Chemistry - A Molecular Science

(^) •
The standard state of a pure subs
tance is its most stable form at 1 atm pressure and the
specified temperature. Unless
indicated otherwise, solids and liquids are considered to be
pure. Gases can be assumed pure even in mi
xtures because the molecules in the gas
interact only weakly. Thus, the standard state
of a gas is a partial pressure of 1 atm.

• Recall from Chapter 7 that 1 M is read ‘one molar’ and implies one
  mole of the substance per liter of solution.
  (^) •
  The standard state of a substance in solution is a concentration of 1 M* at 1 atm pressure and the specified temperature.
  Thus, the standard state
  of a sugar solution is a solution in
  which the sugar concentration is 1 M.
  The enthalpy change of a reaction in which a
  ll of the reactants and products are in their
  standard states is called the
  standard enthalpy of reaction
  and given the symbol
  ΔH
  o.
  All enthalpy values given in this chapter are standard enthalpies.
  There is no standard temperature
  , so the temperature should be specified. However,
  if no temperature is specified, then exactly 25
  oC (298.15 K) should be assumed. While 25
  oC is the commonly used temperature, it is not the standard state temperature. The standard state of a substance depends upon its temperature.
  For example, the standard state of water
  is a liquid at 1 atm and 25
  oC, a solid at 1 atm and -25
  oC, and a gas at 1 atm and 125
  oC.
  The state of each substance is important, so it is usually included in parentheses after
  the substance (s = solid; l = liquid; g = gas; aq = aqueous solution
  †). A thermochemical
  equation gives the enthalpy of reaction next to the balanced chemical equation. The following thermochemical equation for the
  thermite
  reaction at 298 K states that 852 kJ of
  heat are
  liberated
  (Δ
  H < 0) when one mole of solid Fe
  O 2
  reacts with two moles of solid 3
  Al to produce one mole of solid Al
  O 2
  and two moles of solid Fe. 3
  †
  Aqueous solutions are those in whic
  h water is the dominant substance.
  Fe
  O 2
  (s) + 2Al(s) 3
  →
  Al
  O 2
  (s) + 2Fe(s) 3
  ΔH
  o = -852 kJ
  All substances are in their standard states at
  298 K, so the superscript zero is included to
  indicate that the enthalpy change is the
  standard
  enthalpy of reaction.
  PROPERTIES OF ENTHALPY We frequently need to determine the enthalpy
  of a process from the tabulated enthalpy of a
  closely related process. The following two prope
  rties of enthalpy can help us do that:

• The enthalpy of reaction is directly proportio

nal to the number of moles of each substance

reacting.

C

H 8

(l) + 18

25 /

O 2

(g) 2

→

8CO

(g) + 9H 2

O(l) 2

ΔH

o = -5500 kJ

2C

H 8

(l) + 25O 18

(g) 2

→

16CO

(g) + 18H 2

O(l) 2

ΔH

o = 2(-5500) = -11,000 kJ

Octane, C

H 8

, is a component of gasoline, so the 18

above two equations indicate that the

amount of energy released is doubled wh

en the amount of gasoline is doubled.

Chapter 9 Reaction Energetics

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