At this point we know how to determine the number of
electrons, protons and neutrons in an isotope of an element. We
also know that the number below the symbol of the element in the
periodic table is called the relative, weighted average atomic
mass of the element. The number has units of atomic mass units or
Daltons.
We also know the symbols and formulas for the first twenty
elements and some additional 'common' elements.
What about when atoms combine to form compounds? First of all
we can write formulas of compounds. For example, water is a
compound because it is composed of two diferent elements,
hydrogen and oxygen. The formula for water is H2O. The
formula of another compound everyone is familiar with is sodium
chloride, NaCl. How do we make these two compounds?
To make these two compounds we need to do a chemical
reaction. We can do a chemical reaction by mixing substances.
How do we tell if a chemical reaction occurs? The best
evidence is a difference in the physical and chemical properties
of the reactants and the products. Characteristics to watch for
when mixing two or more substances to know whether a chemical
reaction occurs is;
heat and light (flames)
explosion
color change
evolution of a gas
formation of a solid (precipitate)
Lets look at two reactions; the reaction between hydrogen (H2)
and oxygen (O2) and the reaction between sodium (Na)
and chlorine (Cl2). To describe these reactions
chemists use a symbolic representation, called a chemical
equation. The reaction between hydrogen (H2) and
oxygen (O2) can be described using the equation,
H2(g) + O2(g)
--heat--> H2O(g)
In the reaction between hydrogen and oxygen a
balloon was filled with a 50/50 mixture of the two gases. Notice
that the mixture appears to be stable. But if we bring a lighted
match close to the balloon and touch it...blam!! The mixture
reacts, rapidly and explosively! The product of the reaction is
good old water-H2O.
The reaction between sodium (Na) and chlorine
(Cl2) can be described using the equation,
Na(s) + Cl2(g) --heat--> NaCl(s)
In the reaction between sodium and chlorine gas, we heated a
sample of sodium in a deflagrating spoon and then placed the
sodium into a flask containing chlorine gas. It took a moment for
the reaction to get started but once it did, we could see a bring
light, and a white cloud of sodium chloride was formed. A cloud
is simply a fine dispersion of solid particles in a gas. The fine
dispersion of solid is the solid chloride, and the gas is air.
Both of these two reactions were very violent, explosive,
loud, and intensely bright. Yet the products are just sodium
chloride and water. Stuff we depend on to live and we all eat
everyday. That is amazing chemistry, hazardous elements combined
together to form life giving compounds!
The form of a chemical equation involves writing the formulas
of the reactants (the substances that are mixed together) on the
left, using '+' when more than one substance is involved and the
formula(s) of the product(s) on the right. The reactants and the
products are separated by an arrow '--->'. Sometimes
additional information about the reaction is placed above or
below the arrow which separates the reactants and products. Such
information include;
heat (using a delta symbol)
temperature (at which the reaction is run at)
pressure
time (length of time the reaction is allowed to
proceed)
Chemists alway try to be sure the chemical equation is
balanced.
There are some simple rules for balancing equations:
Things not to do when balancing an equation:
Change a subscripted number
Add or remove an element arbitrarily
Add or remove a compound arbitrarily
Things to do when balancing an equation:
Change only the coefficients preceding an
element or compound.
Try to balance non-oxygen and non-hydrogen
elements first.
Balance substances in their elemental form
last.
So to balance the first equation between hydrogen and oxygen,
H2(g) + O2(g)
--heat--> H2O(g)
we want to be sure equal numbers of hydrogen and
oxygen atoms are on both sides. Notice on the products (right
side) there is only one oxygen atom, but on the reactants (left
side) there are two oxygen atoms. To balance the equation we use
a coefficient preceeding the water.
H2(g) + O2(g)
--heat--> 2H2O(g)
When we change the coefficient for water we
change the number of hydrogen atoms. So the products now have
four hydrogen atoms. To get four hydrogen atoms on the reactants
side we change the coefficient from one to two.
2H2(g) + O2(g)
--heat--> 2H2O(g)
To balance the reaction between sodium (Na) and
chlorine (Cl2)
Na(s) + Cl2(g) --heat--> NaCl(s)
we change the coefficient preceeding NaCl to a
two and also preceeding Na.
2Na(s) + Cl2(g) --heat-->
2NaCl(s)
You should practice writing and balancing
chemical equations by doing Question 27 in the textbook on page
81.
Write a chemical equation which describes how to
prepare each of the following compounds from their elements; CaO,
MgCl2, Al2O3, and SO2.
The two reactions we observed in class (shown
above) are examples of formation or synthesis reactions. We mixed
two elements and produced a new compound.
Another interesting reaction is a combustion
reaction. One that we are all familiar with is the combustion of
methane (natural gas). Combustion reactions are characterized by
a compound reacting with oxygen in the atmosphere. Anytime we use
a gas stove to heat or cook we are depending on the combustion of
methane to produce heat. How do we write an equation to describe
the combustion of methane? First we'll begin with the
reactants....methane and oxygen.
CH4(g) + O2(g)
--heat-->
What are the products? In this case, we are
combusing a hydrocarbon, the products are always carbon dioxide
and water.
CH4(g) + O2(g)
--heat--> CO2(g) + H2O(g)
Now comes the challenge...can you balance this
equation? (we'll do this in class)
Can you write the balanced combustion equation
for propane? The chemical formula for propane is C3H8(g).