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Strong and Weak Bases

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Strong and Weak Bases and Base Ionization Constant (Kb)

Ammonia for what?

All the complex electronics and apparatus in a space shuttle generate heat, as do the astronauts. The shuttles have a complex arrangement of systems to dissipate that heat into outer space. One of the components of this system is a series of coils filled with ammonia that are located on the outside of the shuttle. Ammonia absorbs the heat and then releases it into space as the gas circulates through the coils. This approach is both inexpensive and effective.

Strong and Weak Bases and Base Ionization Constant, K_b

As with acids, bases can either be strong or weak, depending on their extent of ionization. A strong base is a base, which ionizes completely in an aqueous solution. The most common strong bases are soluble metal hydroxide compounds such as potassium hydroxide. Some metal hydroxides are not as strong simply because they are not as soluble. Calcium hydroxide is only slightly soluble in water, but the portion that does dissolve also dissociates into ions.

A weak base is a base that ionizes only slightly in an aqueous solution. Recall that a base can be defined as a substance, which accepts a hydrogen ion from another substance. When a weak base such as ammonia is dissolved in water, it accepts an H + ion from water, forming the hydroxide ion and the conjugate acid of the base, the ammonium ion.

\text{NH}_3(aq)+ \text{H}_2\text{O}(l) \rightleftarrows \text{NH}^+_4(aq) + \text{OH}^-(aq)

The equilibrium greatly favors the reactants and the extent of ionization of the ammonia molecule is very small.

An equilibrium expression can be written for the reactions of weak bases with water. Because the concentration of water is extremely large and virtually constant, the water is not included in the expression. A base ionization constant  (K_b) is the equilibrium constant for the ionization of a base. For ammonia the expression is:

K_b=\frac{[\text{NH}^+_4][\text{OH}^-]}{[\text{NH}_3]}

The numerical value of  K_b is a reflection of the strength of the base. Weak bases with relatively higher  K_b values are stronger than bases with relatively lower  K_b values. The table below is a listing of base ionization constants for several weak bases.

Base Ionization Constants at 25°C
Name of Base Ionization Equation K_b
Methylamine \text{CH}_3\text{NH}_2 + \text{H}_2\text{O} \rightleftarrows \text{CH}_3\text{NH}^+_3 + \text{OH}^- 5.6 \times 10^{-4}
Ammonia \text{NH}_3 + \text{H}_2\text{O} \rightleftarrows \text{NH}^+_4 + \text{OH}^- 1.8 \times 10^{-5}
Pyridine \text{C}_5\text{H}_5\text{N} + \text{H}_2\text{O} \rightleftarrows \text{C}_2\text{H}_5\text{NH}^+ + \text{OH}^- 1.7 \times 10^{-9}
Acetate ion \text{CH}_3\text{COO}^- + \text{H}_2\text{O} \rightleftarrows \text{CH}_3\text{COOH} + \text{OH}^- 5.6 \times 10^{-10}
Fluoride ion \text{F} + \text{H}_2\text{O} \rightleftarrows \text{HF} + \text{OH}^- 1.4 \times 10^{-11}
Urea \text{H}_2\text{NCONH}_2+ \text{H}_2\text{O} \rightleftarrows \text{H}_2\text{NCONH}^+_3 + \text{OH}^- 1.5 \times 10^{-14}

Notice that the conjugate base of a weak acid is also a weak base. For example, the acetate ion has a small tendency to accept a hydrogen ion from water to form acetic acid and the hydroxide ion.

Summary

  • Strong and weak bases are defined.
  • The base equilibrium constant is stated.

Practice

Read the material at the link below and answer the following questions:

http://www.chemguide.co.uk/physical/acidbaseeqia/bases.html

  1. In the presence of hydroxide ions, which way does the equilibrium shift for the equation showing the dissociation of water?
  2. What percentage of ammonia in solution actually produces hydroxide ions?
  3. How does the position of equilibrium relate to the strength of a weak base?

Review

  1. Define a strong base.
  2. Define a weak base.
  3. Which is the stronger base: pyridine or urea?

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