NANIK DWI NURHAYATI,S.Si, M.Si

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NANIK DWI NURHAYATI,S.Si, M.Si
CHEMICAL THERMODYNAMICS

• www.nanikdn.staff.uns.ac.id
• [phone], 081556431053

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Enthalpy and Enthalpy Change of Chemical Reactions

• We have already investigated energy transfers
  during physical transformations, what about
  chemical transformations reactions?
• ?H Hproducts Hreactants
• An endothermic reaction (?H gt 0) is a reaction in
  which heat is absorbed from the surroundings.
• An exothermic reaction (?H lt 0) is a reaction in
  which heat is given off to the surroundings.

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Thermochemical Equations

• Thermochemical Equation a balanced chemical
  equation (including phase labels) with the molar
  enthalpy of reaction written directly after the
  equation
• N2 (g) 3 H2 (g) ? 2 NH3 (g) ?H -91.8 kJ
• a) Molar Interpretation When 1 mol of nitrogen
  gas reacts with 3 mol of hydrogen gas to form 2
  mol of ammonia gas, 91.8 kJ of energy is given
  off.

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Heat Transfer

• Heat Capacity (C) quantity of energy required to
  increase the temperature of a sample by one
  degree
• C q/?T
• The magnitude of the heat capacity depends on
• Mass of the sample
• Composition of the sample

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• Calculate the heat capacity of an aluminum block
  that must absorb 629 J of heat from its
  surroundings in order for its temperature to rise
  from 22 ?C to 145 ?C.
• Heat Capacity (C) C q/?T
• C 629 J / (145 - 22 ?C)
• 629 J / 123 ?C
• 5.11 J/ ?C

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Heat Capacity

• When comparing the heat capacities of different
  substances with different masses, it is more
  useful to compare specific heat capacities.
• Specific Heat capacities (c) quantity of energy
  needed to increase the temperature of one gram of
  a substance by one degree Celsius
• Molar Heat Capacity (cm) related to specific
  heat, but for one mole of substance

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Heat Capacity

• When dealing with specific heat capacities
  (c)
• Given c q/m?T
• Derive q cm?T
• ?T Tfinal Tinitial q/cm
• m q/c?T
• q thermal heat
• c specific heat
• m mass
• ?T change in temperature

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• What will be the final temperature of a 5.00 g
  silver ring at 37.0 ?C that gives off 25.0 J of
  heat to its surroundings
• (c 0.235 J/g ?C)?
• ?T Tfinal Tinitial q/cm
• Tfinal 37.0 ?C -25.0 J / (0.235 J/g
  ?C)(5.00 g)
• Tfinal 37.0 ?C -21.3 ?C
• Tfinal 37.0 ?C - 21.3 ?C
• Tfinal 15.7 ?C

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• 148 J of heat are transferred to a a piece of
  glass (c 0.84 J/g?C), raising the temperature
  from 25.0 ?C to 49.4 ?C. What is the mass of the
  glass?
• m q/c?T
• m (148 J)/(0.84 J/g?C)(24.4 ?C)
• m 7.2 g

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Phase Changes

• We just saw that energy transfers ALWAYS
  accompany temperature changes.
• Energy transfers also accompany physical and
  chemical changes, even when there is no change in
  temperature.
• eg. Energy is always transferred into or out of a
  system during a phase change.

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Melting/Freezing

• Heat of Fusion quantity of thermal energy that
  must be transferred to a solid as it melts
  (qfusion - qfreezing)

Water Heat of fusion 333 J/g at 0
?C. Specific Heat (l) 1.00 cal/g?C Specific
Heat (s) depends on T ? 0.5 cal/g?C near 0 ?C