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2.1.4 Acids

Definitions

Term Definition
Acid When dissolved in water an acid releases \(H^+\) ions (protons) into the solution.
Bases React with acid by accepting \(H^+\) ions (protons) and neutralising the acid to form a salt.
Alkalis Bases that dissolve in water and release \(OH^-\) ions into the solution.
Salt Formed when the \(H^+\) in an acid is replaced by a positive ion.

Common chemicals

Common acids

  • \(HCl\)
  • \(H_2SO_4\)
  • \(HNO_3\)
  • \(CH_3COOH\)

Common bases

  • Carbonates
  • Hydrogencarbonates
  • Metal oxides
  • Metal hydroxides
  • Ammonia (accepts \(H^+\) and forms \(NH_4^+\) ions)

Common alkalis

  • \(NaOH\)
  • \(KOH\)
  • \(NH_3\)

Acid-base reactions

Strong and weak acid

  • Both release \(H^+\) ions / \(H^+\) donor in aqueous solutions
  • Strong acid
    • Completely dissociates (> 99%) in aqueous solutions / releases all hydrogen atoms as \(H^+\) ions
    • e.g. \(HCl(aq) \rightarrow H^+(aq) + Cl^-(aq)\)
  • Weak acid
    • Partially dissociates (< 1%) in aqueous solutions / only releases a portion of available hydrogen atoms as \(H^+\) ions
    • e.g. \(CH_3COOH(aq) \rightleftharpoons H^+(aq) + CH_3COO^-(aq)\)

Neutralisation

  • The reaction of acids with bases (including carbonates, metal oxides and alkalis) to form salts
  • Ionic equation: \(H^+(aq) + OH^-(aq) \rightarrow H_2O(l)\)

Experimental techniques

Preparing standard solution

  • Solid weighed accurately using a digital balance with 2 dp or more
  • Dissolve solid in a beaker using distilled water
    • Use less distilled water than needed to fill the volumetric flask to the mark
  • Transfer the solution to (250 \(cm^3\)) volumetric flask
    • Rinse the beaker and transfer washings to the flask so the last traces of the solution are transferred to the volumetric flask
  • Volumetric flask is filled to the graduation line
    • Add distilled water a drop at a time using a dropping pipette
    • Keep adding until the bottom of the meniscus lines up exactly with the mark
  • Mix the solution thoroughly
    • Volumetric flask is sealed with a stopper and inverted several times

Titration

  • Add measured volume of one solution to conical flask using pipette
    • Typical tolerances: 10 \(cm^3\): \(\pm 0.04 cm^3\), 25 \(cm^3\): \(\pm 0.04 cm^3\), 50 \(cm^3\): \(\pm 0.10 cm^3\)
  • Add other solution to burette, record initial reading
  • Add a few drops of indicator to conical flask
Formula Acidic Neutral Alkaline
Phenolphthalein Colourless Colourless Pink
Methyl orange Red Yellow Yellow
  • Run solution from burette into conical flask until it reaches the end point (indicated by colour change)
    • Swirl the flask while the solution is added
  • Record final reading
  • Titre = final reading - initial reading
  • First titre carried out quickly to get approximate titre
  • Repeat accurately by adding solution dropwise as the end point is approached
  • Carry out until two accurate titres are concordant (within 0.1 \(cm^3\))
  • Only use concordant results for calculating the mean titre