acids

• Neutralisation Reaction


  Reaction where an acid and a base form a salt and 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


  A substance that releases H+ ions in aqueous solution
• Common strong acids


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


  An acid that only slightly dissociates when dissolved in water, giving an equilibrium mixture
• Strong acid


  An acid that completely dissociates when dissolved in water
• Salt


  A compound 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


  A substance that neutralises acids
• Common bases


  • Metal oxides
  • Metal hydroxides
  • Ammonia
• Bronsted-Lowry base


  A proton (H+) acceptor
• Neutralisation Reaction


  Acid + Base → Salt + Water
• Titrations are used to determine the concentration of an acid or base
• Titration procedure


  1. Rinse equipment

  2. Pipette alkali into conical flask

  3. Add acid from burette

  4. Add indicator

  5. Observe color change at end point

  6. Record burette readings

  7. Repeat until concordant results
• Acids and alkalis are corrosive, wear eye protection and gloves
• Sodium hydrogen carbonate (NaHCO3) and calcium carbonate (CaCO3) are good for neutralising excess acid
• Magnesium hydroxide is suitable for dealing with excess stomach acid
• 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
• Concentration of diluted CH3CO2H


  Amount/Volume
• Calculating concentration of diluted CH3CO2H

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

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


  conc in gdm-3 = conc in mol dm-3 x Mr = 1.08 x 60 = 64.8 g dm-3
• Calculating percentage of CaCO3 by mass in the tablet

  1. amount = conc x vol = 0.30 x 0.0111 = 0.00333 mol

  2. Moles of HCl left in 10.0 cm3 = 0.00333 mol

  3. Moles of HCl left in 100 cm3 of solution = 0.00333 x 10 = 0.0333

  4. Moles of HCl that reacted with the indigestion tablet = 0.05 - 0.0333 = 0.0167

  5. Moles of CaCO3 = 0.0167/2 = 0.00835

  6. Mass of CaCO3 = 0.00835 x 100 = 0.835 g

  7. Percentage of CaCO3 by mass in the tablet = 0.835/0.950 x 100 = 87.9 %
• Calculating Mr of M2CO3 and identifying the metal M

  1. amount of HCl used = 0.175 x 0.0328 = 0.00574 mol

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

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

  4. Mr of M2CO3 = 3.96/0.0287 = 138.0

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


  • Balance: 0.001 g
  • Volumetric flask: 0.1 cm3
  • 25 cm3 pipette: 0.1 cm3
  • Burette (start, end, end point): 0.10 cm3
• Calculating percentage error for each piece of equipment

  % uncertainty = uncertainty/measurement made x 100
• Reducing apparatus uncertainties


  • Decrease sensitivity uncertainty by using apparatus with greater resolution
  • Increase size of measurement made
• Readings


  Values found from a single judgement when using equipment
• Measurements


  Values taken as the difference between judgements of two values (e.g. using a burette in a titration)
• Uncertainty of analogue scale reading


  At least ±0.5 of the smallest scale reading
• Uncertainty of analogue scale measurement


  At least ±1 of the smallest scale reading
• Uncertainty of digital scale

  Resolution of the apparatus in each measurement
• Reducing uncertainties in a titration

  1. Replace measuring cylinders with pipettes or burettes

  2. Increase titre volume to reduce % uncertainty in burette reading
• Reducing uncertainties in measuring mass

  1. Use balance that measures to more decimal places

  2. Use larger mass

  3. Weigh sample before and after addition and calculate difference