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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)
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:
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.
 
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.
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chemical vs. physical change
