Chemistry Lecture Notes Writing and Balancing Chemical Equations

3H2(g) + N2(g)  2NH3(g) balanced (Dalton) Chemical Equations coefficients 3H2(g) + N2(g)  2NH3(g) balanced (Dalton) reactants products 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. other info

Balancing Chemical Equations Balancing chemical equations is generally done “by inspection,” but here are some common steps: Write the unbalanced equation (often it is written for you). Start by balancing the most complex substance (often but not always). Choose an element that appears in only one reactant and product (always). Balance by adjusting the coefficients so that there are the same number of atoms of each element on both sides. Balance polyatomic ions (if present) as a unit. Balance the remaining atoms, usually ending with the least complex substance, using fractional coefficients if necessary. 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.

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

Balancing Chemical Equations Common polyatomic ions OH–   hydroxide NH4+ ammonium CO32– carbonate HCO3– hydrogen carbonate (bicarbonate) SO32– sulfite HSO3– hydrogen sulfite (bisulfite) SO42– sulfate HSO4– hydrogen sulfate (bisulfate) PO33– phosphite PO43– phosphate HPO32– hydrogen phosphite HPO42– hydrogen phosphate H2PO3– dihydrogen phosphite H2PO4– dihydrogen phosphate NO2– nitrite NO3– nitrate CrO42– chromate Cr2O72– 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

Double Displacement Reactions 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) precipitate CB(aq) weak electrolyte or nonelectrolyte CB(g) gas C+B(aq) ionic (soluble) no net reaction net reaction has occurred

Double Displacement Reactions Precipitate-forming 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 CO32, PO43, OH, O2, and S2 ions except alkali metal and NH4+ salts (hydroxides and sulfides of Ca2+, Sr2+, and Ba2+ are slightly to moderately soluble)

Double Displacement Reactions Precipitate-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 Reactions Acids and bases 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% dissociated strong 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 Reactions Acid-base neutralization reactions 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 Reactions Gas-forming 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) correct equation correct equation

Double Displacement Reactions Gas-forming reactions Complete and balance the following reactions. CuS(s) + HCl(aq)  HCl(aq) + BaCO3(s)  NaOH(aq) + NH4NO3(aq) 