Chapter 10
Chemical Reactions
ALABAMA 8TH GiAnE SCIENCE STANDARDS COVERED IN TIllS CHAPTER INCLUDE:
4
State the law of conservation of matter.
.
7
Balancing chemical equations by adjusting coefficients
Describe states of matter based on kinetic energy of particles in matter.
.
Explaining effects of temperature, concentration, surface area, and
catalysts on the rate of chemical reactions.
Recall from Chapter 6 that a chemical
reaction occurs when an atom within a
iNKS substance exchanges, transfers or lends
energy (in the form of electrons) to
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another atom within that substance. A
chemical reaction is expressed using a
chemical equation. In a chemical reaction, one or more
elements or compounds form new elements or compounds.
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If you think back to Chapter 7, you’ll realize that you
already discovered this! Recall that sometimes elements
join together to become compounds, sometimes Figure 10.1 Mixing Chemicals
compounds break down to their individual elements, and
sometimes there’s a combination of both. Most of the materials we come into contact
with are compounds.
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DESCRIBING REACTIONS
In a chemical reaction, we call the materials that are going to react (be reorganized)
reactants. The materials that result from the reaction are called the products. There will
be the same amount ofeach type ofelement before the reaction as after the reaction. This
is shown as a balanced chemical equation. To demonstrate this, let’s take a look at a
molecule you are very familiar with: water.
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81
Chemical Reactions
As you know, the chemical formula for water is H20. All “H20” means is that you have two
atoms ofhydrogen (H), chemically bound to one atom ofoxygen (0). Ifwe were to look at this
combination as a mathematical equation, it might look like Equation 10.1.
H+H +
0 = HHO
Equation 10.1
If we look at this as a chemical equation, we write it a little differently, adding coefficients to
the atomic symbols. The coefficients (the large numbers in front of each symbol) tell us how
many of each kind of atom there are. If an element is represented by only one atom, there is no
need to write in a coefficient of 1 . So, two atoms of hydrogen react with one atom of oxygen to
form one molecule ofwater. Equation 10.2 shows this chemical equation.
2H + 0
—
H20
Equation 10.2
The arrow tells you that something happens to one set of atoms that results in their
reorganization into something else. On the left side of the arrow are the reactants, and on the
right side are the products.
When read aloud, you say the reactantsyield theproducts. The arrow represents the “yield” part
ofthe equation.
2H + 0
reactants
—.
H20
yield products
Equation 10.3
Remember that the product of a chemical reaction may have some physical properties similar
to the physical properties of the reactants, but it will ALWAYS have different chemical
properties. This is because the product has a different arrangement of atoms than the reactants.
Activity
Practice writing and reading aloud the chemical equations described in each sentence.
1.
One atom of iron reacts with one atom of sulfur to form one molecule of iron sul
fide.
2.
Two atoms of hydrogen react with two atoms of chlorine to form two molecules of
hydrochloric acid.
3.
Two atoms of magnesium react with two atoms of oxygen to form two molecules
ofmagnesmm oxide
4.
One atom of sulfur reacts with two atoms of oxygen to form one molecule of sulfur
dioxide
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Challenge: Two atoms ofnitrogen react with six atoms ofhydrogen to form two molecules
(I ofammonia.
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Chapter 10
BALANCING CHEMICAL EQuATIoNs
The law of conservation of matter (or mass) states that in a chemical reaction there is no loss
of mass. This means you end up with as many atoms of each element at the end of a chemical
reaction as you had before the reaction took place. Nothing is gained; nothing is lost. The
organization of the atoms (what is, and is not, bonded together) simply changes.
Look at the following equation in Figure 10.2. There are 2 atoms ofhydrogen reacting and 2
atoms ofhydrogen as products, so the hydrogen in the equation is balanced. However, there are
2 atoms ofoxygen reacting but only 1 atom ofoxygen shown as products. This is not a balanced
equation.
a,
+
-
O,—+H,O
-
2 atoms
2 atoms
2 atoms
I atom
H
0
H
0
balanced
not equal
equal
figure 10.2 Example of an Unbalanced Equation
How can we balance the equation in Figure 10.2? We balance the equation by inspection. To
do this, we compare the number of atoms of each element on the product side to that on the
reactant side and then add coefficients where necessary to balance the sides.
Step 1 : Put a 2 in front of the H20. Now, we have 2 atoms of oxygen to balance the oxygen,
but the number ofhydrogen atoms increases to 4.
Step 2: Put a 2 in front ofthe H2. Now, we have 4 atoms ofhydrogen on each side of the
equation, and the equation is completely balanced.
2H+O
2
2
4r
0
II
4r
2HO
2 \
balanced
,
4 atoms 2 atoms
4 atoms
2 atoms
equal
Figure 10.3 Example of an Balanced Equating
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Chemical Reactions
REACTION ATTRACTION
How can we make chemical reactions speed up or slow down? What gets them started in the
first place? The answer to these questions involves knowledge of a very important science
concept: reaction rate.
The rate of reaction describes how fast or slow the reactants are converted into a product. In
other words, it is the rate of disappearance of one of the reactants, or it is the rate of change in
concentration of the reactant. The rate of reaction depends on several factors, including
temperature, surface area, concentration and the presence of catalysts.
Increasing both the temperature and surface area will increase the amount of interaction
between atoms, which will then increase the rate of the overall reaction.
How does raising the temperature of reactants speed up a reaction?
When two chemicals react, their molecules have to collide with each other with enough energy
for the reaction to take place. By raising the temperature, you raise the energy levels (kinetic
energy) of the molecules in the reaction
and speed up the rate of the reaction!
. . .
How does raising the surface area of the reactants speed up the reaction?
Let’s look at an actual chemical reaction to demonstrate the answer. When you make a fire, you
start out with tiny pieces of tinder, like pine needles and leaves. These are then used to light
kindling, like small branches and twigs. finally, a mature fire is made by adding logs. The point
is to start the fire with low-bulk, high-surface area materials, which will undergo a chemical
change (burning) quickly. Left alone, a tinder fire will quickly bum out. So, at each ofthe next
steps, higher-bulk, lower-surface area materials (like kindling, then logs) are added. Decreasing
the surface area slows the reaction, so you get a slow-burning fire.
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Figure 10.4 Three Stages of a Fire
Increasing the concentration of the reactants will also increase the rate of reaction.
Concentration describes how much ofeach chemical is present in a solution per unit of volume.
As you know, in order for any reaction to occur, particles in a solution must collide. If the
concentration of particles is high, the chances for their collision are greater. Conversely,
decreasing the concentration would decrease the rate of reaction, because the chances for
collision would be reduced.
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Chapter 10
A catalyst is a substance that increases the rate ofreaction. It does this by decreasing the energy
needed to start the reaction (called the activation energy). The graph in Figure 10.5 shows the
amount of energy of a reaction needed over time with and without a catalyst. Notice that the
energy at the beginning and the end are the same for both reactions; however, the amount of
energy needed for the reaction to occur is less with a catalyst.
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A: without catalyst
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,‘B’\B:with catalyst
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reactants
Time
Figure 10.5 Effect ofa Catalyst
Activity
Balance the following equations by adding new coefficients or adjusting existing ones:
1.
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.
2.
Na
3.
N2+
4Fe
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c
H202
+
+
“20 +
Cl2 -)
02
NaC1
2H2-
2NH3
302
3Fe203
———————=============
The point about surface area may be a little difficult to comprehend, so let’s demonstrate.
I Sugar cubes are granular sugar that has been pressed into a larger cube shape, usually 5 mm
! across. Regular granular sugar is made up of smaller sugar crystals, typically 0.5 mm
I across. Granular sugar is measured with a teaspoon or tablespoon. Predict which will
dissolve faster, the sugar cube or the tablespoon of granular sugar. Get 2 cups of water at
room temperature. Place the sugar cube into one cup and the tablespoon of granular sugar
H into the other. Time how long it takes for each to dissolve without stirring.
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Was your prediction correct? Which dissolved first? Why did this happen?
85
Chemical Reactions
CHAPTER
1.
1 0 REVIEW
What does the phrase “the reorganization ofreactants into products” describe?
A
a physical reaction
C
a chemical formula
B
a chemical reaction
B
a chemical bond
2. Coal is made of carbon. Coal bums at high temperatures in the presence of oxygen.
The result of this chemical reaction is
A
B
C
B
more of the same coal.
coal with different physical properties.
new chemical compounds.
the creation of new elements.
Use the following chemical reaction to answer questions 3 —5:
$02+H20
3.
How many atoms of reactant are there?
A
2
B
4
C
6
D
12
4. Gillian mixes 5 grams of sulfur dioxide (SO2) with 6 grams of water, in a 20-gram,
stoppered test tube. After the reaction, she weighs the test tube and its contents. What
product mass (ffb + H$03) does she record in her laboratory report?
A
B
5g
6g
C
D
hg
31g
5. The Law of Conservation of Mass states that which of the following will always be
equal before and after a chemical reaction?
A
B
C
p
the number of atoms of product and atoms of reactant
the number ofmolecules ofproduct and molecules ofreactant
density of the reactants
the surface area of the productand the reactant