Chapter 9Balancing Equations

Parts of a Reaction• H2SO3 (aq) H2O (l) + SO2 (g)

Reactants Products

•l = liquid

•g = gas

•aq = aqueous (water solution)

•Mixture

•NaCl (aq)

•s or cr = crystalline solid

Arrows• Ex. O2

• -- gas given off

• Ex. Ba(OH)2 – -- precipitate formed (solid) in double displacement

-- yields or forms

-- reversible reaction (rxn)

Types of ReactionsVideo

Classifying Reactions• Synthesis

– Element + element compound

A + X AX

Ex. 2H2 + O2 2 H2O

4 Fe + 3 O2 2 Fe2O3

• Decomposition– Compound 2 or more simpler substances

AX A + X

Ex. 2 H2O2 2 H2O + O2

2 H2O 2H2 + O2

• 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 AgNO3 Cu(NO3)2 +2Ag• Ex. Cl2 + 2 KBr 2 KCl + Br2

• Double Displacement– compound+compound compound + compound– Normally in aqueous solution– Forms a precipitate

+ - + - + - + -

AX + BY AY + BX

Ex. CuCl2 + 2 NaOH 2NaCl + Cu(OH)2

• Combustion– Also called oxidation– This is a burning reaction w/ oxygen

– Produces heat, light and CO2 and H2O

– Hydrocarbon + oxygen CO2 + H2O

– CH4 + 2 O2 CO2 + 2 H2O

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: H2, O2, N2, I2, F2, Br2, Cl2

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

• CaCO3 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)

C8H18 + O2 CO2 + H2O

C8H18 + O2 8 CO2 + 9 H2O

25 O’s

2 C8H18 + 25O2 16 CO2 + 18H2O

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