Predicting the outcomes of chemical reactions is not only key to chemistry, but also a skill that some kids view as impossible. How are they supposed to know what will happen in this jumble of letters and numbers? For some kids, this can be really defeating. A few years ago, I started using the Hallmark Christmas Movie Method. This helps the kids think in terms of change, which is what chemical reactions are. It helps them identify what they are actually seeing in the alphabet soup of a reaction. And it’s fun. I tell the kids that I have been writing a new script for a Hallmark Christmas movie and it goes like this…
There is a hard-charging, up and coming lawyer from the big city. She has been promised that she will make partner in the firm as long as she goes to Christmasville and gets the signature of the old man Mr. Holly. Once they have his signature, they will own his Christmas tree farm and be able to turn it into high rise apartments.
At this point in the story, I will ask my students if they know who lawyer lady is engaged to? Can you describe him? Of course, they can. With uncanny similarities, each period describes her fiancé. Can you describe him? Then the story continues. I will abbreviate the story for two reasons. You already know how it goes, and I maybe someday I will sell the script to Hallmark.
Lawyer Lady goes to small, quaint Christmasville where she is hated. She served the papers to Mr. Holly, but he won’t sign them until after the Christmas Festival. In the meantime, Lawyer Lady meets Mr. Holly’s strapping, wholesome, plaid-clad, single grandson. They hate each other at first, but in short order, they fall in love, she discovers the real meaning of Christmas, she tears up the legal papers, she breaks up with her fiancé, she goes on a romantic walk through the Christmas tree farm, and they embrace while the wonder of Christmas and a few snowflakes spin around their heads. Bam. Hallmark movie.
But you know what? The kids already knew the end of that movie. They tease me that my script will never sell because it has already been written. And then we dig in to why they think that. And it is in that digging that they discover how to predict chemical reactions.
Look at the cover
Just by looking at the cover of the movie, they pretty much already knew how it was going to go. Similarly, just by looking at the reactants, they can predict how it will go. Train students to look at the “cover” of a reaction. Look for the generic pattern. They don’t even need all the details, just the big idea. Is there going to be a breakup? Is someone going to get together? Find the big idea.
Find out who is together
In many reactions, a compound is part of the reactants. Help the students look for who is together. For me, the majority of first year chemistry is in inorganic chem, frequently reactions of ionic compounds. Therefore, have them start to identify what ions are bonded. Why are ionic compounds bonded together? Obviously, it’s their opposite charge. If they are intentionally looking for particular bonding patterns, it will become easier to identify the key players in the reaction.
Is a breakup going to happen?
If a reaction is between a compound and an element (like a single displacement reaction) the compound will have to break up. For aqueous ionic reactions, this is accomplished by dissolving in water. If the students recognize that a break up must occur before the new couple forms, they will be able to focus their attention in the right spot. And why does a breakup happen in a Hallmark movie? To become more stable. And why does a breakup happen in chemistry? To become more stable.
Who gets together?
It is important that the kids begin to recognize that they can predict which ions will combine during the reaction. Opposites attract. Therefore, cations will react with anions. If you have taught for very long, you have seen all sorts of bad pairings in student work. Remind the kids of the Hallmark movie. They know who needs to get together in the movie in order to become more stable. Find what elements need to get together to become more stable.
How do they complete each other?
In the Hallmark movie, the kids know who is going to get together and what changes in personality have to occur before they can become stable. Lawyer Lady has to discover the true meaning of Christmas before she could marry someone from Christmasville. There are particular ways that ions complete each other too. The charges have to balance each other out. All the negatives have to cancel all the positives. With that in mind, it gives the students some freedom to change the subscripts to make the new compound stable. They don’t have to be the same as the reactants.
At this point, the students are hopefully starting to recognize that many reactions are clearly predictable, like a Hallmark movie. They just have to dig in to see the smaller concepts in the reaction. But even in all the letters and numbers, there is predictability.
These last couple of points are specific to single and then double displacement reactions. Yes, they reinforce traditional gender stereotypes. No, I don’t make anyone uncomfortable using them.
Who is more attractive?
In single displacement reactions, a breakup occurs and then there must be a more reactive metal (usually) to react with the anion. In Hallmark terms, the wholesome Christmasville guy has to be more attractive than her big city, money-hungry fiancé. The way that we quantify this is by an activity series. The metals at the top of the list (if you give them a scale of ease of oxidation) are the most reactive. The anion (the lady) has to move up the list in order for a reaction to occur. If the plaid-wearing Christmas guy was actually a one-eyed axe murdering ugly guy, then a reaction isn’t likely to happen. Just as if a student was trying to react potassium chloride with lead. No reaction will occur, since potassium is significantly more likely to oxidize than lead is. In Hallmark terms, potassium is just so much more attractive.
Did the relationship last?
In double displacement reactions, which typically take place in aqueous solution, at least one of the “couples” have to stay together. Generally, a reaction will occur if a solid, a gas, or a new molecular compound is formed (generally water). A solid, in Hallmark terms, is a couple that stays together. They were made for each other and no matter the strains of this world, they will stick it out forever. Isn’t that wholesome? But, if a chemical reaction is between sodium chloride and magnesium nitrate, neither of the possible products will be insoluble. They break up in water. So, if no new stable products are formed, then no reaction has occurred.
Hopefully, using something as familiar as the cheesy Hallmark movies will help your students decrease their anxiety for predicting reactions. Have them look at the cover, discover the plot, and then play it out in chemical terms. It’s possible. In fact, it’s as easy as describing the wholesome, plaid-wearing, grandson… You just pictured him didn’t you. It’s not that hard.
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