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MgCl2(s) is: Thermochemical Standard States ∆Horxn Thermochemical standard state conditions
Standard T = 298.15 K (depends on the temp condition of
Standard P = 1.0000 bar.
Be careful not to confuse these values with STP. Thermochemical standard states of matter
Standard state of pure matter – the pure liquid or solid form.
Gases –1.00 bar of pressure.
For gaseous mixtures - partial pressure =1.00 bar. Aqueous solutions – 1.00 M concentration. Standard Molar Enthalpies of
• Standard molar enthalpy of elements in their most stable
forms at 298.15 K and 1.000 atm is zero.
– Reference state for all enthalpy calculations are the constituent
elements of the compound. Elements
Hg(?) ∆Hfo kJ/mole
0 ∆Hfo of
states: 0 0
0 5 1/7/2014 • ∆Hfo of all compounds, ions and elements in
their non-standard states: Nonzero value
Cu2+(aq) ∆Hfo kJ/mole
+64.77 Using Enthalpies of Formation Standard Molar Enthalpies of
Example: The standard molar enthalpy of formation for
phosphoric acid is -1281 kJ/mol. Write the thermochemical
balanced equation for the formation of H3PO4 P in standard state is P4
Phosphoric acid in standard state is H3PO4(s) Using Enthalpies of Formation • For any process (can be reactions, physical
changes, or transformations)
xA + yB nC + mD • Set-up the equation for Enthalpy of Reaction in:
2H2(g) + O2(g) 2H2O(l) at 25oC
∆Hrxno = (2 • ∆Hf, H2O(l)o ) – (2 • ∆Hf, H2(g)o + ∆Hf, O2(g)o ) ∆Hprocess =
∑(coeﬃcients•∆Hfproducts) – ∑(coeﬃcients•∆Hfreactants) Set-up the equation for Enthalpy of Vaporization in:
H2O(l) H2O(g) at 25oC
o = (1 • ∆H
f, H2O(g) ) – (1 • ∆Hf, H2O(l) ) ∆Hprocess = (n•∆HfC + m•∆HfD) – (x•∆HfA + y•∆HfB) Set-up the equation for Enthalpy...
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