• PRACTICE 5.15

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Measuring heat and energy.

• The two units most often used to measure heat and
  energy are the joule (J) and calorie (c).
• Calories in food are actually designated with a
  C, not c, and stand for kilocalories, 1000
  calories each.

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Measuring heat

• A joule is the amount of energy needed to lift 1
  kg (about 2.2 lbs) one meter off the floor
  against the force of gravity.
• We use this to describe work done by chemical
  reactions.
• A calorie is the amount of energy needed to heat
  up one gram of water exactly 1C.
• This unit is usually used to describe heating and
  cooling processes.

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Heat

• Heat (q) is the energy that is transferred from
  one object to another.
• () heat means energy goes into the system.
  (ENDOthermic)
• (-) heat means energy goes out of the system.
  (EXOthermic)

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Heats of Reaction

• Enthalpy (?H) is a measure of the energy absorbed
  or released by a chemical reaction. Same as q,
  but for chemical rxns only.

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• Enthalpy is calculated under constant pressure,
  so work is 0. (in other words, q ?H).
• Found by subtracting the energy of the reactants
  from that of the products.
• PRACTICE 5.27

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Specific Heat and Heat Capacity

• Heat capacity (C) is the energy needed to raise
  the temperature of an ENTIRE OBJECT by 1C.
• Specific heat (c) is the energy needed to raise
  the temperature of 1 GRAM of an object by 1C.

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• Objects with a LOW heat capacity heat up more
  quickly than objects with HIGH heat capacity.
• Metals, being better conductors, heat up more
  quickly than water, styrofoam, and wood.

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• Thus, heat capacity and specific heat are a
  reflection of how fast something heats up or
  cools off.
• High c/C slow to heat/slow to cool (water)
• Low c/C fast to heat/fast to cool (metals)

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Why the difference?

• Temperature is NOT energy, but is the result of
  energy causing atoms to vibrate.
• All the different atoms and compounds react when
  heated, but in varying amounts.
• More vibration more TEMPERATURE, but NOT
  necessarily more energy.

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• Glass and water molecules vs. metal atoms. Note
  that glasses and metals form connections between
  each other.

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Calorimetry Methods

• Constant Pressure (coffee-cup calorimetry)
• Solution rxns and others where expansion is not
  an issue.
• q mc?T
• Constant Volume (bomb calorimetry)
• Used when the system expands non-negligibly.

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Practical use in the lab

• The study of heat changes in chemical and
  physical processes is called calorimetry.
  (literally, measurement of calories.)
• The formula is
• q mc?T
• q heat, or enthalpy
• m mass
• c specific heat
• ?T change in temperature

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Example

• How much heat would it take to boil a full pot of
  water (about 4000 g) if you start at room
  temperature (25C)?

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Solution

• q mc?T
• q ?
• m 4000 g
• c for water, 4.184 J/g C
• ?T Water boils at 100C so, 100-25 75C
• Put it all together q (4000)(4.184)(75)
• q 1,255,200 J (alot of energy)

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Practice

• 5.39
• 5.44

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Conductors and Insulators

• Conductors HELP energy flow while insulators SLOW
  energy down.
• Heat travels through many objects through the
  vibrations of atoms. The more tightly packed
  together atoms are (more dense), the more easily
  energy can flow.

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What it all means

• The more dense something is, the easier it can
  conduct energy. Metals, for example, do this
  very well.
• Insulators use less dense materials such as air
  to keep you warm, such as a coat (keep your heat
  from escaping.)

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Question 1

• A gas is a good _______, and has a _____ heat
  capacity.
• A. Conductor, low
• B. Conductor, high
• C. Insulator, low
• D. Insulator, high

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Question 2

• Given a block of metal and a pan of water
  weighing the same amount.
• Which would heat up most quickly and which would
  cool off the most slowly?
• A. Water/Water
• B. Metal/Metal
• C. Metal/Water
• D. Water/Metal

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Question 3

• What best describes this object?
• A. High heat capacity, insulator
• B. Low heat capacity, conductor
• C. High heat capacity, conductor
• D. Low heat capacity, insulator

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Heat flow in chemical reactions

• Heat flow is given by q, but for chemical
  reactions we refer to the term enthalpy, ?H
  (pronounced as delta H.
• A closely related value, known as entropy, is
  given by ?S and stands for the amount of energy
  lost to disorder and chaos.

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Phase changes

• All substances must gain or lose energy in order
  to undergo phase changes.
• Gas ? Liquid ? Solid (lose energy, exo)
• Solid ? Liquid ? Gas (gain energy, endo)
• Dissolving solids into liquids (usually endo)

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Heats of Fusion/Solidification

• The heat of fusion (?Hfus) is the energy gained
  by a solid to go to a liquid, energy lost to
  return to a solid is known as heat of
  solidification (?Hsolid) .

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Heats of Vaporization/Condensation

• The heat of vaporization (?Hvap) is the energy
  gained to change from a liquid to a gas and the
  reverse is the heat of condensation (?Hcond) .

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Dissolving

• Any liquid containing a dissolved substance is
  known as a homogeneous mixture, or solution.
• The energy needed to dissolve something is called
  the heat of solution, (?Hsoln) and is always
  endothermic, .

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Phase Changes and Temperature- Water

• A- solid (ice) warming up to 0C
• B- ice melting and STAYING at 0C (solid and
  liquid together)
• C- liquid water warming to 100C
• D- liquid boiling at 100C to steam (gas)
• E- steam heating beyond 100C

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Phase changes

• Every substance has its own unique heats for each
  type of phase change. The higher the heat
  capacity, the higher these values will be.
• Phase changes are isothermal, meaning that the
  temperature stays the same until the change is
  complete.

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Question 4

• The study of heat flows in physical and chemical
  processes is known as?
• A. Chemistry
• B. Biothermistry
• C. Thermometry
• D. Calorimetry

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Question 5

• Can liquid water exist at 0C?
• A. Yes
• B. No
• C. Only if you add chemicals to it
• D. Not enough information

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Question 6
(Rain)

• Which enthalpy would be used here?
• A. ?Hfus
• B. ?Hcond
• C. ?Hsoln
• D. ?Hsolid

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Homework!

• Read 5.1-5.3 (Pages 145-154)
• Key Terms and a brief summary (3-4 sentences) for
  each section.
• 5.8, 5.16, 5.19, 5.20, 5.23
• Due to be handed in Tomorrow!

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Hesss Law

• Hesss Law states that reactions that occur in a
  series of steps have a total enthalpy equal to
  the sum of the individual steps.
• Practice- 5.53

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Heats of Formation

• Also known as enthalpies of formation (?Hf)
  measure of the energy change when a substance is
  formed from raw elements.
• Values are noted as standard ( ex. ?H,
  pronounced delta H not) when elements are in
  their standard states.