The Canadian Chemistry Contest — examples of questions

If you have a student who really likes chemistry and excels in your class, you may want to know more about the kind of questions asked in our national competition. This article presents four examples of questions that were given in part A (multiple choice) and part B (essay-type) of the 2012 Canadian Chemistry Contest (CCC). Solutions to part A examples are provided. To access former contests and their answers, visit the CCC website.

The CCC is available in either English or French to all secondary schools and cégeps in Canada, and it is intended for the top 10% of chemistry students. Its part A consists of 25 multiple-choice questions. Students must also answer two essay questions (chosen from four possibilities) in part B. Money prizes for CCC (national and regional 1st, 2nd and 3rd positions) are awarded in six regions of Canada: Atlantic, Québec, Ontario, Manitoba-Saskatchewan-Nunavut, Alberta-NWT, and BC-Yukon. You can consult the CCC website for more information.

If you have a student who may be tempted by the CCC challenge, mark down April 16 as the date for the 2013 edition of CCC. Watch for information in future issues of Chem 13 News or go to our website.

Part A examples:  multiple-choice questions

Example 1 (question 13 of CCC 2012)

The sour taste of vinegar is due to acetic acid, CH3COOH. When aqueous acetic acid reacts with aqueous sodium hydroxide in a titration, the products of the net ionic equation are:

A.  CH₃COOH₂⁺, Na₂O

B.  CH₃COOH, H₂O, Na⁺

C.  CH₃COO^–, Na⁺

D.  CH₃COO^–, H₂O

E.  CH₃COOH, NaOH

(ANSWER D)

Detailed solution

Acetic acid (CH₃COOH) is a weak acid, but if it reacts with a strong base like sodium hydroxide (NaOH), the reaction between them is complete. Sodium acetate (CH₃COONa) and water are produced. The molecular equation (or formula equation) representing the reaction is:

CH₃COOH(aq)  +  NaOH(aq)  → CH₃COONa(aq)  +  H₂O(l)

For a better description of the reaction, we can write the complete ionic equation in which strong electrolytes are shown as fully ionized species while weak electrolytes stay in their molecular form. In our reaction, strong electrolytes are NaOH (a strong base) and CH₃COONa (a sodium salt). These are fully ionized in water. Acetic acid is a weak electrolyte (weak acid) so it is represented by its entire formula, CH₃COOH, in the equation. Altogether, we get:

CH₃COOH(aq) + Na⁺(aq) + OH^–(aq)  →    CH₃COO^–(aq) + Na⁺ (aq) + H₂O(l)

One can see that we have three product species: CH₃COO^–, Na⁺ and H₂O. However, the Na⁺ ion appears on both sides of the equation. It remains unchanged during the process; it is a spectator ion. The net ionic equation essentially represents the species that are actually reacting:

CH₃COOH(aq) + OH^–(aq)  → CH₃COO^–(aq) + H₂O(l)

So the products are CH₃COO^– and H₂O (answer D).

Example 2 (question 18 of CCC 2012)

In 1774, Swedish chemist Carl Wilhelm Scheele produced molecular chlorine gas from the reaction between sodium chloride, sulfuric acid, and manganese(IV) oxide shown below:

4NaCl(aq) + 2H₂SO₄(aq) + MnO₂(s)  →  2Na₂SO₄(aq)  +  MnCl₂(aq)  +  2H₂O(l)  +  Cl₂(g)

To reproduce Scheele’s experiment, 50.0 mL of 2.00 mol L–1 NaCl is mixed with 25.0 mL of 6.00 mol L–1 H₂SO₄, and 4.35 g of MnO₂. What volume of Cl₂ gas will be produced at  101.325 kPa pressure and a temperature of 25.0 °C?

A.   0.611 L Cl₂ (g)       B.  1.22 L Cl₂ (g)          C.  1.83 L Cl₂ (g)

D.  2.44 L Cl₂ (g)          E.  3.67 L Cl₂ (g)          (ANSWER A)

Detailed solution

This is a typical “limiting reactant” situation in which the focus is put on one of the products: dichlorine gas, Cl₂. Stoichiometric details of this reaction can be represented in the following table.

Where M = molar mass, C = Concentration, V = volume, and n.r. = not required

With these initial amounts of reactants in our table, we can clearly see that:

• NaCl is the limiting reactant since it would react with
  0.0500 mol H₂SO₄, and 0.0250 mol MnO₂; and produce 0.0250 mol Cl₂. The 4:2:1:1 molar proportions for NaCl:H₂SO₄:MnO₂:Cl₂ are respected this way, as shown in the table.
• there are 0.100 mol H₂SO₄ and 0.0250 mol of MnO₂ in excess, and
• the theoretical yield of Cl₂ is 0.0250 mol.

From this last value, we can calculate the volume of Cl₂ gas produced using the ideal gas law. The answer 0.611 L Cl₂(g) using pressure as 101.325 kPa and the temperature as 25.0oC (298.1 K).

Part B examples:  essay questions

Example 1 (question 1 of CCC 2012)

Experiment Design: Determining the K_sp of sodium chloride

Describe an experiment you could perform to determine the solubility product constant (K_sp) of sodium chloride. For this experiment, you are provided with an unlimited supply of pure solid sodium chloride and all standard high school laboratory equipment.

You should begin with an introductory paragraph which outlines the experiment and associated theory and includes an explanation of the following terms: salts, solubility, and solubility product constant. Following this, include a procedure which indicates the required steps, the results that need to be collected and an outline of the required calculations. Given that the actual K_sp of sodium chloride is 37.71, indicate the results you would expect to find if your proposed procedure produced very accurate results. You should conclude with a paragraph outlining any problems you may encounter, and applications of determining solubility product constants or molar solubility values in everyday life. Your application examples may be specific to sodium chloride or for the values of other salts.

Example 2 (question 3 of CCC 2012)

Fundamental chemical principles

Throughout your study of chemistry, you have explored a variety of chemical principles. Identify what you believe to be the three most important chemical principles. You should start with an introductory paragraph describing what a principle is and identifying the three principles you believe are most important in chemistry, follow with at least one descriptive paragraph for each principle highlighting why the principle is fundamental to chemistry with specific examples of the use of the principle, and finish with a concluding paragraph. Include diagrams, structures and chemical equations to illustrate your points.