Acids and Bases

• What are acids?

  Proton donors.
• What happens when we mix acids with water?

  H+ ions are released. They form H3O+ ions as H+ ions don't exist on their own in water.

  Acid(aq) + H2O(l) > H3O+(aq) + Acid-(aq)
• What are bases?

  Proton acceptors.
• What happens when we mix bases with water?

  It reacts with H+ ions to form hydroxide ions.

  Base(aq) + H2O(l) > Base H+(aq) + OH-(aq)
• Weak acid?

  Any carboxylic acid eg ethanoic acid.

  CH3COOH > CH3COO- + H+

  Backwards reaction is favoured so not many H+ ions produced.
• Strong acid?

  HCl, H2SO4, HNO3

  HCl > H+ + Cl-

  Forwards reaction strongly favoured so many H+ ions produced.
• Weak base?

  NH3

  NH3 + H2O > NH4+ + OH-

  Backwards reaction favoured so not many OH- ions produced.
• Strong base?

  NaOH, KOH

  NaOH > Na+ + OH-

  Forwards reaction strongly favoured so many OH- ions produced.
• What happens when acids react with bases?

  Protons are exchanged.

  eg acid + base > Base H+ + Acid-
• Water behaving as a base?

  This occurs when water is added to an acid. It accepts a proton to form H3O+.

  eg acid + H2O > H3O+ + Acid-
• Equilibrium based on acids?

  Strong acid- equilibrium lies right.

  Weak acid- equilibrium lies left.
• What does water dissociate into?

  Its OH- ions and H3O+ ions.

  eg H2O > H+ + OH-
• Water dissociating?

  Water dissociates weakly so we assume its conc has a constant value.
• Calculating Kw?

  Kw= [H+] x [OH-]
• Kw important points?

  Kw value is the same in a solution at a given temp.

  Kw value= 1 x 10^-14
• Pure water Kw?

  Pure water has equal conc of H+ and OH-.

  Pure water Kw= [H+]^2
• What does the pH equation measure?

  The conc of H+ ions in a solution.
• pH equation?

  pH= -log[H+]
• Calculating the conc from knowing the pH?

  Press shift then log. Minus pH.
• Calculating Strong Monoprotic acids pH?

  HCl, HNO3.

  They dissociate to produce one H+ ion for every acid molecule.
• Calculating Strong diprotic acids?

  H2SO4

  Diprotic acids dissociate to produce 2 H+ ions for every acid molecule.

  Calculating pH= -log(H+ x 2)
• Calculating pH of a strong base?

  do Kw (1 x 10^-14) x [OH-] to= [H+]

  Then -log[H+]
• How do weak acids dissociate?

  They only slightly dissociate in aq solutions. Therefore we can't assume [H+]=[acid].

  They exist in the equilibrium HA>< H+ + acid-
• Assumptions of weak acids?

  1: Only a small amount of a weak acid (HA) dissociates. So we assume [HA]equilibrium ~ [HA]start.

  Ka= [H+][A-] / [HA]start.
• Assumption of weak acids?

  2: Dissociation of acid is greater than dissociation of water in the solution. We assume all H+ ions come from the acid.

  Ka= [H+]^2 / [HA]
• Calculating pH of a weak acid?

  1: write down Ka expression. [H+]^2 / [HA].

  2: Rearrange equation to calculate [H+]^2.

  3: Calculate [H+] by square rooting [H+]^2.

  4: Calculate pH by doing -log[H+]
• Calculating the conc of a weak acid?

  1: Calculate [H+] by rearranging the pH equation. 10^-pH

  2: Write down Ka expression. Ka= [H+]^2 / [HA]

  3: Rearrange to calculate [HA]. [H+]^2 / Ka.
• Whats pKa?

  Another way of measuring the strength of an acid. The lower the value, the stronger the acid.
• pKa equation?

  pKa= -log10Ka
• Calculating the pKa of an acid with the Ka value of 7.52x10^-3 moldm-3?

  pKa= -log10 7.52x10^-3= 2.12
• Calculating the pH of 0.025 moldm-3 of CH3COOH acid with 4.75 pKa?

  1: Ka= 10^-4.75= 1.78x10^-5 moldm-3.

  2: Calculate [H+] from the Ka expression.

  [H+]^2= Ka x [HA]. 1.78x10^-5 x 0.025= 4.45x10^-7

  [H+]= Square root of 4n45x10^-7= 6.67x10^-4.

  3: Calculate pH -log10[H+]= 3.18
• Titration curves?

  SA/SB, SA/WB, WA/SB, WA/WB

  
• SA/SB example?

  This is a strong acid as we start at pH 1. The end point is when the acid has been neutralised fully by the base.

  The half neutralisation point is the halfway point between 0 and the end point. It can be used to calculate the pKa of a weak acid by taking the pH at this point.

  
• Indicators?

  A suitable indicator must change colour within the vertical part of the titration curve to accurately determine the end point.
• Methyl orange?

  Red at low pH.

  Yellow at high pH.

  Used for SA/SB, SA/WB
• Phenolphthalein?

  Colourless at low pH.

  Pink at high pH.

  Used for WA/SB
• WA/WB

  Has no sharp pH change. We measure using a pH meter.
• What is a buffer?

  A chemical that resists changes in pH when small amounts of acid/base are added.
• Acidic buffers?

  Keep solution below pH 7.

  Made from a weak acid and its salt.
• Example of acidic buffer?

  Ethanoic acid and sodium ethanoate.

  CH3COOH > CH3COO- + H+

  Weak acids dissociate weakly so equilibrium lies left.

  CH3COO-Na+ > CH3COO- + Na+

  Salts dissociate strongly so equilibrium lies right.