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.

  

An Exothermic Synthesis Reaction

A Quick and Simple Exothermic Synthesis Reaction

(that students always guess is decomposition)

This week I started working on types of chemical reactions.  My chem students are covering this right about now in their online course and students of all varieties like to do chemical reactions.

The reaction is quite simple: we made steel wool rust.  That's right.  We watched stuff rust in lab.  We also measured the rise in temperature from the reaction.

Why would I spend my time on rust?  Well, there are several good pedagogical reasons:  almost all students think that rust is a decomposition reaction and this is a good way to explain oxidation; it is a great exothermic reaction that is safe for all ages; and it is a nice way to introduce real world chemistry, since huge amounts of time and money are spent preventing or stopping rust.  Actually, I usually use this lab to introduce the charming theory of phlogiston, (pronounced flow-gist-on).

Pedagogy aside, here is the REAL reason for this lab.  It is a crisis lab, one of several I always have ready.  The idea came from a homemaking book I read a long time ago, Sidetracked Home Executives.  The authors, (sisters who were sidetracked home executives), recommended making what they called a "crisis" casserole for those days when dinnertime sucker punched your schedule and you weren't prepared.  

The rust lab is one of my crisis labs.  It only requires vinegar, steel wool, (the plain kind from the hardware store not the soapy kind), aluminum foil, Styrofoam cups and thermometers, things which I keep on hand in the lab.

The added bonus of this lab is that it takes about 5 minutes to get results.  This is five minutes for you, (the teacher), to tell the charming, but cautionary tale of phlogiston. (Or whatever you need to talk about instead.)

Here's what I did step by step.  Remember that I have everything set up before each group comes in, and do most of the clean up by myself.  Before the lab, I tear each steel wool pillow into approximately 4 pieces. 

Warm Up, (5 minutes)

I explain that we will be making rust.  Then, I wait for the groans.  I love to play up on the absolutely boring aspect of watching rust form.  You can also tease them by telling them that they will also get to hear the charming, yet cautionary tale of phlogiston.  (Then you will get to explain what "cautionary" means.)

Next, I set up one of the reactions for the class, so the know exactly what they should be doing.

The Set Up, (5 minutes)

• Have each pair of students pour about 1/2 inch of vinegar into the bottom of their Styrofoam cup.  (I use the cheapest grocery store white vinegar I can find, Styrofoam since you want to trap heat.) 

 

• Then, have the students place about 1/4 pillow of steel wool into the cup and squish the steel wool into the vinegar using the tip of a pencil or pen.  (The vinegar is used to speed up the reaction.)

 

• The students will then have to dump out the excess vinegar, either into a sink or into another cup or bowl.  (I use the plastic tubs salad greens come in as slop buckets.  It keeps movement down in the lab, which speeds things up.)

 

• Have one of the students in each pair use a pencil or pen to make a little hole in the center of the steel wool for the tip of the thermometer.  The student will place the thermometer in the steel wool and use the pencil to pile the steel wool around the thermometer.  The steel wool needs to be as close to the thermometer as possible.

• The students will then cover the cup with foil, squishing it around the thermometer tightly to trap as much heat as possible.

 

• The students need to take the initial temperature and write it down.

 

• Have the students set aside the cup, put down their equipment, and get ready to listen to your charming, but cautionary tale of phlogiston.

The Charming, but Cautionary Tale of Phlogiston, (5 minutes)

In the 18th century, chemistry and alchemy were still rather entangled.  Chemist/Alchemists might seem crazy today, (lead into gold?), but they were making excellent observations.  

One of these observations is something that you have also observed, but maybe haven't thought about.  Have you ever burned wood in a campfire?  What is left over?  Wood ash.  Wood ash is the part of the wood that doesn't burn.  It follows that wood is made up of stuff that burns and the wood ash that doesn't burn.  In fact, anything that burns must also be made up of the stuff that burns and the ash that doesn't burn.

For example, Mrs. Stephenson is made up of stuff that burns and Stephenson ash, although we will not be verifying this experimentally.  Ever.

Chemist/Alchemists came up with a really science-y name for the stuff that burns that is inside wood or Mrs. Stephenson.  They called it phlogiston.  Based on many, many observations, they developed phlogiston theory which said, (among other more complicated things), everything that burns is made up of phlogiston and ash.  Further, the ash weighs less than the original thing you put in the fire.

Sounds pretty fair, doesn't it?  For quite a while, it was the go-to theory of burning.  Chemist/Alchemists made their reputations on understanding and testing the phlogiston theory.

Until someone burned magnesium.  

Of course, magnesium does leave ash, but magnesium ash weighs MORE than the original magnesium.

What did this mean for phlogiston theory?  Well, maybe nothing.  Maybe the chemist/alchemists didn't measure magnesium ash carefully and their results were wrong.  However, when all sorts of very careful experiments were done burning magnesium, it was always shown to have ash that weighed more than the original magnesium.

How could that be?  This kind of puts a big hole in the phlogiston theory. 

At this stage, Chemist/Alchemists should have either modified or dumped phlogiston theory.  What some did instead was clearly an attempt to cover themselves.  They announced that the phlogiston of magnesium has negative weight.

Negative weight?  Have you ever known someone on a diet?  Even after they have lost 5, 10, maybe 20 pounds they still have weight.  What would something with negative weight look like?  Would it suck weight out of things it came near?

Unfortunately, some of those Chemist/Alchemists were not acting like scientists.  

It took several years before the phlogiston theory was abandoned.

Does this kind of non-science-y behavior still happen today?  Of course.  This is a snippet from an article in The Guardian, (a British newspaper), by Ben Goldacre.
 
…In 2010, researchers from Harvard and Toronto found all the trials looking at five major classes of drug – antidepressants, ulcer drugs and so on – then measured two key features: were they positive, and were they funded by industry? They found more than 500 trials in total: 85% of the industry-funded studies were positive, but only 50% of the government-funded trials were.

Hmm..  In other words, if a scientist is PAID by the drug industry to do a study, it is more likely to show that a drug works.

Wrap Up (10 minutes)

After your charming, but cautionary tale, the students should be ready to take another temperature reading.  The average temperature rise is about 3C, but some reactions really take off.  If students leave a pool of vinegar in the bottom of their cups, there won't be any rise.  This is a nice time to talk about heat sinks.

Once the students see the rust on the steel wool, ask them if this is a decomposition reaction or a formation, (synthesis) reaction.  Many students, thinking about a rusty old car will guess decomposition.  

Not all is as it seems, however.  This is a formation reaction.  The iron is oxidizing to form rust.  I typically write out the chemical reaction for rust and then balance it. 

Here is the reaction for those of you who are, (ahem) rusty.

Unbalanced:

Fe + O2 --> Fe2O3

Balanced:

4Fe + 3O2 --> 2Fe2O3

 Students can usually see why magnesium is heavier when you burn it, if they know that burning is just rapid oxidation.  Here is the reaction.

Unbalanced:

Mg + O2 --> MgO 

Balanced:

2 Mg + O2 --> 2MgO

Possible Extensions

Rust is actually quite fascinating.  Here are some questions students could research for longer projects.

• Once the steel wool has started to rust, is there any way to stop it?
• Does it stop by itself?
• If not, how long does it take for an entire steel wool pillow to rust completely?
• What can you do to speed up the rusting process? 
• Does plain water make the steel wool rust as quickly as vinegar?  Salt water? 

 
 

Extensive or Intensive? A Simple, Guided Inquiry

A Simple Guided Inquiry 

to Highlight the Difference Between Extensive and Intensive Properties

Student often don't understand the difference between extensive and intensive properties.  This guided inquiry activity helps them explore the difference.  Students will be deciding if the density of tap water is intensive or extensive.

The only way I know how to do this is to determine the density of a small quantity of water and compare it to the density of a large quantity of water.  That's what I expect students to do in this lab. 

You will need equipment to measure the mass and volume of water.  I use dollar store measuring cups to measure out the volume of water for squirrley groups, glassware for the mellow classes.  (1 cup = 0.24 liters)

I mass the tap water on a triple beam balance by pouring the water in a paper cup, and massing the cup + water.

WARNING:  many students try to fudge on the mass of water by "knowing that 1 ml = 1 g water".  This is only true for pure water, not tap water.  Besides, this is more or less what the students are trying to prove.

Here my half hour activity, step by step.



5 minutes

Warm Up:

Define "intensive" and "extensive" and give examples. 

• An intensive property does not change when the amount of material changes.

1. The freezing point of water is an intensive property of water, since a little water freezes at 0C and a lot of water freezes at 0C.
2. The conductivity of copper is an intensive property of copper, since a small amount of copper conducts and a large amount of copper also conducts.
3. The number of pennies per dollar is an intensive property of a dollar, since one dollar is worth 100 pennies per dollar and $1,000,000 is worth 100 pennies per dollar.

• An extensive property does change when the amount of material changes. 

1. The volume of water is an extensive property of water, since a little water has a smaller volume than a lot of water.
2. The mass of copper is an extensive property of copper, since a small amount of copper has less mass than a large amount of copper.
3. The total number of pennies is an extensive property of money, since one dollar has fewer pennies than $1,000,000.

Check to see if the student know what you are talking about:

• Is the air temperature right now extensive or intensive?

(intensive, since a little air will have the same temperature as a lot of air)

• Is your weight extensive or intensive?

(extensive, since your weight increases as you grow bigger)

 25 minutes

The activity

Group up the students.  I like pairs, but sometimes use groups of 3.

You, (the teacher), say:  Is the density of tap water extensive or intensive?  You have 25 minutes to design and perform an experiment to determine which.

The panic

Here is the guided part.  Have the students tell you explicitly what it would mean if the density of tap water were extensive and what it would mean if the density of tap water were intensive.  Sooner or later, someone in the group will figure out that you have to change volumes.  If no one can, have them talk with another group.  Since it takes about 15 minutes to do two density measurements, you can let some groups struggle for a few minutes until you get to them.  If you are working with 30 students, however, you might have them all take time to talk through how to do the experiment for a few minutes and then do some guiding. If none of your student panic, hooray!  You are an amazing teacher.  Almost all of my students stare at me like deer in the headlights.

Here's what I do to guide the needier groups.

I say:

"Suppose I want to know if the weight of water is extensive or intensive.  I would take a little bit of water and find out how much it weighed.  I would take a lot of water and find out how much it weighed.  If the weight changed, it would be extensive.  If it didn't change, it would be intensive."

If the light still doesn't turn on, I would continue:  "Suppose I want to know if the volume of water is extensive or intensive.  I would take a little bit of water and find its volume.  I would take a lot of water and find its volume.  If the volume changed, it would be extensive.  If the volume didn't change, it would be extensive."

I have even resorted to:  "Suppose I want to know if the property X is extensive or intensive.  I would take a little bit of water an determine its X.  I would take a lot of water and find its X.  If property X changed, it would be extensive.  If property X didn't change, it would be intensive." 

What the students should do once they "get it"

Students should take a small volume of water, maybe 1/2 cup and take its mass.  Then, they should take a larger volume of water, maybe 1 cup and take its mass.  They should compute the density of the small volume, (by dividing small mass/small volume).  They should also compute the density of the large volume.

Typical Problems

The two densities won't be the same, most likely.  This could be because they don't use significant digits correctly.

• 1.0004 is actually equal to 1.01 to two significant digits, and most likely you are only able to get two significant digits.

This can also result from bad measuring.  If a group is goofing around or working way too quickly, I will challenge them if their results are bad.
 
The Write Up

I typically have the students take data during the activity and write it up to turn in after the activity is over.  The data you are looking for, of course, is the mass and volume of a small quantity of water, and the mass and volume of a larger quantity of water.  I use a simplified lab write up form, which I will post in the future.  I have groups turn in their original data sheet with their lab write up. 

Occasionally, I just have groups talk with me about what they have done and learned.

I Teach Science a Half Hour at a Time

(Once a week)

Why?  That's how our school works.

I teach at a small, hybrid school in Carson City, Nevada.  The hybrid part means our students do most of their course work on line at home, but come on campus once a week to meet with their teachers.  I have a dedicated half hour to work with them each week.*

Um, that's not a half hour for Biology and a different half hour for Chemistry.  That's a half hour for all the science students together.

What do I do in my half hour?  I can get a surprising amount done, actually.  Instead of working on specific details in each course, I do activities that address big ideas that cross science curriculum.  (I do admit to a bias towards Chemistry, since it is my specialty.)

This blog keeps track of my weekly learning activities.  Please feel free to use any of them as is, or adapt them to meet your needs.  (I probably stole the ideas from somewhere myself.)

Most of the activities here can be used as part of a larger lesson in the classroom.  In fact, they will probably take more than a half hour.  I compress teaching time by having everything set up before the students come into my lab.  I also do most of the clean up by myself.

I am only posting the activities that students understood fairly well, were relatively cheap, and didn't require any special equipment. 

Since this is my first adventure in the blog-o-sphere, I hope my posts are useful.  If you have any suggestions to improve the content, please let me know.

-Lynn
 

*Many of our students come in every day, actually, and I coach those students through their course material one-on-one.  Also, all my students don't come in on the same day.  I have the luxury of only working with about 12 per half hour session at a time.