Answers for the reactions

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Reaction Answers by Year:

1985 1986 1987 1988 1989 1990
1991 1992 1993 1994 1995 1996
1997 1998

Credit: 1 point for each set of reactants, 2 points for products. (If two products, 1 point for each product.)


a) Na + H2O Na+ + OH- + H2

b) H+ + HCO3- H2O + CO2 (Part credit for H2CO3)


d) OH-+ Zn(OH)2 Zn(OH)42- (or Zn(OH)3- or ZnO22- + H2O)

e) BF3 + NH3 BF3NH3

f) Sn2+ + Fe3+ Sn4+ + Fe2+

g) POCl3 + H2O H3PO4 + H+ + Cl-

h) MnO4- + SO32- + H+ Mn2+ + SO42- + H2O

HSO3- and HSO4- were accepted.

(Part credit if H+ and H2O were omitted)


a) Li + N2 Li3N

b) H+ + SO32- HSO3- (or H2SO3 or SO2 + H2O)

c) Na2O + H2O Na+ + OH-

d) Zn2+ + S2- ZnS

(or Zn2+ + HS- ZnS + H+)

e) NH3 + HC2H3O2 NH4+ + C2H3O2-

f) Fe3+ + Fe Fe2+

g) C2H4 + Br2 C2H4Br2

h) Cl2 + I- I2 + Cl-


a) Ca + H2O Ca(OH)2 + H2

Ca2+ + OH- earns one point

b) MgO + CO2 MgCO3

c) H2S + Ni2+ NiS + H+

d) OH- + Al(OH)3 Al(OH)4-

other acceptable answers: Al(OH)63-, AlO2-, Al(OH)4(H2O)2-

e) Ag + H+ + NO3- Ag+ + NO (or NO2) + H2O

1 product = 1 pt, 2 products = 1 pt, 3 products = 2 pts

f) OH- + H2PO4- PO43- + H2O

g) H2O2 + Fe2+ Fe3+ + H2O

Fe(OH)3 only as a product earns one point (Note: the scoring standard on this question has a two next to the formula, but in context of below, a one seems more appropriate)

h) C3H7OH + O2 CO2 + H2O


a) H+ (or H3O+) + I- + Cr2O72- I2 + Cr3+ H2O

(Any reasonable I- oxidation product accepted.)

b) OH- + NH4+ H2O + NH3 (or NH4OH)

c) Mg + Ag+ Mg2+ + Ag

d) KClO3 KCl + O2

(MnO2 is also shown over the reaction arrow in the original.)

e) H+ + CO32- CO2 + H2O (or H2CO3 or HCO3-)

f) SO3 + H2O H+ + HSO4- (or H+ + SO42-)

g) Ba2+ + SO42- BaSO4

(or Ba2+ + HSO4- BaSO4 + H+)

h) Cu2+ + NH3 Cu(NH3)42+

(Partial credit for other logical Cu(II) ammonia complexes and for Cu(OH)2)


a) Zn2+ + PO43- Zn3(PO4)2

b) Ag+ + Br- AgBr

c) Cl2 + OH- OCl- + Cl¯ + H2O

d) H+ + SO32- H2O + SO2 (or H2SO3)

e) Sn2+ + H+ + MnO4- Sn4+ + Mn2+ + H2O

f) Fe3+ + SCN- Fe(SCN)2+ (or Fe(SCN)63-)

g) BCl3 + NH3 Cl3BNH3

h) CS2 + O2 CO2 + SO2 (or SO3)


a)* Pb2+ + 2I- PbI2

b) Fe3+ + NH3 + H2O Fe(OH)3 + NH4+ (NH4OH is OK in place of NH3 + H2O)

Fe2+ + others above Fe(OH)2 + NH4+ (given two points)

c) H2O2 H2O + O2

d)* Ag+ + CrO42- Ag2CrO4

e) H2S + OH- S2- (or HS¯) + H2O

f) N2O5 + H2O H+ + NO3- (HNO3 = one point)

g) Bi + O2 Bi2O3 (Bi2O5 = one point)

h) Cu + H+ + HSO4- (or SO42-) Cu2+ + SO2 + H2O (any two products is given one point; all three must be there for 3 points; omitting H+ and/or H2O is minus one point)

(*Correct equation but wrong charge on ion(s) = one point)


a) Al2O3 + OH- Al(OH)4-


Al2O3 + H2O Al(OH)3

(Personal note by John Park: I think the H2O in the second equation above comes from the fact that the NaOH concentration was not specified in the original problem. For example, suppose [OH-] were 10¯12 M. Then the second equation becomes the predominate one.)

b) CaO + SO3 CaSO4

c) H+ + OH- H2O

d) Ca + N2 Ca3N2

e) CuS + O2 CuO + SO2

f) H+ + Cl- + MnO2 Mn2+ + Cl2 + H2O (one pt. for either redox product, two pts. for all three products)

g) Zn2+ + NH3 Zn(NH3)42+


Zn2+ + NH3 + H2O Zn(OH)2 + NH4+

h) Cu2+ + SO42- + Ba2+ + OH- Cu(OH)2 + BaSO4

A rare double precipitation. Partial credit was allowed for some alternate solutions, e.g.

Cu2+ + OH- Cu(OH)2

Ba2+ + SO42- BaSO4


a) Mg2+ + 2 OH- Mg(OH)2

b) LiH + H2O Li+ + OH- + H2

OH- or H2 earns one point, all three for two points.

c) NH3 + HF NH4+ + F-

NH3 + H+ NH4+ earns two points

d) Al + Ag+ Al3+ + Ag

e) I- + H2O I2 + H2 + OH-

I2 or H2 (one point); all three products for two points

f) K2O + H2O K+ + OH-

KOH product alone is one point.

g) H+ + Cu(NH3)42+ Cu2+ + NH4+

h) CaCO3 + CO2 + H2O Ca2+ + HCO3-


a) Cu + H+ + NO3- Cu2+ + NO + H2O (1 pt for either Cu2+ or NO; NO2 also accepted; 2 pts for all three.)

b) MnO4- + H2O2 Mn2+ + O2 + H2O (1 pt for either Mn2+ or O2; 2 pts for all three)

c) H+ + MnS H2S + Mn2+

d) Fe + Cl2 FeCl3

e) Mg3N2 + H2O Mg(OH)2 + NH3 (Mg2+ + OH- also accepted)

f) SO2 + OH- HSO3-

g) AgCl + NH3 Ag(NH3)2+ + Cl- (other coordination numbers also accepted)

h) Zn2+ + PO43- Zn3(PO4)2


a) CN- + Ag+ Ag(CN)2-

Note: any complex ion of Ag+ with cyanide with consistent charge earns 3 points; AgCN given as product earns one product point.

b) Mn2+ + S2- MnS

Note: If Mg is used instead of Mn, maximum possible score is two points.

c) P4O10 (or P2O5) + H2O H3PO4

Note: Acidic species (H+ or oxyacid of phosphorous) earns one product point; P in +5 oxidation state in oxyanion earns one product point; anions of oxyacids of phosphorous require H+ for full credit for products.

d) (NH4)2CO3 NH3 + H2O + CO2

Note: any one product earns one point; all three earn two points. NH4OH + CO2 earns one product point. NH3 + H2CO3 eans one product point.

e) CO2 + OH- HCO3-

Note: CO32- + H2O as products earns two product points. CO32- alone as product earns one product point. HCO3- + H2O earns one product point.

f) H+ + Cl- + KMnO4 K+ + Mn2+ + Cl2 + H2O

Note: HCl and MnO4- acceptable as reactants. Any valid redox product earns one point. All four product earns two points. K+ and/or H2O only as products earns no credit. If both H+ and H2O omitted, then maximum of two points possible.

g) Na + H2O H2 + Na+ + OH-

Note: all three products earn two product points. Any valid redox product earns one product point.

h) Cr2O72- + Fe2+ + H+ Cr3+ + Fe3+ + H2O

Note: All three products earn two product points. Any valid redox product earns one product point. H2O only earns no credit. If Cl- Cl2 instead of Fe2+ Fe3+, then maximum of two points possible.


a) C2H5OH + O2 CO2 + H2O

CO acceptable as oxidized form of carbon; C acceptable as product if accompanied be CO or CO2

b) BaO + H2O Ba2+ + OH-

Only 1 product point awarded for Ba(OH)2

c) Cl2 + OH- Cl- + ClO¯ (+ H2O)

both an oxidized and a reduced form of Cl necessary for 2 product points; H2O not necessary as product; both ClO- and ClO2- acceptable as oxidized forms of Cl 1 point deducted if acidic products shown (e.g., H+, HClO, HCl)

d) Fe2+ + O2 (+ H2O) Fe2O3 or FeO(OH) or Fe(OH)3

Fe3+, Fe3+ + OH-, and FeO2 or Fe3O4 awarded only 1 product point

e) H2SO4 + Ca3(PO4)2 H3PO4 + CaSO4

no ionized form of H3PO4 acceptable; CaSO4 may appear as Ca2+ + SO42-, or Ca2+ + HSO4-, or Ca(HSO4)2

f) H2S + Hg2+ HgS + H+


H2S + HgCl2 HgS + H+ + Cl-

If reactant is HgCl2, products must include Cl-

g) CaH2 + H2O Ca2+ + OH- (or Ca(OH)2) + H2

no ionized form of CaH2 is acceptable; no H- as a reactant

h) Zn + Cu2+ Zn2+ + Cu


(a) CaCO3 CaO + CO2

(b) Ni + Cu2+ Ni2+ + Cu

hydrated ions acceptable with correct charge; 1 point for Ni(OH)2 as product

(c) HPO42- + H+ H2PO4-

incorrect charge on H2PO4- when only one product occurs, 1 point only; 1 product point for transfer if H+ from an ionic reactant to product when a phosphate species is incorrectly but consistently written.

(d) Cl2 + Br- Cl- + Br2

no credit for monatomic Cl as reactant or Br as product

(e) NH3 + HC2H3O2 C2H3O2- + NH4+

1 product point for NH4C2H3O2; 1 point for NH3 + H+ NH4

(f) (NH4)2CO3 + Ba2+ + OH- NH3 + BaCO3 + H2O

1 product point for either NH3 or BaCO3; 2 product points for all three species correct

(g) N2O3 + H2O HNO2

1 product point for H+ + NO2-

(h) MnO4- + C2O42- MnO2 + CO2

no penalty for OH- or H2O in equation; no point earned for Mn2+ as product


(a) Al3+ + OH- Al(OH)3

Other acceptable products: Al(OH)4-; Al(OH)4(H2O)2-; Al2O3xH2O; AlO2-

(b) H+ + Br- + BrO3- Br2 + H2O

(c) SO2 + H2O H2SO3 (or H+ + HSO3- or H+ + HSO3- + SO32-)

(d) PH3 + BCl3 H3PBCl3

(e) H2 + FeO H3O + Fe

(f) KNH2 + H2O NH3 + OH- + K+ (or NH4OH + OH- + K+)

(g) Mg + N2 Mg3N2

(h) Ni2+ + S2- NiS

Ni2+ + H2S NiS + H+

Ni2+ + HS- NiS + H+


(a) Sn2+ + Fe3+ Sn4+ + Fe2+

(b) Co2+ + OH- Co(OH)2 (s)

(c) C2H4 + O2 CO2 + H2O

(d) H3PO4 + OH- H2PO4- + H2O

(e) CaSO3 CaO + SO2

(f) H+ + Cl- + Ag(NH3)2+ NH4+ + AgCl

(AgCl2- as a product is possible if the HCl is concentrated and in excess)

(g) Na2O + H2O Na+ + OH-

(h) Zn + H+ H2 + Zn2+

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