Acids, Bases and Buffers

• What is a Bronsted-Lowry acid?

  A proton donor
• What is a Bronsted Lowry base?

  A proton acceptor
• Give two equations where water is acting as either an acid or a base.

  • HCl + H2O --> H3O+ + Cl -
  • H2O + NH3 --> OH- + NH4+
• What is the name for H3O + ?

  Hydroxonium ion
• Water is slightly...

  Ionised
• What is the Kw equation?

  Kw = [H+] + [OH-]
• What is Kw?


  The ionic product of water
• What is the value of Kw?

  1 x 10^-14 mol2dm-6
• What is Kw also equal to?

  [H+]^2
• What is the formula for pH?

  pH = -log[H+]
• How do we work out the concentration of [H+] from pH?

  10^-pH
• What is a weak acid?

  It only partially dissociates in water
• What is the equation for the partial dissociation of a weak acid?

  HA $\Rightarrow$ H$^+$ + A$^-$
• What does it mean if there is a large Ka value?

  The equilibrium is shifted right and the acid is more dissociated
• The smaller the value of pK$_a$...


  The stronger the acid
• how do we define pK$_a$?


  pK$_a$= -log K$_a$
• how is an acidic buffer made?

  A mixture of weak acid and soluble salt of that acid
• When do we use $\frac{\left[H^+\right]^2}{\left[HA\right]}$instead of $\frac{\left[H^+\right]\left[A^-\right]}{\left[HA\right]}$?


  When we have a solution of only weak acid and water.
• What do we assume in acid-base calculations?

  • [H$^+$] = [$A^-$]
  • [HA]$_{eqm}$= [HA]$_{initial}$
• How do we convert pKa to Ka?

  $10^{-pK_a}$
• What does it mean if pKa is bigger?

  Weaker acid
• Why is H2O not shown in the Kw equation?

  It is almost constant
• why must we wash the pH probe with distilled water between each of the calibration measurements?


  Prevent solutions contaminating each other
• What is the formula for Ka?

  $Ka\ =$$\ \frac{\left[H^+\right]\left[A^-\right]}{\left[HA\right]}$
• What does a strong acid-strong base graph look like?


  This

  
• What does a strong acid-weak base graph look like?


  This

  
• What does a weak acid and weak base titration look like?


  This

  
• What does a weak acid and strong base titration look like?


  This

  
• Why do weak acid-strong base and weak acid-weak base titration graphs have an initial curve?

  They are reversible reactions
• What is the equivalence point?

  The point in a titration at which the volume of one solution has reacted exactly with the volume of the second. Moles of acid added equals moles of alkali present.
• What is the end point?

  The point at which the indicator just changes colour and should coincide with the equivalence point
• What are the factors of a suitable indicator?

  • Colour change must be sharp rather than gradual
  • End point given by indicator must be same as equivalence point
  • Indicator should give a distinct colour change
  • Soluble in water
  • Intensely coloured
• What is the colour change in methyl orange if the base is in the conical flask?


  Yellow to red
• What is the colour change in methyl orange if the acid is in the conical flask?


  Red to yellow
• What is the colour change in phenolphthalein if the base is in the conical flask?


  Pink to colourless
• What is the colour change in phenolphthalein if the acid is in the conical flask?


  Colourless to pink
• What is the colour change in bromomythol blue if the base is in the conical flask?


  Blue to yellow
• What is the colour change in bromomythol blue if the acid is in the conical flask?


  Yellow to blue
• What is the pH range of methyl orange?

  3.1-4.4
• What is the pH range of phenolphthalein?

  8.2-10.0