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Chemical Formulas TEKS: 8.8A, 8.9A, 8.9B, 8.9C

Chemical Formulas All of the elements have symbols that are listed on the periodic table. Some elements have 1 letter and some have 2 letter symbols. It is very important to pay attention to the capitalization of elemental symbols. Elemental symbols are case sensitive!

Can you see how this could get confusing? C - carbon O - oxygen Co - cobalt CO – carbon monoxide

Combinations of element symbols are a short way to show chemical formulas. An example of this is water or H 2 O.

The small number 2 after the H is called a subscript. When you see a subscript after an elemental symbol you know how many of that atom are in the formula. If there is no subscript after an element in a formula, then you know that there is only one of that atom.

H 2 O This formula tells you that the compound has 2 hydrogen atoms and 1 oxygen atom.

Some elements bond to each other and form simple molecules. Hydrogen and oxygen are examples of this. They rarely occur as single atoms. When these molecules are in formulas they look like this --- H 2 or O 2

Chemical Equations A chemical equation is a description of a chemical reaction using symbols and formulas. Example: 2H 2 + O 2  2H 2 O

This equation tells us that 2 molecules of hydrogen and 1 molecule of oxygen react to form 2 molecules of water. 2H 2 + O 2  2H 2 O

8NaCN + 4Au + 2H 2 O +O 2  4NaAu(CN) 2 + 4NaOH Here is the equation used in refining gold using sodium cyanide and zinc.

When writing chemical equations, scientists use an arrowto show that a reaction has occurred.

The substances that you begin with are shown on the left hand side of the equation and are called reactants. The substances that you end up with are shown on the right hand side of the equation and are called products. Reactant + reactant  product CaO + CO 2  CaCO 3

The following formula can be read several ways. CaO + CO 2  CaCO 3

CaO + CO 2  CaCO 3 Calcium oxide (CaO) and carbon dioxide (CO 2 ) react to form calcium carbonate (CaCO 3 ). Or One molecule of CaO and one molecule of CO 2 react to form one molecule of CaCO 3.

Instructions for Counting Atoms Before you can do anything with a formula, you have to know how much matter you have and what it is.

Counting the atoms in a molecule is very important. Here are the rules.

When there is a number in front of a molecule it means that there are multiple molecules present. This number is called a coefficient. 2H 2 O means that there are 2 molecules of water present.

This means that there are 4 hydrogen and 2 oxygen atoms present in the molecules. 2H 2 O = 4H and 2O H 2 O + H 2 O = 4H + 2O =

NH 3 N=1, H=3

Practice: 3H 3 PO 4 H =3 x 3 = 9 P =3 x 1 = 3 O =3 x 4 = 12

Practice: 2Fe 2 O 3 Fe =2 x 2 = 4 O =2 x 3 = 6

Practice: 3C 2 H 4 O 2 C =3 x 2 = 6 H =3 x 4 = 12 O =3 x 2 = 6

Sometimes chemists put parentheses in their formulas. If there is a subscript after a parenthesis, then you know that there are multiple sets of everything in the parentheses. Mg(OH) 2 Mg = 1 O = 2 x 1 = 2 H = 2 x 1 = 2

Practice: Ca(H 2 PO 4 ) 2 Ca =1 H =2 x 2 = 4 P = 2 x 1 = 2 O =2 x 4 = 8

Practice: Ca(OH) 2 Ca =1 O =2 x 1 = 2 H =2 x 1 = 2

Practice: (NH 4 ) 3 PO 4 N =3 x 1 = 3 H = 3 x 4 = 12 P =1 O = 4 x 1 = 4

Practice: 3Mg(OH) 2 Mg =3 x 1 = 3 O = 3 x (1 X 2) = 6 H =3 x (1 X 2) = 6

Balanced Equations

No matter what, in any chemical reaction, we cannot create or destroy matter. When you burn firewood you are left with ashes.

It seems that the wood has disappeared, but if you were to measure the mass of the ashes plus all the gases and smoke it would equal the mass of the wood before you burned it.

This idea is called the conservation of mass.

In a chemical reaction the total mass of the reactants always equals the total mass of the products. The atom counts don’t change either. reactantsproducts

This means that the total number of each atom in the equation must be equal on both sides.

Sometimes we have an equation that is not balanced. That is, there are not the same number of each atom on either side of an equation. H 2 + O 2  H 2 O

To balance an equation follow these steps.

Remember, we cannot change any of the compounds or elements in an equation. That would change the reaction. But we can change the number of molecules in the reaction to balance it.

1. Count the atoms of each element in the reactants and in the products. You can see that there are fewer oxygen atoms in the products than in the reactants. Reactants Product H 2 + O 2  H 2 O

Reactants Product H 2 + O 2  H 2 O H = 2 2 O = 2 1

2. To balance the oxygen atoms, place the coefficient 2 in front of the formula for water. This gives you two oxygen atoms in both the reactants and the products. But now there are too few hydrogen atoms in the reactants.

ReactantsProducts H 2 + O  2H 2 O H = 2 4 O = 2 2

3. To balance the hydrogen atoms, place coefficient 2 in front of hydrogen’s formula. To be sure your answer is correct, always double-check your work!

ReactantsProducts 2H 2 + O  2H 2 O H = 4 4 O = 2 2

Now you have a balanced equation! Remember this is a trial and error process. If you try one way that does not work, try something else until you get it right!

Let’s practice some more. Get out some scratch paper and let’s play!

Zn + HCl  ZnCl 2 + H 2

Fe 2 O 3 + C  Fe + CO

H 2 O 2  H 2 O + O 2

SO 2 + O 2  SO 3

Education is Gold! Go get ‘ya some!