AcidsAndBases

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Arrhenius Theory - Year 11 Stuff, not applicable in Year 12

• Acids are substances that ionise into $H^{+}$ ions
• Bases are substances that disassociate into $O{H}^{-}$ ions
• Electrolytes: substances that ionise/disassociate in an aqueous solution
  • Strong electrolytes completely ionise/disassociate
  • Weak electrolytes only partially ionise
• Pros:
  • Nice and easy
• Doesn’t cover anything

Brønsted-Lowry

• Acid is a proton donor ($H^{+}$ is a proton)
• Base is a proton acceptor
• Conjugate Acid-Base pair: A pair of substances that differ by an $H^{+}$ ion
  • When $H^{+}$ is accepted/donated, it turns into the other substance
• Amphoteric: A substance that can act as both an acid and a base
  • “Self ionisation” of water
• Mono-protic Acid: it can only donate one proton
• Poly-protic: it can donate more than one proton

Acid Strength

• Ability to donate a proton
• Often measured through $K_a$ value
• This is the K value for equilibrium in an aqueous solution

$\frac{[C{H}_{3}CO{O}^{-}][H_3{O}^{+}]}{[]C{H}_{3}COOH]},{\text{ K}}_a=1.76\ast 10^{-}5,\text{ at 25˚C}$

• If $K_a$ is greater than 1, it is a strong acid
• The stronger the acid, the weaker its conjugate base
• Often shown in table
  • Strong acid is top left
  • Strong base is bottom right
  • IT’S LIKE THE SRP TABLE VALUES
    • well, not really
    • but kinda same structure????
  • Oxide ion is stronger base than $O{H}^{-}$
  • Thus it doesn’t exist in a solution
• Larger $K_a\to$ more ionisation $\to$ stronger acid

• Strength of acid/base
  • Strong
    • Fully ionised/disassociated
  • Weak
    • Partially
  • Stronger - more ionisation
• Concentration: mol/L of substance
• If an acid/base is strong, concentration does not matter, it is still strong

Hydrolysis

• Gives pH of a solution
• Brønsted-Lowry with $H_{2}O$ molecule
• basically asking if $H_{2}O$ will act as a base or an acid

pH of ionic solutions

• Will the ion act as a base/acid when dissolved
• Which is it more likely to do
• Neutral: $S{O}_4^{2-}$, $N{O}_3^{-}$, Group I and II metals
  • Tail of a strong acid forms a neutral solution
  • Lucarelli says sulfate is basic but Pearson says it’s neutral
• Some metals (transition) such as $A{l}^{3+}$ and $F{e}^{3+}$ are acidic
  • They form hydrated complex ions and undergo hydrolysis
  • Best described as Lewis Acids
    • Not in syllabus

pH

• Logarithmic scale quantifying the concentration of hydrogen ions

$\text{pH}=-{\log }_{10}[H^{+}]=-{\log }_{10}[H_3{O}^{+}]$

$[H^{+}]=10^{-\text{pH}}$

$\text{At 25˚ C: }{K}_w=10^{-14}=[H^{+}][O{H}^{-}]$

• If a solution is neutral $[H^{+}]=[O{H}^{-}]$
• More of both as equilibrium shifts right with temperature
• pH of 7 is neutral at 25˚ C

Buffers

• Restrict changes to pH

  • When small amounts of an acid/base are added

• Solution of a weak acid and its conjugate base

  • E.g. acetic (ethanoic) acid mixed with acetate (ethanoate) solution $C{H}_{3}COO{H}_{(aq)}+H_2{O}_{(l)}\rightleftharpoons H_3{O}_{(aq)}^{+}+C{H}_{3}CO{O}_{(aq)}^{-}$

• Buffer capacity: ability to resist pH changes

  • Relative concentrations: equimolar concentrations of acid and conjugate base is best
  • High concentrations: enable more $H_3{O}^{+}$ or $O{H}^{-}$ to be absorbed/neutralised]

• Add 1 mole of strong base for every 2 moles of weak acid

  • 1 mole of weak acid is neutralised
  • Thus, there is 1 mole of conjugate base for every mole of weak acid

Acidic Buffer solution pH < 7

$C{H}_{3}COO{H}_{(aq)}+H_2{O}_{(l)}\rightleftharpoons H_3{O}_{(aq)}^{+}+C{H}_{3}CO{O}_{(aq)}^{-}$

Add $H_3{O}^{+}$???

• By LCP, reverse reaction is favoured, $H_3{O}^{+}$ (not all of it - partial) is consumed, equilibrium shifts left
• Net: $[H_3{O}^{+}]$ partially increases
• pH remains constant(ish) (slightly lower) Add $O{H}^{-}$???
• $[H_3{O}^{+}]$ decreases, thus by LCP, forward reaction is favoured, produces more $H_3{O}^{+}$ , equilibrium shifts right
• Net: $[H_{3}O]$ partially decreases
• pH remains constant (slightly higher - PARTIAL increase)

$O{H}^{-}+C{H}_{3}COOH\to H_{2}O+C{H}_{3}COOH$

• Direct reaction with molecule can occur

Basic Buffer solution pH > 7

$N{H}_3+H_{2}O\rightleftharpoons N{H}_4^{+}+O{H}^{-}$

Add $H_3{O}^{+}$

• $[O{H}^{-}]$ decreases, thus by LCP, forward reaction is favoured, produces more $O{H}^{-}$, equilibrium shifts right
• Net: $[O{H}^{-}]$ partially decreases
• pH remains reasonably constant (slightly lower)

Add $O{H}^{-}$???

• By LCP, reverse reaction is favoured, $O{H}^{-}$ (not all of it - partial) is consumed, equilibrium shifts left
• Net: $[O{H}^{-}]$ partially increases
• pH remains constant(ish) (slightly higher)

Systems

• Biological: blood
  • $H_{2}C{O}_3/HC{O}_3^{-}$
  • $H_{2}P{O}_4^{-}/HP{O}_4^{2-}$
  • keeps pH constant
• Ocean Acidification
  • $C{O}_{2(g)}\rightleftharpoons C{O}_{2(aq)}$
  • $C{O}_{2(aq)}+H_2{O}_{(l)}\rightleftharpoons H_{2}C{O}_{3(aq)}$
  • $H_{2}C{O}_{3(aq)}+H_2{O}_{(l)}\rightleftharpoons H_3{O}_{(aq)}^{+}+HC{O}_{3(aq)}^{-}$
  • $HC{O}_{3(aq)}^{-}+H_2{O}_{(l)}\rightleftharpoons H_3{O}_{(aq)}^{+}+C{O}_{3(aq)}^{2-}$

Titrations

Indicators

• Equilibrium influenced by $[H^{+}]$ ($[H_3{O}^{+}]$)

$I{n}^{-}+H_3{O}^{+}\rightleftharpoons HIn+H_{2}O$

• $In$ is the conjugate base of an indicator (A particular colour, e.g. colourless)
• $HIn$ is the conjugate acid of an indicator (A distinct colour from its conjugate base, e.g. tartar)
• Increases $[H_3{O}^{+}]$ (more acidic/lower pH)
• Favours forward reaction, Equilibrium shifts right
• Indicator is tartar at low pH
• Colourless at high pH

Volumetric Analysis

• Volumetric - solution concentrations - titrations (acid - base)
• Make up a solution of known concentration (Primary Standard)
  • Weigh solid
  • Known mass = known moles
  • Wash beaker with distilled water
  • Mix solid with distilled water (dissolve)
  • Transfer to volumetric flask
  • Measure the volume from the bottom of the meniscus
  • Transfer to storage bottle
    • Clean and dry
      • If not, rinse out thoroughly
      • Pour some of the solution in, swirl, and tip out if there is water left
    • School setting: must not be waterx
• Primary standard
  • High purity
• Low affinity for water
  • Not hygroscopic
  • Not deliquescent
• Standardise $HCl$ using primary $N{a}_{2}C{O}_{3(aq)}$ by titration
  • Rinsing:
    • Conical flask: rinse with $H_{2}O$ (known number of moles)
      • If it is rinsed with $N{a}_{2}C{O}_3$, volume of added $HCl$ increases, as concentration would be too low
    • Burette: Final rinse with $HCl$ (known concentration)
    • Pipette: Final rinse with $N{a}_{2}C{O}_{3(aq)}$ (known concentration)
  • Strong acid weak base titration, at equivalence point, solution is acidic
    • pH is below 7
  • Thus, an indicator with an equivalence point below 7 is used
• end point
  • You stop titrating; first permanent colour change of indicatorD

Indicator Choice

• When a weak acid and strong base are titrated, the resulting salt will be basic
  • Thus, an indicator with an equivalence point above 7 is used
• When a strong acid and weak base are titrated, the resulting salt will be acidic
  • Thus, an indicator with an equivalence point below 7 is used
• When a strong acid and strong base are titrated, the resulting salt is neutral
  • Thus, an indicator with an equivalence point of 7, or very close to 7, is used
• When a weak acid and weak base are titrated, it is not favourable, as there must be large amounts of both present for a neutralisation to occur

Calculations

• Titration situation is a common context for problems
• Tracking solutions/dilutions/samples

Uncertainty

• Error for burette is ±0.05 mL
• Burette reading
  • If read from the top of the meniscus, it doesn’t actually cause any significant error
  • This is because the difference is still the same, as the reading is taken significantly, and thus the amount of titrate added is the same
• Pippetting
  • Technique
  • Random error
• End point
  • Systematic uncertainty (always over)
  • Impacts accuracy
• Indicator Choice
  • Systematic
  • (wrong choice - technique)
• Incorrect rinsing, parallax
  • Dirty glasswear
  • Bad technique
  • CAN BE AVOIDEDs