Acids and bases

• acid

  proton donor
• base

  proton acceptor
• acid base equilibria

  transfer of protons
• acids

  
• bases

  
• strong acid

  acid that completely dissociates to ions when dissolved in water (aqueous conditions)
• strong base

  base that completely dissociates to ions when dissolved in water (aqueous conditions)
• weak acid

  acid that only partly dissociates to ions in water

  equilibrium lies more on the left
• weak base

  base that only partly dissociates to ions in water

  equilibrium lies more on the left
• weak acids and bases

  
• acid base reactions

  acid loses proton

  base accepts proton

  
• pH scale

  measure of the hydrogen ion concentration in an aqueous solution

  covers wide range 0-14 so logarithmic scale used
• pH =

  -log$_{10}$(H$^+$)
• monoprotic acids

  each molecule of an acid will release one proton when it dissociates
• diprotic acids

  each molecule of an acid will release two protons when it dissociates
• ionic product of water, K$_w$


  affected by temperature

  at 298K, K$_w$= $1\times10^{-14}$$mol^2dm^{-6}$

  
• pure water

  always one $H^+$ for every $OH^-$

  so $K_w=$$\left(H^+\right)^2$
• weak acid dissociation constant


  pKa = –log 10 Ka

  
• pH titration

  shows how pH of solution changes during acid base reaction.

  when they react, neutralisation point is reached which is identified as a large vertical section through neutralisation/equivalence point
• pH curves

  base slowly added to acid and pH measured using pH probe

  smaller added volumes, more accurate curve

  
• calculating concentrations using pH graph

  find neutralisation point

  
• buffer solution

  maintains an approximately constant pH, despite dilution or addition of small amounts of acid or base

  resists change in pH
• Acidic buffer solutions

  contain a weak acid and the salt of that weak acid

  eg ethanoic acid and sodium ethanoate

  pH less than 7
• Basic buffer solutions

  contain a weak base and the salt of that weak base

  eg ammonia and ammonium chloride

  pH more than 7
• how does an acidic buffer maintain pH

  has proton acceptor that can react with added acid = salt of weak acid

  has proton donor that can react with added base = acid
• adding acid to acidic buffer

  H$^+$ ions react with salt of weak acid ($CH_3COO^-$) to remove excess H$^+$