IGCSE Chemistry 0620/51 Practical Test Study Notes (May/June 2024)

General Examination Instructions and Paper Information

• Paper Identification: CHEMISTRY 0620/51 Paper 5 Practical Test, May/June 2024.
• Duration: 1 hour 15 minutes.
• Materials Required: Confidential instructions list, black or dark blue pen, HB pencil (for diagrams/graphs), calculator.
• Formatting and Conduct Rules:
  • Answer all questions in the provided spaces.
  • Erasable pens or correction fluids are strictly prohibited.
  • Write name, centre number, and candidate number at the top of the document.
  • Show all working and use appropriate units.
• Total Marks: The total marks for the paper is 40. Marks for each part are indicated in brackets $[ ]$.

Investigation: Temperature Change in the Reaction of Magnesium and Dilute Sulfuric Acid

Objective

To investigate the temperature change occurring during the reaction between magnesium ribbon ($Mg$) and dilute sulfuric acid ($H_2SO_4$) under varying concentrations.

Experimental Procedures

• Experiment 1 (Control):

  • Use a $25.0\,cm^3$ measuring cylinder to deliver $20\,cm^3$ of dilute sulfuric acid into a boiling tube.
  • Measure the initial temperature of the acid and record it in Table 1.1.
  • Add a coiled $5\,cm$ length of magnesium ribbon to the acid and simultaneously start a timer.
  • Continually stir the contents with the thermometer.
  • After exactly $45\,seconds$, measure the final temperature of the mixture and record it.
  • Rinse the boiling tube with distilled water before subsequent experiments.

• Experiments 2 through 5 (Variable Dilution):

  • Process: Maintain the volume of dilute sulfuric acid at constant $20\,cm^3$. Add varying volumes of distilled water prior to the reaction to decrease the acid concentration.
  • Experiment 2: Add $2.0\,cm^3$ of distilled water.
  • Experiment 3: Add $4.0\,cm^3$ of distilled water.
  • Experiment 4: Add $6.0\,cm^3$ of distilled water.
  • Experiment 5: Add $10.0\,cm^3$ of distilled water.
  • Mixing: In Experiments 2-5, place a bung in the tube and invert it to ensure the acid and water are thoroughly mixed before adding the magnesium ribbon.

Data Recording and Analysis

• Table 1.1 Parameters:

  • Volume of dilute sulfuric acid / $cm^3$.
  • Volume of distilled water / $cm^3$.
  • Initial temperature / $^{\circ}C$.
  • Temperature after $45\,s$ / $^{\circ}C$.
  • Temperature increase / $^{\circ}C$.

• Rate Calculation:

  • The average rate of temperature increase is calculated using the following equation:
  • $\text{average rate of temperature increase} = \frac{\text{temperature increase}}{45\,seconds}$
  • Units: $^{\circ}C/s$ (or specified as per calculation).

• Graphing and Deductions:

  • Plot a graph on Fig 1.1 with the volume of distilled water on the x-axis and temperature increase on the y-axis.
  • A line of best fit must be drawn.
  • Deduction: Using the line of best fit, the temperature increase for $7.5\,cm^3$ of distilled water can be predicted.

Accuracy and Evaluation

• Smallest Temperature Change: Occurs in Experiment 5 because it has the largest volume of distilled water ($10.0\,cm^3$), resulting in the lowest concentration of sulfuric acid. A lower concentration reduces the frequency of collisions between particles, slowing the reaction and resulting in less energy released per unit of time.
• Instrumentation Limitations:
  • A $25.0\,cm^3$ volumetric pipette cannot be used to measure varying volumes of water (like $2.0\,cm^3$ or $4.0\,cm^3$) because it is designed to deliver a single, fixed volume accurately.
• Improvements to Accuracy:
  • Thermal Insulation: Wrapping the boiling tube in cotton wool minimizes heat loss to the surroundings, ensuring measured temperature changes are closer to theoretical values.
  • Apparatus Change: Using a polystyrene cup (which is a better insulator than glass) instead of a boiling tube.
• Hypothetical Variable Change: Reducing the magnesium ribbon length from $5\,cm$ to $2\,cm$ would significantly decrease the temperature rise, as magnesium is the limiting reactant. The new graph line (labeled 'g') would be lower than the original.

Qualitative Analysis of Solid E

Procedure and Observations

• Initial Processing: Divide Solid E into two portions.
• Portion 1 (Thermal Decomposition): Gently heat the solid in a boiling tube and observe changes (e.g., color changes, condensation on tube walls).
• Portion 2 (Solution Formation): Dissolve the remaining solid in $5\,cm$ of distilled water to form Solution E.

Chemical Tests on Solution E

1. Aqueous Sodium Hydroxide (NaOH): Add dropwise and then in excess. Observe precipitate color and solubility.
2. Gas Identification: Warm the product of the NaOH test. Test the gas with damp red litmus paper. If it turns blue, the gas is Ammonia ($NH_3$).
3. Sodium Hydrogencarbonate ($NaHCO_3$): Add to the second portion. Effervescence indicates the presence of an acid ($H^+$ ions).
4. Acidified Potassium Manganate(VII): Add to the third portion. If the purple color turns colorless, a reducing agent (like Sulfite, $SO_3^{2-}$) is present.
5. Sulfate Test: Add dilute nitric acid ($HNO_3$) and aqueous barium nitrate ($Ba(NO_3)_2$). A white precipitate after standing indicates Sulfate ($SO_4^{2-}$) ions.

Conclusion

Identify the three ions in Solid E based on the cation, anion, and gas tests performed.

Separation of a Three-Component Mixture

Components and Solubility Data

Separation Steps

1. Filtration of the Mixture: Filter the original mixture.
   • Residue: Contains solid Sodium Chloride and Zinc Carbonate.
   • Filtrate: Pure Ethanol (separated from the solids).
2. Solvent Extraction of Residue: Add distilled water to the solids remaining in the filter paper and stir.
3. Second Filtration: Filter the resulting aqueous mixture.
   • Residue: Pure Zinc Carbonate (unaffected by water).
   • Filtrate: Aqueous Sodium Chloride solution.
4. Evaporation: Heat the aqueous Sodium Chloride filtrate in an evaporating basin until the point of crystallization or dryness to recover pure solid Sodium Chloride.

Reference Tables for Qualitative Analysis

Tests for Anions

• Carbonate ($CO_3^{2-}$): Add dilute acid; result: effervescence, carbon dioxide ($CO_2$) produced.
• Chloride ($Cl^-$): Acidify with dilute nitric acid, then add aqueous silver nitrate; result: white ppt.
• Bromide ($Br^-$): Acidify with dilute nitric acid, then add aqueous silver nitrate; result: cream ppt.
• Iodide ($I^-$): Acidify with dilute nitric acid, then add aqueous silver nitrate; result: yellow ppt.
• Nitrate ($NO_3^-$): Add aqueous sodium hydroxide, then aluminum foil; warm; result: ammonia produced.
• Sulfate ($SO_4^{2-}$): Acidify with dilute nitric acid, then add aqueous barium nitrate; result: white ppt.
• Sulfite ($SO_3^{2-}$): Add acidified aqueous potassium manganate(VII); result: purple to colorless.

Tests for Aqueous Cations

• Aluminum ($Al^{3+}$): NaOH: white ppt., soluble in excess; Ammonia: white ppt., insoluble in excess.
• Ammonium ($NH_4^+$): NaOH: Ammonia produced on warming.
• Calcium ($Ca^{2+}$): NaOH: white ppt., insoluble in excess; Ammonia: no ppt. or very slight white ppt.
• Chromium(III) ($Cr^{3+}$): NaOH: green ppt., soluble in excess; Ammonia: green ppt., insoluble in excess.
• Copper(II) ($Cu^{2+}$): NaOH: light blue ppt., insoluble in excess; Ammonia: light blue ppt., soluble in excess (dark blue solution).
• Iron(II) ($Fe^{2+}$): NaOH/Ammonia: green ppt., insoluble in excess, turns brown on standing.
• Iron(III) ($Fe^{3+}$): NaOH/Ammonia: red-brown ppt., insoluble in excess.
• Zinc ($Zn^{2+}$): NaOH/Ammonia: white ppt., soluble in excess to colorless solution.

Tests for Gases

• Ammonia ($NH_3$): Turns damp red litmus paper blue.
• Carbon dioxide ($CO_2$): Turns limewater milky.
• Chlorine ($Cl_2$): Bleaches damp litmus paper.
• Hydrogen ($H_2$): 'Pops' with a lighted splint.
• Oxygen ($O_2$): Relights a glowing splint.
• Sulfur dioxide ($SO_2$): Turns acidified aqueous potassium manganate(VII) from purple to colorless.

Flame Tests for Metal Ions

• Lithium ($Li^+$): Red
• Sodium ($Na^+$): Yellow
• Potassium ($K^+$): Lilac
• Calcium ($Ca^{2+}$): Orange-red
• Barium ($Ba^{2+}$): Light green
• Copper(II) ($Cu^{2+}$): Blue-green