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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 ) = solidheat produced: exothermic

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Chemistry Lecture Notes Writing and Balancing Chemical Equations

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Chemistry lecture notes writing and balancing chemical equations

Chemistry Lecture Notes

Writing and Balancing

Chemical Equations


Chemistry lecture notes writing and balancing chemical equations

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) = solidheat produced: exothermic

(l) = liquidheat consumed: endothermic

(g) = gas

(aq) = aqueousother info: temperature, pressure, catalyst, etc.


Chemistry lecture notes writing and balancing chemical equations

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.


Chemistry lecture notes writing and 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)


Chemistry lecture notes writing and balancing chemical equations

  • Balancing Chemical Equations

    • Common polyatomic ions


Chemistry lecture notes writing and balancing chemical equations

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


Chemistry lecture notes writing and balancing chemical equations

  • 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)


Chemistry lecture notes writing and balancing chemical equations

  • 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


Chemistry lecture notes writing and balancing chemical equations

  • 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


Chemistry lecture notes writing and balancing chemical equations

  • 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) 


Chemistry lecture notes writing and balancing chemical equations

  • 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


Chemistry lecture notes writing and balancing chemical equations

  • 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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