CHEM 1050 Lecture Notes - Lecture 3: Bond-Dissociation Energy, Spontaneous Process, Covalent Bond
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Week 3 and 4
We need access to tables of delta H values
Pure and standard states are required
Standard conditions are required
Standard enthalpy of formation (delta H knot) values are the enthalpy changes seen when we
form one mole of substance from its elements in their standard states at 1.0 x 10 ^ 5 Pa (1 atm)
Delta H formation for pure elements in the standard state at a chosen temperature is equal to 0.
Vap (vaporization reaction): liquid goes to a gas
Ions in aqueous solution:
To deduce the delta H formation values for all ions in an aqueous solution, we need a method to
separate the contribution of the ions
Our solution, we define the following
o Delta H formation (H+, aq) = 0
Bond Enthalpies (BE):
The energy required to break one mole of bond in the gas phase only – it requires energy to
break the bonds
BE is always LARGE AND POSITIVE
BE is the measure of the STRENGTH of a covalent bond
NOTE: Bond dissociation enthalpy calculations are for gasses only!!!!
Bond length gets shorter as bond enthalpy get larger!!
Lattice Enthalpy - the change in the energy occurs when an ionic crystalline solid is separated into
isolated gaseous ions
MX (s) M+ (g) + X- (g)
Magnitude of Lattice Enthalpies:
Depends on the charge of the ions and their separation
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