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Balancing Chemical Equations A Chemical Reaction Reactants Products A process in which at least one new substance is produced as a result of chemical change. Writing a Chemical Equation Chemical symbols give a “before-and-after” picture of a chemical reaction Reactants MgO + Products C magnesium oxide reacts with carbon Bonds are broken CO to form + Mg carbon monoxide and magnesium New bonds form Reading A Chemical Equation 4 NH3 + 5 O2 bonds broken 4 NO + 6 H2O new bonds form Four molecules of NH3 react with five molecules O2 to produce four molecules NO and six molecules of H2O A Balanced Chemical Equation Same numbers of each type of atom on each side of the equation. Aluminum reacts with sulfur to form aluminum sulfide. Al + S 2Al + 3S Al2S3 1Al2S3 Not Balanced Balanced If a coefficient is “1”, write the “1”. Don’t leave it blank. Translate word equation into formula equation Calcium chloride reacts with aluminum hydroxide to produce calcium hydroxide and aluminum chloride. “reacts with” “to produce” “and” => => => + + Translate word equation into formula equation Calcium chloride reacts with aluminum hydroxide to produce calcium hydroxide and aluminum chloride. CaCl2 + Al(OH)3 Ca(OH)2 + Why is this Not Balanced ? AlCl3 Translate word equation into formula equation Hydrogen reacts with chlorine to form hydrochloric acid. H2 + Cl2 HCl Not Balanced ! Dicarbon tetrahydride reacts with oxygen to form carbon dioxide and water. C2H4 + O2 CO2 + H2O Not Balanced ! Balance Equations with Coefficients Coefficients in front of formulas balance each type of atom. 4NH3 + 5O2 4NO + 6H2O 4N = 4N 12 H = 12 H 10 O = 10 O DO NOT CHANGE SUBSCRIPTS TO BALANCE AN EQUATION ! Balance Equations with Coefficients Do not put coefficients in the middle of a formula. 4NH3 is OK, but N4H3 is NOT OK. 4NH3 + 5O2 4NO + 6H2O DO NOT CHANGE SUBSCRIPTS TO BALANCE AN EQUATION ! Steps in Balancing An Equation 1) Use coefficients to balance equations. Never change or add subscripts. 2) Place a coefficient of “1” in front of the most complex formula, especially those with polyatomic ions. 3) Balance each element in the formula. It’s helpful to leave oxygen compounds and pure elements until last. 4) Balance odd-even problems by placing a “2” in front of the “odd” formula. 5) When a polyatomic ion occurs on both side, treat it as a group. 6) Do a final check to make sure that there are the same number of atoms for each element on each side of the equation. If a coefficient is “1”, write the “1”. Don’t leave it blank. Steps in Balancing An Equation Not on Handout Fe3O4 + H2 __Fe3O4 + __H2 Fe + H2O __Fe + __H2O Fe: 1 Fe3O4 + __H2 3 Fe + __H2O O: 1 Fe3O4 + __H2 3 Fe + 4 H2O H: 1 Fe3O4 + 4 H2 3 Fe + 4 H2O Treat PI’s as a Group Mg + H3PO4 + __H3PO4 __Mg Mg3(PO4)2 + H2 __ Mg3(PO4)2 + __H2 __Mg + __H3PO4 1 Mg3(PO4)2 + __H2 __Mg + 2 H3PO4 1 Mg3(PO4)2 + __H2 __Mg + 2 H3PO4 1 Mg3(PO4)2 + 3 H2 3 Mg + 2 H3PO4 1 Mg3(PO4)2 + 3 H2 Balancing: PI doesn’t stay as a group Not on Handout Sodium reacts with water to form sodium hydroxide and hydrogen. Translate into chemical equation: Na + H 2O NaOH + H2 Hydroxide on the left does not appear as hydroxide on the right. Balance oxygens and hydrogen separately. Also notice product side: hydrogen occurs in both formulas. Balancing: PI doesn’t stay as a group __Na + __H2O __NaOH + __H2 Use the “odd-even” strategy. There is an odd number of hydrogens on the right side. Make the “odd” side even by putting a coefficient of “2” in front of NaOH __Na + __H2O 2 NaOH + __H2 2 NaOH + __H2 Then balance oxygens on the left. __Na + 2 H2O Be careful counting the hydrogens: 4 on the left On the right, 2 hydrogens in 2NaOH + ? H2 Balancing: PI doesn’t stay as a group __Na + 2 H2O 2 NaOH + __H2 Be careful counting the hydrogens: 4 on the left On the right, 2 hydrogens in 2NaOH + ? H2 You need 1 group of H2 to get a total of 4 on the right. __Na + 2 H2O 2 NaOH + 1 H2 Finish with sodium 2 Na + 2 H2O 2 NaOH + 1 H2 If a coefficient is “1”, write the “1”. Don’t leave it blank. Strategy for Combustion Reactions Combustion rxns have a hydrocarbon reacting with oxygen to from carbon dioxide and water. C8H18 + O2 → CO2 + H2O Put a coefficient of “1” in front of the hydrocarbon. Balance the carbons, then balance the hydrogens. Save oxygen until last. If a “1” doesn’t work, erase and try a “2”. See board. More Practice Online: http://www.sciencegeek.net/APchemistry/APtaters/EquationBalancing.htm