5.1.3 Acids, Bases and Buffers

• What are monobasic, dibasic and tribasic acids?

  HCl - monobasic

  H2CO3 - dibasic - reacts with twice as many moles of metal or base than monobasic

  H3PO4 - tribasic

  CH3COOH - monobasic
• What is the ionic equation for the reaction of an acid with a metal?

  2H+ (aq) + Mg (s) ---> Mg2+ (aq) + H2 (g)

  (Mg is an example this applies for any metal)

  • This is a redox reaction
• What is the ionic equation for the reaction of an acid with a carbonate?

  Insoluble (group 1 are insoluble):

  2H+ (aq) + MgCO3 (s) ----> Mg2+ (aq) + CO2 (g) + H2O (l)

  Soluble:

  2H+ (aq) + CO3^2- (aq) ----> CO2 (g) + H2O (l)

  (Not redox)
• What is the Arrhenius model?

  • Acids dissociate and release H+ ions in aqueous solution
  • Alkalis dissociate and release OH- ions in aqueous solution
• What is the Bronsted - Lowry model?

  • Acids are proton donors
  • Bases are proton acceptors
• What are conjugate acid - base pairs?

  • Conjugate acid - base pairs contain 2 species that can be interconverted by the transfer of a proton

  
• How is the hydronium ion formed?

  • Adding a proton to H2O to form H3O+
• What must be present for the dissociation of an acid to take place?

  • Water
• Label the conjugate acid - base pairs

  • Acids on the left will have a conjugate base on the right
  • Bases on the left will have a conjugate acid on the right

  A) Acid 1

  B) Base 1

  C) Acid 2

  D) Base 2

  
• What does the pH scale measure?

  • pH measures H+ ion concentration
  • Decrease in one pH unit is a tenfold increase in [H+]
• State the equation for calculating pH

  A) -log

  B) H+

  
• State the equation for calculating the concentration of H+ ions from a pH

  A) 10

  B) -pH

  
• What is the acid dissociation constant, Ka?

  • Ka is temperature dependent and units are mol dm-3
  • Increase in temperature increases the value of Ka, therefore the forward reaction must be endothermic
  • The larger the value of Ka, the greater the degree of dissociation and the stronger the acid

  
• State the equations to convert between pKa and Ka

  A) -log Ka

  B) 10

  C) -pKa

  
• State how to calculate the concentration of H+ ions in a weak acid

  A) Ka

  B) HA

  
• State how to calculate the pH of a strong base

  • Calculate the concentration of H+ ions using Kw
  • Use the concentration to find the pH

  How to calculate concentration of H+:

  A) Kw

  B) OH-

  
• What is a buffer solution?

  • A system that minimises changes in pH when small amounts of acid or base are added
  • An acidic buffer consists of a weak acid and a soluble salt of its conjugate base e.g. CH3COOH and CH3COONa
• How are buffer solutions prepared?


  Ethanoic acid partially dissociates:

  CH3COOH (aq)  ⇌ H+ (aq) + CH3COO- (aq)

  • Equilibrium lies well over to the left creating a large reservoir of CH3COOH

  CH3COONa dissolves and completely dissociates into its ions:

  CH3COONa (s) + aq --> CH3COO- (aq) + Na+ (aq)

  • The salt of the conjugate base completely dissociates creating a large reservoir of CH3COO- ions and pushes equilibrium further left
• How does the addition of H+ ions affect a buffer solution?

  CH3COOH (aq)  ⇌ H+ (aq) + CH3COO- (aq)

  • Concentration of H+ increases
  • H+ ions react with the conjugate base CH3COO-

  CH3COO- (aq) + H+ (aq) ---> CH3COOH (aq)

  • The equilibrium shifts to the left, therefore removing most of the H+ ions
• How does the addition of OH- ions affect a buffer solution?

  CH3COOH (aq)  ⇌ H+ (aq) + CH3COO- (aq)

  • Concentration of OH- ions increases

  OH- (aq) + H+ (aq) ---> H2O (l)

  • The ethanoic acid dissociates, shifting the equilibrium to the right, therefore restoring most of the H+ ions
• Which buffer solution should be chosen?

  • Buffer solutions are most effective when the concentration of the weak acid and its conjugate bases are the same
  • Therefore Ka = [H+]
  • The pH of the buffer solution is the pKa of the weak acid
  • The pH of the buffer solution can be set by changing the concentrations of the acid and the conjugate base
• What is the importance of buffer solutions?

  • Controls pH for the effective functioning of enzymes
  • In blood plasma (pH needs to be maintained around 7.35 and 7.45)

  
• How is the pH of a buffer solution calculated?

  • Calculate concentration of H+ ions
  • -log [H+]

  A) Ka

  B) Ha

  C) A-

  
• Do stronger acids have a higher or lower pKa?

  • Lower pKa
• What is a pH meter?


  • Used to monitor pH of a solution during an acid-base titration
  • Used in conjunction with an acid-base titration indicator or in isolation
  • Used to determine equivalence point
• What is the equivalence point?


  Equivalence point - point where a volume of one solution reacts exactly with a volume of another solution
• What titration curve is this?

  • Strong acid / strong base

  
• What titration curve is this?

  • Strong acid / weak base

  
• What titration curve is this?

  • Weak acid / strong base

  
• What titration curve is this?

  • Weak acid / weak base

  
• How does a titration curve change as acid / base is added?

  • At the beginning there is excess acid so the pH increases slowly as base is added
  • The steep part of the curve is where there is a rapid increase in pH with a small addition of base
  • At the end there is excess base so the pH increases slowly as base is added

  A) Equivalence point

  
• How are acid-base indicators used to determine the end point of a titration?

  • Indicators are weak acids (red)
  • Conjugate base (yellow)
  • At the end point of the titration [HA] = [A-]
  • For this indicator the end point will be orange

  
• How do we determine which indicator to use during an acid-base titration?

  • Different indicators have different Ka values
  • At the end point [H+] = Ka and pH = pKa
  • The end point must coincide with the sharp rise in pH of the titration curve that includes the equivalence point
• What indicator should be used for an acid-base titration with a strong acid and a strong base?

  • Any indicator
• What indicator should be used for an acid-base titration with a strong acid and a weak base?

  • Methyl orange
• What indicator should be used for an acid-base titration with a weak acid and a strong base?

  • Phenolphthalein
• What indicator should be used for an acid-base titration with a weak acid and a weak base?

  • No indicator
  • A pH probe should be used instead
• What happens to the equilibrium and colour when H+ ions are added?

  • Shifts left
  • Yellow -> Orange -> Red

  
• What happens to the equilibrium and colour when OH- ions are added?

  • Weak acid dissociates
  • Shifts right
  • Red -> Orange -> Yellow

  
• What colour is phenolphthalein at the end point?


  • Light pink