Transcript 2 O
Balancing Chemical Equations 10/21/08 Part I: Parts of a Balanced Chemical Equation •remember, chemical reactions are described by chemical equations. •chemical equation = represents, with symbols and formulas, the identities and molar amounts of the reactants and products in a chemical reaction. For example: •“The reactant ammonium dichromate yields the products nitrogen, chromium (III) oxide, and water.” is the same as: (NH4)2Cr2O7(s) N2(g) + Cr2O3(s) + H2O(g) •the equation above is UNBALANCED. There are: •2 N, 8 H, 2 Cr, and 7 O on the left, and •2 N, 2 H, 2 Cr, and 4 O on the right, so H and O are unbalanced. •you will learn how to balance it using coefficients today. •coefficients = large numbers in front of a chemical formula which multiply every element in the formula. For example: •Al2(SO4)3 = 2 Al, 3 S, and 12 O •5Al2(SO4)3 = 10 Al, 15 S, and 60 O •coefficients = large numbers in front of a chemical formula which multiply every element in the formula. For example: •Al2(SO4)3 = 2 Al, 3 S, and 12 O 5Al2(SO4)3 = 10 Al, 15 S, and 60 O •you can only insert coefficients to change the number of elements on either side of an equation, NEVER add, delete or change subscripts! 2 H2O2 right way 2 H2 + __O 1 2 → __ •Ex: balance __ (use coefficients)! wrong way! •now you have 4 H and 2 O on both sides of the arrow, and the equation is balanced. •check coefficients to see if they need to be reduced (if they are all divisible by the same number) •let’s look at a few more tips for balancing equations: 1. start with the most complicated compound, or, find a compound that contains odd numbers. Odd numbers are bad! Try putting a 2 or a 4 in front of it to make it even. Even = good! 2 6 1 6 CuSO4 → __ 2 Al2(SO4)3 + __ 4 Al + __ 3 Cu 3 • Ex: __ 2. leave PAIs alone! Do not break them up into their component elements. They balance much easier if you leave them together. 2. leave PAIs alone! Do not break them up into their component elements. They balance much easier if you leave them together. 3. balance one element in the compound at a time. • insert coefficients in front of the other compound that contains that element, then see how that addition affects the other elements in that compound. Then balance those elements. 4. leave oxygen until the end if you can, since it occurs in almost everything polyatomic and lots of covalent compounds, too. 6 2 3 K2S2O3 + __ 1 Cr2(S2O3)3 +12 • Ex: __ 6 __ K → __ 4 2 Cr 5. for combustion reactions, there is a special set of rules to follow: • look at the number of H in the hydrocarbon. • if H 4 = yes, start with a 1 in front of the hydrocarbon. • if H 4 = no, start with a 2 in front of the hydrocarbon. • balance the C first, then the H, then the O. 5. for combustion reactions, there is a special set of rules to follow: • look at the number of H in the hydrocarbon. • if H 4 = yes, start with a 1 in front of the hydrocarbon. • if H 4 = no, start with a 2 in front of the hydrocarbon. • balance the C first, then the H, then the O. Remember, O is in both the CO2 and the H2O! 3 • Ex: 1 __C3H8 + __O __H2O 6 + 4 = 10, so… 5 2 → __CO 2+4 8 • Ex: __C 2 4H10 +13 __O2 → __CO __H2O 16 + 10 = 26, so… 2 +10 ___ Al2O3 → ___ Al + ___ O2 ___ C7H14 + ___ O2 → ___ CO2 + ___ H2O ___ Na + ___ Cl2 → ___ NaCl ___ C2H4 + ___ O2 → ___ CO2 + ___ H2O ___ SO2 + ___ O2 → ___ SO3 ___ Al2S3 + ___ KCl → ___ AlCl3 + ___ K2S ___ NaMnO4 + ___ Ba → ___ Ba(MnO4)2 + ___ Na ___ C8H18 + ___ O2 → ___ CO + ___ H2O ___ CaO + ___ CrN → ___ Ca3N2 + ___ Cr2O3 ___ Al + ___ CuSO4 → ___ Al2(SO4)3 + ___ Cu