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CHEM 1310 - Notes 4.docx

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Department
Chemistry
Course
CHEM 1310
Professor
Sarrah Vakili
Semester
Summer

Description
 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 = mCspec T∆ 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  ∆Hᵒ f  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  ∆Hᵒ fxample:  ∆ Hᵒf[C, graphite] = 0 = 0 reference, standard  ∆ Hᵒf[C, diamond] = 1.895 kJ/mol = standard state but not reference state  ∆ Hᵒf[N 2 = 0 = 0 reference and standard state  May calculate using Hess's Law ( Hᵒ) fr look up in tables  ∆ Hᵒf- standard reaction enthalpy of formation of one mole of molecules form their constituting elements when all the substances in their standard state at the temperature of interest ∆Hᵒ fnd Hess's Law  Hess - father of thermochemistry  Standard enthalpy of reaction - Hᵒ or Hᵒ rxn  May be determined indirectly by Hess's law or by Hᵒf  ∆ Hᵒrxn= sum of products - sum of reactants Hess's Law  May indirectly solve for H by the manipulation of chemical equations (and there associated H) for process that occur in steps/stages  Divide equation by x - divide H by x  Reverse equation - changes sign o
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