Chemistry Lecture Notes

Writing and BalancingChemical Equations

3H2(g) + N2(g) 2NH3(g) balanced (Dalton)

reactants products

coefficients

general:

A(l) + B(s) C(g) + D(aq) + heat

(s) = solid heat produced: exothermic(l) = liquid heat consumed: endothermic(g) = gas(aq) = aqueous other info: temperature, pressure, catalyst, etc.

otherinfo

Chemical Equations

Balancing chemical equations is generally done “by inspection,” but here are some common steps:

1. Write the unbalanced equation (often it is written for you).2. Start by balancing the most complex substance (often but not always).3. Choose an element that appears in only one reactant and product (always).4. Balance by adjusting the coefficients so that there are the same number of

atoms of each element on both sides.5. Balance polyatomic ions (if present) as a unit.6. Balance the remaining atoms, usually ending with the least complex

substance, using fractional coefficients if necessary.7. If fractional coefficients have been used, multiply both sides of the

equation by the denominator to obtain whole numbers.

Do not alter any formulas (change subscripts). Do not add or delete any reactants or products. Use the smallest set of whole numbers. Double check to be sure there are the same number of atoms of each

kind on both sides of the equation.

Balancing Chemical Equations

Balancing Chemical Equations

P4O10(s) + H2O(l) H3PO4(aq)

Cu(s) + AgNO3(aq) Cu(NO3)2(aq) + Ag(s)

Ca(s) + NH3(g) CaH2(s) + Ca3N2(s)

K(s) + H2O(l) KOH(aq) + H2(g)

The combustion of butane (C4H10)

OH–   hydroxide NH4+   ammonium

CO32–   carbonate HCO3

–   hydrogen carbonate (bicarbonate)

SO32–   sulfite HSO3

–   hydrogen sulfite (bisulfite)

SO42–   sulfate HSO4

–   hydrogen sulfate (bisulfate)

PO33–   phosphite PO4

3–   phosphate

HPO32–   hydrogen phosphite HPO4

2–   hydrogen phosphate

H2PO3–   dihydrogen phosphite H2PO4

–   dihydrogen phosphate

NO2–   nitrite NO3

–   nitrate

CrO42–   chromate Cr2O7

2–   dichromate

MnO4–   permanganate SCN–   thiocyanate

C2O42–   oxalate C2H3O2

–   acetate

CN–   cyanide OCN–   cyanate

ClO–   hypochlorite   ClO2–   chlorite   ClO3

–   chlorate ClO4– perchlorate

BrO–   hypobromite   BrO2–   bromite   BrO3

–   bromate BrO4– perbromate

IO–   hypoiodite   IO2–   iodite   IO3

–   iodate IO4– periodate

Balancing Chemical EquationsCommon polyatomic ions

Double displacement (metathesis) reactions: reactions in which anions and cations exchange partners

general: A+B(aq) + C+D(aq) A+D(aq) + CB(?)

if: CB(s) precipitateCB(aq) weak electrolyte or nonelectrolyteCB(g) gas

C+B(aq) ionic (soluble) no net reaction

net reactionhas occurred

Double Displacement Reactions

Solubility Rules: • Soluble compounds

• all salts of alkali metal ions and the NH4+ ion

• all salts of NO3, C2H3O2

, ClO4 ions

• most salts of Cl, Br, and I ions• except Ag+, Pb+2, and Hg2

+2 salts

• most salts of the SO42 ion

• except Sr+2, Ba+2, Pb+2, and Hg2+2 salts

(CaSO4 is slightly soluble)• Insoluble compounds

• most salts of CO32, PO4

3, OH, O2, and S2 ions

• except alkali metal and NH4+ salts

(hydroxides and sulfides of Ca2+, Sr2+, and Ba2+ are slightly to moderately soluble)

Double Displacement ReactionsPrecipitate-forming reactions

Complete and balance the following reactions in aqueous solution, indicating the state of each reactant and product:

AgNO3 + NaBr

CuCl2 + KOH

Ba(NO3)2 + Na2SO4

Zn(C2H3O2)2 + Na2S

Double Displacement ReactionsPrecipitate-forming reactions

Arrhenius: acid - produces H3O+(aq) in water (formula starts with an H)base - produces OH(aq) in water (OH– salts or amines)

HCl(aq) H3O+(aq) + Cl(aq) 100% dissociatedstrong acid

HC2H3O2(aq) H3O+(aq) + C2H3O2(aq) 0.4% dissociated

weak acid

NaOH(aq) Na+(aq) + OH(aq) 100% strong base

NH3(aq) NH4+(aq) + OH(aq) 0.4% weak base

Double Displacement ReactionsAcids and bases

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l) acid base salt water (nonelectrolyte)

Complete and balance the following reactions:

HClO4(aq) + KOH(aq)

HNO3(aq) + Ca(OH)2(aq)

Double Displacement ReactionsAcid-base neutralization reactions

Reactions that produce gases: H2S, NH3, CO2

K2S(aq) + 2HNO3(aq) H2S(g) + 2KNO3(aq)

NH4Cl(aq) + KOH(aq) NH4OH(aq) + KCl(aq)

but: NH4OH(aq) NH3(g) + H2O(l)

NH4Cl(aq) + KOH(aq) NH3(g) + H2O(l) + KCl(aq)

Na2CO3(aq) + 2HCl(aq) H2CO3(aq) + 2NaCl(aq)

but: H2CO3(aq) CO2(g) + H2O(l)

Na2CO3(aq) + 2HCl(aq) CO2(g) + H2O(l) + 2NaCl(aq)

correctequation

correctequation

Double Displacement ReactionsGas-forming reactions

Complete and balance the following reactions.

CuS(s) + HCl(aq)

HCl(aq) + BaCO3(s)

NaOH(aq) + NH4NO3(aq)

Double Displacement ReactionsGas-forming reactions