Describing Chemical Reactions

Review Physical change – The atomic structure stays the same. – No bonds are broken, and no new bonds are formed. – Their can be a change in appearance. – Examples; Cutting, boiling, freezing, thawing, condensing, melting, smashing, bending, etc. – Usually (not always) reversible.

Review Chemical Change – The atomic structure is changed – Bonds are broken, or new bonds are formed, or both. – Like physical change the appearance can change. – Examples; burning, rusting, exploding, formation of a gas, if heat is absorbed, if something gets cold, change in color, cooking, etc.

Formulas = Abbreviated or shortened versions of a compound. – Easier and quicker to write – Give you more information – Examples Carbon Dioxide vs CO 2 Ammonia vs NH 3

Equation = A way of writing what happens during a chemical reaction. – Uses formulas – Tells you what you begin with before a reaction. – Tells you what you end up with after the reaction.

Reactants are what you begin with before the reaction occurs. They are always written on the left side. If you have more than one reactant they are separated by a plus sign just like a math problem. – Wood + Oxygen  – C 6 H 12 O 6 + O 2 

Products are what you end up with after the reaction. They are always written on the right side. If you have more than one product they are separated by a plus sign. –  Carbon dioxide + Water Vapor –  CO 2 + H 2 O

Reactants Wood + Oxygen  C 6 H 12 O 6 + O 2  Products Carbon Dioxide + Water CO 2 + H 2 O

Conservation of Mass – During a chemical reaction, matter is not created or destroyed. All the atoms present at the start of the reaction are present at the end. – The total mass of the reactants must equal the total mass of the products. –

Open system – Matter can enter and leave the reaction. Closed system – Matter is not allowed to enter or leave.

Balancing Chemical Equations Conservation of mass means the same number and type of atoms must be present before and after a chemical reaction. There are 4 steps to balancing a chemical equation

Step 1: Write the equation Reactants Yields Products H 2 + O 2  H 2 O

Step 2: Count the Atoms Reactants Yields Products H 2 + O 2  H 2 O Reactants – Hydrogen = 2 – Oxygen = 2 Products – Hydrogen = 2 – Oxygen = 1

H 2 + O 2  H 2 O H = 2, O=2  H = 2, O=1 Step 3: Use Coefficients to balance the atoms 2 H 2 + O 2  2 H 2 O Step 4: Look back and Check (Recount atoms) H = 4, O = 2  H = 4, O = 2 The equation is balanced