Thermodynamics

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Thermodynamics

• The study of energy transfers and chemical
  driving forces

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Heat flow

• Heat flowing into or out of a system always
  results in some kind of change to the system
• The temperature of the system could change
• There could be some other change, like a change
  in physical state, for instance

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Heat flow

• When heat flowing into or out of a substance
  results in a ?T, we can calculate the amount of
  heat with the equation
• q ms ?T
• But sometimes heat flowing into or out of a
  substance results in a different kind of change
  without a temperature change
• Melting, freezing, chemical reactions, etc.
• Measured as ?H a change in enthalpy

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Enthalpy

• Enthalpy (H) ? the total E (KE PE) of a system
  at constant P
• when a system reacts, ?H Hfinal -
  Hinitial
• for a chemical reaction
• ?Hrxn Hproducts - Hreactants

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The only problem is...

• The enthalpy of a system (H) cannot actually be
  measured
• KE 1/2mv2
• the velocity of any object is always relative to
  a frame of reference
• the absolute velocity of the earth cannot be
  determined

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But, we do know...

• For an endothermic reaction, ?H is ()
• For an exothermic reaction, ?H is (-)
• so, ?H is all that is really important, and it
  can be measured if we assume all the energy
  gained or lost is heat

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?H q/n

• At constant pressure

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?H

• ?H is a state function -
• that is, what is the absolute difference?
• the history of how it got there isnt important
• ex T, P, V, etc...

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Measuring ?H

• Because ?H q/n, the heat lost or gained per
  mole, if we can measure the heat lost or gained,
  we can know the value of ?H

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What are all the ?Hs?

• Any energy change for a system that doesnt
  result in a ?T for the system is measured as a ?H
• Ex melting/freezing, boiling/condensing,
  dissolving, or the energy that flows into or out
  of a reacting system

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Changes in state require changes in energy (?H)
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What do all the ?Hs mean?

• ?Hfus the heat that must be added to change
  1.0mol of a substance from a solid to liquid at
  its melting point
• For freezing ? use a (-) number
• Freezing is exothermic
• For melting ? use a () number
• Melting is endothermic

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What do all the ?Hs mean?

• ?Hvap the heat that must be added to change
  1.0mol of a substance from a liquid to gas at
  its boiling point
• For condensing ? use a (-) number
• condensing is exothermic
• For boiling ? use a () number
• boiling is endothermic

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What do all the ?Hs mean?

• ?Hsoln the heat that is either absorbed (
  ?Hsoln ) or released by (- ?Hsoln ) a substance
  when it dissolves
• ?Hrxn the heat that is either absorbed ( ?Hrxn
  ) or released by (- ?Hrxn ) the reactants during
  the course of a chemical reaction

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What do all the ?Hs mean?

• Note
• all ?Hs are usually kJ/mol
• divide the number of kJ that flow by the of
  moles
• reverse process same , opposite sign

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Heat flow can result in several things

• If the heat flow results in a ?T, the equation
  used is
• q ms ?T
• IF the heat flow results in a different change
  like melting or freezing, the equation is
• q n?H

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Thermodynamics

• The study of energy transfers and chemical
  driving forces

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Enthalpy

• Enthalpy (H) ? the total E (KE PE) of a system
  at constant P
• when a system reacts, ?H Hfinal -
  Hinitial
• for a chemical reaction
• ?Hrxn Hproducts - Hreactants

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Heat of reaction

• The entire energy change (?E) for a reaction is
  often called the heat of reaction (?Hrxn )
• The energy may be absorbed ( ?Hrxn) or released
  (- ?Hrxn )
• the energy may be heat, sound, light,
  electricity, etc.

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Heat of reaction ?Hrxn

• Chemical changes involve breaking bonds in the
  reactants (endothermic) and forming bonds in the
  products (exothermic)
• for a chemical reaction
• ?Hrxn Hproducts Hreactants
• Whether the ?Hrxn is a or - number depends on
  the relative strengths of the bonds in the
  reactants and products.

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• endothermic reaction
• heat is a reactant
• ?H is ()

products
Energy
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• exothermic reactions
• heat is a product
• ?H is (-)

reactants
Energy
?H (-) number kJ
products
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Thermochemical Equations

• Since reactions involve the gain of heat from or
  loss of heat to the surroundings, an energy term
  may be included on the reactant or product side
  of a chemical equation

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Example...

• When four moles of ammonia burns in air, 1170kJ
  of heat are produced.
• 4 NH3(g) 5 O2(g) ?
  4 NO(g) 6 H2O(l) 1170 kJ
• ?Hrxn -1170 kJ

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• How many kJ are released into the surroundings
  for each gram of ammonia that reacts?
• 1.00gNH3
• x 1 mol/17.04g
• x 1170kJ/4 mol NH3
• 17.2 kJ

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• How many grams of nitric oxide have been produced
  according to the above reaction if 5000 kJ of
  heat has been generated?
• 5000 kJ
• x 4 mol NO/1170 kJ
• x 30.01g/1 mol
• 513g NO

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Thermodynamics

• The study of energy transfers and chemical
  driving forces