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Electrolytes: definition
Let's start by looking at the definition of electrolytes. These electrolyte solutions are able to carry an electric current.
- Conductivity is referred to as the ability of electric current to flow through a material.
An electrolyte is a compound that dissociates into ions when dissolved in water. Electrolytes conduct electric current in aqueous solutions or in a molten state.
For example, when sulfuric acid dissolves in water, it dissociates into hydrogen ions (H+) and sulfate anions (SO42-).
$$ \text{H}_{2}\text{SO}_{4}\text{(aq)} \longrightarrow \text{2 H}^{+}\text{ (aq) + SO}_{4}^{2-}\text{(aq)} $$
Usually, electrical conductivity testers are used to test a solution's conductivity (figure 1). This test involves adding the aqueous solution to a beaker containing two electrical plates with opposite charges connected to a power source and a light bulb. Basically, when the charge runs to the plates, the positive ions are attracted to the negative plate, while the negative ions are attracted to the positive plate.
Types of Electrolytes
There are three types of electrolytes you should remember, and they are based on the degree to which they dissociate in water.
Strong Electrolytes
First up are strong electrolytes. In the category of strong electrolytes, we have soluble ionic compounds, strong acids, and strong bases.
Strong electrolytes are those that, when dissolved in water, dissociate completely into ions.
Ionic compounds are usually composed of a metal and a non-metal or a metal and a polyatomic ion. A common example of a soluble ionic compound is sodium chloride (NaCl). However, not all ionic compounds are soluble in water (H2O). Table 1 shows the Solubility rules for ionic compounds in water.
| Compounds containing | Soluble or Insoluble in H2O | Exceptions |
| NO3- | soluble | none |
| CH3COO- (acetate) | soluble | none |
| Cl-, Br- and I- | soluble | Ag+, Hg22+ and Pb2+ |
| SO42- | soluble | Sr2+, Ba2+, Hg22+ and Pb2+ |
| S2- | insoluble | NH4+, Li+, Na+, K+, Rb+, Cs+, Ca2+, Sr2+, and Ba2+ |
| CO32- | insoluble | NH4+, Li+, Na+, K+, Rb+, and Cs+ |
| PO43- | insoluble | NH4+, Li+, Na+, K+, Rb+, and Cs+ |
| OH- | insoluble | NH4+, Li+, Na+, K+, Rb+, Cs+, Ca2+, Sr2+, and Ba2+ |
Let's look at an example.
Is the ionic compound potassium nitrate (KNO3) soluble or insoluble in water?
When KNO3 is allowed to dissociate in water, it forms K+ and NO3- ions. The Solubility rules states that all ionic compounds containing NO3- are soluble in water. Therefore, KNO3 is soluble in water.
Is the ionic compound barium sulfate (BaSO4) soluble or insoluble in water?
According to the solubility rules for ionic compounds in water, compounds containing SO42- tend to be soluble in water unless it has one of the metals in the "exceptions" column. Notice that Ba2+ is indeed in the "exceptions" column, meaning that BaSO4 is actually insoluble in water.
Strong acids are acids that 100% dissociate in water to produce positive hydrogen ions (H+) and negative chlorine ions (Cl-). The seven strong acids are HCl, HBr, HI, HClO4, HClO3, H2SO4, and HNO3.
The figure below shows the dissociation of hydrochloric acid (HCl) in water.
Similar to strong acids, strong bases also completely dissociate in water. But, instead of forming H+ ions, they form hydroxide ions (OH-) in the solution. For example, aqueous sodium hydroxide (NaOH) completely dissociates in water into aqueous sodium ions (Na+) and hydroxide (OH-) ions.
$$ \text{NaOH (aq) } \longrightarrow \text{ Na}^{+}\text{(aq) + OH}^{-}\text{(aq)} $$
The strong bases include Group 1 metal hydroxides (LiOH, NaOH, KOH, RbOH, CsOH) and Group 2 metal hydroxides (Ba(OH)2, Sr(OH)2, and Ca(OH)2).
Weak Electrolytes
Next, we have weak electrolytes, and these are either weak acids or weak bases.
Weak electrolytes are those that do not completely dissociate in water (H2O). In other words, they partially dissociate in water and can only conduct a weak current in aqueous solutions.
When dealing with weak electrolytes, we use half-arrows pointing in opposite directions, indicating that the reaction moves significantly in both directions.
$$ \text{CH}_{3}\text{COOH }\text{(aq)} \rightleftharpoons \text{CH}_{3}\text{COO}^{-}\text{ (aq) + H}^{+}\text{(aq)} $$
Table 2 shows a list of some common weak acids and weak bases.
| Weak Acids | Weak Bases |
| Hydrogen fluoride (HF) | Ammonia (NH3) |
| Hydrogen cyanide (HCN) | Methylamine (CH3NH2) |
| Hydrogen peroxide (H2O2) | Diazane (NH2NH2) |
| Hydrogen sulfide (H2S) | Hydroxylamine (NH2OH) |
| Fulminic acid (CHNO) | Ammonium hydroxide (NH4OH) |
| Formic acid (HCOOH) | Aluminum hydroxide (Al(OH3)) |
| Hypochlorous acid (HOCl) | Iron (II) hydroxide (Fe(OH)2) |
| Acetic acid (CH3CO2H) | Aniline (C6H5NH2). |
| Boric acid (B(OH)3) | Pyridine (C5H5N) |
Basically, if the acid or base you are dealing with is not a part of the 7 strong acids and 8 strong bases we learned above, then it will be a weak acid or base!
Non-electrolytes
Non-electrolytes are actually molecular compounds. Molecular compounds are compounds made of non-metals that are not considered acids or bases.
Non-electrolytes are those that do not dissociate into ions but are still able to dissolve in water. Therefore, they do not make the aqueous solution electrically conductive.
Common examples of non-electrolytes include alcohol methanol (CH3OH) and glucose sugar (C6H12O6). Glucose is highly soluble in water, but even though it dissolves into water, it does not dissociate into ions. In other words, a non-electrolyte solution does not conduct electricity.
Electrolytes: properties
The properties of electrolytes depend on the type of electrolyte. Strong electrolytes completely dissociate in water, forming an aqueous solution with a high conductivity.
Weak electrolytes partially dissociate into its ions in water, so its aqueous solution has a low conductivity.
Non-electrolytes, on the other hand, are soluble in water, but they do not dissociate into ions. Therefore, an aqueous solution with a non-electrolyte does not conduct electric current.
Electrolytes: functions
Now that we know the different types of electrolytes that exist, let's dive into the functions of electrolytes. In chemistry, the most common use of electrolytes is probably in the process of Electrolysis.
Electrolysis involves using electricity to split up compounds into its ions.
The basic set-up for the electrolysis of sodium chloride (NaCl) can be seen in the figure below. Here, notice that we need a power source (for example, a battery), and two electrodes. These electrodes are then placed in the electrolyte (substance containing positive and negative ions, which is NaCl in this case).
When electricity is applied to the electrolyte, the positive ions from the electrolyte (Na+) are attracted to the negative electrode, whereas the negative ions (Cl-) are attracted to the positive electrode.
For a more in-depth explanation on electrolysis and its uses, check out "Electrolysis"!
Electrolytes are also very important in physiology. Salts containing sodium, potassium, and calcium ions are essential for the conduction of nerve impulses, muscle contraction, and osmotic regulation! When an electrolyte imbalance occurs, these very high or very low levels of electrolyte disrupt the cell function, leading to mild or life-threatening complications.
For example, low levels of sodium (Na+) ions might cause headaches, confusion, disrupted attention, and even cerebral edema (brain swelling), whereas high levels of sodium ions can cause agitation, and tachycardia (fast heart beat).
Chemical Equation of Electrolytes
Lastly, let's take a look at a chemical equation involving electrolytes. Let's start with calcium chloride (CaCl2). The first thing we need to do is figure out whether CaCl2 is soluble in H2O. So, if you scroll up to the table containing the solubility rules for ionic compounds, you will notice that CaCl2 is soluble in water, so it will dissolve in it.
Now, is it going to be a strong, weak or a non-electrolyte? Well, since CaCl2 is an ionic compound, then it will be a strong electrolyte, and therefore, readily dissociate in H2O into its ion equivalents.
$$ \text{CaCl}_{2}\text{(aq) } \text{ = Ca}^{2+}\text{(aq) } \text{ + Cl}^{-}\text{(aq) } $$
Now, I hope that you feel more confident in your understanding of electrolytes and how to determine whether a compound is a strong, weak or non-electrolyte!
Electrolytes - Key takeaways
- An electrolyte is a compound that dissociates into ions when dissolved in water. Electrolytes conduct electric current in aqueous solutions or in a molten state.
- To test a solution for conductivity, an electrical conductivity tester is mostly used.
- Strong electrolytescompletely dissociate in water, forming an aqueous solution with a high conductivity.
- Weak electrolytespartially dissociate into its ions in water, so its aqueous solution has a low conductivity.
- Non-electrolytes, on the other hand, are soluble in water, but they do not dissociate into ions. Therefore, an aqueous solution with a non-electrolyte does not conduct electric current.
References
- Zumdahl, S. S., Zumdahl, S. A., & Decoste, D. J. (2019). Chemistry. Cengage Learning Asia Pte Ltd.
- Theodore Lawrence Brown, Eugene, H., Bursten, B. E., Murphy, C. J., Woodward, P. M., Stoltzfus, M. W., & Lufaso, M. W. (2018). Chemistry : the central science (14th ed.). Pearson.
- N Saunders, Kat Day, Iain Brand, Claybourne, A., Scott, G., & Smithsonian Books (Publisher. (2020). Supersimple chemistry : the ultimate bite-size study guide. Dk Publishing.
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