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Chemistry Lecture Notes Writing and Balancing Chemical Equations

Chemistry Lecture Notes Writing and Balancing Chemical Equations. other info. Chemical Equations. coefficients. 3H 2 ( g ) + N 2 ( g )  2NH 3 ( g ) balanced (Dalton). reactants. products. general: A( l ) + B( s ) C( g ) + D( aq ) + heat ( s ) = solid heat produced: exothermic

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Chemistry Lecture Notes Writing and Balancing Chemical Equations

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  1. Chemistry Lecture Notes Writing and Balancing Chemical Equations

  2. other info 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.

  3. 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.

  4. 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)

  5. Balancing Chemical Equations • Common polyatomic ions

  6. 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

  7. 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)

  8. 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

  9. 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

  10. 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) 

  11. 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

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

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