ionic equlibria- PHAY0002

• Define an electrolyte solution?

  Solution containing ions, thus conducting an electric current
• What's a strong electrolyte?

  Substances that are completely ionized when dissolved in a solvent
• Give some examples of strong electrolytes?

  Sodium chloride, hydrochloric acid, sulfuric acid, nitric acid, sodium hydroxide
• Whats a weak electrolyte?

  Substances that are not completely ionized when dissolved in a solvent.

  This solution contains ions and molecules of a solute

  As shown by the double arrow, it represents that the reaction is reversible, so the acid hasn't completely ionized and hasn't completely gone into solution, an equilibrium is established.

  
• what are non-electrolytes?

  substances that dont ionize when dissolved in a solvent, no ions are produced.

  Example: glucose in water
• Whats an effective concentration?

  Effective concentration (or activity) is the adjusted concentration of a substance in a solution, accounting for interactions between particles, as this will reduce their ability to participate in reactions.
• Why is the actual concentration of ions in solution different to their activity?

  Because ions are not completely free to move about due to the electrostatic interactions between charged particles.

  This can be from attractive forces between oppositely charged ions or repulsive forces between same charged ions.
• What is the ionic atmosphere?

  Electrostatic cloud of counter ions surrounding a charged particle.

  So around a Na+ ion there will be a cluster of Cl- ions, this means theres more cl- ions than Na+ ions which will shield the central ions charge.
• What is the ionic strength?

  Ionic strength is a measure of the total ion concentration, accounting for both concentration and charge. This will affect its solubility and activity.
• How do you calculate ionic strength?

  Ci = molar conc of each ion

  Zi is the charge number of each ion squared

  Then you take the sum of this

  
• Calculate the ionic strength of 0.1M NaCl solution?

  Na+ : c= 0.1 , z= +1

  Cl- : c= 0.1, z = -1

  Ionic strength = ½ [(0.1 × 1²) + (0.1 × 1²)] = 0.1 M
• Calculate the ionic strength of a 0.1M CaCl2?

  Calcium conc: 0.1m

  Charge: +2

  Cl-:

  Conc: 0.2 - 2 ions

  Z = -1

  1/2 x [(0.1 x 4)] + [0.2 x1)] = 0.3M

  2^2 = 4

  1^2 = 1
• Describe the typical behaviour of ions in a concentrated solution?

  ions behave as if they were present at lower concentrations than they actually are, due to the shielding effects of their ionic atmospheres.
• Whats the equation to calculate the pH of strong acids?

  pH = -log10 [H+]
• Calculate the pH of 0.0050M HCL?

  As HCI is a strong electrolyte, [H+] = 0.0050 M

  pH = –log10 (0.0050)

  pH = 2.30
• How do you calculate the pH of strong bases?

  Kw= [H+]x[ OH-]
• Calculate the pH of a 0.0050 M sodium hydroxide solution?

  As NaOH is a strong electrolyte it is fully dissociated, so [OH-] = 0.0050 M.

  
  

  pOH : -log( 0.0050) = 2.30

  
  

  pH = 14 - 2.30 = 11.7
• How do you calculate the pH of weak acids?

  In order to work out Ka from pKa you need to:

  Ka = 10–pKa

  Then rearrange the equation to calculate [H30]:

  [H3O+]^2 = Ka x cHA

  Then find pH:

  pH = -log10 [H3O+]

  
• Calculate the pH of 0.1M of acetic acid? The pKa is 4.76

  Convert pKa to Ka:

  Ka = 10–pKa

  Ka = 1.738 x 10-5 M

  Then calculate the hydroxonium concentration:

  [H3O+]2 = Ka x cHA

  [H3O+]2 = (1.738 x 10-5) x 0.1

  [H3O+] = 1.318 x 10-3 M

  Finally, calculate the pH:

  pH = – log10 [H3O+]

  pH = 2.88
• What happens when the conc of H+ is higher than the original conc of acid when calculating for a weak acid?

  
  

  Ka​=10−0.7 ≈0.2M.

  Ka = x² / [AH] - x

  Rearange to put it in a quadratic form:

  x²+Ka​x−Ka​cHA​=0.

  x² + 0.2x - (0.2 x 0.1) = 0

  x² + 0.2x - 0.02 = 0

  Use the quadratic formula:

  • a=1,b=0.2,c=−0.02.

  Concentrations cannot be negative so e 𝑥 = [H3O+] = 0.0732 M.

  pH = - log10 [H3O+] = 1.135

  ] = 1.135

  
• How would you calculate the pH of a weak base?

  pKw = pKa + pKb
• Distinguish between a conjugate acid and base?

  is what forms when a base accepts a proton (H+), while a conjugate base is what remains after an acid donates a proton
• The pKb for aqueous ammonia is 4.74. What is the pKa for its conjugate acid NH4+?

  pKa = pKw – pKb :

  14.00 - 4.74 = 9.26
• What is produced when you react a strong acid with a strong base?

  Neutral salt:

  Example: NaCl

  • Neither the Na+ ion nor the Cl− ion reacts with water, so the solution stays neutral -pH 7
• What is produced when you react a strong acid with a weak base?

  Acidic salt:

  Example: NH₄Cl.

  • NH4+(ammonium ion) reacts with water:
  • NH4+ + H2O→ NH3 + H3O+
  • This releases H3O+(hydronium ions), making the solution acidic
• What is produced when you react a weak acid and a strong base?

  Basic salt:

  Example: Sodium acetate (NaCH₃COO).

  • CH3COO- (acetate ion) reacts with water:
  • CH3COO− + H2O⇋ CH3COOH + OH−
  • This produces hydroxide ions, making the solution basic.
• What is produced when you react a weak acid and a weak base?

  Neutral salt:

  Weak Acid + Weak Base → Neutral Salt

  • Example: Ammonium acetate (NH4+CH3COO−).
  • Both ions (e.g., NH4+ & CH3COO− )react with water but their effects cancel out, resulting in a neutral pH.
• What is a buffer solution?

  A buffer solution is a solution that resists changes in pH when small amounts of acid or base are added.
• Why is the use of buffers important?

  To maintain pH of fluids in the human body to ensure the proper functioning of enzymes
• What is the physiological pH of blood?

  7.4
• What is the physiological pH of cerebrospinal fluid?

  7.35
• What is the pH of the stomach?

  1-3.5 it has a range to accommodate to the different substances we ingesr
• What is the pH of small intestine?

  8-9
• What is the pH of urine?

  4.6-8
• What is a buffer typically composed of?

  A mixture of a weak acid and its salt (conjugate base, A-)

  Example: acetic acid and sodium acetate
• How do you calculate the pH of a buffer solution?

  Henderson-Hasselbalch equation

  
• In a buffer solution, why does the [A-] equal to the initial amount of salt added?

  The salt completely dissociates in solution

  Its a weak acid so very little additional A- comes from the dissociation of acid

  The presence of a high concentration of A− from the salt shifts the equilibrium of the acid dissociation reaction to the left due to Le Chatelier's princple
• Why does the [HA] equal the initial amount of acid added?

  Very little of the acid dissociates (because it is a weak acid)

  Very little of the A– from the salt converts back to HA
• A solution contains 0.01 M acetic acid and 0.02 M sodium acetate. Calculate the pH of the buffer. pKa(acetic acid) = 4.76?

  ph = 5.06

  
• What happens if you add a strong acid to a buffer solution?

  It will react with the salt to form more weak acid

  HA + H2O ⇌ H3O+ + A-

  Salt reacts with acid (proton) to form more of the weak acid

  𝐻+ + 𝐴− → 𝐻A