CHM-DSM-251 Unit 3: Physical Chemistry Experiments
Course Code: CHM-DSM-251
Paper Name: Practical (Inorganic, Organic and Physical Chemistry)
Section Marks: 20 Marks
1. Surface Tension Determination
Surface tension is the property of a liquid surface that causes it to act like a stretched elastic membrane.
- Experiment: To determine the surface tension of glycerol or acetic acid solutions at various concentrations.
- Method: Usually performed using a Stalagmometer (drop number or drop weight method).
- Data Analysis: A graph is constructed to show the variation of surface tension with the concentration of the solute.
2. Viscosity Measurement
Viscosity is the measure of a fluid's resistance to flow.
- Experiment: To determine the viscosity of glycerol or acetic acid solutions at different concentrations.
- Instrument: Performed using an Ostwald Viscometer.
- Procedure: The time taken for a fixed volume of liquid to flow between two marks on the viscometer is measured and compared to a reference liquid (usually water).
- Graphing: A plot is made showing the relationship between viscosity and concentration.
3. Instrumental Titrations
Instrumental methods offer higher precision and can be used for colored or very dilute solutions where visual indicators fail.
pH-metric Titration
- Reaction: Strong acid (e.g., HCl) vs. Strong base (e.g., NaOH).
- Principle: Measurement of the pH change as the titrant is added to determine the equivalence point.
Conductometric Titration
- Reaction: Strong acid vs. Strong base.
- Principle: Based on the change in electrical conductivity of the solution as ions are replaced during the reaction.
- Observation: For a strong acid-strong base titration, the conductance initially decreases sharply as highly mobile H+ ions are replaced by less mobile cations, then increases after the equivalence point as excess OH- ions are added.
4. Solubility and Enthalpy of Dissolution
The solubility of a substance is temperature-dependent, which allows for the calculation of thermodynamic properties.
- Experiment: To determine the solubility of benzoic acid at different temperatures.
- Goal: To determine the ΔH (Enthalpy) of the dissolution process.
- Method: Saturated solutions are titrated at different temperatures, and the results are analyzed using the Van't Hoff equation.
5. Exam Focus: Tips and Viva Questions
Practical Exam Tips
- Cleanliness: For surface tension and viscosity experiments, the glassware must be absolutely clean (grease-free) as impurities significantly affect the results.
- Temperature: Record the room temperature accurately, as physical properties like viscosity are highly sensitive to temperature changes.
- Consistency: Take at least three readings for flow time or drop numbers to ensure the reliability of your data.
Common Viva Questions
Q: How does temperature affect the viscosity of a liquid?
A: For liquids, viscosity generally decreases as temperature increases because the kinetic energy of molecules increases, overcoming intermolecular forces.
Q: What is the unit of viscosity?
A: The SI unit is Pascal-second (Pa·s), but it is commonly expressed in Poise or Centipoise.
Q: Why do we use NH4Cl in Group III cation analysis?
A: To suppress the ionization of NH4OH via the common ion effect, ensuring the hydroxide ion concentration is only high enough to precipitate Group III cations.