Chapter 5 Thermochemistry

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Chapter 5 - Thermochemistry
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Topics to be addressed

• Nature of energy and forms it takes
• Interconversion between forms of energy
• Work
• First law of thermodynamics
• Internal energy and state functions
• Enthalpy
• Calorimetry
• Food values

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Themodynamics / Thermochemistry

• Thermodynamics - Study of energy and its
  transformations
• Thermochemistry Study of relationship between
  chemical reactions and energy changes (in the
  form of heat)

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Nature of Energy

• Kinetic energy energy associated with motion
• Ek ½ mv2
• Potential energy energy that is a function of
  the position of an object with respect to others.
  Arises when force (a push or pull) is operating
  on object
• Gravity
• E mgh
• Electrostatic forces between charged particles
• Eelectrostatic ?Q1Q2 / q
• Unit of energy is the Joule (old unit was
  calorie)

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System vs Surroundings

• System what we are interested in
• Surroundings everything else
• Example Reactants and solvents in container vs
  container and everything outside
• Closed systems can exchange energy, but not
  mass
• This exchange can be in the form of work or heat

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Work and heat

• We define energy as the capacity to do work or
  transfer heat
• Work transfer of energy involving motion
  against a force
• W F x d
• Example lifting a free weight a distance d ?
• W F x d mgd
• Example pushing on a plunger with a constant
  pressure
• to compress a gas
• W P x ?V F/A x ?V
• (where ? change in)

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First Law of Thermodynamics

• ENERGY IS CONSERVED
• If energy is lost by system, it must be gained
  by surroundings.
• What is included in the energy of a system?
• Internal energy sum of all potential and all
  kinetic energies of the system.
• Internal energy is defined relative to some
  reference, so report ?E

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Changes in energy

• ?E has a
• Magnitude
• Units
• Sign
• Efinal gt Einitial , E is and energy has been
  gained by the system. For a system which is
  defined as a chemical reaction, this means that
  the products have a greater total energy than the
  reactants
• Efinal lt Einitial , E is and energy has been
  lost by the system. For a reaction, this means
  that the products have a lower internal energy
  than the reactants.

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?E, work and heat

• A change in internal energy of a system must be
  accompanied by work on (or by) the surroundings
  and/or by heat transferred to (or from) the
  surroundings. Mathematically, this can be
  expressed as
• ?E w q
• if q is , heat has been transferred into the
  system
• if w is , work has been done on the system
• Example Suppose a chemical reaction produces
  both a flow of heat to the surroundings and
  pushes a piston to give an increase in volume of
  a cylinder
• q is negative, w is negative, and ?E is
  negative

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Heat changes due to chemical reactions

• The sign is based on whether the heat energy is
  released to the surroundings (an exothermic
  reaction) or heat is absorbed from the
  surroundings (an endothermic reaction)
• Example reaction between protons and hydroxide
  in a neutralization reaction makes the water (and
  therefore the container) become warmer
• Example dissolution of ammonium chloride in
  water causes heat to be absorbed from the
  surrounding water molecules and the beaker
  becomes cool

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Internal energy and state functions

• The internal energy of a system under a certain
  set of conditions (temperature, pressure, etc) is
  the same, relative to some reference, no matter
  how that state is achieved. Internal energy is a
  state function.
• Since this statement applies to both a given
  initial set of conditions and a given final set
  of conditions, ?E is also a state function.
• Note this does not mean that q and w are state
  functions. (figure 5.10 in book where a battery
  discharges)

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Enthalpy

• Define a quantity enthalpy, denoted H which
  corresponds to the sum of the internal energy of
  a system and the product of the pressure and
  volume
• H E PV
• The change in enthalpy is then
• ?H ? E ? (PV)
• and will be a state function since all of the
  variables are state functions

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Enthalpy

• ?H ? E ? (PV)
• If P is constant, then ? (PV) P ?V -w (the
  sign is consistent with our observation that
  expansion would be work done by the system, thus
  decreasing the energy of the system)
• We defined ?E q w, so
• ?H q w w q
• ?H is the heat change under conditions of
  constant pressure.