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Unit I: CHEMICAL REACTIONS I.5. Balancing Chemical Equations Chemical Equations Remember: Formulas must BE CORRECT Ionic – use charges Angela goes fishing Chris uses the crisscross method Others look at net overall charge Covalent [molecular] – use prefixes Phases Sometimes a chemical reaction will also show the phases in which the reactants and products exist (s) – solid (l) – liquid (g) – gas (aq) – aqueous (water solution) Typical Reaction 2Mg ( s ) O2( g ) 2MgO( s ) REACTANT(S) REACTS TO FORM PRODUCTS(S) Typical Reaction Phases 2Mg ( s ) O2( g ) 2MgO( s ) Subscripts Coefficients General Rules 1. 2. 3. 4. 5. 6. Always check your formulas Count the number of each kind of atom. Treat polyatomics like atoms. Put in coefficients to equalize atoms. If an atom appears in more than one place on the same side of the equation, do it last. Do ‘singletons’ LAST… Balancing Equations Even-Odd Trick An odd number x 2 = an even number K + O2 K2 O Balancing Equations Even-Odd Trick Ca + O2 CaO Balancing Equations Two’s and Three’s N2 + H2 NH3 Balancing Equations Two’s and Three’s Al + Cl2 AlCl3 Balancing Equations PAI’s Trick is to look for a PAI on both sides of the equation A PAI can be thought of as a single “thing” Start with the most complex compound Balancing Equations PAI’s Ca(NO3)2 + Na2S CaS + NaNO3 Balancing Equations PAI’s Ca(NO3)2 + Na2SO4 CaSO4 + NaNO3 Balancing Equations PAI’s AlBr3 + SrCO3 Al2(CO3)3 + SrBr2 Balancing Equations PAI’s Fe(NO3)3 + (NH4)2CO3 Fe2(CO3)3 + NH4NO3 Balancing Equations PAI’s AgNO3 + K2CrO4 Ag2CrO4 + KNO3 Hebden Do Exercises 11, 13, 14, 16, 17, 18, 20, 21, 23, 25 on page 110-111 Balancing Equations C, H, O Balance in order C then H then O Can’t Have Order Balancing Equations C, H, O CH4 + O2 CO2 + H2O Balancing Equations C, H, O C2H6 + O2 CO2 + H2O To get whole number coefficients = Must get rid of the fraction so you times all coefficients by 2 C2H6 + O2 CO2 + H2O Balancing Equations C, H, O C4H10 + O2 CO2 + H2O C4H10 + O2 CO2 + H2O Balancing Equations C, H, O C24H50 + O2 CO2 + H2O C24H50 + O2 CO2 + H2O Tricky Question #18 Ca(OH)2 + NH4Cl NH3 + CaCl2 + H2O Balancing Equations C, H, O- alcohols CH3OH + O2 CO2 + H2O Balancing Equations C, H, O- alcohols CH3OH + O2 CO2 + H2O Balancing Equations C, H, O- alcohols CH3OH + O2 CO2 + H2O Balance These C14H29OH C6H12O6 C9H50O4S2 + + O2 O2 CO2 + CO2 + H2O H2O + O2 CO2 + SO2 + H2O Worksheet Do Problem Set #1 on page 4 in your Student Workbook