Chemistry Structured Questions (Internal) Exam

Topics to work on

• Moles and quantitave chemistry
  • Converting mass to particles via Avogadro’s constant
  • Multi-step titration calculations and volumetric unit conversions
• Practical titrations and salt preparation
  • Distinguishing chemical reaction steps (neutralization) from physical separation steps (crystallization)
  • Purification techniques: Using activated charcoal/carbon to absorb indicator dye before crystallization.
• Enthalpy and energetics
  • Counting total individual bonds broken/formed
• Collision theory and exam terminology
  • Explicitly including activation energy, Eₐ, and collision frequency (collisions per unit time) when explaining temperature effects on reaction rate.
• Experimental design and thermometric titrations
  • Describing specific trends *(e.g. “higher temperature increase”) rather than vague impact terms like “affects the temperature”.
  • Explaining post-endpoint temperature drops (reaction completion + excess cold reactant/heat loss).
  • Equipment trade-offs: Burette vs. measuring cylinder

Structured questions

1. (c) Complete Table 1.2.
   • Correct answers (format: x(atomn) y(protn) El(elem) e+/-(electrons))
   • Feedback: You wrote negative for the electrons, which is wrong because there are more protons than electrons. Protons give a positive charge.

2. (e) Calculate the number of atoms in 2.00g of argon. Give your answer in standard form.

   • Correct working:
     • First, find the molar mass (Mr) of Argon. It is approximately 39.95 g/mol.
     • Second, calculate the number of moles. Use the formula:
       • mol = mass / Mr
       • mol = 2.00g / 39.95g/mol = 0.0501 mol
     • Third, calculate the number of atoms. You must multiply the number of moles by Avogadro’s constant, 6.02 x 10²³ atoms/mol.
       • no. of atoms = no. of mol x 6.02 x 10²³
       • no. of atoms = 0.050065 mol x (6.02 x 10²³) = 3.0149 x 10²² atoms
     • Fourth, express in standard form.
       • 3.01 x 10²² atoms
   • Feedback: You went straight into calculating the number of atoms using 2.00g instead of calculating the number of moles. You can’t calculate the number of atoms using the value 2.00g because Avogadro’s constant uses atoms/mol, not atoms/g.

3. (a) Complete the symbol equation for the reaction. Include state symbols.

   • Correct answer:
     • H₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) +2H₂O(l)
   • Feedback: You were missing the 2 in 2H₂O.Your symbol equation would be unbalanced without the 2 in front of the H₂O.

4. (b) State the type of exothermic reaction taking place.

   • Correct answer: Neutralization.
     • Neutralization is an acid reacting with a base to form a salt and water is specifically called a neutralization reaction.
     • This fits the symbol equation of sulfuric acid reacting with sodium hydroxide as there is an acid reacting with a base to form sodium sulfate, the salt, and the water (2H₂O).
   • Feedback: Initially, you wrote “crystallization” due to the phrase “dry crystals” in step 4. This would be right IF the question specifically asked for the process in step 4. The question asked for the type of REACTION. The actual chemical reaction occurs in step 3, where the H₂SO₄ is mixed with NaOH to form the salt and water.

5. (e) Calculate the volume of H₂SO₄(aq), in cm³, added in step 3.

   • Correct answer:
     • First, find the moles of H₂SO₄ reacted:
       • From the balanced equation, the molar ratio of H₂SO₄ : NaOH is 1 : 2.
       • mol of H₂SO₄ = mol of NaOH / 2
     • Second, calculate the volume in dm³.
       • Given that the concentration of H₂SO₄ is 0.200 mol/dm³, we can use the volume formula to find the answer.
       • vol = mol / conc = 0.00500 mol / 0.200 mol/dm³ = 0.0250 dm³
     • Third, convert the volume to cm³.
       • vol in cm³ = 0.0250 x 1000 = 25.0 cm³
     • The answer is 25.0 cm³.

6. (g) The dry crystals formed in step 4 are colored and not white. This is because the student should do an additional step between step 3 and step 4. Suggest what the student should do in this additional step to produce white crystals.

   • Correct answer: Add activated charcoal (carbon) to the solution to absorb the indicator, and filter the mixture to remove the charcoal before evaporating the solution.
     • In order to obtain pure white crystals, the student needs to remove the indicator before evaporating.
   • Feedback: You wrote “heat the crystals so it becomes anhydrous.” Both hydrated sodium sulfate (Na₂SO₄ • 10H₂O) and anhydrous sodium sulfate (Na₂SO₄) are NATURALLY white. The crystals turned colored due to the methyl orange indicator added in step 2, which remained in the solution when it was evaporated. Therefore, heating the crystals will NOT remove or decolorize the indicator dye.

7. (b) The equation for the reaction can be represented as shown in Fig 4.2. Table 4.1 shows some bond energies. Use the bond energies in Table 4.1 and the value of ΔH of the reaction to calculate the S=O bond energy in kJ/mol.

   • Correct answers:
     • Calculate the energy needed to break the bonds in the reactants.
       • S-F bonds: 4 x 330 = 1320 kJ
       • O-H bonds: 4 x 460 = 1840 kJ
       • Total reactants = 1320 + 1840 = 3160 kJ
     • Calculate the energy released when the bonds in the products form.
       • Let x equal S=O bond energy.
       • S=O bonds: 2 x x = 2x
       • H-F bonds: 4 x 570 = 2280 kJ
       • Total products = 2280 + 2x kJ
     • Calculate the S=O bond energy.
       • Using the fundamental bond energy enthalpy formula, ΔH = Energy to break bonds (reactants) - Energy released forming bonds (products)
       • Substitute all known values into the equation. -54 = 3160 - (2280 + 2x)
       • Simplify and solve for x.
       • -54 = 3160 - 2280 - 2x
       • -54 = 880 - 2x
       • 2x = 880 + 54
       • 2x = 934
       • x = 934 / 2 = 467 kJ/mol.
   • Feedback: When counting the O-H bonds in water, you wrote that there are 2 total bonds to break instead of 4 O-H bonds, which resulted in a wrong calculation. The product SO₂ contains 2 S=O bonds. You need to account for the S=O bonds in products to prevent miscalculations.

4. (c) The equation for the reaction is shown (see photos directory). State the effect, if any, on the position of equilibrium when the following changes are made. Give a reason for each of your answers.

   • Correct answers:
     • The temperature is increased: Equilibrium shifts to the left because the reverse reaction is endothermic (absorbs heat).
   • Feedback: Since ΔH = -54 kJ/mol, the forward reaction is EXOTHERMIC (gives off heat). Le Chatelier’s principle states that when you increase temperature, the system tries to cool down by shifting in the endothermic direction to absorb heat.

5. (d) Explain, in terms of collision theory, why reducing the temperature decreases the rate of the reverse reaction.

   • Correct answer:
     • Lower temperature means particles have less kinetic energy (and move slower).
     • There are fewer collisions per second (less frequent collisions).
     • Fewer particles have greater energy than or equal to the activation energy (Eₐ).
   • Feedback: Exam mark schemes usually demand the term activation energy (Eₐ) for temperature-rate questions.

6. (d) Explain why the temperature change decreases towards the end of each experiment.

   • Correct answer: The reaction is complete (all the alkali has reacted), so adding excess cold acid cools down the solution (or heat is lost to the surroundings).
   • Feedback: You wrote “temperature decreases because the OH- ions in the sodium hydroxide and the H+ ions in hydrochloric acid have dissociated.” Saying ions have dissociated does NOT explain why cooling occurs once the peak is passed.

7. (f) Explain how the results would be different if a polystyrene cup is used instead of the beaker.

   • Correct answer:
     • The temperature changes would be higher/greater.
     • Polystyrene is a better insulator than glass (or reduces heat less to the surroundings).
   • Feedback: You wrote “the polystyrene cup would affect the temperature changes of solution G and H.” Exam schemes require you to state the direction of change rather than using vague words like “affect,” followed by the scientific reason.

8. (g) Give an advantage and a disadvantage of using a burette rather than a measuring cylinder to add the dilute hydrochloric acid to solution G and solution H.

   • Correct answer:
     • Disadvantage: It is harder to read or set up.
   • Feedback: You left the disadvantage blank.