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Unit 4: Introduction to Chemical Equations
First, there is the distinction between chemical reactions and chemical equations. A chemical reaction is
____________. For example, when magnesium burns, a shiny, lustrous piece of flexible metallic ribbon
is heated in the presence of a colorless, transparent gas and changes into a white-to-grey, opaque,
crumbly, powdery solid. That is a ________________________. The chemical equation is a way of
representing that reaction by using___________ to represent each of the chemicals involved.
*In your own words describe a chemical
formula______________________________________________________________________________
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When water is electrolyzed, we can write that H2O becomes H2 and O2. _________ + O2" is an equation
that represents the reaction. The reaction itself is the formation of two colorless gases, hydrogen and
oxygen, when an electric current is passed through the colorless transparent liquid, water. It is
important that you realize the distinction between reactions and equations.
Second, there are a few important terms that you may already know, but let's take this time to formalize
it. The term "reactant" is the same as the term "reagent." It refers to each of the chemicals that are
reacting with one another; in other words, what you have when you ____________ the reaction. The
chemicals that you get as the reaction proceeds are called the "____________." So, as a chemical
reaction proceeds, you start with reactants and you end up with products. When magnesium burns, you
have magnesium as one reactant or reagent and oxygen as the other reactant or reagent. The
magnesium oxide that you end up with is the product.
*What is the end of the reaction called?
*What is the beginning of the reaction called?
There are three types of equations with which you need to become familiar. They are word equations,
skeleton equations and balanced equations. They are discussed below.
Word Equations
The first of the three types of equations are word equationsThe first example deals with water being
electrolyzed and becoming hydrogen and oxygen. One way of representing that reaction is to use the
word equation, "water hydrogen + oxygen," which is read as "water becomes hydrogen and oxygen."
We use an arrow to show which way the reaction is going. The arrow is usually read as "becomes" or
"yields." The "+" sign is read as "and." The water is changing into hydrogen and oxygen. Sometimes you
will see ___________________ instead of an arrow. I prefer arrows because that emphasizes that a
reaction is proceeding in a particular direction. Some people like to use equal signs to emphasize the
idea that this is a chemical equation and that _________________________________. The mass of
water that reacts is equal to the mass of hydrogen and oxygen that is formed. In this course, you will see
(and use) the arrow rather than the equal sign.
This second example shows the word equation for magnesium reacting with oxygen to become
magnesium oxide: "magnesium + oxygen  magnesium oxide." Word equations are simply a shorthand
way of writing down what chemicals were the
___________________________________________________ in the chemical reaction.
Skeleton Equations
Word equations can be very useful. Generally, however, skeleton equations, in which we have
substituted chemical formulas for the chemical names, are even more useful. Skeleton equations are
also sometimes called __________________________________________.
Continuing with the electrolysis of water, we have a skeleton equation, "H2O -> H2 + O2." The formula
for water is H2O; the formula for hydrogen is H2; and the formula for oxygen is O2. A skeleton equation
is just a way of using the formulas to indicate the chemicals that were involved in the chemical reaction.
"Mg + O2 -> MgO." This skeleton equation shows that magnesium reacts with oxygen to form
magnesium oxide. It also shows that the formula for magnesium is Mg, for oxygen is O2, and for
magnesium oxide is MgO. Throughout this lesson you will be presented the formulas of chemicals in
equations.
In the examples presented so far you know that the formulas have been established by experimental
data such as mass and volume measurements, calculations including composition, and reasoning using
Avogadro’s Law. Similar work has gone into determining the other formulas you will see. In a few
lessons you will know enough about the nature of atoms of different elements to be able to predict the
formulas of the compounds they will form.
Balanced Equations
Next, we need to deal with balanced equations. A balanced equation shows
____________________________because it shows how atoms are conserved.
*What is the law of conservation of matter?
_____________________________________________________________________________________
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A skeleton equation does not show conservation. Consider this skeleton equation.
Notice that water consists of two hydrogen atoms and one oxygen atom. On the right hand (or product)
side, the hydrogen consists of two hydrogen atoms hooked together and oxygen consists of two oxygen
atoms hooked together.
Now notice that we have two oxygen atoms on the right and only one on the left side of the arrow -that's not balanced.
word equation
Fill in the equation here.
Balanced equations take into account what substances are involved in the reaction and how much of
each is involved in the reaction. The balanced equation for this reaction shows that you actually started
with two molecules of water, 2 H2O. You have to have two water molecules in order to get the two
oxygen atoms you need for the oxygen molecule on the right.
Notice that on the left side of the arrow there are altogether four hydrogens and two oxygens. On the
right there are also a total of four hydrogens and two oxygens--the same number of atoms.
Word equation
They have been rearranged, they don't have the same bonding patterns, they are not hooked up
together in the same way; but they are the same in number, so that is a balanced equation.
Same thing is true in the balanced equation for magnesium burning. Magnesium combines with oxygen
to form magnesium oxide. Note that the oxygen starts as O2, two oxygen atoms hooked together. When
magnesium oxide is formed, those oxygen atoms are split apart and each one hooks up with a
magnesium. There is a magnesium for every oxygen. Well, if you start with two oxygens, then you need
to end up with two oxygens. And if you have a magnesium for every oxygen, then you have to start with
two magnesiums.
Word equation
magnesium
+
skeleton equation
oxygen ->magnesium oxide
Mg
+
O2
->
MgO
->)
2 MgO
balanced equation
2 Mg
+
O2
So the balanced equation is 2 Mg + O2 -> 2 MgO.
These examples illustrate the three kinds of equations you will be dealing with: (1________________,
(2) _______________________, and (3) _____________________________. Note that there is a
progression--each one tells you a little bit more.
The word equations tell you what chemicals are reacting by name. The skeleton equations tell you the
same thing but use the formulas, and the formulas tell you about the composition of the chemicals that
you are working with. The balanced equations tell you how much of each chemical is involved -- that is,
the proportions in which they react. For example, two H2O's become two H2's and one O2.
Summarize the above paragraph in your own words.
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