Question
Answer FIVE of the eight options in this part. (Answers to more than five options will not be scored.) Give the formulas to show the reactants and the products for FIVE of the following chemical reactions. Each of the reactions occurs in aqueous solution unless otherwise indicated. Represent substances in solution as ions if the substance is extensively ionized. Omit formulas for any ions or molecules that are unchanged by the reaction. In all cases a reaction occurs. You need not balance.
Example: A strip of magnesium is added to a solution of silver nitrate.
Mg + Ag+ → Mg2+ + Ag
(a) Excess sodium cyanide solution is added to a solution of silver nitrate.
(b) Solutions of manganese(II) sulfate and ammonium sulfide are mixed.
(c) Phosphorus(V) oxide powder is sprinkled over distilled water.
(d) Solid ammonium carbonate is heated.
(e) Carbon dioxide gas is bubbled through a concentrated solution of potassium hydroxide.
(f) A concentrated solution of hydrochloric acid is added to solid potassium permanganate.
(g) A small piece of sodium metal is added to distilled water.
(h) A solution of potassium dichromate is added to an acidified solution of iron(II) chloride.
▶️Answer/Explanation
Ans:
Guiding principles :
1) Each reaction is worth a total of 3 points
2) Reactants +1 point; products +2 points
3) Ignore balancing and states
4) Inappropriate ionization – maximum 1 point penalty per equation
(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 1 product point
(b) Mn2+ + S2- → MnS
Note: If Mg used instead of Mn, maximum possible score is 2 points
(c) P4O10 (or P2O5) + H2O → H3 PO4
Note: Acidic species (H+ or oxyacid of phosphorus) earns 1 product point; P in +5 oxidation state in oxyanion earns 1 product point; anions of oxyacids of phosphorus require H+ for full credit for products
(d) (NH4 )2CO3 – NH3 + H2O + CO2
Note: Any one product earns 1 point; all three earns 2 product points
NH4OH + CO2 earns 1 product point
NH3 + H2CO3 earns 1 product point
(e) CO2 + OH– → HCO3–
Note: CO32- + H2O as products earns 2 product points
CO32- alone as product earns 1 product point
HCO3– + H2O earns 1 product point
(f) H+ + Cl– + KMnO4 → K+ + Mn2+ + Cl2 + H2O
Note: HCl and MnO4– acceptable as reactants Any valid redox product earns 1 point All four products earns 2 points
K+ and/or H2O only as products earns no credit
If both H+ and H2O omitted, then maximum of 2 points possible
(g) Na + H2O → H2O + Na+ + OH–
Note: All three products earns 2 product points Any vaid redox product earns 1 product point
(h) Cr2O72- + Fe2+ + H+‘ → Cr3+ + Fe3+ + H2O
Note: All three products earns 2 product points
Any valid redox product earns 1 product point
H2O only earns no credit
If Cl– → Cl2 instead of Fe2+ → Fe3+, then maximum of 2 points possible