Acids

• Neutralisation Reaction


  Acid + Base → Salt + Water
• Common Neutralisation Reaction Equations


  • H2SO4 + 2NaOH → Na2SO4 + 2H2O
  • HCl + NaOH → NaCl + H2O
  • 2HCl + Na2CO3 → 2NaCl + CO2 + H2O
  • 2HCl + CaCO3 → CaCl2 + CO2 + H2O
• Alkali


  A soluble base that releases OH- ions in aqueous solution
• Common alkalis


  • Sodium hydroxide (NaOH)
  • Potassium hydroxide (KOH)
  • Aqueous ammonia (NH3)
• Acid


  Releases H+ ions in aqueous solution
• Common strong acids


  • Hydrochloric (HCl)
  • Sulfuric (H2SO4)
  • Nitric (HNO3)
• Weak acid


  Ethanoic acid (CH3COOH)
• Salt


  Formed when the H+ ion of an acid is replaced by a metal ion or an ammonium ion
• In carbonate reactions there will be effervescence due to the CO2 gas evolved and the solid carbonate will dissolve
• Base


  Neutralises acids. Common bases are metal oxides, metal hydroxides and ammonia.
• Bronsted-Lowry base

  Defined as a proton (H+) acceptor
• Neutralisation Reaction Equations


  • 2HNO3 + Mg(OH)2 → Mg(NO3)2 + 2H2O
  • 2HCl + CaO → CaCl2 + H2O
  • H2SO4 + K2CO3 → K2SO4 + CO2 + H2O
  • HCl + NH3 → NH4Cl
• Strong acid


  Completely dissociates when dissolved in water
• Weak acid


  Only slightly dissociates when dissolved in water, giving an equilibrium mixture
• Titration Method


  1. Rinse equipment

  2. Pipette alkali into conical flask

  3. Add acid from burette

  4. Add indicator

  5. Observe colour change at end point

  6. Record burette readings

  7. Repeat until concordant results
• Acids and alkalis are corrosive, wear eye protection and gloves
• In quality control, titrations/testing will be done on several samples as the amount/concentration may vary between samples
• If titrating a mixture, need to consider if it contains other substances with acid-base properties
• Sodium hydrogen carbonate (NaHCO3) and calcium carbonate (CaCO3)

  Good for neutralising excess acid as they are not corrosive and have no toxicity
• Magnesium hydroxide


  Suitable for dealing with excess stomach acid as it has low solubility and is only weakly alkaline
• Common Titration Equations


  • CH3CO2H + NaOH → CH3CO2-Na+ + H2O
  • H2SO4 + 2NaOH → Na2SO4 + 2H2O
  • HCl + NaOH → NaCl + H2O
  • NaHCO3 + HCl → NaCl + CO2 + H2O
  • Na2CO3 + 2HCl → 2NaCl + CO2 + H2O
• Distilled water can be added to the conical flask during a titration to wash the sides of the flask
• Indicators are generally weak acids so we only add a few drops, too much will affect the titration result
• If the jet space is not filled properly prior to commencing the titration it will lead to errors
• Example 1: Titration of vinegar


  1. Work out amount of NaOH

  2. Use balanced equation to get moles of CH3CO2H

  3. Calculate concentration of diluted CH3CO2H

  4. Calculate concentration of original concentrated CH3CO2H

  5. Calculate concentration of CH3CO2H in original 25 cm3 in g/dm3
• Example 2: Titration of impure calcium carbonate tablet

  1. Calculate moles of NaOH used

  2. Work out number of moles of HCl left in 10.0 cm3
• Calculating concentration of diluted CH3CO2H

  Concentration = 0.00250 / 0.0231 = 0.108 mol dm-3
• Calculating concentration of original concentrated CH3CO2H

  Concentration = 0.108 x 10 = 1.08 mol dm-3
• Calculating concentration of CH3CO2H in original concentrated 25 cm3 in g dm-3

  Concentration in g dm-3 = 1.08 x 60 = 64.8 g dm-3
• Calculating percentage of CaCO3 by mass in the tablet

  Amount = Concentration x Volume

  Moles of HCl that reacted = 0.05 - 0.0333 = 0.0167

  Moles of CaCO3 = 0.0167/2 = 0.00835

  Mass of CaCO3 = 0.00835 x 100 = 0.835 g

  Percentage = 0.835/0.950 x 100 = 87.9%
• Calculating Mr of M2CO3 and identifying the metal M


  Moles of HCl used = 0.175 x 0.0328 = 0.00574

  Moles of M2CO3 in 25.0 cm3 = 0.00574/2 = 0.00287

  Moles of M2CO3 in 250 cm3 = 0.00287 x 10 = 0.0287

  Mr of M2CO3 = 3.96/0.0287 = 138.0

  Ar of M = (138-12-16x3)/2 = 39, so M = Potassium
• Apparatus uncertainty


  Balance: 0.001 g

  Volumetric flask: 0.1 cm3

  Pipette: 0.1 cm3

  Burette: 0.10 cm3
• Calculating percentage error


  % uncertainty = uncertainty/measurement x 100
• To decrease apparatus uncertainties, use apparatus with greater resolution or increase the size of the measurement
• Experiments with smaller readings will have higher experimental uncertainties
• Calculating percentage difference


  % = (actual value - calculated value)/actual value x 100
• If %uncertainty due to apparatus < percentage difference, there is a discrepancy in the result due to other errors
• If %uncertainty due to apparatus > percentage difference, all errors can be explained by the sensitivity of the equipment
• Readings


  Values found from a single judgement when using equipment
• Measurements


  Values taken as the difference between two judgements (e.g. using a burette)