0910 Electrolytes - strong or weak?

If a weak electrolyte, how weak?

Preview. In Module 0909 Solutes: Electrolytes or non-electrolytes?, the distinction was made between solutes that are classified as either electrolytes or non-electrolytes (in aqueous solutions).
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Here we make a further distinction among solutes that are electrolytes: some are classified as strong electrolytes, and some as weak electrolytes. What is the difference?

KEY IDEAS

This module concerns covalent molecular substances which are electrolytes (in aqueous solution): that is, when dissolved in water, the resultant solution conducts electricity because of the presence of ions. We say that the solute ionises. See Module 0909 Solutes: Electrolytes or non-electrolytes?

In aqueous solutions, solutes which are electrolytes may be classified as either (i) strong electrolytes or (ii) weak electrolytes:

• Solutes which are strong electrolytes: In aqueous solution, all of the solute molecules ionise to form ions (and no un-ionised solute molecules remain).
• Solutes which are weak electrolytes: In aqueous solution, only some of the solute molecules ionise to form ions (and the others remain as un-ionised molecules).

 
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Strengths of weak electrolytes

Weak electrolytes differ in their degree of “weakness” or “strength”: In comparison of various weak electrolytes, in solutions of the same concentration and at the same temperature:

• the greater the fraction of solute molecules that are ionised at equilibrium, the stronger is said to be the weak electrolyte.
• the lower the fraction of solute molecules that are ionised at equilibrium, the weaker is said to be the weak electrolyte.

 

We need to be able to interpret the language of chemists, such as the following statements:

• Solutes A and B are both weak electrolytes, but A is a stronger electrolyte than B. That is, in aqueous solutions of both A and B, only some of the molecules are ionised at equilibrium, but in solutions of A and B of the same concentration and at the same temperature, the fraction of A molecules that are ionised is greater than the fraction of B molecules that are ionised.
• Solutes A and B are both weak electrolytes, but A is a weaker electrolyte than B, That is, in aqueous solutions of both A and B, only some of the molecules are ionised at equilibrium, but in solutions of A and B of the same concentration and at the same temperature, the fraction of A molecules that are ionised is less than the fraction of B molecules that are ionised.

 

In the following table, some data about selected weak electrolytes (all of which are acids) illustrate the key ideas stated above:

The degree of ionisation of the molecules of various solutes, each in 0.0100 M aqueous solutions at 25 C.
For example, in the hydrogen fluoride solution, [HF] = 0.0092 M, [H+] = [F-] = 0.0008 M.
Dichloracetic acid is a stronger weak electrolyte than is hypochlorous acid.
Hydrogen cyanide is a weaker weak electrolyte than is hydrogen fluoride.

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Specifying the conditions ....

When comparing the strength of weak electrolytes, why do we need to specify the condition “of the same concentration and at the same temperature”? This is because, for any solute that is a weak electrolyte:

• The fraction of molecules that are ionised depends on the concentration of the solution: The more dilute the solution, the greater is the fraction of molecules that are ionised (and the more concentrated the solution, the lower is the fraction ionised).
• The fraction of molecules that are ionised depends on the temperature of the solution: If the solution process is exothermic, the higher the temperature, the lower is the fraction ionised (and if the solution process is endothermic, the higher the temperature, the greater is the fraction ionised).

 

 Foreshadowing …
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The “extent” of ionisation of a weak electrolyte in solution can be estimated experimentally. It can also be calculated from experimental values of the equilibrium constants (in this particular context, called ionisation constants) for ionisation of the solute. This is discussed in Module 0913: Weak electrolytes: Getting quantitative.
 
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Language, language, language ....

The definition of a weak electrolyte given above is that “only some of the solute molecules ionise to form ions”. Good one!

However, in almost every chemistry textbook (except those with Prof. Bob as an author), the wording is something like “The molecules of a weak electrolyte are only partially ionised”.

Well, the authors know what they mean ………..

Warning about interpretation: This does NOT mean that all of the molecules are a little bit ionised! It is rusted-on chemists’ language to mean that only some of the molecules are ionised (and the others not at all).

Here is a highly stylised portrayal of an instantaneous snapshot (at the sub-microscopic level) of a solution containing dissolved weak electrolyte:
 

Each solute molecule is represented as two segments – one as a rectangle and one as a circle. Ionisation is the interaction with water molecules that gives rise to separation of the two segments, giving rise to a positively charged ion (the circles) and a negatively charged ion (the rectangles).

At this moment in time, only some of the molecules are ionised, and most molecules have retained their identity. 
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While this portrayal tries to convey the notion that at any moment only some of the solute molecules are ionised, you should be aware that it has serious deficiencies in other aspects:

• The water molecules in the solution are not shown – neither those in the bulk of he water, nor those that would be in the “hydration shell” around each ion.
• The solute molecules are far closer together than would be the case in a dilute solution.
• The portrayal cannot give a sense of the huge number of molecules and ions in a real system. To make 1 L of a 0.010 M solution of a solute, 6 000 000 000 000 000 000 000 molecules of solute are dissolved.
• By being an instantaneous snapshot, the rapid motion of the molecules, and the collisions with water molecules, is not visible.
• This portrayal cannot indicate the condition of dynamic equilibrium: all the while, molecules are splitting into anions and cations, at the same time (and number) that anions and cations are coming together to form uncharged molecules. Nor that it is highly unlikely that the same two ions formed in an ionisation event find each other again to form the same solute molecule.

 

​SELF CHECK

1.         Refer to the above depiction of a snapshot, at the sub-microscopic level, of a  solution of a weak electrolyte.

(a)        If the solute were acetic acid (ethanoic acid), what particles are represented by (i) the rectangles, and (ii) the circles? Note the charges on the ions.
(b)        What percentage of the weak electrolyte molecules are ionised at any point in time?
(c)        Suppose that in a defined part of the solution, 1 000 000 solute molecules ionise every second. In that same volume, how many ions join together to form solute molecules per second?
 
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2.         In a 0.05 M aqueous solution of hydrogen chloride:
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(a)        What is the concentration of hydrogen chloride molecules?
(b)        What is the concentration of hydronium ions (H+)?
(c)        What is the concentration of chloride ions (Cl-)?
 
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3.         Suppose that in a 0.20 M solution of the covalent molecular solute AB, 10% of the molecules are ionised when the ionisation reaction is in a condition of dynamic chemical equilibrium.

(a)        What is the concentration of AB molecules?
(b)        What is the concentration of A+ ions?
(c)        What is the concentration of B- ions?
(d)        What can we conclude about the rate (molecules per second) at which AB molecules ionise, in comparison with the rate at which A+ and B- ions associate to form AB molecules?

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4.         Dichloracetic acid is a strong-ish weak electrolyte in aqueous solutions. Explain what that means.
 

5.  Reference is made above to the common use of an ambiguous definition of weak electrolytes: “Those solutes which only partially ionise in solution”.
Suppose, by way of analogy, that we had a cupboard with many doors, and someone said that “the doors are partially open”. This can be interpreted in two ways.
Which one of the following diagrams representing the cupboard doors best corresponds with the equilibrium condition in a solution of a weak electrolyte?

                     (A) All of the doors are partly open.                                     (B) Some of the doors are wide open and most are shut.

ANSWERS TO SELF-CHECK QUESTIONS

1.         (a) Rectangles are acetate ions (CH3COO-), circles are hydrogen ions (H+)
            (b) 20%. Four of the twenty solute molecules are ionised.
            (c) 1 000 000 of each. Necessarily so to be in dynamic chemical equilibrium.
 
2.         (a) 0 M (all of the molecules of a strong electrolyte are ionised).
            (b) 0.05 M
            (c) 0.05 M
 
3.         (a) 0.18 M     (b) 0.02 M     (c) 0.02 M
 
4.         Not all of the molecules ionise in solution, but more than for other weak electrolytes.
 
5.         B
 
 

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