Chemical Equilibrium

Dynamic Equilibrium

- The reactions continue in both directions but concentrations stop changing once equilibrium is reached.

- characteristics:

  (1) characterized by constant large-scale properties e.g. mass, colour, volume, pressure

  (2) small-scale processes continue

  (3) rate of forward change = rate of backward change

  (4) can be attained from either direction

  (5) achieved in a closed system

Le Chatelier's Principle

- If a system is at equilibrium, and a change is made in any of the conditions, then the system responds to

  counteract the change as much as possible.

Effects of changes in concentration, temperature, pressure and the presence of catalyst

on the equilibrium position

Haber Process (99 essay question)

- manufacture of ammonia

- 3 Stages:

  (1) Production and purification of N2 and H2

        (a) N2 is prepared by fractional distillation of liquid air.

        (b) H2 is prepared by reacting naphtha with steam. CH4 + H2O ---> CO + 3 H2

        (c) Air is not used because oxygen can react with Fe to give Fe2O3 and Hydrogen can react with

             Oxygen to cause explosion.

        (d) The purpose of purification is to prevent the poisoning of catalyst by impurities.

   (2) Catalytic reduction of N2 to NH3

         N2 (g) + 3 H2 (g) <=== (Fe, 500*C, 200 atm) ===> 2 NH3 (g), enthalpy change = - 92kJmol-1

         Optimum Conditions:

         (a)  Temperature: 500*C. From the equation N2 (g) + 3 H2 (g) <=====> 2 NH3 (g) + heat, the

               reaction is exothermic. Lower temperature increases the % yield. However, lower temperature

               results in too low reaction rate. Therefore higher temperature (500*C)

          (b) Pressure: 200 atm. From the equation N2 (g) + 3 H2 (g) <=====> 2 NH3 (g), there is a decrease

               in number of mole. Therefore increasing pressure can have higher % yield.

    (3) Liquefaction of NH3 (g): NH3 (g) is easily liquefied by cooling / compression because of the strong

         intermolecular hydrogen bond. The purpose of liquefaction is to save space.

- Some uses include making of fertilizers, explosive, nylon, window cleaner and refrigerant.

Contact Process (97 essay question)

- manufacture of Sulphuric (VI) acid

- 3 stages:

  (1) Production and purification of SO2

        (a) SO2 is prepared by burning sulphur [S (s) + O2 (g) ---> SO2 (g)] or by roasting sulphur-containing

            ores in air [4 FeS (s) + 7 O2 (g) ---> 2 Fe2O3(s) + 4 SO2 (g)].

        (b) Air is used because it is cheaper and N2 does not react with SO2 and catalyst. Also, air contains

              20% oxygen which is in excess.

        (c) SO2 and air are purified to prevent the poisoning of catalyst by impurities.

  (2) Catalytic oxidation of SO2 to SO3

       2 SO2 (g) + O2 (g) <=== (V2O5, 450*C, 1 atm) ===> 2 SO3 (g), enthalpy change = - 196kJmol-1

       V2O5 = vanadium (V) oxide

       Optimum Conditions:

       (a) Temperature: From the equation 2 SO2 (g) + O2 (g) <=====> 2 SO3 (g) + heat, the reaction is

            exothermic. Lower temperature increases the % yield. However, lower temperature results in too

            low reaction rate. Therefore higher / moderate temperature (450*C) is used.

      (b) Pressure: From the equation 2 SO2 (g) + O2 (g) <=====> 2 SO3 (g), there is a decrease in number

           of mole. Therefore higher pressure can have higher % yield. However, at 1 atm / 450*C already

           give 98% yield. Hence it is not justified to use higher pressure because it required a plant with

           very high cost to build and maintain.

      (c) Conversion of SO3 (g) to H2SO4 (l)

           SO3 (g) + H2SO4 (l) ---> H2S2O7 (l) [oleum]

           H2S2O7 (l) + H2O (l) ---> H2SO4 (l)

- Some uses include the manufacture of fertilizers, soapless detergents and paints / pigments.

Dissociation of Water

- Ionic product of water, Kw = [H+ (aq)][OH- (aq)]

  Kw = 10^(-14) mol^(2)dm^(-6) at 25*C

- pH = - log10 [H+]

  Methods to measure pH values include:

  (1) Indicator paper - by comparing colour of paper with colour chart

  (2) pH meter: It is calibrated by dipping the electrode in buffer solution of known pH.

Concept of acids and bases

- Bronsted-Lowry Theory: Acid is a proton donor while base is a proton acceptor.

- acid + base ---> conjugated base + conjugated acid

- strong acids and bases: fully or highly ionized in water

- weak acids and bases:

  (1) Concentration measures the amount of substance in a given volume of solution

  (2) Strength is the measure of extent to which acid can donate (base can accept) H+ (aq)

  (3) Dissociation constant of acid (Ka) and Dissociation constant of base (Kb)

  (4) Greater Ka means stronger acid but greater pKa means weaker acid

Hydrolysis of Salt

- Salt of strong acid and strong base e.g. NaCl

  NaCl (aq) ---> Na+ (aq) + Cl- (aq)

  There is no reactions between Na+ or Cl- with water, thus [H3O+] = [OH-], i.e. neutral solution.

- Salt of weak acid and strong base e.g. NaHCO3

  NaHCO3 (aq) ---> Na+ (aq) + {HCO3}- (aq)

  {HCO3}- reacts with water to form excess hydroxide ions, hence alkaline solution

  {HCO3}- (aq) + H2O(l) <===> H2CO3 (aq) + OH- (aq)

- Salt of strong acid and weak base e.g. NH4Cl

  NH4Cl (aq) ---> {NH4}+ (aq) + Cl- (aq)

  {NH4}+ reacts with water to form excess H3O+/H+, hence acidic solution

  {NH4}+ (aq) + H2O(l) <===> NH3 (aq) + H3O+ (aq)

- Salt of weak base and weak acid e.g. CH3CO2NH4

  CH3CO2NH4 (aq) ---> {CH3CO2}- (aq) + {NH4}+ (aq)

  Both of these ions react with water:

  {NH4}+ (aq) + H2O(l) <===> NH3 (aq) + H3O+ (aq) 

  {CH3CO2}- (aq) + H2O(l) <===> CH3COOH (aq) + OH- (aq)

  Whether the final solution is slightly acidic, slightly alkaline or neutral depends on the 2 equilibria. In

  fact, the solution is almost neutral.

Acid-base titration

- Acid-base indicator

  It changes colour with the change of pH.

  e.g. methyl orange: red ---> yellow

        methyl red: yellow ---> red

        phenolphthalein: colourless ---> pink

- Choice of indicators:

  If an indicator is to be used to determine the end-point of an acid-base titration, its pH should fall on the

  'vertical portion' of the titration curve and this give a sharp colour change at the end-point.

- End point: the point at which the reaction is shown to complete by indicator.

- Equivalent point: the point at which the reaction is shown to complete by mole ratio of equation.

Buffer Solution

- to calibrate pH meter

- one which resist the change in pH on addition of small amount of strong acid or alkai.

- presence of H+ (aq) acceptor: to traps added H+ (aq) ions

- presence of H+ (aq) donor: to supply H+ ions

- importance of buffers in biological system:

  Many processes in living system must take place under precise pH conditions. If the pH changes to a

  value outside a narrow range, the process will not occur at the correct rate, or it may not take place at all,

  and the organism will die.

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