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?