Sodium (Na) Reaction Demonstration
Lesson 5 of 11
Objective: Students will be able to observe the transfer of electrons on a large scale using models then compare that to the actual chemical reaction.
This lesson is based on California's Middle School Integrated Model of NGSS.
NGSS Performance Expectation (PE): (MS-PS1-2) Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occured.
Science and Engineering Practice (SP) 2: Developing and Using Models
Disciplinary Core Ideas (DCI): PS1.A: Structure and Properties of Matter - Substances are made from different types of atoms, which combine with one another in various ways. Atoms form molecules that range in size from two to thousands of atoms.
Crosscutting Concepts (CCC) 3: Scale, Proportion, and Quantity - Time, space, and energy phenomena can be observed at various scales using models to study systems that are too large or too small.
With this lesson I want to show the students what a sodium reaction looks like on a macro level (Teacher Demonstration) then what is would look like as an actual chemical reaction. My goal is to show the kids what the electrons are doing and how that generates such an exciting display (MS-PS1-2).
By using models beforehand my students get a sense of what is transpiring at the atomic level (SEP2). since the actual chemical reaction yields no evidence of the actual transfer of electrons (PS1.A)(CCC3).
This lesson is design to allow my students to use models to develop and engage scientific thinking, while improving their use of scientific vocabulary.
This lesson involves me first demonstrating the reaction of Sodium (Na) and water (H2O) in the classroom using students as the individual atoms and racket balls as valance electrons. I previously introduced valance electrons in the lesson Atom Model Demonstration.
Set Up: To identify the students as specific atoms, I have decorated ten 12" tall disposable paper chef hats. The hats are labeled with the atom's name, symbol, atomic number, atomic mass, and electron configuration, making it easy for students to follow the demonstration. You should make four hats represent hydrogen (H), two hats representing oxygen (O), and two representing sodium (Na). The remaining two hats in this set I keep as spares. Label eight racket balls with a lower case 'e' and a negative '-' sign, which will be used to represent electrons.
At the beginning, tell the students hydrogen (H) has only one electron in its outer shell, identify that electron as a valance electron. Choose one student to wear the hydrogen hat and give them one racket ball (electron) to hold. Choose another student to represent a second hydrogen (H) atom. Pick a third student to wear the oxygen (O) hat and tell the kids that oxygen (O) has six valance electrons. Give this student six racket balls (electrons) to hold. Make sure you explain that oxygen (O) has two electrons locked up in the inner shell, but those electrons cannot be shared and are not part of chemical reactions.
Build a water molecule using two hydrogens and one oxygen (H2O). Explain to the students that hydrogen (H) desires 2 electrons to be as close to helium (He) as possible. Also tell the kids that oxygen (O) is searching for two electrons to become like helium (He). Arrange the kids in a triangle and have the hydrogens (H) give their single electron to oxygen (O). In turn have oxygen (O) return the favor and give hydrogen (H) two electrons respectively. Have this sharing go on for several minutes as you build a second water molecule (H2O) right next to it.
Choose two students to represent sodium (Na) and have them across the room, ready to stroll into the picture. I have each sodium (Na) atom walk in between the water molecules (H2O) trading their electrons and explain that sodium (Na) will disrupt this nice game of sharing and build new molecules of hydrogen gas (H2) and sodium hydroxide (NaOH). Have one set of hydrogen (H) combine and share one pair of electrons and have the a sodium (Na) atom combine with an oxygen (O) and a hydrogen (H), called a hydroxide (OH-). Do this with the second sodium (Na) and repeat the process with the remaining oxygen (O) and a hydrogen (H).
I place the chemical reaction on the board to reenforce the concept.
2H2O + 2Na -> H2 + 2NaOH
I walk students through the chemical reaction to explain that sodium (Na) moves in a replace a hydrogen (H) atom (single replacement reaction). This transfer of atoms produces a lot of energy and this cause a flame to be produced. I explain that fire is the evidence of a chemical reaction (this is a little simplistic, but appropriate for eighth graders).
I then drop a small portion of sodium (Na) in COLD water, directing students to watch the reaction (WEAR GLOVES - the sweat from your hands can ignite the sodium! Use a dry plastic knife to cut the sodium in small chunks). I explain that this reaction is producing hydrogen gas (H2) and the flame being produced could ignite the hydrogen gas (H2) and produce an explosion. I very nonchalantly ask if this is something they would like to see, never explaining that's always been my intension. I let them beg and plead for a few seconds before I reluctantly give in. (DANGER - use cold water only at this point. Hot water WILL cause an explosion!)
We then go outside to the grass and I perform the same reaction with HOT water (DANGER - explosion). I heat up some water in the microwave and pour it in a disposable aluminum baking tray. I place it in the center of our athletic field, position the kids a safe distance away and add the pieces of sodium (Na) to the warm water. The resulting explosion is very memorable.
TIP: Using an aluminum tray with it's low sides allow more students to see the reaction without fear of danger if the reaction were to destroy the container (aluminum ruptures without shards - don't ask...I know. I once used glass and luckily no shards hit any kids).
As a concluding assignment my students have to provide two drawing on what they learned today which must include a minimum of three colors, labels, and at least two sentences describing their picture. They are comparing what they saw in the classroom using large scale models to what they saw in the field as the sodium (Na) reacted with water (H2O).