Law of Conservation of Mass & Balancing Equations LG: I can use the law of conservation to explain why chemical reactions need to be balanced

Law of Conservation of Mass During a chemical reaction, the atoms of the reactants are rearranged to form the products. H2 + Cl2  2 HCl No atoms are gained or lost; therefore no mass is gained or lost. This principal is known as the law of conservation of mass.  

Law of Conservation of Mass Some reactions appear ‘unbalanced’ at first:  Chemical Equation: Mg + HCl  MgCl2 + H2 Particle Picture: Chemists fix this problem by ‘balancing’ the equation to show that all atoms in the reactants are still present in the products: Chemical Equation: Mg + 2 HCl  MgCl2 + H2 Unbalanced equations are called skeleton equation “because thy show the bare bones” of what’s happening

Balancing Equations Example 1 Skeleton NaCl + BeF2 → NaF + BeCl2 Equation Balanced Particle Picture Balanced Equation Discuss rules of balancing – if you need more of a particular atom, you have to add more reactant or product MOLECULES. Can only change coefficients, not subscripts.

Balancing Equations Strategies: Balancing by inspection - Balance particle in this order: Atoms that are not part of a polyatomic ion, and not H or O Next, balance polyatomic ions Last, balance H and O Example 2 Cu  +  O2  →  CuO Example 3 Al(NO3)3 + H2SO4 → HNO3 + Al2(SO4)3

Balancing Equations Example 4 Sodium phosphate reacts with calcium chloride to produce calcium phosphate and sodium chloride Word Equation Chemical Equation (& balance it!)

Balancing Equations Practice! H2 + N2  NH3 Cu + AgNO3  Ag + Cu(NO3)2 NH4 + O2  H2O + NO2 C3H6 + O2  CO2 + H2O CaCO3 + HCl  CaCl2 + H2O + CO2 Show how to balance using ‘half an oxygen’

Homework! Old Book: Pg. 215 # 3, 4 (don’t worry about ‘mole ratio’) New Book: Pg. 155 # 3ab, 5 (don’t worry about states in #3)