Acids 'n' Bases

Question 1

Bronsted-Lowry acids and bases

Answer 1

• B-L acid = a proton donor, gives H+
• B-L base = a proton receiver, takes H+
• strong B-L acids are fully dissociated in water, weak ones are only partially dissociated
• weak acids from reversible reactions e.g CH3COOH <=> CH3COO- + H+
• acid/base equilibria involves the transfer of protons

Question 2

pH calculations

Answer 2

• monoprotic acid = releases 1 H+ per molecule, diprotic acid = releases 2 H+ per molecule
• pH = -log[H+], [H+] = 10^(-pH), this is used because the concentration of hydrogen ions covers a very wide range
• [H+] = Kw/[OH-]
• diluted pH = [H+] in original solution * old volume/new volume

Question 3

Ionic product of water

Answer 3

• water is slightly dissociated so it forms a reversible reaction H2O <=> H+ + OH-
• equilibrium constant = [H+][OH-]/[H2O]
• Kw = equilibrium constant*[H2O] = [H+][OH-]
• in pure water [H+] = [OH-], so Kw = [H+]^2
• under standard conditions, Kw = 10^-14, it varies with temperature

Question 4

Mixing strong acids and bases

Answer 4

• calculate mols of H+ and OH-
• find the excess of either H+ or OH-
• calculate [H+] or [OH-]
• find pH

Question 5

weak acids

Answer 5

• weak acids dissociate slightly in aqueous solution
• acid dissociation constant for weak acids = Ka = [H+][A-]/HA] = [H+]^2/[HA]
• [HA] is the concentration on the label of the bottle
• pKa = -logKa, Ka =10^(-pKa)
• small pKa = stronger acid

Question 6

useful ionic equations

Answer 6

• H+ + OH- -> H2O
• 2H+ + CO3(2-) -> H2O + CO2
• H+ + HCO3- -> “”
• H+ + NH3 -> NH4+

Question 7

Weak acids mixing with strong bases

Answer 7

• for every mol OH- added, one mol HA is used up and one mol A- is produced
• find pH by calculating mols HA and OH- then finding which is in excess
• if HA is in excess, calculate mols formed of A-, then use [HA] leftover and [A-] to find [H+] and find pH
• if OH- is in excess, calculate [OH-], then find [H+] and then pH
• if mols HA = mols OH-, pH = pKa

Question 8

Indicators for titration

Answer 8

• HA and A- are different colours in solution
• Methyl Orange changes colour at a higher pH than phenolphthalein
• indicators must be suitable for certain titrations, as they change colour at around the equivalence point of the neutralisation reaction, which should be the end of the titration

Question 9

pH curves

Answer 9

• show the results of acid/base titration, plotting pH against amount of base added to acid
• equivalence point = when mol acid = mol base, facilitates a rapid change in pH
• diprotic acids have two pH curves, one on top of the other

Question 10

shapes of pH curves

Answer 10

• strong acid/strong base = low pH to high pH, rapid change at equivalence point
• weak acid/strong base = medium pH to high pH, rapid change at equivalence point
• strong acid/weak base = low pH to medium pH, rapid change at equivalence point
• weak acid/weak base = medium pH to medium pH, gradual change at equivalence point. Harder to find end point of titration with indicators as it is slow, so use a pH meter

Question 11

Buffer solutions

Answer 11

• resist small changes in pH, by shifting equilibrium so they only change pH a tiny amount
• [H+] = Ka[HA]/[A-]
• calculate pH of buffer by finding mols A-, then [A-], [HA] and Ka, find [H+] or [OH-] and then pH

Question 12

Acidic Buffers

Answer 12

• weak acid + weak acid salt solution
• [acid]+[salt]&raquo_space;[H+]
• made by mixing weak acid with its own salt, or mixing an excess of weak acid in a strong base
• adding H+ causes more [HA] to form and equilibrium to shift in opposite direction, adding OH- decreases [H+] so equilibrium shifts back

Question 13

Basic Buffers

Answer 13

• weak base + weak base salt solution
• [base]+[salt]&raquo_space; [OH-]
• made by mixing weak base with its salt, or by adding excess of weak base to strong acid
• adding H+ causes [OH-] to decrease and equilibrium to shift in opposite direction, adding OH- decreases [HA] and causes equilibrium to shift back

Question 14

Applications of buffers

Answer 14

• in shampoo to keep pH constant in hair
• biological washing powders keep pH right for enzymes, systems in our body do the same thing to keep processes with enzymes effective
• e.g blood must have pH of about 7.4