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Strength of Bases (Alkalis) Tutorial

Key Concepts

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Base Dissociation (Ionisation or Ionization) in Water

Strong Base Dissociation (Ionisation or Ionization)

The hydroxides of Group 1 metals, MOH, and the hydroxides of Group 2 metals, M(OH)2, are strong bases.
These bases dissociate completely in water to form hydroxide ions and a hydrated metal cation in solution.

Group 1 Hydroxides   Group 2 Hydroxides
General
form
basehydroxide ions+cation
General
equation
MOHOH-(aq)+M+(aq)
examplesNaOHOH-(aq)+Na+(aq)
examplesKOHOH-(aq)+K+(aq)
 
basehydroxide ions+cation
 
M(OH)22OH-(aq)+M2+(aq)
 
Ca(OH)22OH-(aq)+Ca2+(aq)
Ba(OH)22OH-(aq)+Ba2+(aq)

When a Group 1 or Group 2 hydroxide dissolves in water, only hydrated metal cations and hydroxide ions will be found in the solution, that is, the undissociated Group 1 or 2 hydroxide will not be found in the solution.
For a Group 2 hydroxide that is not very soluble in water, such as calcium hydroxide, Ca(OH)2, some undissociated solid Ca(OH)2(s) will be found floating in the water or lying at the bottom of the solution, but the species that are found as part of the solution will be Ca2+(aq) and OH-(aq), not Ca(OH)2(aq)#.

For 0.010 mole of a water soluble strong base, MOH, dissolved in 1 L of water:

  base hydroxide
ions
+ hydrated metal cations
  MOH OH- + M+
Moles of species before
base is added to water
0.010 mol   0 mol   0 mol L-1
Moles of species after
base is added to water*
0 mol   0.010 mol   0.010 mol

For 0.010 mole of a water soluble strong base, M(OH)2, dissolved in 1 L of water:

  base hydroxide
ions
+ hydrated metal cations
  M(OH)2 2OH- + M2+
Moles of species before
base is added to water
0.010 mol   0 mol   0 mol L-1
Moles of species after
base is added to water
0 mol   2 × 0.010 mol
= 0.020 mol
  0.010 mol

Weak Base Dissociation (Ionisation or Ionization)

Aqueous solutions of ammonia (NH3), amines (general formula R-NH2), and phosphine (PH3) are examples of weak bases.
Weak bases do not react completely with water.
In aqueous solution, the undissociated weak base is in equilibrium with its conjugate acid and hydroxide ions.

Brønsted-Lowry Definition
general formbase+water hydroxide ions+conjugate acid
general equationB+H2O(l) OH-(aq)+BH+(aq)
ammoniaNH3+H2O(l) OH-(aq)+NH4+(aq)
methanamine
(methylamine)
CH3NH2+H2O(l) OH-(aq)+CH3NH3+(aq)
ethanamine
(ethylamine)
CH3CH2NH2+H2O(l) OH-(aq)+CH3CH2NH3+(aq)

The concentration of the undissociated base, hydroxide ions, and the conjugate acid of the base, will depend on how much of the base reacts with the water.

For 0.010 mole of a weak base, R-NH2, dissolving in 1 L of water:

  base + water hydroxide
ions
+ conjugate acid
Moles of species before
base is added to water
0.010 mol       0 mol   0 mol
Moles of species after
base is added to water
0.010 - n mol       n mol   n mol

where n is a number less than 0.010

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Comparing the Strength of Bases

  1. Electrical Conductivity of Dilute Aqueous Solutions

    A strong Arrhenius base dissociates fully, producing the maximum number of hydroxide ions and metal cations in solutions.
    Only a small amount of a weak base reacts with water so it produces fewer ions in solution than a strong base.

    How well an aquous solution conducts electricity is dependent on the number of mobile ions available##.
    The more mobile ions there are in solution, the better the electrical conductivity of the solution.

    In order to determine the comparative (or relative) strengths of bases which accept only one proton, we could measure the electrical conductivity of each solution as long as we use the same concentration for each base, and all the measurements are taken at the same temperature.

    Bases at the same concentration and temperature
    stronger base ← ← ← ← weaker base
    better electrical conductor ← ← ← ← poorer electrical conductor

    Note that the solutions must have the same concentration. For a water soluble base, the more concentrated the solution, the more hydroxide ions there will be in solution and the better the solution will conduct electricity.
    Similarly the solubility of a base is dependent on temperature, and the mobility of ions in solution is temperature dependent, so the electrical conductivity measurements must be made at the same temperature.

  2. pH of Dilute Aqueous Solutions

    In order to determine the comparative (or relative) strengths of bases that accept only one proton, we could measure the pH of each solution as long as we use the same concentration for each base, and all the measurements are taken at the same temperature.

    Monobasic bases at the same concentration and temperature
    stronger base ← ← ← ← weaker base
    higher pH ← ← ← ← lower pH

    Note that the solutions must have the same concentration. For a water soluble base, the more dilute the solution, the lower its pH will be.The more concentrated the solution, the higher its pH will be.
    Similarly the solubility of a base is dependent on temperature which will effect the number of ions in solution, so the pH measurements must be made at the same temperature.

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Worked Examples

Question 1. Which substance forms the weaker base in water, ammonia or sodium hydroxide?

  1. Aqueous solutions of all Group 1 hydroxides are strong bases.
    Sodium is a group 1 metal.
    Sodium hydroxide is a strong base.
  2. Ammonia in aqueous solution is a weak base.
  3. Ammonia is therefore a weaker base than sodium hydroxide.

Question 2. Which aqueous solution, 0.010 mol L-1 potassium hydroxide or 0.010 mol L-1 methanamine (methylamine), will have the highest pH at 25°C?

  1. Since both substances only accept one proton and are at the same concentration and temperature, the stronger base will have the higher pH.
  2. Aqueous solutions of Group 1 hydroxides are strong bases.
    Potassium is a Group 1 metal.
    Potassium hyroxide is a strong base.
  3. Aqueous solutions of amines, such as methanamine (methylamine), are weak bases.
  4. The stronger the base, the higher the pH.
    Aqueous potassium hydroxide is the stronger base so it will have the higher pH.

Question 3. At 25°C, the pH of an aqueous solution of sodium hydroxide is 9.0 while the pH of an aqueous ammonia solution is 10.5
Which of these solutions is the stronger base?

  1. Aqueous solutions of Group 1 hydroxides are strong bases.
    Sodium is a Group 1 metal.
    Sodium hydroxide is a strong base.
  2. Aqueous solutions of ammonia are weak bases.
  3. The stronger base is sodium hydroxide.
    Note that the difference in pH could be caused by the sodium hydroxide solution being quite dilute while the ammonia solution could be quite concentrated.

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#It is possible to determine the concentration of ions in solution for a base that is only slightly soluble in water if we know the value of its solubility product, Ksp.

*It is true that the self-dissociation of water also contributes hydroxide ions to the solution, but the concentration of these hydroxide ions is only 10-7 mol L-1 at 25°C, so as long as the concentration of hydroxide ions produced by the base is much greater than that produced by the water, the contribution made by the dissociation of water can be neglected.

##As long as we are discussing dilute aqueous solutions at 25oC, this is generally true.