Single Replacement Reactions and Activity Series

Single Replacement Reactions

And Activity Series

Oh, Thanksgiving week at school.  It's hard to keep students' attention when a four day weekend is on their minds.  This activity is an example of a greatest hit.  It's easy and impressive.

Basically, the students will 

• learn what a single replacement reaction is, 
• how to read an activity series to predict what will happen in a single replacement reaction 
• and then will perform several single replacement reactions of their own.  

It's the reactions that are the impressive part.  They bubble and hiss.  One of the reactions can even be forced to make a rather loud pop.

Unfortunately, you can't get impressive results without a price.  This activity uses more stuff than most.  Since the students will be performing chemical reactions, you will need some chemicals and safety equipment.

Note:  I use lots of plastic water and soda bottles in lab.  I ask people to save them for me, and generally get plenty donated to keep me going throughout the year.  I simply store my stash in garbage bags in a storage closet.

However -- and this is crucial -- to be able to use water bottles, you need to establish the rule that NO FOOD or DRINK is EVER allowed in lab.  This is a no exception, no mercy rule for me.  During the first session, I simply have any offending student place their food or drink in a container I have set up outside the lab.  They can have it back when they are done.  After that, students are asked to leave the lab if they bring in any food or drink.  I've never had a problem, since students pass the container on their way in.

You need:

• hydrochloric acid  (I use muriatic acid from the local home improvement store.  It costs about $8 per gallon in Carson City, and you only need about 3 tablespoons per student/pair of students.  That's enough for 85 set ups.)
• steel wool as a source of iron, (optional if you can't use magnesium)
• galvanized staples as a source of zinc  (You can also use galvanized nails, but I like to break the staples in half.)
• aluminum foil
• magnesium turnings  (These are optional since magnesium is so expensive, but fun.)
• small disposable water bottles, (I use old, empty 8 fl oz Costco water bottles.  These are just for students to safely handle the acid.)
• safety goggles for each student
• baking soda for neutralizing spills
• an eyewash station in your lab

Here's what I do step by step.




Warm Up (10 minutes)


Unless you have already covered single replacement reactions, you will need to do some talking.


A single replacement reaction has the form 

AX + B --> BX + A.  

The letters A, B and X stand for arbitrary elements, since none of these letters appears on the periodic table.  The best analogy to explain single replacement reactions is the old standby of dancing partners.

Suppose A is dancing with X and B sees the couple enjoying themselves.  B is not dancing with anyone.  B is jealous of A who is having all the fun and cuts in.  Once B cuts in, A is now alone, while B is having fun dancing with X.  

In the sexist version, you can assume A and B are male and X is a female.  Under what conditions can B cut in?  Well, if B is a snotty-nosed brat, he probably won't be able to cut in.  However, if B is the biggest, baddest bully on campus he probably will be able to.


An activity series separates the bullies from the brats.  In case you haven't seen an activity series in a while, here is one.

lithium
potassium
strontium
calcium
sodium
magnesium
aluminum
zinc
chromium
iron
cadmium
cobalt
nickel
tin
lead
hydrogen
antimony
arsenic
bismuth
copper
mercury
silver
paladium
platinum
gold


(I put the elements that will be used in this activity in bold, so they are easy to find.)

All activity series are just a list of elements, usually metals.  The point of the list is to distinguish who can cut in.  The bullies are at the top of the list and the brats are at the bottom.  Specifically, the elements on top can replace elements below them in a single replacement reaction.

For example, if you consider calcium, Ca, and sodium, Na, then since calcium is on top of sodium in the activity series

CaX + Na  --> no reaction

since Na is more of a wimp than Ca and can't cut in.  However,

NaX + Ca --> CaX + Na 

since Ca is on top of Na in the activity series, so Ca can cut in on NaX.

Did you notice that gold is at the very bottom of the series?  Gold can't cut in at all;it is the ultimate anotty nosed brat.  Lithium, at the top of the series is the biggest, baddest bully around.  It will replace everything else in the series.

The activity series also tells you what reaction will be more vigorous.  In general, the higher up an element is on the activity series, the more vigorous the reaction it will make.

I like to test students at this point with a short worksheet on single replacement reactions.  You can find it here

You can find the answers to the worksheet here

The Activity (15 minutes)

Each student needs to put on safety goggles and wear them for the rest of the lab.  

Once the goggles are on, I go over the protocol for what to do in case of an acid spill.  (Let me know, cover the spill with baking soda if it is on the lab bench.  Calmly wash any skin that got splashed.  Check for splashes on clothes and calmly apply splash with baking soda and rinse with water.)

Also, I have a exhaust vent over the work area.  Hydrochloric acid has nasty fumes, so I make sure that the students don't get near the acid while it is reacting.  I also run fans and vent out the door.

Each student or pair has three capped small water bottles, each with a small amount of hydrochloric acid in it.  I label the bottles with a warning.  Please reread the safety note about NO FOOD OR DRINK.  I don't want a student casually picking up a water bottle full of acid and drinking it.

The student will place the galvanized staple in the first bottle.  Do NOT have the student cap the bottle, since gas will build up in the bottle.  You should only have a tablespoon or so of acid, but it is enough to break the sides of the bottle.  

If students place a flaming match over the mouth of the bottle, it will sometimes make a popping sound.  

(If it doesn't, you can have students pop the hydrogen gas by setting up the same reaction with acid in the bottom of a test tube, (in a rack) and adding the staple. Have a student hold a lighted match over the test tupe opening after the reaction has started.  This will make a moderately, satisfyingly loud pop. I let any student who wants to pop the gas try it.  I just keep a line of test tubes waiting and let them try it one at a time.)

In the next bottle, have the students add a piece of wadded up aluminum foil.  This one will take a few seconds to start reacting.  See if the students can guess why.  (The oils on the surface of the aluminum foil protect it.)

In the last bottle, have the students add a very small pinch of magnesium turnings.  Otherwise, they can add a little steel wool, although this is a little anticlimactic after the aluminum foil.
 
Wrap Up (5 minutes)


If we have time, I have the students tell me what products they made in each of the reactions.  More advanced students will have no trouble writing and balancing the equations, since they are simple.