Chemistry - A Molecular Science

Chapter 9 Reaction Energetics

Reversing the direction of a reaction ch•

anges the sign of the enthalpy of reaction.

H

O(s) 2

→ H

O(l) 2

ΔH

o = +6 kJ at 0

oC

H

O(l) 2

→

H

O(s) 2

ΔH

o = -6 kJ at 0

oC

Thus, 6 kJ are absorbed (

ΔH > 0) when one mole of ice at its melting point, but 6 kJ are

released (

ΔH < 0) when one mole of water freezes.

9.3

ENTHALPIES OF COMBUSTION

Combustion

is the reaction of a substance with

oxygen, and it is the major source of

harnessed energy. It is the energy released in

combustion reactions that fuels our vehicles,

heats our homes, and powers our bodies. The

products of combustion are usually, but not

always, the oxides of the elements comprising the molecule being combusted.

The

enthalpy or heat of combustion

of a substance is defined as the heat absorbed

when one mole

of the substance reacts with oxygen at constant pressure.

Combustion reactions are all exothermic, so h

eat is given off rather than absorbed, and

enthalpies of combustion are negative in this text.* Consider the following thermochemical equations that deal with some common combustion reactions: •

C^6

H^12

O^6

(s) + 6O

(g) 2

→

6CO

(g) + 6H 2

O(l) 2

HΔ

o = -2816 kJ

†^

Our bodies derive energy from the combustion of glucose (a sugar).

(^) •
6CO
(g) + 6H 2
O(l) 2
→
C
H 6
O 12
(s) + 6O 6
(g) 2
HΔ
o = +2816 kJ
Plants make glucose in photosynthesis by re
versing the glucose combustion reaction. The
energy required to make this endothermic reaction proceed is supplied by the sun.
4Fe(s) + 3O•
(g) 2
→
2Fe
O 2
(s) 3
Δ
oH
= -1648 kJ
Rusting of iron is the basis of corrosion.
Example 9.2 a) The heat of combustion of sucrose (table sugar) is -5650 kJ/mol
§. How much heat is
released when one teaspoon of sucrose (~4.8 g) is burned?
Mm
(C
H 12
O 22
) = 342 g/mol, so one teaspoon (4.8 11
g) of sucrose contains

• Care must be taken when using tables in other sources because they
  may list heats of combustion as posit
  ive. Just remember that heat is
  always liberated in a combustion reaction.
  † When
  ΔH appears with a chemical equation, it has units of kJ
  because the amounts are specified by the equation. However, when an enthalpy is given without a chemical equation, the units are kJ/mol to specify that one mole of the s
  ubstance is involved. For example,
  ΔH
  o = -2816 kJ for this reaction, but the heat of combustion of
  glucose would be given as -2816 kJ/
  mol in a table to specify the
  reaction of one mole of glucose.
  § No chemical equation is given, so
  the units are kJ/mol
  to specify that
  the enthalpy is for the combusti
  on of one mole of sucrose.
  4.8 g sucrose
  1 mol sucrose×342 g sucrose
  = 0.014 mol sucrose
  H = (0.014 mol)(-5650 kJ/mol) = -79 kJ,Δ
  so 79 kJ of heat are released
  b) Verify the statement on the label of a bag of sugar that “one teaspoon of sugar
  contains 19 calories.” The calorie (cal) is an older unit of energy th
  at is still commonly used.
  It is the amount of
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  State
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