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 CO2

• Ammonia vs NH3

• 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 – C6H12O6 + O2

• 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– CO2 + H2O

Reactants

Wood + Oxygen C6H12O6 + O2

Products

Carbon Dioxide + WaterCO2 + H2O

• 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.

– http://www.pearsonsuccessnet.com/ebook/products/0-13-181250-5/view1_sx05_lart58.pdf

• Open system– Matter can enter and leave the reaction.

• Closed system– Matter is not allowed to enter or leave.

• http://www.pearsonsuccessnet.com/ebook/products/0-13-181250-5/view1_sx05_lart59.pdf

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• H2 + O2 H2O

• Step 2: Count the Atoms • Reactants Yields Products• H2 + O2 H2O• Reactants– Hydrogen = 2– Oxygen = 2

• Products– Hydrogen = 2– Oxygen = 1

• H2 + O2 H2O• H = 2, O=2 H = 2, O=1• Step 3: Use Coefficients to balance the atoms• 2 H2 + O2 2 H2O• Step 4: Look back and Check (Recount atoms)• H = 4, O = 2 H = 4, O = 2• The equation is balanced