PHAY002; Ionic Equilibria

• Explain what is meant by

  -strong electrolytes

  -weak electrolytes

  -non-electrolytes

  Strong electrolytes=dissociate completely in solution

  Weak electrolytes= doesnt dissociate completely, forms equilibrium of neutral molecules & charged species

  Non-electrolytes=uncharged molecules
• why are ions in solution not completely free to move about?

  -they do not interact with other molecules

  -this is due to electrostatic interactions between charged paticles (attractive forces between oppositely charged ions & repulsive forces between similarly charged ions)

  -these affect ion mobility & reactivity, making their ‘effective concentration’ different from their ACTUAL concentration which can be calculated in stoichiometry.
• what is the ‘activity’ of a substance

  -the effective concentration of a substance

  -has the symbol a
• what is ‘ionic atmosphere’

  -Due to electrostatic interactions, ions of opposite charge will tend to cluster together, and so in the vicinity of any particular ion there will be a greater number of ions of opposite charge (counter ions).

  -The environment of a particular ion therefore carries an opposite charge, and this diffuse, charged environment is known as the ionic atmosphere.

  -This atmospheric shell of counter-ions partially shields the central ion’s charge. The strength of this effect depends on the total ionic concentration in the solution.

  
• what is ionic strength and how is it calculated?

  The ionic strength (I) of a solution is a measure of the total ion concentration, accounting for both concentration and charge

  ci is the molar concentration of each ion

  zi is the charge number of each ion

  the sum is taken over all ions i in solution

  
• -calculate the ionic strength of a 0.1M NaCl solution

  -calculate the ionic strength of a 0.1M CaCl2 solution

  Note; ionic strength of solutions containing divalent or trivalent ions is much greater than for solutions of monovalent ions.

  
• what does the activity of an ion represent?

  -its effective concentration; how the ion actually behaves in solution rather than its analytical concentration
• how is activity calculated


  a = γ ⋅ c

  c = concentration

  y= activity coefficient

  For very dilute ionic solutions, γ approaches 1 and activity equals concentration
• what happens to activity coefficient (y) as ionic strength increases


  As ionic strength increases, γ typically decreases. This means that in more concentrated solutions, ions behave as if they were present at lower concentrations than they actually are, due to the shielding effects of their ionic atmospheres
• what is pH defined by

  pH=-log10[H+]

  where [H+] is expressed in mol L-1 or M.
• how do you calculate the pH of strong acids

  -For strong acids the hydrogen ion concentration is equal to the stoichiometric concentration – these acids dissociate completely in solution

  CALCULATE USING pH=-log10[H+]
• what is the ionic product of water (general ean)

  Kw = [H3O+][OH-]

  In dilute solutions the concentration of water is essentially constant

  ([H2O] ≈ 55.6 M).
• What is the ionic product of water at 298K

  Kw= 1.008x10^-14

  PKw=14
• how to calculate the pH of strong bases

  -use Kw=[h3o+][OH-] to find H3O+

  -use kw = 10^-14

  -use [H3O+] to find pH
• how to find pH of weak acids

  use dissociation constant Ka

  Ka=[A][H3o++]/[HA]

  [H3O+] = [A-]

  *[HA] = cHA in this case as the acid is weak so very little will dissociate
• How to calculate pH of weak bases

  Kb=[OH]^2/cB

  If the base is weak [B] ≈ cB where cB is the stoichiometric concentration of the base.

  *only works for weak bases
• what is the relationship between pkA and pkB

  pKw = pKa + pKb
• NOTE

  Strong acid + Strong base -> Neutral salt

  Strong acid + Weak base -> Acidic salt

  Weak acid + Strong base -> Basic salt

  Weak acid + Weak base -> Neutral salt
• describe what buffers are

  -buffer solutions resist changes in pH when an acid or base is added

  -The pH of human body fluids must be maintained within narrow limits for the proper functioning of enzymes, the balance of osmotic pressure

  -The pH in these various environments can have a profound effect on drug absorption, distribution, activity and excretion.

  -a buffer is typically a mixture of a WEAK ACID and its SALT e.g. acetic acid & sodium acetate
• what is Henderson hasselbach’s equation


  *

  
• how do buffer solutions react to additions of strong acid/strong bases?

  -When a strong acid is added to a buffer solution it will react with the salt to form more weak acid.

  If we add an acid (H+) to our buffer: H++ A- → HA

  -On adding a strong base, it reacts with the weak acid to form more salt.

  If we add a strong base (OH-) to our buffer:

  OH- + HA → A- + H2O
• what is buffer capacity

  The amount (in moles) of acid or alkali that must be added to one litre of the buffer to produce a unit change in pH.

  
• what is isoelectric point

  This is the pH at which the net charge on the amino acid is zero and at which the amino acid shows minimum migration in an electric field.

  pHiso= pka1+pka2/2
• what is a species diagram

  A visual picture of how the concentrations of species in solution vary with pH can often be very useful