Some Useful Equations for CHEM 1120

Normality and Titration*footnote1
 
Definition of Normality: NA = [#H+, #OH- or #e - ] CA
Use for Titration: N1V1 = N2 V 2


Equations on thermodynamics:
 
 

Definition of molar heat capacity: q = Cn ΔT
First Law of Thermodynamics: ΔE = q + w
Definition of enthalpy: ΔH = ΔE + PΔV
Definition of Gibbs' free energy : ΔG = ΔH - TΔS
Obtaining the ΔrHo from ΔHof : ΔrHo   = νΔH of products - nu νΔHof reactants
Obtaining the ΔrSo from So : ΔrSo  = νSo products - νSo reactants
(for ions these are ΔSo)
Note the SI symbol to designate one mole of reaction is the subscript r after the Δ so: Δr .  Examples: Δr H o and ΔrSo .
 
Summary of the criteria for equilibrium and spontaneity
Condition For an Isolated System For a Closed System at Constant Pressure
Spontaneous Process ΔS > 0 ΔG < 0
Equilibrium ΔS = 0 ΔG = 0
Non spontaneous Process Impossible ΔG > 0


Equations on Equilibria:

    The equilibrium constant from thermodynamic data:

        K = e Go/RT                     (very important)

 The van't Hoff plot uses the linearized version of this equation in the form:

ln K =  - (ΔHo/R)(1/T) + ( ΔSo/R)

 i.e

  y  =   m x   +  b

Where the slope, m, is:   m = -(ΔHo/R )    (NOTE NEGATIVE!)
and the intercept, b, is:    b = (ΔSo/ R)
where x is:                      x = (1/T)
and y is:                          y = ln K

 Definitions of some Ks

Ksp is the equilibrium constant between a slightly soluble ionic compound (reactant) and its ions in solution (product).  Example:

    CaF2  <-->  Ca2+   +  2F-

    Ksp = [Ca2+][F- ]2

Kd is the equilibrium constant of a complex ion (in a Lewis acid-base reaction) with its dissociated simple ion and ligands.  Example:

    Co(NH3)62+  <-->  Co2+  +  6NH3

    Kd  [Co2+ ] [NH3]6
                [Co(NH3)62+]



Equations on pH:

p function:   p( ) = -log10( )
    Examples:
                    pH = -log [H3O+]
                    pOH = -log[OH-]
                    pCl = -log[Cl-]
                    pKa = -log Ka




 Types of equilibrium problems encountered (see handout about type I and type II equilibrium problems):
 
pH of a strong acid:

pH = -log(Cacid)

pH of a strong base:


pOH = -log(Cbase)

pH = 14.00 - pOH

pH of a weak acid:


Ka = x2/( Cacid - x)             x underlined can usually be ignored

solve for x

pH = -log(x)

pH of a weak base:


Kb = x2/( Cbase - x)             x underlined can usually be ignored

solve for x

pOH = -log(x)

pH = 14.00 - pOH

pH of an acid buffer:


Ka = x( Csalt - x) /( Cacid - x)             x underlined can usually be ignored

solve for x:   x = Ka( Cacid ) / ( Csalt )

pH = -log(x)

pH of a base buffer:


Kb = x( Csalt - x)/( Cbase - x)             x underlined can usually be ignored

solve for x:   x = Kb( Cbase ) / ( Csalt )

pOH = -log(x)

pH = 14.00 - pOH

pH of a salt of a strong acid and weak base:


Ka = x2/( Csalt - x)             x underlined can usually be ignored

where Ka = Kw/Kb(of the conjugate base)

solve for x

pH = -log(x)

pH of a salt of a strong base and a weak acid:

Kb = x2/( Csalt - x)             x underlined can usually be ignored

where Kb = Kw/Ka(of the conjugate acid)

solve for x

pOH = -log(x)

pH = 14.00 - pOH

Molar solubility:

note that you need to know how to write the equilibrium equation for the dissolution of the salt to get the values for
νcation and νanion.

Ksp = ( νcationx )νcation( νanionx )νanion

solve for x

pH for the dissolution of a slightly soluble hydroxide:

Ksp = ( νcationx )νcation( νhydroxidex )νhydroxide

solve for x

pOH = -log( νhydroxidex )

pH = 14.00 - pOH

dissociation of a complex ion:




 Equations on Kinetics:

Zero order kinetics:
    rate equation:                    -ΔCt = k

    integrated rate equation:    C = -kt + Co

First order kinetics:
    rate equation:                    - ΔCt = kC

    integrated rate equation:    lnC = - kt + lnCo

Second order kinetics:
    rate equation:                    - ΔCt = kC2

    integrated rate equation:    1/C = kt + 1/Co

Arrhenius equation:            k = A eH*/RT
                                                     where ΔH* is the "activation energy" or "enthalpy of activation"

Equations on Electrochemistry:

In the stoichiometry of the electrochemical cell, one can convert from coulombs to moles of electrons using the Faraday constant, F.  F = 96 487 C mol-1

Cell diagram:

Anode | Anolyte | (salt bridge) | Catholyte | Cathode
   OXIDATION                        REDUCTION

Standard Potentials, Eo, are reduction potentials

Eocell = Eooxidation, anode + Eocathode

Eooxidation =   -E ostd

Eoreduction =  + Eo std

For non-standard conditions:

Nernst Equation:

    Ecell = Eo cell - (RT/nF) lnQ                     n is the number of electrons transferred

    Ecell = Eo cell - (0.0592/n) lnQ  at 25oC

The relationship between Gibbs' free energy and potential:

    ΔG = - nFE

*footnote 1:  SIO and IUPAC have recommended elimination of normality.  Thus, the algebraic symbol, N, and the unit symbol, N, are not standard SI.  I disagree with this decision which was was based upon the understanding the normality is a convenience and not a necessity.  The real reason for the use of normality is if one has a totally unknown substance when doing a titration, in which case the answer can be reported only in normality (or something equivalent.)  The convenience of the equation: N1V1 = N 2V2  is only a side issue.