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Lecture 3

CHEM 1050 Lecture Notes - Lecture 3: Bond-Dissociation Energy, Spontaneous Process, Covalent Bond


Department
Chemistry
Course Code
CHEM 1050
Professor
Harvey Marmurek
Lecture
3

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Chemistry 1050
Lectures summarized
Week 3 and 4
Hess’ Law:
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)
of pressure
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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