Chemical Equilibrium

1
Chemical Equilibrium

• Experiment 26

2
Chemical Equilibrium

• At the point of equilibrium, the concentrations
  of all reactants remain constant with time.
• At equilibrium, the original reaction is still
  taking place but is opposed by the reverse
  reaction.
• The equilibrium constant, Keq, is the ratio of
  the concentrations of all product species present
  at the point of equilibrium to the concentrations
  of all original reactant species.

3
The Equilibrium Constant

• For the general reaction,
• aA bB ?? cC dD
• The equilibrium constant will be,
• Keq CcDd / AaBb

4
The Spectrophotometer

• In choice II, you will use a spectrophotometer to
  measure the transmittance of mixtures of ferric
  nitrate and potassium thiocyanate at different
  concentrations.
• Using the equation for absorbance, A
  -logT , and a calibration curve for the reaction,
  you will be able determine the concentration of
  your products.

5
Le Châteliers Principle

• Le Châteliers principle states that if we
  disturb or stress a system that is already in
  equilibrium, then the system will react so as to
  minimize the effect of the disturbance.

6
Solubility Equilibria

• Suppose you have a saturated solution of sodium
  chloride.
• NaCl(s) ??Na(aq) Cl-(aq)
• If you added HCl, Cl-, the equilibrium would
  shift to consume the added ion.
• The equilibrium would shift to the reactants.
• This can be observed by the appearance of NaCl
  precipitate.

7
Complex Ion Equilibria

• Remember from last week that metal ions will
  react with certain substances to produce brightly
  colored ions.
• By monitoring the color of the solution you can
  determine the position of equilibrium.
• Fe3 SCN- ?? FeSCN2
• Yellow colorless
  red
• Using the equilibrium, you can add Fe 3 or SCN-
  ions or remove SCN- ions and observe the effects
  this has on the equilibrium.

8
Acid/Base Equilibria

• Many acids and bases, particularly weak ones,
  exist in equilibrium like conditions.
• For example, an aqueous solution of ammonia,
• NH3 H ?? NH4
• Using an acid-base indicator to observe pH
  changes, you will add NH3 and H ions to see what
  effects it has on the equilibrium.

9
Procedure Notes

• For choice II, make sure your test tubes, cuvets,
  are clean and dry.
• Use concentrated ammonia and hydrochloric acid in
  the fume hoods only!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
  !!!!!!!