Acids & Bases

• What is a Bronsted-Lowry acid?

  A proton donor (E.g. NH4+)
• What is a Bronsted-Lowry base?

  A proton acceptor (E.g. OH- ions)
• State the meaning of the term strong acid:

  An acid that completely dissociates in water
• State the meaning of the term weak acid:

  Acid that partially dissociates in water
• Explain why [H2O] is not shown in the Kw expression

  H2O is constant
• Explain why the value of Kw increases as temperature increases

  Equilibrium is endothermic, equilibrium shifts to the RHS to oppose the temperature increase
• Suggest why the pH probe is washed with distilled water between each of the calibration measurements

  Different solutions must not contaminate each other
• Explain why the volume of sodium hydroxide solution added between each pH measurement is smaller as the end point of the titration is approached

  To avoid missing the end point
• State why all three of the indicators are suitable for this titration

  All have a pH range within the steep part of the titration curve
• State the meaning of the term weak acid

  Acid that partially dissociates (in water to form H+ ions)
• Which acid-base pair has the highest pH at the equivalence point?

  CH3COOH & NaOH
• Give an equation to show each stage in the dissociation of sulfuric acid in aqueous solution

  1. H2SO4 ---> HSO4- + H+
  2. HSO4- --><-- SO42- + H+
• Suitable Indicators for pHs

  Methyl orange: Red on acid side. pH: 3-5. Yellow on basic side

  Litmus: Red on acid side. pH: 5-8. Blue on basic side

  Phenolpthalein: Colourless on acid side. pH: 8-10. Pink on basic side
• Define what is meant by a buffer solution

  A solution which is able to resist changes in pH when small volumes of acid or base are added
• Explain why the expression for Kw does not include the concentration of water

  Concentration of H2O is much greater than [H+] or [OH-] so water is constant
• Salt bridge


  Allows for ions to flow, soaked in KNO3 solution
• Voltmeter


  High resistance - prevents current from flowing in circuit
• Rules for equations


  - NO PR

  - More negative half feel is oxidised, more positive is reduced

  - Reduction is forward direction

  - Oxidising agent gets reduced and reducing agent gets oxidised
• Acids and bases equations


  pH = -log [H+] or [H+] = 10^-pH

  Ka = [H+] [A-]/ [HA]

  Kw = [H+] [OH-] so kw = [H+]^2

  Or kw = [H+][A-]

  To find [H+], simply do [H+] = 10-pH

  The higher the conc of H+ ions in acidic solution - the lower pH
• Key Points


  Acids with water: HA + H2O >< H3O+ + A-

  Bases with water: B + H2O >< BH+ + OH-
• Weak acids


  CH3COOH - Ethanoic acid and other carboxylic acids

  Backward reaction is favoured - not many H+ protons produced

  CH3COOH >< CH3COO + H+
• Strong acids


  HCl, H2SO4, HNO3

  Forward reaction favoured strongly - more H+ produced

  HCl >< H+ + Cl -
• Indicators


  Methyl - Red acid and yellow basic- 3-5pH

  Litmus - Red acid and blue basic - 5-8pH

  Phenolphthalein - Colourless acid and pink basic - 8-10pH
• Equilibrium Constant kP


  Mole fraction = Number of moles of component / Total number of moles

  Partial pressure = Mole fraction X Total Pressure