Thermochemistry and Energy

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Thermochemistry and Energy

• Water freezing and boiling at the same time,
  really?!

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• Thermochemistry is the study of energy changes
  that occur during chemical reactions and phase
  changes (changes of state)

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The Nature of Energy

• Energy is the ability to do work or produce heat
• Heat (q or Q) is energy flows due to temperature
  differences (always hot to cold)

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System
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Law of Conservation of Energy

• Energy cannot be created nor destroyed only
  converted into different types.

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Kinetic vs. Potential Energy

• Two main types of energy kinetic and potential
• Kinetic energy of motion
• Potential energy due to position or energy
  stored in chemical bonds
• Chemical potential energy - the energy stored in
  a substance because of its composition
• Example gasoline

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Kinetic vs. Potential Energy

• Two main types of energykinetic and potential
• Kineticenergy of motion
• Potentialenergy due to position or energy stored
  in chemical bonds
• Chemical potential everythe energy stored in a
  substance because of its composition
• Example gasoline

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Temperature vs. Heat

• Temperature is a measure of the motion in a
  sample
• Temperature is a measurement of heat.
• Heat is the total energy of molecular motion,
  dependent upon amount, size, and type of
  particles. Heat is energy.

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Units of Heat

• calorie - the amount of heat required to raise
  the temperature of one gram of pure water by one
  degree Celsius
• Calorie nutritional calorie
• 1 Calorie 1000 calories 1 kilocalories
  (kcal)
• Joule SI unit of heat
• 1 calorie 4.184 J

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Converting Energy Units

• Calorie/calorie/kilocalorie calorie/Joule

1 C 1000 cal 1 kcal
1 cal 4.184 J
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Example 1

• A cereal has 155 nutritional Calories per
  serving. How many calories, kilocalories and
  Joules is this?

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Example 2

• A person on a diet consumed 1350 Calories in one
  day. How many calories, kilocalories and Joules
  is this?

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System and Surroundings

• Universe system surroundings
• System the specific part of the universe you
  wish to study. In chemistry this is your chemical
  reaction/physical process
• Surroundings Everything else in the universe
• When heat is transferred it can flow in or out of
  the system

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Endothermic vs. Exothermic

• An Exothermic process is one that releases heat
  to its surroundings (feels warm)
• Energyproducts lt Energyreactants
• An Endothermic process is one that absorbs heat
  from the surroundings (feels cold)
• Energyproducts gt Energyreactants

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Exothermic Process
Eproducts lt Ereactants
Potential
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Endothermic Process
Eproducts gt Ereactants
Potential
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Q and heat flow

• Exothermic process, heat is released, q is
  negative (-)
• Endothermic process, heat is absorbed, q is
  positive ()

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Specific Heat Cs

• Specific heat of a substance is the amount of
  heat required to raise the temperature of one
  gram of that substance by one degree Celsius.
• Unit for specific heat is J/goC

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• Each substance has a different specific heat
• Water 4.184 J/goC
• Gold 0.129 J/goC
• Copper 0.386 J/goC
• The lower the specific heat the lower the amount
  energy is required to raise its temperature.

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Calculating Heat Released and Absorbed

• Q mCs(?T)
• Q Heat (Joules or kilojoules)
• m is mass (gram)
• ?T is temperature change (C) Tfinal -Tinitial
• Cs is specific heat at a constant pressure (J/gC)

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Example 3

• If the temperature of 56.6g of ethanol increases
  from 45.0C to 80.0C, how much heat has been
  absorbed by the ethanol? Specific heat of ethanol
  2.44J/gC

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Example 4

• A 4.00g sample of a substance was heated from
  274K to 314K and absorbed 32J of heat, what is
  the specific heat of the substance?

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Example 5

• If 98000J of energy are added to 6200g of water,
  what will the change in temperature of the water
  be? Specific heat of water is 4.184 J/gC