Chemical Formulas

TEKS 8.5 D and 8.5 F• Recognize that chemical formulas are used to

identify substances and determine the number of atoms of each element in chemical formulas containing subscripts.

• Recognize whether a chemical equation containing coefficients is balanced or not and how that relates to the law of conservation of mass.

• Recognize the importance of formulas and equations in representing chemical reactions– 1998 TEKS 8.9C

What do you know about chemical reactions, chemical formulas, and chemical equations?

• Teacher can type answers in here.

Chemical Formulas

What is a chemical formula?• Chemical, or molecular, formulas are a concise

way of expressing information about the atoms that constitute a particular chemical compound.

• Wait…what?

• It is an expression which states the number and type of atoms present in a molecule of a substance.

Do you know what the chemical formulas are for the following substances?

Water

Carbon dioxide

Sodium chloride or salt

Magnesium sulfate (AKA Epsom salt)

MgSO4

For something more complicated….Glucose or sugar

Subscripts

Definition• A subscript is used to represent the number of

each atom being represented. • They are only used when more than one atom is

being represented. If only one atom is represented, there is no subscript.

• In the formula for water, what is the subscript?• There is only one atom of Oxygen, so it does not

have a subscript.

H2OH2OH = HydrogenO = Oxygen

Some exceptions exist

• Usually the subscript just multiplies or shows the number of atoms of a single element. If the subscript exists outside of a set of parenthesis then it will multiply the atoms of all of the elements inside the parenthesis.

• How many of each atom are there now?• Answer: Nitrogen-1, Carbon-3, Hydrogen-9

N(CH3)

3

N = NitrogenC = CarbonH = Hydrogen

Here are some molecules…

• In this image Hydrogen atoms are white and Nitrogen atom is red.

• Write the chemical formula for this molecule.

Answer: NH3

This substance is ammonia

• In this image the Carbon atom is black and the Oxygen atoms are red.

• Write the chemical formula for this molecule.

Answer: CO3

This substance is carbonate

• In this image blue represents Nitrogen atoms, red represents Oxygen atoms, white represents Hydrogen atoms and black represents Carbon atoms.

• Write the chemical formula for this molecule.

Answer: C8H10N4O2

This substance is caffeine

• In this image the Carbon atom is blue and the Oxygen atom is red.• Write the chemical formula for this

molecule.Answer: CO

This substance is carbon monoxide

• A harder one:In this image Nitrogen atoms are blue, the Platinum atom is grey, Chlorine atoms are green, and Hydrogen atoms are white.

• Write the chemical formula for this molecule.

Answer: N2PtH6Cl2

Coefficient

7H2

O

Definition• Coefficients appear on the left side of a chemical

formula.• They are used to multiply all the atoms in a

compound• In the following formula, which is the coefficient?• Earlier we learned that the subscript 2 meant that

there were two Hydrogen atoms. The coefficient 7 means there are 7 times more.

• How many Hydrogen atoms do we have? • How many Oxygen atoms?

7H2

O

Chemical Equations

Chemical Equations• representation of chemical reaction in

equation: a representation, using chemical symbols in a form resembling a mathematical equation, of the process involved in a chemical reaction

• This is an example of a chemical equation. The components on the left combine together to yield (represented by the arrow) the component on the right.

2H2+O22H2O

Yields

Definition• The quantity of a specified product obtained in a

reaction or series of reactions, usually expressed as a percentage of the quantity that is theoretically obtainable

Product

Definition• What is produced. • Which side represents the products?

• They are found on the right side of a chemical equation.

2H2+O22H2O2H2+O22H2O

Reactant

Definition• The starting substances.• Which side represents the reactants?

• They are found on the left side of a chemical equation.

2H2+O22H2O2H2+O22H2O

Law of conservation of mass

• Atoms are neither created, nor destroyed, during any chemical reaction.

• This means that the same number of atoms that are present after a reaction are the same number of atoms that are present before a reaction.

• There is only a rearrangement

Balancing chemical equations

Subscripts are never changed when balancing an equation

Step 1• Write out your “un-balanced” equation using

formulas of reactants and products.

CH4+O2CO2+H2O

Step 2• Count up the atoms in the products and reactants.• How many carbons, hydrogens, and oxygens are

on each side? Are they equal?

CH4+O2CO2+H2O

C=1H=4O=2

C=1H=2O=3

They are NOT equal

Step 3• Since our carbons are ok we will not mess with

those now.• However, we have half the number of hydrogens

in the products than we do in the reactants.• What do we need to add? Where do we add it?

CH4+O2CO2+H2OCH4+O2CO2+2H2

O C=1H=4O=2

C=1H=4O=4

They are still NOT equal

Step 4• Now we have half the number of Oxygens.• What do we need to add? Where do we need to

add it? Is everything equal now?

CH4+O2CO2+2H2OCH4+2O2CO2+2H2

O C=1H=4O=4

C=1H=4O=4

They ARE now balanced

Practice Time

H2+O2H2O2H2+O22H2O

Fe+Cl2FeCl32Fe+3Cl22FeCl3

Cu+AgNO3Cu(NO3)2+AgCu+2AgNO3Cu(NO3)2+2Ag

Zn+HClZnCl2+H2

Zn+2HClZnCl2+H2

Pb(NO3)2+AlCl3PbCl2+Al(NO3)3

3Pb(NO3)2+2AlCl33PbCl2+2Al(NO3)3