Download 10-4 Enthalpy (Section 10.6)

10-4 Enthalpy (Section 10.6)
And you
• Enthalpy, symbolized by H, can be thought of as
the potential energy stored in the bonds of
molecules. Chemists use the change in enthalpy
∆H to measure the heat content of a system
(when the pressure is constant).
• We define the “system” to be the chemicals and
everything else is termed the “surroundings”.
• Applying the First Law of Thermodynamics
(Conservation of Energy), any heat lost by the
system will equal the heat gained by the
surroundings (and vice versa).
• Exothermic (“exo” means released or “exits” and
therm refers to heat)
Exothermic reactions characteristics
• Reactants: high E (H), less stable, weak bonds
• Products: low E (H), more stable, strong
bonds
• System releases PE from bonds to KE of
surroundings (which feel hot).
• ∆H = P – R = negative value (heat released)
Endothermic (“endo” means absorbed or
“going in” and therm refers to heat)
Characteristics:
• Reactants: low E (H), more stable, strong
bonds
• Products: high E (H), less stable, weak bonds
• System absorbs KE from surroundings as PE in
the bonds. Surroundings will feel cold.
• ∆H = P – R = positive value (heat absorbed)
Bond Energies and ∆H:
• It requires energy to break the bonds of the
reactants. It releases energy when new bonds
of the products form. The difference between
these two energies is the ∆H. Note, though, if:
• Energy absorbed to break reactants > Energy
released forming products Endo ∆H = +
• Energy absorbed to break reactants < Energy
released forming products Exo ∆H = -
Example
• ½ H2 + ½ Cl2  HCl Think of bond energies
as KE entering or
leaving the system.
• 216 + 120  427 The change in KE = the
change in PE
•
 91
• H = -91kJ/mol Exothermic
10-5 Enthalpy of Formation
• The enthalpy of formation, ∆Hf, is defined as
the heat absorbed or released when making 1
mole of a compound from its elements (at
25oC and 1 atm).
• By convention, the Hf of any element at this
temperature and pressure is zero.
Hf (kJ/mol)
½ N2(g) + 3/2 H2(g) → NH3
-46
• Reaction
•
• ½ N2(g) + O2 → NO2
• 2 Al + 3/2 O2 → Al2O3
+34
-1676
Stability of Product
stable
unstable
very stable
10-6 Enthalpy of a Rection
•
•
•
•
H = ∑Hf (products) – ∑Hf (reactants)
Hc = enthalpy of combustion
~ defined for the combustion of 1 mole of a fuel
CH4(g) + 2O2(g)
→
CO2(g) + 2H2O(l)
• [-393.5 + 2•(-285.8)]
–
[-74.8] = -890.3 kJ/mol
• Burning fuels is always exothermic
• Hc = Hrxn