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Chemical Reactions
Using valence numbers to write
formulas for chemical compounds
and coefficients to balance reactions.
Let’s review:
Remember unbonded atoms: have the same number of protons as electrons
are unstable – they do not complete the octet rule
can cause ‘trouble’ if they remain unstable
are at a high energy state
will try to become stable – complete that outer shell with
8 electrons
can be classified as metals – less that 4 valence electrons
nonmetals – more than 4 valence electrons
Some vocabulary terms: valence – the ability of an atom to bond in order to become
stable
valence shell – the last energy shell of the atom
valence electrons – the electrons occupying the valence shell
valence number – the number of electrons lost, gained, or
shared in order to become stable or
complete the octet rule
More on valence numbers:
Valence number has to do with the number of electrons that are making
the atom unstable. That number of electrons may be electrons that
the atoms wants to get rid of or lose (metals), it may be the number of
electrons the atoms needs to make 8 electrons on the last shell or the
number of electrons the atoms needs to gain (nonmetals), or the number
of electrons the atom needs to share because of the availability of
atoms it has to bond (carbon, hydrogen, or nonmetal to nonmetal)
Remember:
Metals need to lose electrons – their valence number is positive
Nonmetals need to gain electrons – their valence number is negative
Carbon and Hydrogen because they almost always share electrons – their valence
number is both positive and negative.
We can use these valence numbers to write chemical formulas.
Remember a chemical formula is a way chemists write compounds, using the chemical
symbol for the element and subscripts to show the ratio of atoms bonded.
ex:
MgCl2 shows the formula for magnesium chloride
the formula tells us this compound contains
1 magnesium (subscript of 1 on magnesium, Mg) and
2 chlorines (subscript of 2 on chlorine, Cl)
But how did we get to MgCl2 ?
Mg is a group 2 element meaning it has 2 valence electrons. Its valence number is +2.
Cl is a group 7 element meaning it has 7 valence electrons. Its valence number is -1.
If we crisscross valence numbers, Mg+2 Cl-1 , so that the valence number of Mg
Becomes Cl’s subscript, and the valence number of Cl becomes Mg’s subscript, we
Have the formula Mg1Cl2
We are going to study 4 chemical reactions over the next few weeks.
Starting with the first reaction which is the SYNTHESIS reaction,
we can also learn the
LAW OF CONSERVATION OF MATTER which states that matter
cannot be created nor destroyed, only changed in its form.
SYNTHESIS reaction by its name states that something is being made.
It will follow the general formula:
A
metal
+
B
nonmetal
Let’s look at an example
AB
where
compound – formed by crisscrossing
valence numbers
Li
+
Cl2
Li Cl
First: Li is a group 1 element, has 1 valence electron, has a valence number of +1
Cl is a group 7 element, has 7 valence electrons, has a valence number of -1
When we crisscross valence numbers we get LiCl.
Second: Remember the 7 diatomic molecules: F, Cl, Br, I, H, N, O. These elements must
be written with a subscript of 2 if they are NOT bonded to another element.
That is why on the left side of the arrow, the reactant side, there is a
subscript of 2 on chlorine. Notice on the right side of the arrow, the product
side, the subscript of 2 is gone because chlorine is now bonded to lithium.
Third: There is a problem. And the problem is that we have “destroyed” a chlorine.
On the left side of the arrow, the reactant side, there are 2 chlorines. On the right
side of the arrow, the product side, there is only 1. According to the Law of
Conservation of Matter we cannot leave the reaction this way. We have to
balance the reaction. We balance the reaction by using coefficients.
Just like in math, coefficients mean to multiply.
So let’s see how that works:
The 2nd type of reaction is the reverse of the SYNTHESIS reaction – it is the
DECOMPOSITION reaction. Instead of something being made, something is being
broken down.
It follows the general reaction:
AB
compound
A
metal
+
B
where
nonmetal
In this reaction, you do not have to worry about the valence numbers. Just break the
Compound up between the elements to give you a metal and a nonmetal on the
Product side. BUT REMEMBER, diatoms!!!
The 3rd kind of reaction is a SINGLE REPLACEMENT reaction. It follows the general
Formula:
A
metal
+
BC
compound
AC + B
new compound
where
metal
The metals have replaced each other.
**Reactivity series on the back of your periodic table is necessary for this reaction.
The 3rd reaction is a DOUBLE REPLACEMENT reaction. It follows the general formula:
AB
+
Compound
CD
AD
+
compound
compound
CB
where
compound
Remember: A and C are the metals
B and D are the nonmetals
In order to write new compounds as products on the right side of the arrow, valence
numbers need to be used for each new compound. Because we are writing
new compounds, subscripts from the left side of the reaction are left on the
left side.
MgCl2
+
Al2O3
MgO
+
AlCl3
This reaction is not balanced yet, but it shows the new subscripts for the new compounds.