Chemistry Solutions
May 2015 | Classroom Commentary
A Dialog on Terminology: Double Replacement vs Ion Swap
By W. Patrick Cunningham, Rachel DeYoung, and Kevin Yang
Several decades ago, in my high school chemistry class, I was taught that reactions between ionic salts or acids and bases are called double-replacement reactions. In such reactions, the cation of one reactant is matched with the anion of the second, while the cation of the second is matched with the anion of the first. My teacher demonstrated the reaction type with colorless solutions of lead(II) nitrate and sodium iodide, which react to form the colorful lead(II) iodide precipitate and colorless sodium nitrate solution.
About that time, I was also introduced to what is called single-replacement reactions, in which, usually, an active metal replaces a cation in a salt or acid. The cation is then released as the element and the active metal takes its place as the new cation. The examples I remember from my school days are of copper reacting with silver nitrate to form silver metal and copper(II) nitrate, and the replacement of bromide in sodium bromide by chlorine in chlorine water.
Cu (s) + 2 AgNO3 (aq) → Cu(NO3)2 (aq) + 2 Ag (s)
It was a little confusing, as I later learned that double-replacement reactions feature an exchange of ions, while single-replacement reactions actually involve a movement of electrons from the active metal to the less active cation. In the first reaction type, ions move; in the second, electrons move.
For several years now, I have been exclusively describing the double-replacement reactions as an ion-swap, and avoiding the single-replacement label, and instead refer to them as just redox reactions. Many teachers maintain the terminology that I originally learned in the 1960s. In AP chemistry, I have “disciples” of both traditions. So I invited articulate student spokespersons from both camps to set down their thoughts in a friendly paper debate. Here are their views.
Rachel DeYoung’s view
Double replacement
When it came time to learn about the different types of chemical reactions and the equations that correspond to those reactions, I came across the equation commonly known as double replacement. Similar to its brother equation, the single-replacement reaction, I was somewhat aware of what would happen. When given an equation of two chemical compounds that contain cations and anions, the generic reaction can be expressed like this: AX + BY → AY + BX.
I am a visual learner, so developing a pattern and seeing it work in this reaction made the process easy to remember. In the English language, many words have the same meaning. These synonyms are incredible tools used in literature and science to allow the person behind it all to describe or encode something with a unique and specific meaning. The memorization technique (A + Y and B + X), which could also be called a mnemonic device, helped me remember the order of the double replacement chemical reaction because of its consistency.
Psychologist Elizabeth Loftus argues that the level of encoding put into memorization determines the level of memory—the more emphasis or meaning behind a new piece of information determines whether a person stores it in their long-term or short-term memory. (1) Scientifically speaking, if a person sees something that is memorable, such as a man in a grocery store wearing a banana suit with pink sunglasses, they will associate that grocery store with the man in odd attire. In the case of double-replacement reactions, the reactant sequence of how one compound combines with another allowed me to determine the type of reaction. The consistency of having two compounds combining on the reactant side of the equation to form two different compounds on the product side was easy to remember because of the emphasis of homogeneity in these types of reactions.
After encoding this pattern in my memory, I can now distinguish the type of reaction type and predict the products when I know only the reactants. While I understand the importance of knowing different charges of elements in reactant compounds now that I am in my second year of chemistry, initially I found the visual aspect of a pattern coupled with the auditory stimulus component of the term "double replacement" solidified my understanding of this chemical reaction and tuned me in to the latent meaning.
Kevin Yang’s view
Ion swap
In chemical education, two names can label this aqueous reaction process:
AX + BY → AY + BX
double replacement or ion swap.
But names such as these are not arbitrarily assigned, and for good reason. In this case, double replacement demonstrates a significant ambiguity in its function, and ion swap, in my opinion, is the superior term.
The term ion swap is both succinct and revealing of its purpose. Looking at the written reaction above, two ions swap places. It’s that simple. Like in a ballroom dance, ions swap partners. On the other hand, double replacement delivers a confusing message. Yes, the ions are being replaced; but by what? With what chemical?
Using the similar term single replacement to describe a simpler chemical reaction makes intuitive sense, given the connotation of the word “replace.” But when it comes down to the more complex reaction, double replacement depicts an open-ended process that leaves the reaction process ambiguous.
The last thing the English language needs is more ambiguity in its word choices. Already, we drive on parkways and park on driveways. Idioms such as these fill the English language and make it both frustrating and confusing to nonnative speakers. In a world with so many methods for naming objects, it is imperative that names be clear and concise, as well as ideally reveal a certain amount of truth regarding its addressee.
One argument for double replacement cites its ease of understanding, that the words are friendly to students. But ion swap is arguably just as easy, if not easier to understand—at least from this student’s perspective. What happens when an acid meets a base? Ions swap.
Ion swap is clearly the way to go. It is a conclusive name that accurately describes how the chemicals rearrange. What more can we expect from a name?
Postscript from Mr. Cunningham
The kind of reactions that I experiment with and teach in high school are of three types: electrons move (oxidation–reduction), ions move (acid–base and precipitation), and substances shake apart (dehydration of a hydrated salt). In reality, when we add an active metal like zinc to a strong acid such as hydrochloric, the zinc doesn't “replace” anything. It loses electrons to hydrogen ions and swims out into the solution as a +2 ion.
In reality, when we mix hydrochloric acid with sodium hydroxide, there is no real replacement going on at all. The hydrogen and chloride ions swim around randomly, as do the sodium and hydroxide ions. Once the substances dissolve, the ions have more interaction with the water molecules that surround them than with their original “partners.” When the hydrogen and hydroxide ions collide, as they must, they stick together because of the strong Coulombic attraction between the two ions.
The same can be said about the mixing of silver nitrate solution with sodium chloride solution. Once the salts dissolve, in the process they dissociate, and absent the reaction to form a low-energy precipitate (AgCl), they would stay forever separated by water molecules until the solution dried up.
Now it must be admitted that, with this in mind, the ions don't really “swap” at all. But the reasons I prefer to teach using the language “ion-swap” and “oxidation–reduction” and stay away from calling any kind of chemical reaction a “replacement” is that such language is more descriptive of the actual mechanism of the chemical change. It is ions that react in an AX + BY process. It is electrons that transfer in the A + BY process. Using the same noun—replacement—to describe two fundamentally different processes is deceptive, in my opinion. Two fundamentally different chemical changes should be described with two entirely different terms.
References
- “Elizabeth F. Loftus: Award for Distinguished Scientific Applications of Psychology,” American Psychologist 58, no. 11 (2003): 864-873; DOI: 10.1037/0003-066X.58.11.864.