Thermodynamics

1
Thermodynamics

• Chapter 5
• Thermochemistry

2
Thermochemistry

• Thermodynamics study of energy and its
  transformation.
• Thermochemistry relationship of energy changes
  in chemical reactions.

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The Nature of Energy

• Force
• Work
• Heat
• Energy

• is the push or pull exerted on an object.
• is the energy required to overcome a force. w
  force distance
• the transfer of energy from one object to
  another because of a difference in temperature
• capacity to due work or to transfer heat.

4
Energy

• Kinetic energy due to an objects motion (Ke ½
  mv2) represents the performance of work
• Potential energy due to an objects position
  relative to another object represents the
  capacity to due work

• Usage of Energy

5
Energy Units

• Joule (S.I. Unit) energy of a 2 kg obj traveling
  at a velocity of 1 m/s
• 1 J 1kg m2/s2
• calorie (cal) energy req to raise the temp of 1
  g of water 1 deg C.
• 1 cal 4.184 J
• In foods 1 Cal 1000 cal 1kcal

6
Thermo Environments

• System
• A. Open
• B.Closed
• C.Isolated
• Surrounds

• part of the universe considered
• (reactants, products, etc)
• both energy and matter can be transferred into or
  out of
• energy can be trans into or out but not matter
• neither energy or matter can be trans into or
  out of the system
• all other parts of the universe including the
  vessel

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The First Law of Thermodynamics

• "Energy can neither be created nor destroyed".
  Another approach is to say that the total energy
  of an isolated system remains constant.
• Law of conservation of energy 1st law of thermo
• Internal Energy (U or E) consist of all the
  energy of a system sum of potential and kinetic
  energies

8
First Law Cont.

• Internal Energy can not be measured however
  changes in energy can be
• Delta U or E is defined as the change in internal
  energy
• DU E final E initial
• DU energy is gained by system or
  (endothermic)
• - DU energy is lost by the system of (exothermic)

9
First Law cont.

• Delta U q w is another way of expressing the
  first law
• q heat integral c delta t
• c heat capacity delta t change in
• tempt
• q heat absorbed
• q - heat released
• w work neg integral p delta v
• p external pressure delta v
• change in volume
• w done to system
• w - done by system

10
State Functions

• Depend on the current state of the system and are
  independent on the pathway leading to the current
  state
• Internal Energy U is an example of a state
  function
• q and w are not state functions
• Delta H enthalpy q ( pressure constant)

11
Thermochemical Equations

• Balanced chemical equations that show the
  associated enthalpy change
• ex. 2Na(s) 2H2O(l) ? 2NaOH(aq) H2(g) (delta
  H -367.5kj)
• note the enthalpy change depends on the phase
  of the substances

12
Thermochem eq

• Ex 2H2(g) O2(g) ? 2H2O(g) DH -483.7kj
• 2H2(g) O2(g) ? 2H2O(l) DH -571.7kj
• Molar Interpretation
• 2 moles of H2 and 1 mole of O2 react give
  liquid water and 572kj of heat evolve
• How much heat would evolve if one mole of H2
  reacted
• 1H2(g) 1/2 O2(g) ? 1H2O(l) DH -286kj
• 1H2O(l) ? 1H2(g) 1/2 O2(g) DH 286kj

13
Heat and Enthalpy Change

• The amount of energy in an open system is best
  described by the amount of heat produced ( little
  work is done because the atmosphere produces
  little resistance)
• Enthalpy- is the amount of heat contained by
  substance (cant measure - can only measure
  change in a system at cons. pressure)
• DH q (p) (constant pressure)

14
Delta H enthalpy

• Is a state function
• DH H final H initial
• products reactants
• DH (-) exothermic
• DH () endothermic

15
Calculating the heat of reactions

• How much heat is evolved when 907 kg of ammonia
  is produced according to the following equation
  (-2.45 x 106kj)
• N2(g) 3H2(g) ? 2NH(g) DH -91.8kj

16
Thermo Problems Cont.

• Ammonia burns in the presence of copper catalyst
  to form nitrogen gas.
• What is the enthalpy change to burn 25.6g of
  ammonia? (-475kj)
• 4NH3(g) 3O2(g) ? 2N2 6H2O(g)
• 
  DH -1267kj

17
Calorimetry (measurement of heat flow)

• Heat Capacity of an obj (C) is the amt of heat
  necessary to raise its tempt by 1 deg C
• Is directly port to the amt of the sub.
• q Cdt C q/dt
• So molar heat capacity is often used- the amt of
  heat necessary to raise the temp of one mole 1
  deg C

18
Specific Heat

• heat capacity of 1g of a substance is called its
  specific heat capacity of specific heat
• s q (quantity of heat trans)/ m(grams of sub x
  dt (temp change)
• Calc the amt of heat a substance has gained or
  lost
• q s x grams sub x dt

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Hesss Law

• for a chem eq that can be written as the sum of
  two or more steps, the enthalpy change for the
  overall eq equals the sum of the enthalpy changes
  for the individual parts

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