Today’s analogy is a little bit rare. You might not have encountered this scenario in your life, and it’s an underused trope in Hollywood, mostly appearing in romantic comedies. Ironically, the type of reaction it will be illustrating, double displacement reactions, are the most common type of chemical reactions. Are you ready for the analogy?
Significant other swaps. I’m not talking forcible swaps, I’m talking about consenting swaps that make more sense when you consider the chemistry of the couple and compatibility of personalities.
For example, couple A, a pairing of Pb and NO, is always fighting. NO has been burned by their previous partners and doesn’t trust Pb, a person who is flirtatious and attention-seeking by nature. Those two traits do not mix well, leading to a messy break up that leaves them open to date other people. Meanwhile, couple B, a pairing of K and I, have been slowly inching their way to their own break up. K is goals-oriented and is already thinking ahead to the future, believing I to be their partner for life. This has been scaring I, whose live-in-the-moment philosophy is thrown off by talk of marriage. In an effort to regain some of their freedom, I has been chatting with strangers when I and K go out together. It was not a pretty night when those two split…
But there is hope in this story! Free to date people who compliment their personalities instead of contradicting them, they find happiness with other people instead of the grief they felt with their previous partners. Pb and I, the outgoing and boisterous ones, have a great time living it up in the city while K and NO find comfort and security in each other’s steadfastness. A storybook ending for all four.
Back in the world of chemistry, we find the equation that inspired the above analogy: Pb(NO3)2 + 2KI -> 2KNO3 + PbI2. Looking at it, there are four distinct chemical compounds: lead (Pb), nitrate (NO3), potassium (K), and iodine (I). If you track their locations between the reactants side and the products side, you’ll see they switch partners like in the analogy. Pb ditches NO3 for I, leaving NO3 to pair up with the newly-single K. This is the definition of a double displacement reaction, shown visually by this graphic:
These types of reactions are the most common type in chem labs, generally yielding interesting and easily observable results. In order to make them happen, the solid substances involved are dissolved in water to form a solution of that substance. The water acts as a reaction space, allowing the substances to react without interference from outside elements. Some of these reactions will form a solid substance known as a precipitate, which stands out against the clear solution of most reactions. Those that form a precipitate are known as precipitation reactions, a category of double displacement reactions that is only set apart by its formation of a solid substance.
One example of both double displacement reactions and precipitation reactions is the one described above, between lead nitrate and potassium iodide. It is very often used in textbook illustrations because of its distinctly colored precipitate. The formation of the bright yellow lead iodide is striking and very clear to see against the clear solution it is formed in. To see this reaction in action, click on this representation of swapping: <–>
What’s up next post: An overlapping with double displacement reactions, and the emotions of a break up