Reaction Enthalpy Dialog
Simple bond-making or bond-breaking processes:
aggregate ® deaggregate (break bonds; D H > 0)
deaggregate ® aggregate (make bonds; D H < 0)
Complex reactions
reactant aggregates ® product aggregates (D H > or < 0 ?)
Can't predict qualitatively
But can analyze processes quantitatively in terms of aggregation and deaggregation concepts. Build this up using 4 ideas:
1) Standard conditions
|
Gases |
partial P = 1 atm |
|
Solids, liquids |
subjected to 1 atm pressure |
|
Solutions |
all solutes 1 M |
|
T |
can be any value, but most often 298 K |
Standard state of a substance
The form in which the substance is most stable under standard conditions.
Ex. Std state of oxygen is __________
Std state of carbon is __________
Do GA1, GAS8
Standard Enthalpy (Change) of Reaction, D HoR
The enthalpy change for a reaction run under standard conditions, all participants in their standard states.
Ex: 2H2(g) + O2(g) ® 2H2O(l) (D HoR = -572 kJ/mole)
means P(H2) = 1 atm, P(O2) = 1 atm, H2O(l) subjected to 1 atm, T = 298 K
Do GA2, GAS8
2) Hess's Law
If 2 or more reactions can be combined (added/subtracted) to give a desired reaction, D H for the desired reaction is a similar combination of D H's for the combined reactions.
Ex: Given
|
D Ho, kJ/mole |
|
|
(a) 2O3(g) ® 3O2(g) |
-427 |
|
(b) O2(g) ® 2 O(g) |
495 |
|
(c) NO(g) + O3(g) ® NO2(g) + O2(g) |
-199 |
Calculate D HoR for NO(g) + O(g) ® NO2(g)
Do GA3 GAS 8
3) Formation reaction; D Hof (See text)
4) Atomization reaction; D Hoatom (see handout)
Reaction in which 1 mole of substance in its standard state is atomized (converted to separated atoms in the gas phase); i.e., all bonds of a substance are broken (deaggregation).
Ex. Atomization of ethanol
C2H5OH(l) ® 2 C(g) + 6 H(g) + O(g) (DHoatom = 3269 kJ/mole)
Do GA5 GAS 8 Write atomization reactions and look up atomization enthalpies.
Analysis of D HoR of general, complex reactions
Reactants ® Products (DHo = ?)
Use Hess's Law:
|
Reactants ® atoms |
S D Hoatom (reactants) deaggregation |
|
atoms ® Products |
-S D Hoatom (products) aggregation |
D HoR = S D Hoatom (reactants) - S D Hoatom (products)
Ex: C6H12O6(s) + 6 O2(g) ® 6 CO2(g) + 6 H2O(l)
D HoR =
Do GA6 GAS8