Required practicals 6-10: A2 techniques (A2 only)

Introduction to A2 Required Practicals

Welcome to the final stretch of your practical chemistry journey! While your AS practicals (1-5) focused on the fundamentals, the A2 Required Practicals (6-10) dive into more complex techniques used in research and industry. These practicals are essential for Unit 5 (CH05), where 30 marks are dedicated specifically to practical skills. Don't worry if some of these feel a bit more technical—we will break them down step-by-step so you can master the procedures and the logic behind them.

Note: For guidance on planning, uncertainties, and data handling, please refer to the specific chapters in the "Unit 5: Practical and mathematical skills" section.

Required Practical 6: Measuring the EMF of an Electrochemical Cell

In this experiment, you learn how to harness the flow of electrons between different chemical species to measure Electromotive Force (EMF). Think of this as measuring the "push" behind the electrons in a battery.

The Procedure

1. Prepare the Half-Cells: Clean two metal electrodes (e.g., Copper and Zinc) with emery paper to remove any oxide layers. Place each metal into a beaker containing a \(1.0\text{ mol dm}^{-3}\) solution of its ions (e.g., \(CuSO_4\) and \(ZnSO_4\)).
2. The Salt Bridge: This is the most important part! Soak a piece of filter paper in an inert electrolyte like potassium nitrate (\(KNO_3\)). Place the ends in each beaker. This completes the circuit by allowing ions to flow without the solutions mixing.
3. Measure: Connect the electrodes to a high-resistance voltmeter using crocodile clips and wires. Record the voltage (EMF).

Key Tips for Success

Why a high-resistance voltmeter? We want to measure the maximum potential difference when no current is flowing. If current flows, the concentrations change and the voltage drops.
Common Pitfall: If your voltmeter reads zero, check if your salt bridge has dried out or if the electrodes are properly cleaned.

Required Practical 7: Investigating pH Changes

This practical involves tracking how the acidity or alkalinity of a solution changes during a titration. You will typically investigate a weak acid with a strong base (e.g., ethanoic acid and \(NaOH\)) or a strong acid with a weak base.

The Procedure

1. Calibrate the pH Probe: Before starting, place the probe in buffer solutions of known pH (usually pH 4, 7, and 10). This ensures your readings are accurate (accounting for "probe drift").
2. The Titration: Place a fixed volume of acid in a beaker. Add the base from a burette in small increments (e.g., \(2.0\text{ cm}^3\)).
3. Refining Near the End-Point: As the pH starts to change more rapidly, add the base dropwise until you pass the equivalence point.
4. Plotting: Plot a graph of pH (y-axis) against volume of base added (x-axis).

Key Takeaway

The "vertical section" of your graph contains the equivalence point. For a weak acid/strong base titration, the pH at the equivalence point will be greater than 7. You can also find the \(pK_a\) of a weak acid by looking at the pH at the half-neutralisation point (where volume = half the volume at the equivalence point).

Required Practical 8: Measuring Rates of Reaction

You need to be familiar with two distinct ways to measure how fast a reaction happens: Initial Rate and Continuous Monitoring.

Method A: Initial Rate (The "Clock Reaction")

In a clock reaction (like the Iodine Clock), you measure how long (\(t\)) it takes for a distinct visual change to occur (like a color change to blue-black).
- We assume that the Rate is proportional to \(1/t\).
- By repeating the experiment with different starting concentrations, you can determine the order of reaction.

Method B: Continuous Monitoring

Here, you track the progress of one single reaction over time.
- Gas Collection: If a reaction produces gas (like \(Mg + HCl\)), use a gas syringe to measure the volume at regular intervals (e.g., every 20 seconds).
- Colorimetry: If the reaction changes color, a colorimeter measures "absorbance," which is proportional to concentration.
- Analysis: Plot a graph of Concentration vs. Time. To find the rate at any specific time, draw a tangent to the curve and calculate its gradient.

Required Practical 9: Identifying Transition Metal Ions

Transition metals form beautiful, colored ions in water. This practical uses "test-tube" reactions to identify ions like \(Fe^{2+}\), \(Cu^{2+}\), \(Al^{3+}\), and \(Fe^{3+}\).

The Reactions to Know

You will add sodium hydroxide (\(NaOH\)), ammonia (\(NH_3\)), or sodium carbonate (\(Na_2CO_3\)) to the metal-aqua ions.
- Copper(II) \([Cu(H_2O)_6]^{2+}\): Forms a blue precipitate with \(NaOH\). With excess \(NH_3\), the precipitate dissolves to give a deep blue solution.
- Iron(II) \([Fe(H_2O)_6]^{2+}\): Forms a green precipitate that turns brown on standing (due to oxidation).
- Iron(III) \([Fe(H_2O)_6]^{3+}\): Forms a brown precipitate.
- Aluminium(III) \([Al(H_2O)_6]^{3+}\): Forms a white precipitate with \(NaOH\), which redissolves in excess \(NaOH\) (showing it is amphoteric).

Quick Tip: Carbonate (\(CO_3^{2-}\)) reactions are great for distinguishing \(2+\) and \(3+\) ions. \(3+\) ions are more acidic and will react with carbonates to produce bubbles of \(CO_2\) gas, while \(2+\) ions just form a precipitate.

Required Practical 10: Preparing and Purifying an Organic Solid

This is the "grand finale" of organic synthesis. You synthesize a solid (like aspirin or benzoic acid) and then must ensure it is pure.

Step 1: Preparation and Filtration

After the reaction, the solid is often separated using filtration under reduced pressure (using a Buchner funnel and flask). This is much faster than standard gravity filtration and leaves the solid drier.

Step 2: Purification (Recrystallisation)

1. Dissolve the impure solid in the minimum volume of hot solvent. (This ensures the solution is saturated).
2. Filter hot to remove any insoluble impurities.
3. Leave the filtrate to cool slowly. As it cools, the solubility drops and pure crystals "crash out" of the solution.
4. Filter again and wash with a small amount of ice-cold solvent to remove soluble impurities on the surface.

Step 3: Testing Purity (Melting Point)

To check if your hard work paid off, determine the melting point of the dry solid using a melting point apparatus.
- Pure substances have a sharp melting point that matches the data book value.
- Impure substances melt over a wide range and at a lower temperature than the pure substance.

Summary Key Takeaway: For RP10, remember the phrase "Minimum volume of hot solvent." It is the most common answer in exam questions about why a student's yield was low or why the crystals didn't form!