Thermochemistry

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Thermochemistry
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• When energy is changed from one form to another,
  all of the energy can be accounted for. This is
  called the Law of Conservation of Energy.

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• Heat is energy. If heat is released by a chemical
  system, an equal amount of heat will be absorbed
  by the surroundings

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• (For example) - When your body breaks down sugar
  completely, the body releases the same amount of
  heat as compared to burning the same amount of
  sugar in a flame.

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• Heat is transferred due to a temperature
  difference. Faster molecules speed up the slower
  molecules. This slows the faster molecules.

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• In an endothermic process, the system absorbs
  heat from the surroundings. A process that
  absorbs heat is a(n) endothermic process.

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• In an exothermic process, the system gives off
  heat to the surroundings.

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• If you were to touch the flask in which an
  endothermic reaction was occurring, the flask
  would probably feel colder than before the
  reaction started.

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• In an exothermic reaction, the energy stored in
  the chemical bonds of the reactants is greater
  than the energy stored in the bonds of the
  products. Heat is given off.

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• As perspiration evaporates from your skin, your
  body is cooled.
• With respect to your body, this process is said
  to be exothermic.

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• A calorie is the quantity of heat that raises the
  temperature of 1 gram of pure water by 1?C.
• 1 Calorie 4.18 kJ of heat

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• How many kJ of energy can be released by a banana
  containing 150 Cal?
• (1 Calorie 4.18 kJ)

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• An object's heat capacity is the amount of heat
  required to change the temperature of an object
  by exactly 1C.

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• The specific heat capacity or specific heat is
  the heat divided by the mass and temperature
  change.
• Where c specific heat Q heat m mass and
  ?T change of temperature
• The temperature of a 10. g sample of metal
  changed from 25C to 50C when it absorbed 500. J
  of heat. What is the specific heat of this
  sample?
• c ?
• m 10. g
• ?T 50C-25C 25C
• Q 500. J

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• Determine the specific heat of a material if an
  18 g sample absorbed 75 J as it was heated
  from 15C to 40.C.
• c ?
• m 18 g
• ?T 40.C-15C 25C
• Q 75 J

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• What is the specific heat of a substance if 2000.
  cal are required to raise the temperature of a
  300. g sample by 20.C?
• c ?
• Q 2000. cal
• m 300. g
• ?T 20.C

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• What is the amount of heat required to raise the
  temperature of 200.0 g of aluminum by 10.C?
• (specific heat of aluminum 0.21 cal/g?C)
• Q ?
• m 200. g
• c 0.21 cal/g?C
• ?T 10.C

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• Q mc?T
• Q (200. g)(0.21 cal/g?C)( 10.C) 420 cal

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• Water has the highest specific heat of all common
  substances.

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• The symbol ?H stands for the heat of reaction for
  a chemical reaction.
• This is also called the change in enthalpy. ?H
  Q
• The heat content of a system is equal to the
  enthalpy only for a system that is at constant
  pressure.
• Calorimetry depends on the law of conservation of
  energy.

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• If 27.0 mL of water containing HCl is mixed with
  28.0 mL of water containing NaOH in a calorimeter
  such that the initial temperature of each
  solution was 24.0C and the final temperature of
  the mixture is 33.0C, how much heat (in kJ) is
  released in the reaction? Assume that the
  densities of the solutions are 1.00 g/mL.

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• ?H ?
• m 27.0 g 28.0 g 55.0 g
• ?T 33.0C - 24.0C 9.0C
• c 0.00418 kJ/gC

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• A lead mass is heated and placed in a foam cup
  calorimeter containing 40.0 mL of water at
  17.0C. The water reaches a temperature of
  20.0C. How many joules of heat were released by
  the lead?

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• ?H ?
• m 40.0 g
• ?T 20.0C - 17.0C 3.0C
• c 4.18 J/gC

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• Fusion solid ? liquid
• Solidification liquid ? solid
• Molar heat of fusion
• heat required to melt a mole ?Hfusion
• Molar heat of solidification
• heat required to freeze a mole ?Hsolid
• ?Hfusion -?Hsolid

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• Vaporization is liquid ? vapor
• Condensation is vapor ? liquid
• Molar heat of vaporization
• heat required to vaporize a mole ?Hvap
• Molar heat of condensation
• heat required to condense a mole ?Hcond
• ?Hvap -?Hcond

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• Given the equation
• I2(s) 62.4 kJ ? I2(g) ?H 62.4 kJ

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• How much heat, in kJ, is released when 108 g of
  water at OC freezes to ice at OC if ?Hsolid
• for water -6.01 kJ/mol?

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• How much heat is released in the condensation of
  27.0 g of steam at 100C to water at 100C if
• ?Hcond for water -40.7 kJ/mol?

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• How many grams of ice at 0C can be melted into
  water at 0C by the addition of 75.0 kJ of heat?
• ?Hfus for water 6.01 kJ/mol

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• The heat of solution is the amount of heat
  absorbed or released when a solid dissolves.
• If the molar heat of solution of NaOH is -445.1
  kJ/mol, how much heat (in kJ) will be released if
  80.00 g of NaOH are dissolved in water?

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• Hess's law makes it possible to calculate H for
  complicated chemical reactions.
• Hesss Law If you add 2 or more thermochemical
  equations to give a final equation, then you also
  ADD the heats of reactions to give the final heat
  of reaction.

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• Given the equation
• C2H4 (g) 3O2(g) 2H2O(l) 1411 kJ
• How much heat is released when 8.00 g of O2 react?

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• What is the heat of reaction (?H) for the
  combustion (with O2) of benzene, C6H6 to form
  carbon dioxide and water? Write the balanced
  equation for the reaction.

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• Standard heats of formation
• C6H6 48.50 kJ
• O2 0.0 kJ
• CO2 -393.5 kJ
• H20 -285.8 kJ

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• 2 C6H6 15 O2 ? 12 CO2 6 H2O
• ?H (2) (48.50 kJ) (15)(0.0 kJ)
• (12) (-393.5 kJ) (6) (-285.8 kJ)
• ?H -3266.9 kJ