Chapter 21 quizlet

• What is a buffer solution?

  *A system that minimises pH changes when small amounts of an acid o bases are added.*

  They have two components to remove added acids or alkalis:

  -Weak acid (HA) to remove added alkali

  -Conjugate base A⁻ to remove added acid
• How can a buffer be made from a weak acid and it's conjugate base?

  By mixing a solution of a weak acid with that of one of its salts.

  E.g ethanoic acid and potassium ethanoate.

  When added to water:

  *CH₃COOH*⇌ H⁺ and CH₃COO⁻

  CH₃COONa + H₂O → *CH₃COO⁻* + Na⁺
• How does a buffer behave in equilibrium?

  HA ⇌ H⁺ + A⁻

  The position of equilibrium favours the acid and when more A- ions are added it shifts further to the left which continues to reduce [H⁺] which results in a buffer solution which can remove added acids and alkali by shifting the position of equilibrium.
• How can a buffer be made from the partial neutralisation of a weak acid?

  Adding an aqueous alkali solution to an excess weak acid so the acid is partially neutralised to form the conjugate base.

  Since it's partially neutralised some of the weak acid is left over uncreacted so it has a mixture of the two.
• What happens when an acid is added to a buffer?

  Shifts the position of equilibrium to the left to oppose the change.

  [H⁺] increases and react with conjugate base (A⁻)

  [HA] increases to remove the H⁺ ions
• What happens when an alkali is added to a buffer?

  Weak acids removes the alkali and shifts the position of equilibrium to the right.

  [OH⁻] increases and react with H⁺ to form water. [H⁺] decreases

  HA dissociates to release more H⁺ so that the ratio of HA:A⁻ are the same
• When are buffers most effective?

  When: [HA] = [A⁻]

  The pH of a buffer is = pKa of HA

  The operating pH of a buffer is ⁺⁻1 of pH.
• What are the assumptions when calculating the pH of a buffer?

  All the A- ions come from the salt

  Almost all HA molecules remain unchanged in the solution.
• What thing do you have to remember when calculating pH of a buffer?

  When neutralisation: Use an ICE table to find change in moles.

  When the volumes are the same cancel out volumes
• What is the blood pH?

  The healthy pH of blood is 7.35 to 7.45

  When < 7.24 = acidosis

  When > 7.45 = alkalosis
• What acids are there in the body?

  -Fixed (non-volatile) acids which are produced through metabolism or ingestion. They excrete through the kidney as a liquid.

  -Carbonic acid the only volatile one.

  H₂Co₃ ⇌ H⁺ + HCo₃⁻ (Hydrogen carbonate)
• What happens in the blood carbon acid-hydrogen carbonate buffer system?

  H₂CO₃ ⇌ H⁺ + HCo₃⁻

  Increasing H⁺ shifts the position of equilibrium to the left. [H⁺] increases and reacts with HCO₃⁻ to form H₂CO₃.

  Increasing OH⁻ shifts the position of equilibrium to the right as OH⁻ reacts with H⁺ to form H₂O
• How can you calculate the acid base ratio?

  [Base]/[Weak Acid] = Ka/[H⁺]
• How else can the pH of a buffer solution be calculated?

  pH = pKa + log([A⁻]/[HA])

  When [A⁻]/[HA] = 1 → log1=

  so pH = pKa
• What are the key points of a pH titration curve?

  -Excess of acid and base. This is at the start and end of the titration. pH increases slowly as the opposing solution is added.

  -Equivalence point: the volume of the solution that exactly reacts with the volume of the other.

  
• What is an acid-base indicator and what does the end point show

  It's a weak acid and has distinct colours from its conjugate base. As an acid the indicator could be red and as a base its could be yellow so at the end point the indicator has equal concentrations so the colour will be in the middle.
• What happens to an indicator when a basic solution is added to an acid?

  HA⇌A⁻+H⁺ (all aq)

  OH⁻ react with H⁺ from the indicator to make H₂O

  HA will dissociate to oppose and shift EQM to right.

  Colour will change when EQM is shifted to the right.
• What happens to an indicator when an acidic solution is added to basic one?

  HA⇌A⁻+H⁺ (all aq)

  H⁺ reacts with A⁻

  Shifts EQM to the left.

  Colour will change when EQM is shifted to the left
• What is an indicators endpoint and how would you choose a suitable one?

  Ka = [H⁺] and pKa = pH and [HA] = [A⁻]

  The vertical section of the pH curve matches the indicator pH range.
• What is a buffer solution?

  A system that minimises pH changes when small amounts of an acid or a base are added.
• What do buffer solutions contain?

  Two components to remove acid or alkali - a weak acid and its conjugate base.
• How well do buffer solutions last?

  The two components will eventually be used up. As soon as one component has all reacted, the solution loses its buffering ability towards added acid or alkalis.
• Does pH change at all?

  As the buffer works, the pH does change but only by a small amount.
• How do you prepare a buffer solution from a weak acid and its salt?

  Mix a solution of ethanoic acid with a solution of one of its salts, for example, sodium ethanoate.

  
  

  When ethanoic acid is added to water, the acid partially dissociates and the amount of ethanoate ions in solution is very small. Ethanoic acid is the source of the weak acid component.

  
  

  Salts of the weak acids are ionic compounds and provide a convenient source of the conjugate base. When added to water, the salt completely dissolves. Dissociation into ions is complete and so the salt is the source of the conjugate base component of the buffer solution.
• How do you prepare a buffer solution via partial neutralisation of the weak acid?

  By adding an aqueous solution of an alkali, such as NaOH, to an excess of the weak acid. The weak acid is partially neutralised by the alkali, forming the conjugate base.

  
  

  Some of the weak acid is left over unreacted. The resulting solution contains a mixture of the salt of the weak acid and any unreacted weak acid.
• What happens when acid is added to a buffer solution?

  1) [H⁺] increases.

  2) H⁺ ions react with the conjugate base, A⁻.

  3) The equilibrium position shifts to the left, removing most of the H⁺ ions.
• What happens when alkali is added to a buffer solution?

  1) [OH⁻] increases.

  2) The small concentration of H⁺ ions reacts with the OH⁻ ions to make water.

  3) HA dissociates, shifting the equilibrium position to the right to restore most of the H⁺ ions.
• How do you know which weak acid to use if they result in buffer solutions that operate over different pH ranges?

  A buffer is most effective when there are equal concentrations of the weak acid and its conjugate base.

  
  

  The pH of the buffer solution is the same as the pKa value of HA.

  
  

  The operating pH is typically over about two pH units, centred at the pH of the pKa value.
• How do you work out the pH of a buffer solution?

  [H⁺] = Ka x ([HA]/[A⁻])

  
  

  Provided that Ka and the concentrations of HA and A⁻ are known, [H⁺] and the pH can be calculated.
• What happens if blood pH falls below 7.35?

  People can develop acidosis.

  
  

  Can cause fatigue, shortness of breath, shock or death.
• What happens if blood pH rises above 7.45?

  People can develop alkalosis.

  
  

  Can cause muscle spasms, light-headedness, and nausea.
• How does the carbonic acid-hydrogencarbonate buffer system work on addition of H⁺?

  [H⁺] increases.

  
  

  H⁺ ions react with the conjugate base, HCO₃⁻.

  
  

  The equilibrium position shifts to the left, removing most of the H⁺ ions.
• How does the carbonic acid-hydrogencarbonate buffer system work on addition of an alkali, OH⁻?

  [OH⁻] increases.

  
  

  The small concentration of H⁺ ions reacts with the OH⁻ ions.

  
  

  H₂CO₃ dissociates, shifting the equilibrium position to the right to restore most of H⁺ ions.
• How does the body prevent H₂CO₃ build up?

  By converting it to carbon dioxide gas, which is then exhaled by the lungs.
• What are the main features of a pH titration curve?

  pH increases slowly at first as basic solution is added.

  
  

  At the vertical section, pH increases rapidly on addition of a very small amount of base. Acid and base concentrations will be similar here.

  
  

  The equivalence point is at the centre of the vertical section.

  
  

  The graph then goes back to increasing slowly as pH increases slowly as basic solution is added.
• What is the equivalence point of the titration?

  The volume of one solution that exactly reacts with the volume of the other solution.

  
  

  The solutions have then exacly reacted with one another and the amounts used will match the stoichiometry of the reaction.
• What is an acid-base indicator?

  A weak acid, HA, that has a distinctively different colour from its conjugate base, A⁻.

  
  

  A common indicator is methyl orange. The weak acid is red, the conjugate base is yellow.
• What happens to an indicator at the end point of a titration?

  The indicator contains equal concentrations of HA and A⁻ and the colour will be in between the two extreme colours.
• What colour is methyl orange initially just before a strong base is added to a strong acid?

  It is initially red as the presence of H⁺ ions forces the equilibrium position well to the left.
• What happens to this methyl orange upon addition of a basic solution containing OH⁻ ions?

  OH⁻ ions react with H⁺ in the indicator.

  
  

  The weak acid, HA, dissociates, shifting the equilibrium position to the right.

  
  

  The colour changes, first to orange at the end point and finally to yellow as the equilibrium position is shifted right.