Chapter 4: Acids and Redox

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4.1 Acids, bases and neutralisation

  • Acid: a proton (H+) donor
  • Base: a proton (H+) acceptor e.g. metal oxides, hydroxides, carbonates and ammonium
  • Alkali: a base that is soluble in water and releases OH- ions into the solution
  • Salt: a compound formed when the H+ of an acid is replaced by a positive metal ion

Strong acid: an acid that completely dissociates into it's ions in solution

E.g. HCl (aq) ---> H+ (aq) + Cl- (aq)

Weak acid: an acid that partially dissociates into it's ions in solution

E.g. CH3COOH (aq) <---> H+ (aq) + CH3COO- (aq) (<---> means reaction is incomplete)

Neutralisation: the reaction of an acid and a base to form a salt (chloride, sulfate, ethanoate salts)

  • Acid + metal ---> salt + hydrogen
  • Acid + base ---> salt + water
  • Acid + carbonate ---> salt + water + carbon dioxide
  • Ionic equation for acid + alkali (soluble base) H+ (aq) + OH- (aq) ---> H2O (l)
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4.2 Acid-base titrations

Standard solution: a solution of known concentration

Preparing a standard solution (practical):

  • Solid is weighed accurately
  • Solid is dissolved in a beaker using distilled water
  • Solution is transferred into volumetric flask & last traces rinsed into flask with distilled water
  • Flask filled to graduation line with distilled water, one drop at a time until bottom of meniscus sits on the line
  • Volumetric flask inverted serveral times to throughly mix the solution

Typical tolerances of pipette, burette and volumetric flask:

  • a 10 cm3 pipette: +0.04 cm3
  • a 25 cm3 pipette: +0.06 cm3
  • a 50 cm3 burette: +0.10 cm3
  • a 100 cm3 volumetric flask: +0.20 cm3
  • a 250 cm3 volumetric flask: +0.30 cm3
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4.2 Acid-base titrations

Acid - base titration procedure (practical):

  • Add a measured volume of one solution to a conical flask using a pipette
  • Add the other solution to a burette and reacord the initial burette reading (to 0.05 cm3)
  • Add 2 - 3 drops of an indicator to the flask (methyl orange or phenolphthalein)
  • Run solution from burette into flask, swirling flask throughout to mix the 2 solutions, indicator changes colour at end point, (indicates volume of one solution that exactly with the volume of other solution)
  • Record final burette reading, volume added from burette = titre (final volume - initial volume)
  • A quick trial titration is carried out first to find the approximate titre
  • Titration is repeated accurately by adding the solution dropwise as end point is approached
  • Further titrations carried out until 2 accurate titrations are concordant (agree to within 0.10cm3)

The mean titre only uses closest accurate titres, rejects inaccurate titres (not agreeing to within 0.10 cm3). If inaccurate titres included you lose accuracy of the titration technique.

  • From results you will know: concentration and reacting volume for one solution
  • Only reacting volume for the other solution
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