For each of the following three reactions, in part (i) write a balanced equation for the reaction an — Chemical Reactions Chemistry Question
Problem Context
For each of the following three reactions, in part (i) write a balanced equation for the reaction and in part (ii) answer the question about the reaction. In part (i), coefficients should be in terms of lowest whole numbers. Assume that solutions are aqueous unless otherwise indicated. Represent substances in solutions as ions if the substances are extensively ionized. Omit formulas for any ions or molecules that are unchanged by the reaction. You may use the empty space at the bottom of the next page for scratch work, but only equations that are written in the answer boxes provided will be scored.
Model Answer
### (i) Balanced equation:
HC2H3O2 + OH− → C2H3O2− + H2O
Scoring Guidelines:
- 1 point is earned for the correct reactants.
- 2 points are earned for the correct products.
- 1 point is earned for balancing the equation for mass and charge.
### (ii) Explanation:
The pH will be greater than 7. HC2H3O2 is a weak acid and LiOH is a strong base. At the equivalence point, the C2H3O2− is the pH-determining species because of the reaction, C2H3O2− + H2O ⇄ HC2H3O2 + OH−.
Scoring Guidelines:
- 1 point is earned for the correct answer with an explanation.
Model Answer
### (i) Balanced equation:
2 Fe3+ + Sn2+ → 2 Fe2+ + Sn4+
OR
2 Fe3+ + 3 Sn2+ → 2 Fe + 3 Sn4+
Scoring Guidelines:
- 1 point is earned for the correct reactants.
- 2 points are earned for the correct products.
- 1 point is earned for balancing the equation for both mass and charge.
### (ii) Explanation:
Fe3+ is reduced.
Scoring Guidelines:
- 1 point is earned for correct answer.
Model Answer
### (i) Balanced equation:
3 Ag+ + PO43− → Ag3PO4
Scoring Guidelines:
- 2 points are earned for the correct reactants.
- 1 point is earned for the correct product.
- 1 point is earned for balancing the equation for both mass and charge.
### (ii) Explanation:
Nine moles of Ag+ would be needed to react completely with the three moles of PO43−, so sodium phosphate is present in excess.
Scoring Guidelines:
- 1 point is earned for a correct answer with reference to reactant data.