Illustrated Guide to Home Chemistry Experiments

236 DIY Science: Illustrated Guide to Home Chemistry Experiments

LABORATORY 1 3.2:

qUANTIfy THE CommoN IoN EffECT

In Part I of the preceding laboratory, we

examined the effect on equilibrium of changing

the concentration of one of the reactant ions.

We found that increasing the concentration

of either reactant ion (Na+ or Cl–) by adding

sodium carbonate or hydrochloric acid,

respectively, forced solid sodium chloride to

precipitate from the saturated solution. This

phenomenon is called the common ion effect.

RIREEqU d EqUIpmENT ANd SUppLIES

£ goggles, gloves, and protective clothing

£ balance and weighing paper

£ dropper or beral pipette (1)

£ graduated cylinder, 10 mL

£ test tubes (6) and rack

£ pH meter

£ tap water

£ acetic acid, glacial (6 mL)

£ sodium acetate, anhydrous (4.0 g)

In addition to affecting solubility, the common ion effect reduces
the ionization of weak acids and weak bases in solution. Acetic
acid is a weak organic acid that dissociates only partially in
aqueous solutions, yielding hydronium (H 3 O+) ions and acetate
(CH 3 COO–) ions:

CH 3 CooH(l) + H 2 o ⇔ CH 3 Coo–(aq) + H 3 o+(aq)

Sodium acetate is an ionic compound that dissociates essentially
completely in aqueous solution, yielding sodium (Na+) ions and
acetate (CH 3 COO–) ions.

CH 3 CooNa(s) → CH 3 Coo–(aq) + Na+(aq)

The common ion effect tells us that acetic acid should
dissociate less completely in sodium acetate solution than
in pure water, because the sodium acetate increases the
concentration of acetate ions. Less dissociation means fewer
protons, and an accordingly higher pH. In this lab, we’ll test that
hypothesis by measuring the pH of a fixed concentration of
acetic acid in solutions that contain various concentrations of
sodium acetate.

CUTIOA nS

Glacial acetic acid is corrosive, toxic, and has a very

strong, biting odor. Wear splash goggles, gloves, and

protective clothing.

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SBSTITUTIU oNS ANd modIfICATIoNS

• You may substitute narrow-range pH test paper for the
  pH meter, but the accuracy and precision of your data
  will be lower.


• You may substitute white vinegar for glacial acetic
  acid, but the lower concentration of acetic acid
  reduces the pH variation between samples.


• You may substitute 6.7 g of sodium acetate trihydrate
  for the 4.0 g of anhydrous sodium acetate.


• You may make your own sodium acetate by
  neutralizing baking soda with white vinegar and
  evaporating the solution to dryness.