0909 Solutes: Electrolytes or non-electrolytes?

23/11/2020

Module 0909

Solutes: Electrolytes or non-electrolytes?

Do molecules retain their identity when water dissolves them?

Introduction and overview

What happens to substances during the process of dissolving in water?

This module concerns the fact that when a substance is dissolved in water, the chemical species in the solution may be different from those in the substance before being added to water. This is the subject of Module 0908 Chemical species, speciation.

The observable properties of solutions depend on the nature of the chemical species in the solution. In this module, we discuss just one of those properties (whether the solution conducts electricity or not) and make sense of the observations.

Prof Bob demonstrates that solutions of some substances conduct electricity and some do not.

KEY IDEAS - Solutes: Electrolytes or non-electrolytes?

Which solute species in solution (molecules or ions in a lattice) can resist the attack of the dreaded blue monster water molecules? Those that surrender are called electrolytes.

Let's be clear ....

The terms electrolyte and non-electrolyte are used to distinguish between types of solutes.

It is a common misconception to apply these terms to solutions. No, they are descriptive terms for solutes: the stuffs that we dissolve in solvents to make solutions.

In this discussion we will be concerned only with aqueous solutions (water is the solvent).

Definitions

The distinction between electrolyte solutes and non-electrolyte solutes, demonstrated clearly in the video, is the following:

If we dissolve a substance in water and the resulting solution conducts electricity, that substance (the solute) is said to be an electrolyte.
If we dissolve a substance in water and the resulting solution does not conduct electricity, that substance (the solute) is said to be a non-electrolyte.

A detour: Classification
This way of classifying solutes, is just one way of the many possible ways. Solutes can be classified according to other properties of their solutions in water (containing no other substances), such as the following:

Colour
Whether their solution in water is an oxidising agent or a reducing agent
Whether their solution in water is acidic or alkaline.

For example, aqueous solutions of hydrogen chloride gas (HCl):

conduct electricity (so HCl gas is an electrolyte)
are colourless
are oxidising agents
are acidic.

The reasons that people engage in classification, and possible characteristics used as a basis for classification, are the subject matter of Module 0202 People classifying stuffs: Why?

Prior knowledge required to make sense of this module

Electricity is the movement of electrically charged species: in a copper cable, it is the movement of electrons, and in solution it is the movement of aquated ions (and NOT of electrons).

Ionic solutes (Solutes which are ionic compounds)

Ionic substances are all electrolytes: they conduct electricity (see the demonstration in the video). The competitive role of water in dissolving ionic compounds, giving rise to the formation of aquated (or, hydrated) ions, more or less independent of each other, and able to move through the solution, is discussed in Module 0905 Dissolution of ionic salts in water.

The dissolving process for potassium sulfate, for example, can be described by the chemical equation:

Rationalisation of why solutions of ionic compounds conduct electricity:

Solutions of ionic solutes contain charged species – aquated cations and aquated anions. Under the influence of a d.c. electric potential, the cations move toward one of the electrodes, and the anions move in the opposite direction (toward the other electrode). The movement of these charged species constitutes an electric current, so the circuit that includes the solution and the lamp in the video demonstration is complete, and current flows, illuminating the lamp.

An alternative definition of electrolyte

The definition presented above is based on an observable phenomenon: whether a solution of the solute conducts electricity. Now that we have an explanation for the conduction of electricity, we could define an electrolyte as a substance whose solutions in water contain ions. And a non-electrolyte is a substance whose solutions in water do not contain ions.

What goes on between the electrodes .......

You probably don't believe that this was an actual recorded conversation among ions in solution with an applied electric field?

Solutes that are covalent molecular compounds

A reminder ....

Covalent molecular compounds consist of electrically neutral molecules that are aggregates of atoms held together by covalent bonds (shared electrons). In any of these compounds, all of the molecules are identical, and their composition corresponds with that indicated by the formula of the substance. That is, in the substance methane, whose composition is represented by the formula CH3, every molecule has one carbon atom and three hydrogen atoms joined together in a particular sequence.

This is not like ionic compounds, which consists of ordered arrangements of positively charged ions and negatively charged ions. In the substance magnesium chloride, for example, whose composition is indicated by the formula MgCl2(s), there are no particles consisting of one magnesium atom and two chlorine atoms. The formula MgCl2 means that in the lattice of ions, there are twice as many chloride ions (Cl-) as there are magnesium ions (Mg2+).

See Module 0504 Chemical formulas: How do we interpret them?

[Sorry, the website builder program that I am using does not allow me to do superscripts and subscripts.]

Are solutes which are covalent molecular compounds electrolytes or non-electrolytes?

In other words ....
Do covalent molecular compounds form ions when they dissolve in water - or not?

For a discussion of the competing forces involved in dissolution of a covalent molecular compound in another (including in water), see Module 0902 Miscibility of liquids.

Observations .....

Tests of the electrical conductivity of solutions of various solutes show that some molecular solutes are electrolytes, and some are non-electrolytes - unlike ionic solutes which are all electrolytes.

For example ....

Sucrose (C12H22O11(s)) and ethanol (C2H5OH(l)) are non-electrolytes
Hydrogen chloride (HCl(g)) and ethanoic acid (CH3COOH(l))are electrolytes.

Making sense ....

It is easier to rationalise our observations than it is to predict what our observations will be for each molecular solute.

Here is a simple explanation for the observations:

During dissolution of non-electrolytes in water, the molecules retain their identity as electrically neutral species. There are no charged particles, so there can be no conduction of electricity in an electric field.
During dissolution of electrolytes in water, the interaction between the solute molecules and water molecules causes a covalent bond in the solute molecules to be broken, with formation of charged species called ions - from each solute molecule, one positively charged ion (called a cation), and one negatively charged ion (called an anion). This process is called ionisation.

A sub-classification of molecular electrolytes
....... Foreshadowing the next module 0910 Electrolytes - Strong or weak?

The dissolution of electrolytes in water can be represented by chemical equations. But, to do that accurately, we need to consider a sub-classification of electrolytes. By definition, solutions of molecular electrolytes conduct electricity, but some (called strong electrolytes) conduct electricity very well, while others (called weak electrolytes) conduct poorly.

Making sense ......

In solutions of strong electrolytes, all of the solute molecules ionise as they interact with water molecules during dissolution. An example is hydrogen chloride gas, whose solutions in water are called hydrochloric acid solutions. A chemical equation for ionisation of hydrogen chloride molecules during dissolution is ....

In solutions of weak electrolytes, only some of the solute molecules ionise, but most remain as un-ionised molecules. The ionisation reaction attains a condition of chemical equilibrium (See Module 1100 Equilibrium: an overview, and Module 1101 Visualising dynamic chemical equilibrium). The condition of chemical equilbrium is indicated by the double arrows in the following equations:

So, the concentrations of ions in solutions of weak electrolytes are lower than in solutions of strong electrolytes (with the same solution concentration) - so the solutions of weak electrolytes are not able to conduct electricity so well.

SELF-CHECK: Some thinking tasks

Some are naturally conductors, and some are not. Why? Why not?

1. State whether each of the following is as an electrolyte (or not):

(a) A solution of potassium chloride
(b) Potassium chloride.
(c) Water
(d) Ethanoic acid
(e) Table sugar
(f) Hydrogen chloride gas.

2. This question concerns magnesium sulfate, MgSO4(s).

(a) Is a sample of solid magnesium sulfate (i) positively charged, (ii) negatively charged, or (iii) neutral?
(b) Explain, with a chemical equation, why magnesium sulfate is an electrolyte.
(c) What species “carry” electric current is an aqueous solution of magnesium sulfate?

3. Ethanol (C2H5OH, a liquid) and hydrogen chloride (HCl, a gas) are both covalent molecular compounds.

(a) Are the molecules of ethanol (i) positively charged, (ii) negatively charged, or (iii) electrically neutral?
(b) Are the molecules of hydrogen chloride (i) positively charged, (ii) negatively charged, or (iii) neutral?
(c) Ethanol is a non-electrolyte, but hydrogen chloride is an electrolyte. Explain, using chemical equations.

4. Aqueous solutions of both magnesium sulfate (an ionic compound, MgSO4(s)) and formic acid (a covalent molecular compound, HCOOH(l)) conduct electricity. Aqueous solutions of methanol (a covalent molecular compound, CH3OH(l)) do not conduct electricity.

Decide whether each of the following statements is true or false:

(a) An aqueous solution of magnesium sulfate is an electrolyte.
(b) The substance methanol is a non-electrolyte.
(c) In aqueous solutions of formic acid, there are charged species
(d) During dissolution of some magnesium sulfate in water, covalent bonds in the MgSO4 molecules are broken (as a result of interaction with water molecules), and aquated ions are formed. This is called ionisation.
(e) During dissolution of some methanol in water, no covalent bonds in the CH3OH molecules are broken. So there are no charged species in solution.
(f) The substance magnesium sulfate is an electrolyte.
(g) During dissolution of some formic in water, covalent bonds in the HCOOH molecules are broken (as a result of interaction with water molecules), and aquated ions are formed. This is called ionisation.
(h) Solid magnesium sulfate is believed to consist of cations and anions packed in an ordered network. During dissolution of some magnesium sulfate in water, the ions are separated from each other (as a result of interaction with water molecules), and aquated.
(i) Since formic acid is an electrolyte, we can deduce that its molecules are charged.
(j) An aqueous solution of magnesium sulfate conducts electricity as the result of movement of electrons in the solution.
(k) Since formic acid is an electrolyte, we can deduce that formic acid consists of ions.

5. In a solution made by bubbling HCl gas into water, what are the species that conduct electricity?

Draw a diagram that portrays the conduction of electrical charge in a hydrochloric acid solution.

6. Dissolving an ionic compound in water can be considered to be the result of competing processes.

Using potassium sulfate (K2SO4) as an example, describe what are these competing processes.

7. What is the difference between an electric current in a copper cable and one through an aqueous solution of an electrolyte?

ANSWERS

1.         (a) No. Electrolytes are solutes, not solutions.
            (b) Yes
            (c) No
            (d) Yes
            (e) No
            (f) Yes

2.         (a) neutral
            (b) See K2SO4 corresponding example in key ideas above.
            (c) Aquated ions K+(aq) in one direction, and SO42-(aq) in the opposite direction.

My apologies .... In the formula of the ion SO42-, the 4 is a subscript to the O atom, and the 2- is a superscript indicating the charge on the whole species SO4.

3. (a) Neutral
(b) Neutral
(c) In solution, ethanol molecules retain their identity as uncharged species. On the other hand, when hydrogen chloride dissolves in water, as a result of (i) attraction between the H atom in each H-Cl molecule and the partially negative end of water molecules, as well as (ii) attraction between the Cl atom in each H-Cl molecule and the partially positive end of water molecules, in each molecule the H-Cl bond is broken, with formation of aquated ions: H+(aq) ions, and Cl-(aq) ions. The term aquated (also sometimes called “hydrated” means that each ion has a “shell” of water molecules hanging on to it.

4.         (a) False
            (b) True
            (c) True
            (d) False
            (e) True
            (f) True
            (g) True
            (h) True
            (i) False
            (j) False
            (k) False

5. Aquated ions: H+(aq) ions, and Cl-(aq) ions
The H+(aq) ions and the Cl-(aq) ions move in opposite direction.

6. See Module 0905 Dissolution of ionic salts in water.

7. In a copper cable an electric current is the flow of electrons in one direction. In an aqueous solution, electricity is the flow of aquated ions – both positive and negative, in opposite directions.

LEARNING CHEMISTRY FOR UNDERSTANDING

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