Chemical Formulas

Chemical reactions occur when old bonds break and new bonds form. Energy O O Energy Energy H H C Energy Energy H H Energy O O Old Bonds Break New Bonds Form

Chemical Formulas A chemical formula helps us to see what actually happened in a reaction. Take the easy Sodium Chloride reaction: Na + Cl NaCl *Reactants (what goes in) are ALWAYS listed on the left of the arrow *Products (what comes out) are ALWAYS listed on the right of the arrow

How to read a chemical formula: Ox Ox Ox Ox H H “makes” C C H H H Ox H Ox Ox Ox H CH4 + 2O2 CO2 + 2H2O The formula uses the symbols from the periodic table for each element: C for Carbon, H for Hydrogen and O for Oxygen in this case. The small numbers after the chemical symbol, tell you how many atoms of that element there are in the molecule (no number = only 1 atom). For example; there are 2 Hydrogen's in the water molecule so you see H2 but only 1 O for oxygen = H2O The “+” sign means “and”. The arrow sign means “makes”. You can also think of the arrow sign like an = sign in math. The molecules are written together. CH4 is Methane; O2 is an Oxygen molecule; CO2 is Carbon Dioxide and H2O is Water. The big numbers in front of the molecule, tells you how many of that molecule there are in the reaction (no number = only 1 molecule). For example; there are 2 water molecules in this reaction. Product (What you end with) Reactant (What you start out with)

How to see a chemical formula: Energy Pow O O Energy Pow Pow H H C Energy Pow Pow H H Pow O O CH4 + 2O2 CO2 + 2H2O When Methane and Oxygen are mixed, they react! The atoms of each element let go of their bonds and the chemical energy holding them together comes out. Then the atoms rearrange and new bonds are formed. The formula shows what you start with and what you end with

A new Law I give unto you… The Law of Conservation of Matter: Matter is not created or destroyed in a chemical reaction. What goes in, must come out! Both sides have the same # of elements CH4 + 2 O2 CO2 + 2 H2O Reactants Products

Conservation of Mass Explained: During a chemical reaction, matter is neither created or destroyed. The atoms just trade partners around. All the atoms present at the start of the reaction are still present at the end. They are just someplace else. EX: I can change my seating chart around, but that does not mean I have a new class! You are just in different places

What goes in MUST equal what comes out! If you add up the amounts of Iron and Sulfur you will see that it equals the amount of Iron Sulfide made in the reaction. What goes in MUST equal what comes out! - Describing Chemical Reactions

Step 1: Write the Equation Notice that the number of Oxygen atoms are not equal on both sides of the arrow! The Law of Conservation of Matter says they must be so we have to change up the formula a little… Step 1: Write the Equation How to write a chemical formula In other words, how in the world do we know that a formula uses 2 or more molecules of one type but only one of another? How in the world do we know that? Well, you gotta do a little math to find out! Step 2: Count the Atoms

Step 3: Use Coefficients to Balance But what if the reaction actually made 2 water molecules? Well then the oxygen would be equal! Step 3: Use Coefficients to Balance But count the numbers of atoms again and you will see that now Hydrogen is unequal. We are not done yet…

Step 3: Use Coefficients to Balance So…let’s assume that the reaction needed 2 H2 molecules to work: Step 3: Use Coefficients to Balance So the final chemical formula is: 2H2 + O2 2H2O Count the atoms one last time and you can see that we did it! The number of Hydrogen and Oxygen on both sides are equal! What goes in = what came out

Lets Try a Few Ca + O2 → CaO Where you see an empty □, you CAN add a coefficient. But you don’t HAVE to. Only when you need to, to write a correct formula. Use the steps you just learned.

Lets Try a Few H2O2 → H2O + O2