Answers for the reactions

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Reaction Answers by Year:

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

1985

a) Na + H₂O Na⁺ + OH^- + H₂

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

c) C₂H₅OH + HCOOH HCOOC₂H₅ + H₂O

d) OH^-+ Zn(OH)₂ Zn(OH)₄^2- (or Zn(OH)₃^- or ZnO₂^2- + H₂O)

e) BF₃ + NH₃ BF₃NH₃

f) Sn²⁺ + Fe³⁺ Sn⁴⁺ + Fe²⁺

g) POCl₃ + H₂O H₃PO₄ + H⁺ + Cl^-

h) MnO₄^- + SO₃^2- + H⁺ Mn²⁺ + SO₄^2- + H₂O

HSO₃^- and HSO₄^- were accepted.

(Part credit if H⁺ and H₂O were omitted)

1986

a) Li + N₂ Li₃N

b) H⁺ + SO₃^2- HSO₃^- (or H₂SO₃ or SO₂ + H₂O)

c) Na₂O + H₂O Na⁺ + OH^-

d) Zn²⁺ + S^2- ZnS

(or Zn²⁺ + HS^- ZnS + H⁺)

e) NH₃ + HC₂H₃O₂ NH₄⁺ + C₂H₃O₂^-

f) Fe³⁺ + Fe Fe²⁺

g) C₂H₄ + Br₂ C₂H₄Br₂

h) Cl₂ + I^- I₂ + Cl^-

1987

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)₆^3-, 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^- PO₄^3- + 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

1988

a) H⁺ (or H₃O⁺) + I^- + Cr₂O₇^2- 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⁺ + CO₃^2- CO₂ + H₂O (or H₂CO₃ or HCO₃^-)

f) SO₃ + H₂O H⁺ + HSO₄^- (or H⁺ + SO₄^2-)

g) Ba²⁺ + SO₄^2- 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)

1989

a) Zn²⁺ + PO₄^3- Zn₃(PO₄)₂

b) Ag⁺ + Br^- AgBr

c) Cl₂ + OH^- OCl^- + Cl¯ + H₂O

d) H⁺ + SO₃^2- H₂O + SO₂ (or H₂SO₃)

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

f) Fe³⁺ + SCN^- Fe(SCN)²⁺ (or Fe(SCN)₆^3-)

g) BCl₃ + NH₃ Cl₃BNH₃

h) CS₂ + O₂ CO₂ + SO₂ (or SO₃)

1990

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⁺ + CrO₄^2- Ag₂CrO₄

e) H₂S + OH^- S^2- (or HS¯) + H₂O

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

g) Bi + O₂ Bi₂O₃ (Bi₂O₅ = one point)

h) Cu + H⁺ + HSO₄^- (or SO₄^2-) 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)

1991

a) Al₂O₃ + OH^- Al(OH)₄^-

OR

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⁺

OR

Zn²⁺ + NH₃ + H₂O Zn(OH)₂ + NH₄⁺

h) Cu²⁺ + SO₄^2- + 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⁺ + SO₄^2- BaSO₄

1992

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^-

1993

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⁺ + PO₄^3- Zn₃(PO₄)₂

1994

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⁺ + S^2- 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: CO₃^2- + H₂O as products earns two product points. CO₃^2- 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) Cr₂O₇^2- + 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.

1995

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⁺ + SO₄^2-, or Ca²⁺ + HSO₄^-, or Ca(HSO₄)₂

f) H₂S + Hg²⁺ HgS + H⁺

OR

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

1996

(a) CaCO₃ CaO + CO₂

(b) Ni + Cu²⁺ Ni²⁺ + Cu

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

(c) HPO₄^2- + 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^- + C₂O₄^2- MnO₂ + CO₂

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

1997

(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^- + SO₃^2-)

(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⁺ + S^2- NiS

Ni²⁺ + H₂S NiS + H⁺

Ni²⁺ + HS^- NiS + H⁺

1998

(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⁺

Some of these practice problems came from a great site called The Chem Team. Go visit The Chem Team Site for many more practice problems, tutorials, and resources.