3 Enthalpy

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3 Enthalpy
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• Units SI unit joule
• 1KJ 1000J 239.0 cal
• 1st law of Thermodynamics
• The total energy of the universe is constant
• i.e energy cannot be created or destroyed but can
  be changed from one form to another.

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• Heat (or thermal energy) q is the energy
  transferred between a system and its surroundings
  as a result of a difference in temperature only.
• All other forms of energy transfer (mechanical,
  electrical etc) involve some type of work, w, the
  energy transferred when an object is moved by a
  force.
• State functions A state function is a property
  dependant on the current state of the system
  (e.g. its composition, volume, temp etc)
• It is independent of the path the system took to
  reach that state.

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• Energy of a system is a state function
• So ?E is a constant for any given change but q
  and w may vary
• (q and w are not state functions)
• and ?E q w

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• Energy can be converted from one form to another
• e.g. from mechanical ? heat ? electrical ? light
  etc.
• Energy is released when bonds are made
• Energy is used when bonds are broken
• Chemical reactions involve
• Bond breaking
• Bond forming

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• Energy changes for an exothermic reaction one
  where heat is released to the surroundings

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Energy changes for an endothermic reaction one
where heat is absorbed from the surroundings
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• Examples of exothermic reactions
• neutralisation
• burning hydrocarbons
• respiration
• Examples of endothermic reactions
• photosynthesis
• dissolving ammonium nitrate in water

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• The chemical energy which a system possesses is
  its enthalpy. Enthalpy is the change in energy at
  constant pressure
• symbol H
• And ?H H products H reactants
• if energy is absorbed by a system ?H is positive
• if energy is released by a system ?H is
  negative

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Energy changes for an endothermic reaction
A B C D
E
Bond forming
Bond breaking
AC BD
Products
AB CD
Overall energy change
Reactants
Products have more energy than reactants
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• Energy changes for an exothermic reaction

A B C D
E
Bond forming
Bond breaking
AB CD
Reactants
AC BD
Overall energy change
Products
Products have less energy than reactants
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• Energy Diagrams

E
Activation energy
Reactants
?H
Products
Time
An exothermic reaction
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• Reactants

E
Activation energy
Products
?H
Reactants
Time
An endothermic reaction
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• Standard enthalpy of formation is the heat
  absorbed when 1 mole of a substance is formed
  from its elements in their standard state.
• The standard state of a substance is 1 mole of
  the substance in a specified state (solid,
  liquid, gas) at 1 atmosphere of pressure. The
  value of an enthalpy change is quoted for
  standard conditions gases at 1 atm, solutions at
  unit concentration and substances in their normal
  states at a specified temperature. (usually 273K
  or 00C)
• All elements in their standard state are assigned
  an enthalpy of formation of 0

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• Standard enthalpy of reaction is the heat
  absorbed in a reaction at constant pressure
  between the number of moles of reactant shown in
  the equation for the reaction.
• Standard enthalpy of combustion is the heat
  absorbed when 1 mole of a substance is completely
  burned in oxygen at constant pressure.
• Standard enthalpy of solution is the heat
  absorbed when 1 mole of a substance is dissolved
  at constant pressure in a stated amount of
  solvent. This may be 100g or 100ml or an
  infinite amount, i.e. a volume so large that on
  further dilution there is no further heat change.
  

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?Hr? depends only on the difference between the
standard enthalpy of the reactants and the
standard enthalpy of the products, not on the
route by which the reaction occurs. This is
Hesss Law If a reaction can proceed by more
than one route the overall enthalpy is the same
regardless of which route is followed.
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• Find the enthalpy change for the reaction
• CH2 CH2(g HCl(g) ?
  C2H5Cl(g)
• Given the following data
• ?Hf? standard enthalpy change of formation
• ?Hf? CH2CH2 52.3 KJ mol-1
• ?Hf? HCl - 92.3 KJ mol-1
• ?Hf? C2H5Cl -105 KJ mol-1
• ?H ?Hf? products - ?Hf?
  reactants

?Hr? -105 (52.3) (-92.3)
-65KJ mol-1
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• Note that the formula ?H ?Hf? products - ?Hf?
  reactants
• Applies when the data given is the enthalpy of
  formation.
• If the data given is the enthalpy of combustion
  the following formula is used ?H ?Hc? reactants
  - ?Hc? products
• It doesnt matter what the reaction is called. It
  may be called a combustion reaction but if the
  data given is the enthalpy of formation use the
  formula ?H ?Hf? products - ?Hf? reactants !

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• Calculate the standard enthalpy of reaction for
  the following
• 2C 2H2(g) O2 ? CH3CO2H(l)
• Given the following enthalpies of combustion
• C -394 KJ/mol
• H2(g) -286 KJ/mol
• CH3CO2H(l) -876 KJ/mol
• ?r (2x -394) (2x-286) (-876) KJ/mol
  -484 KJ/mol