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Chemical Formulas and Equations
8.5D recognize that chemical formulas are used to identify substances and determine the number of atoms of each element in chemical formulas containing substances; 8.5F
recognize whether a chemical equation containing coefficients is balanced or not and how that relates to the law of conservation of mass
Chemical Compounds
Chemists have identified over 10 million chemical compounds
Some are molecular – like the proteins and hormones in your body
Some are ionic compounds – such as the salts in your body fluids
A chemical formula shows the kinds and numbers of atoms in the smallest representative unit of the substance (recall this from the last lesson?)
Formulas
H2O shows that a water molecule has 2 hydrogen and 1 oxygen atom
CO2 – carbon dioxide 1 carbon and 2 oxygen atoms
C2H6 – ethane (natural gas) 2 carbon and 6 hydrogen atoms
What they don’t show: If we look at NH3, it does show how many atoms
and elements and the ratio that make up that molecule.
What a formula doesn’t show is the structure You had a bit of practice with this idea during the
bonding practice
So why the formula?
There are many reasons, but the best reason is to demonstrate on paper, chemical reactions!
Chemical reactions are shown with chemical equations.
Learning how to read one is very important in chemistry
Chemical formulas practice:
Make sure to complete the front page and ask questions for now. When everyone is done, we will continue on with equations.
Chemical Equations
A chemical equation is a symbolic representation of a chemical
reaction
2 2
Equation Example:
The burning of methane gas in oxygen is:
CH4 + 2 O2 → CO2 + 2 H2O
Reading Chemical Equations
Each side of an equation represents a combination of chemicals
The combination is written as a set of chemical formulas, separated by +symbols.
CH4 + 2 O2 → CO2 + 2 H2O
Reading Chemical Equations
The two sides of the equation are separated by an arrow
•The combination of chemicals before the reaction are on the left side of the arrow
•The right side indicates the combination of chemicals after the reaction
Language of Chemical Equations
In this reaction, sodium (Na) and oxygen (O2) react to produce a single molecule, Na2O (this together is known as “sodium oxide”
4Na + O2 → 2Na2O
Reactants Products
Yields
Reactants
Arrow (yields)
Products
Conservation of Mass and Balancing Equations
8.5F recognize whether a chemical equation containing coefficients is balanced or not and how that relates to
the law of conservation of mass
Law of Conservation of Matter
Mass is neither created nor
destroyed (during a chemical
reaction)
Conservation of Mass and/or Matter
What this means: 1. Matter is conserved type of atoms does
not change
•Nothing is created or destroyed
2. Mass is conserved amount of atoms cannot change
•Nothing is created or destroyed
So…IN A CHEMICAL REACTION:
Elements can combine to form new substances
Substances can be broken down into simpler substances (down to the elemental level)
Atoms are re-arranged; NOT created or destroyed
Reactants Products
Mass of Reactants = Mass of Products
In a chemical formula this law is demonstrated
Balancing Equations
To show conservation of mass We Balance equations
•Make sure there are the same number of each type of atom in the products (right side) and on the reactants side (left)
Take a look at this reaction. How many
hydrogen atoms are there on each side of the reaction?
• Now, look at oxygen.
Balancing Equations
If there is not a subscript or coefficient number shown, then it is understood to be 1
CH4 + 2O2 → CO2 + 2H2O =C1H4 + 2O2 → C1O2 + 2H2O1
1 C C 4 H 4 H4 O 4 O
Complete (and correct) equations signifying reactions are always balanced
Are the following balanced?
Let’s look at the number of atoms on each side of the reaction and then decide if the equation is balanced.
Counting Atoms – remember the rules from the chemical formulas practice?
2CuO2
2NH4
3S2O2
Copper - 2Oxygen - 4
Nitrogen - 2Hydrogen - 8
Sulfur - 6Oxygen - 6
Counting Atoms – remember the rules from the chemical formulas practice?
4Mg2O4H2
2NaN3HO Sodium – 2Nitrogen – 6Hydrogen – 2Oxygen - 2
Magnesium – 8Oxygen – 16Hydrogen - 8
BALANCED or NOT BALANCED?
Cu + O2 CuO
2Cu + O 2CuO
S + O2 SO2
Mg + O2 MgO
2Mg + O2 2MgO
Not Balanced
Not Balanced
Balanced
Balanced
Not Balanced
Now, in your notes, there is a space to write examples and to explain why the examples are balanced or not.
Now let’s practice! Steps: 1. Work with a
partner to fill out this page
2. When done, check your answers with another group and then with the teacher