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Physical Science: Session 4

A Closer Look:
The Chemical Reactions Shown in the Session 4 Video

Chemical reactions can be very different from one another, giving off different gases, different amounts of heat, and even different smells. At a microscopic scale, hundreds of thousands or even millions of encounters between pairs of particles are responsible for creating the macroscopic changes that we can observe. (Not surprisingly, Dalton, the 18th century English scientist, often called “the father of modern chemistry,” referred to this process as the “dance of the atoms.”) The key to a chemical reaction is that the attractive forces between different particles are strong enough to tear the particles apart from each other.

Molecules that have two or more atoms are called compounds. These compounds are held together by the forces between their component atoms and/or ions, which are atoms with either an excess or deficit of electrons. When we mix two different compounds together (the “reactants”), the molecules of each will be composed of two or more different atoms or ions.

If when the molecules of the two compounds collide, they are in contact for only a split second, they simply bounce off each other, no reaction takes place, and the original materials remain. However, if during a collision, an atom or ion of one molecule feels a force from one of the atoms/ions in a second molecule that is stronger than the force that held it to its original molecule, it will combine with the more attractive atom/ion, forming a new molecule (the “product”). At this point, a chemical reaction has occurred.
This process is also consistent with the principle of conservation of matter presented in Session 3: atoms and ions can rearrange into different molecules, but they never just disappear. As a result, in a chemical change the total number of particles is not changed. Our microscopic definition of chemical change is now, as our host Sallie Baliunas states in the video: “a dissociation, recombination, or rearrangement of [particles].”

How does this apply to the two different reactions that appear in the video?

1. Lye (drain cleaner) and Epsom salt
Sodium hydroxide + magnesium sulfate ----> magnesium hydroxide and sodium sulfate
2(NaOH) + MgSO_4 ---–> Mg(OH)_2 + Na_2SO_4)

nah and epsom salts

We begin by dissolving the compound Epsom salts in water, thereby splitting the molecule into its component atoms and ions.

The second compound, the lye (or liquid drain cleaner) is already dissolved in water.

When these two reactants are mixed together, the ions that make up the sodium hydroxide and magnesium sulfate collide. Because the forces attracting these particles to each other are greater than the forces between them and the water molecules, new molecules of magnesium hydroxide and sodium sulfate are formed. We see white products "precipitate" out of the clear mixture and the solution that remains is different from either of the original reactants:

nah and epsom mix

2. Baking soda and Vinegar
Sodium bicarbonate + acetic acid gives sodium acetate, water, and carbon dioxide.
NaHCO3 + HOOCCH3 ------> NaOOCCH3 + H2O + CO2 )
In this reaction, we mix the baking soda (sodium bicarbonate), a solid powder, with vinegar.

baking sodavinegar

When we bring these compounds together, two baking soda particles and one vinegar particle collide. In this case, the attractive forces between two reactants come together and three products are created, a molecule of sodium acetate that remains in solution, a water molecule, and a molecule of carbon dioxide. Carbon dioxide is a gas at room temperature, so it bubbles up out of the mixture as the reaction takes place.

baking soda and vinegar mixed

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