AQUEOUS

AQUEOUS

1. SOLUBILITY

   • Def: the number of grams or moles of the solid it will dissolve to make 1L of saturated solution at that temperature
   • mol L-1

2. SOLUBILITY PRODUCT

   • Def: the equilibrium constant for equilibrium between an undissolved salt and its ions in a saturated solution
   • Solid and ions in a saturated solution are in equilibrium
   • Solids and water are not included in the calculation
   • Ks = products / reactants
   • Calculating Sol Product given Sol: S2 or 4S3

3. PRECIPITATES

   • Def: a formation of a solid from solution
   • Will form if IP > KS
   • Combining two solutions with different ions will dilute both of them

4. COMMON ION EFFECT

   • Def: when the solubility of one salt is reduced by the presence of another having a common ion
   • Can calculate the solubility of any salt in a soln containing a common ion if the conc of the ion in the soln and the Ks is known
   • If there is more product present, the equilibrium moves to the left - side of the precipitated salt

5. FACTORS AFFECTING CONC

   • If the pH is low, there will be more H+ ions, so OH- ions will react with the H+ ions (acid base neutralisation) and this removes some of the OH- ions the equilibrium shifts to the right, so the amount that can be dissolved increases, so solubility increases

6. ACIDS & BASES

   • WEAK
     • BASE
       • Calculate pH: need to work out kb
         • Kb = 1x10-14 / Ka
           • Kb = [OH-]2 / [A-]
         • Value must be >7
     • ACID
       • All organic acids
       • Calculate pH: need to know conc of acid and Ka
         • HA = any weak acid
         • Reaction with water: HA(aq) + H2O = A-(aq) + H3O+(aq)
         • Ka = [H+]2 / [HA]
           • Assume two cons are equal
             • Actually small amount of H+ ions from the dissociation of water present but can be ignored as long as H+ ions from weak acid is a much larger value
           • Assume amount of acid that has reacted with water is so small that the conc of acid is essentially unchanged
         • Value must be <7
     • Only partially dissociates - almost all still present
   • STRONG
     • ACID
       • e.g. HCl, HNO3, H2SO4
     • BASE
       • e.g. KOH, NaOH and all hydroxides
     • Fully dissociates - all changed to products
   • Amphiprotic: can behave as both acids and bases
   • Conjugate Pairs: Species that differ by one proton
   • SALTS
     • Salts: compound that consists of positive and negative ions ionically bonded together
       • When put into water, they separate into the positive and negative ions
         • Ions that do not react with water: Na+, Cl-, Br-, K+, SO42-, NO3-, F-, I-
           • Conc will be the highest after water
         • If it is a weak acid or base, will not react much with water - conc will be less than the unreactive ion, but not that much less
       • pH
         • All cations that are conjugate acids of weak bases act as weak acids and lower the pH of the soln
           • Conc of the salt willl be the conc of the base

7. ELECTROLYTE

   • Def: a soln that contains ions that are free to move therefore can conduct electricity
   • The more ions present the better the soln acts as an electrolyte
   • CONDUCTIVITY
     • Def:

8. BUFFER SOLUTIONS

   • Def: a combination of a weak acid and its conjugate base, or a weak base and its conjugate acid, whose function is to resist small changes to pH when a strong acid or base is added
   • Product side has the conjugate of the acid or base plus water
   • MAKING BUFFERS
     • By adding a reasonable volume of equal amounts of a weak acid and its conjugate base
     • By half neutralising a weak acid or a weak base with a strong base or strong acid
   • DIFFERENTIATING
     • Weak Acids: only partially dissociates so the conjugate base is only present in very small amounts so can't act as a buffer - needs to be a reasonable amount of acid and conjugate base. This only has acid present
     • Buffer: has equal amounts of acid and its conjugate base - this way it can respond if an acid or base is added
   • EFFECTIVENESS
     • Equal Amounts: equally effective at buffering acid or base
     • More Weak Acid: better at buffering when a base is added
     • More Conjugate Base: better at buffering when an acid is added
   • CALCULATIONS
     • HA(aq) + H20(l) = A-(aq) + H3O+(aq)
       • Ka = [A-][H3O+] / [HA]
         • Cannot assume [H3O+] = [A-]
     • Find pH: find H3O+ then turn into pH
       • Concs will be diluted
     • Find Base or A-: this is a conc and can be multiplied by the molar mass of the salt used to make the base
     • pH = pka + log [A-] / [HA]
       • If the conc of the acid and its conjugate base are equal (e.g. halfway to equivalence point) pH = pka
       • Ratio [A-] / [HA] or [b] / [a] tells you how much base to acid
     • Find Mass of Salt: multiply conc of base by molar mass of salt
   • Adding water does not change pH because conc of the weak acid and weak base are both changed by the same factor so the ratio does not change

9. TITRATIONS

   • Acids are neutralised by bases
     • Weak acids: produces small amounts of H+ when it dissociates however as they are neutralised by a base, the equilibrium shifts to the right to produce more H+ ions
       • Therefore a weak acid and strong acid of the same conc and volume are neutralised by the same base
   • END POINT
     • Def: the point where the acid-base indicator changes colour
   • EQUIVALENCE POINT
     • Def: point when the amount of added base is exactly equal to the initial amount of acid in the soln
       • However pH not necessarily neutral as salt soln formed may contain an ion which is acidic or basic
   • WEAK ACID TITRATIONS
     • pH at equivalence point is greater than 7
       • All of the weak acid at the start of the titration has been converted into the conjugate base in the titration. The conjugate base dissociates slightly creating hydroxide ions
         • Results in basic pH
     • Can use weak acid formula
   • WEAK BASE TITRATIONS
     • pH at equivalence point less than 7
       • All of the weak base has been converted into the conjugate acid in the titration. The conjugate acid dissociates creating hydrogen ions
         • Results in slightly acidic pH
     • Same as weak acid but reversed
   • CALCULATIONS
     • pH at equivalence point
       • Weak acid has now been changed into its conjugate base
         • Use weak base formula
           • However weak base has been diluted so need to calculate the changed concentration
             • New Conc = Original conc x original volume / total volume
             • C1V1 = C2V2