6.2 – NOTES Representing Reactions

D.3 Keeping Track of Atoms When a car’s gas tank empties, where do the atoms in the gasoline go? The atoms don’t just disappear   Law of conservation of matter Matter can’t be created or destroyed in a reaction Molecules get broken apart and rearranged but atoms will remain

Balanced chemical equation Number of atoms of each element is the same on the reactant and product sides The reactant is on the left side of the arrow The product is on the right side of the arrow The number of atoms must be equal on both sides

Subscripts The number printed below the line of type Tell you how many elements are present in a molecule or formula No number means that there is one atom   Coefficients Big numbers put in front of formulas Tell you how many units are present in an equation The only number you can change You do not write a coefficient of 1

Molecules are the smallest unit of a covalently bonded substance   Formula unit are the smallest unit of an ionically bonded substance

Letters after a molecule or formula unit are for the state of matter (s) = solid (l) = liquid (aq) = aqueous, dissolved in water (g) = gas

D.4 Accounting for Atoms Burning propane gas is a common source of heat for campers. The chemical equation is: C3H8(g) + O2(g)  CO2(g) + H2O(g) Let’s interpret this equation several ways: Words   Propane Gas reacts with Oxygen gas to produce Carbon Dioxide and Water

Molecular Models 1 propane molecule + 1 oxygen molecule  1 carbon dioxide molecule + 1 water molecule An Atom Inventory Reactant Side Product Side C= C = O = O = H = H =   Is this a balanced chemical equation?

D.5 Nature’s Conservation: Balanced Chemical Equations Law of conservation of matter is based on the idea that atoms are indestructible Extremely small amounts of matter may be converted to energy

YOU CANNOT CHANGE SUBSCRIPTS WHEN BALANCING- ONLY COEFFICIENTS!!!! Steps for balancing a chemical equation Check to see if equation is balanced (atom inventory)   Balance Polyatomic Ions (Usually in Parenthesis) Balance Metals Balance Non-metals except for H and O Balance H Balance O Check to see if atoms are equal in both sides

Examples KClO3  KCl + O2  

 Ba(NO3)2 + Na2SO4  BaSO4 + NaNO3

Zn + HCl  H2 + ZnCl2

Al + Ni(NO3)2  Al(NO3)3 + Ni

Reaction types A + B  AB 2 Na + Cl2  2 NaCl Reactions are classified based on the number of reactants and products present in the reaction Synthesis/composition reaction Reactants- multiple reactants (2 or more) Products- One Product   Example  A + B  AB 2 Na + Cl2  2 NaCl

AB  A + B 2 KBr  2K + Br2 Decomposition reaction Reactants- one Products- multiple (2 or more) Example AB  A + B 2 KBr  2K + Br2 

Single replacement reaction Reactants- element and compound Products- element and compound Follows rules of the activity series Example: A + BC  B + AC Li + AuCl  Au + LiCl

AB + CD  AD + CB or CB + AD (order doesn’t matter) Double replacement reaction Reactants- 2 compounds Products- 2 compounds Examples  AB + CD  AD + CB or CB + AD (order doesn’t matter) NaCl + HBr  HCl + NaBr

Combustion reaction CH4 + 2O2  CO2 + 2H2O C3H8 + 5O2  3CO2 + 4H2O Reactants- Hydrocarbon (CxHy) and Oxygen (O2) Products- CO2 and H2O Examples CH4 + 2O2  CO2 + 2H2O C3H8 + 5O2  3CO2 + 4H2O