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Lecture

CHEM 1310 Lecture Notes - Jmol, Dihydrogen Cation, Joule


Department
Chemistry
Course Code
CHEM 1310
Professor
Sarrah Vakili

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Q. How much heat is required to bring H2O in the form of ice at 0 C to steam at 100 C?
A.
Step 1: melt ice fusion
o Turns to liquid
Step 2: heat
o Turns to H2O at 100 C
Phase change from liquid to gas vaporization
Q = quantity of heat
o =∆Hfus + ∆H + ∆Hvap
o =step 1 + step 2 + step 3
∆H = q = mc∆T
Solve:
o Hfus = 6.02 kJ/mol
o Hvap = 40.7 kJ/mol
o Look these up in table of values
o T = mCspecT
o =18.02g/mol(4.184 J)(100 K)
o =7539.6 J/mol = 7.54 kJ/mol
o Q = step 1 + step 2 + step 3
=6.02 kJ/mol + 7.54 kJ/mol + 40.7 kJ/mol = 54.2 kJ/mol needed
Standard Enthalpy of Formation
Hᵒf
A substances enthalpy change in going from the standard state to the reference state
The value is 0 if the substance is a pure element in the reference state or it is the ion H+
Standard state - the solid or liquid of the pure element or compound when the pressure is 1
bar - commonly found in table format - 1 bar, 298 K
Pure + crystalline = solids
Liquids = normal
Gases = assumed ideal
Reference state - the most stable form (state) of the element or compound
Formation Reaction
Makes mol of cpd/product of interest
Make from standard state or elemental reference
States standard (reference) temperature, pressure + concentrated
Maybe endothermic or exothermic
Hᵒf example:
Hᵒf [C, graphite] = 0 = 0 reference, standard
Hᵒf [C, diamond] = 1.895 kJ/mol = standard state but not reference state
Hᵒf [N2] = 0 = 0 reference and standard state
May calculate using Hess's Law (Hᵒf) or look up in tables
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