Chapter 9 Balancing Equations

Parts of a Reaction H 2 SO 3 (aq)  H 2 O (l) + SO 2 (g) ReactantsProducts l = liquid g = gas aq = aqueous (water solution) Mixture NaCl (aq) s or cr = crystalline solid

Arrows Ex. O 2  -- gas given off Ex. Ba(OH) 2  –-- precipitate formed (solid) in double displacement  -- yields or forms  -- reversible reaction (rxn)

Types of Reactions Video

Classifying Reactions Synthesis –Element + element  compound A + X  AX Ex. 2H 2 + O 2  2 H 2 O 4 Fe + 3 O 2  2 Fe 2 O 3

Decomposition –Compound  2 or more simpler substances AX  A + X Ex. 2 H 2 O 2  2 H 2 O + O 2  2 H 2 O  2H 2 + O 2

Single Displacement ( or replacement) –Element + compound  element + compound Nonmetal displaces nonmetal CY + D  CD + Y Metal displaces metal AX + B  BX + A Ex. Cu + 2 AgNO 3  Cu(NO 3 ) 2 +2Ag Ex. Cl KBr  2 KCl + Br 2

Double Displacement –compound+compound  compound + compound –Normally in aqueous solution –Forms a precipitate AX + BY  AY + BX Ex. CuCl NaOH  2NaCl + Cu(OH) 2 

Combustion –Also called oxidation –This is a burning reaction w/ oxygen –Produces heat, light and CO 2 and H 2 O –Hydrocarbon + oxygen  CO 2 + H 2 O –CH O 2  CO H 2 O

Balancing Equations Video

Balancing Equations Reaction equations give correct ratios between reactants and products Follows Law of Conservation of Mass (what was there b/4 must be there after)

Steps in Balancing: 1.Determine the reactants and products 2.Assemble the parts of the chemical equation Write the formulas properly (w/ the proper oxidation numbers) and (diatomics) Diatomics: H 2, O 2, N 2, I 2, F 2, Br 2, Cl 2

3. Balance the equation Use coefficients (# b/4 the symbol) to show equal # of atoms for each element on both sides of the equation Change only COEFFICIENTS NEVER, EVER touch the subscripts CaCO 3  subscript NEVER place coefficients in middle of formula Na3Cl You may need to use the least common multiple

4. Check: count up each element and make sure the # atoms in each of the reactants equals the # of atoms in each of the products

Water  hydrogen + oxygen

Lead (II) nitrate + sodium hydroxide  lead (II) hydroxide + sodium nitrate

Balancing combustion: Balance C (carbons) then H (hydrogens) then O (oxygens) If that doesn’t work and you get an odd number of O’s –Erase – double the hydrocarbon and do again (C, H, O)

C 8 H 18 + O 2  CO 2 + H 2 O C 8 H 18 + O 2  8 CO H 2 O 25 O’s 2 C 8 H O 2  16 CO H 2 O

Precipitates Check every double displacement reaction for precipitates (check products) Use chart, anything other than a “s” on chart is a precipitate place a down arrow (  ) after that compound –AgCl 