Lecture 2 (pH, acids & bases)

• What is the pH of pure water?


  7
• Equation for pH:


  $pH =$$-log10 (H+)$
• The larger the Ka...


  The stronger the acid
• The smaller the pka...


  The stronger the acid
• pka equation:


  $pka =$$-log10 (Ka)$
• Ka equation:


  (H+)(A-)/(HA)
• Strongest acid?

  HCl
• Weakest acid?

  H3PO4 (H2PO4- + H+) phosphoric acid
• What is the pka?


  pH at which the acid is half dissociated into H+ & A-
• When pH=pka...


  (HA)=(A-)
• What is a buffer?


  Substance that resists pH change on addition of acid or base. Weak acids or bases can act as buffers.
• When can weak acids & bases act as buffers?

  When pH is close to its pka (+/-1 pH unit)
• Roughly equal amount acid & base in...

  Buffering region
• What happens when you add more H+ to a buffer solution?


  Base can accept H+, so equilibrium shifts left.
• What happens when you add more OH- to a buffer solution?


  Acid can release H+, so equilibrium shifts right.
• Why are buffers used in lab experiments?


  Keep solutions at right pH. Buffer must have pka close to pH needed. Biologically & chemically inert.
• Physiological pH:


  Biological systems work best at their optimum pH.
• Acidosis:


  pH < 7.35
• Alkalosis:


  pH > 7.45
• What is blood buffered by?

  Phosphate (HPO4 2-) & bicarbonate (HCO3-) ions

  Carbonic acid (weak) so acts as buffer
• Compensatory respiratory alkalosis:


  If blood gets too acidic, equilibrium shifts left to remove H+. Shifts left again to remove CO2 by exhaling at lungs.

  (May breathe heavily while exercising as blood pH to acidic so lungs respire more quickly so can exhale CO2)
• What is blood buffered by?

  Phosphate (HPO4 2-) & bicarbonate (HCO3-) ions.

  Carbonic acid (weak) so buffer

  
• LO:


  :

  
• How do you calculate [H+]?


  :

  
• What do acids do?

  Acids donate protons.
• What do bases do?


  Bases accept protons.
• What do acids ionise to form?


  Proton & a base
• When pH < PKa...


  There's more HA (mostly acid)
• When pH > PKa...


  There's more A- (mostly base)
• Stomach pH:


  2
• Blood pH:


  7.4
• Pancreatic juice pH:


  8
• Milk pH:


  6.6
• Examples of digestive proteases:


  • pepsin (pH 1-2 Stomach)
  • Chymotrypsin (pH 8- upper intestine)