Introduction to AS Required Practicals

Welcome to your guide on the first five required practicals of your OxfordAQA International A-level Chemistry course! While Unit 5 is an A2 exam, it tests your knowledge of all practical work from both years. These five AS-level practicals form the foundation of your laboratory skills. Don't worry if you find the details a bit overwhelming at first—we will break down each procedure, the "why" behind the steps, and the common pitfalls to avoid. Mastering these will give you a huge advantage in the 30 marks of practical-based questions in Unit 5!

Required Practical 1: Volumetric Solutions and Titrations

The goal here is to find the exact concentration of an unknown solution. This is a two-step process: first, you make a standard solution (one with a known, precise concentration), and then you use it in a titration.

1. Making a Volumetric (Standard) Solution

To make \(250\text{ cm}^3\) of a solution:

  1. Weigh the solid: Weigh a clean, dry weighing boat with the solid on a digital balance. Transfer the solid to a beaker and re-weigh the empty boat. The difference is the exact mass transferred (this is called weighing by difference).
  2. Dissolve: Add a small volume of distilled water (less than \(100\text{ cm}^3\)) and stir with a glass rod until all the solid dissolves.
  3. Transfer: Pour the solution into a \(250\text{ cm}^3\) volumetric flask using a funnel.
  4. Rinse: Rinse the beaker, glass rod, and funnel with distilled water and add the washings to the flask. This ensures every last molecule of solute is included.
  5. Make up to the mark: Add distilled water until the bottom of the meniscus sits exactly on the graduation line. Use a pipette for the last few drops!
  6. Invert: Stopper the flask and invert it several times to ensure the concentration is uniform throughout.

2. Carrying out the Titration

  • Rinse the burette with the solution you are going to fill it with, then fill it.
  • Use a pipette filler and pipette to transfer exactly \(25\text{ cm}^3\) of your other solution into a conical flask.
  • Add a few drops of a suitable indicator.
  • Add the solution from the burette, swirling the conical flask constantly. As you get near the end-point, add the solution drop-wise.
  • Record the final volume and calculate the titre (final volume minus initial volume).
  • Repeat until you have at least two concordant results (results within \(0.10\text{ cm}^3\) of each other).

Quick Review: Why use a conical flask instead of a beaker? The sloped sides prevent splashing, which would lose some of your reactants! Also, always read the burette at eye level to avoid parallax error.

Required Practical 2: Measuring Enthalpy Changes

In this practical, we measure the heat energy change (\(q\)) during a reaction, usually in an aqueous solution. We use a technique called calorimetry.

The Procedure

For a reaction between a solid and a liquid (or two liquids):

  1. Place a polystyrene cup inside a glass beaker (this provides extra insulation).
  2. Measure a known volume of the liquid into the cup and record its temperature every minute for a few minutes to establish a steady starting temperature.
  3. At a specific minute, add the second reactant but do not record the temperature at that exact moment.
  4. Stir the mixture and record the temperature every minute until the temperature change levels out or starts to reverse.

Calculations and Graphs

To find the temperature change (\(\Delta T\)) accurately, plot a graph of temperature vs. time. Draw two lines of best fit: one for the temperatures before mixing and one for after. Extrapolate both lines to the minute when the reactants were mixed. The vertical distance between these lines at the mixing time is your "corrected" \(\Delta T\).

Use the formula: \(q = mc\Delta T\)

  • \(q\) = heat energy (J)
  • \(m\) = mass of the solution (usually assumed to be \(1\text{ g}\) per \(1\text{ cm}^3\))
  • \(c\) = specific heat capacity (you will be given this value)
  • \(\Delta T\) = change in temperature (K or \(^\circ\text{C}\))

Then, calculate the enthalpy change (\(\Delta H\)) in \(\text{kJ mol}^{-1}\) by dividing \(q\) (in kJ) by the number of moles of the limiting reactant.

Common Error: Heat loss to the surroundings is the biggest error. Using a lid and a polystyrene cup helps minimize this!

Required Practical 3: Identifying Ions

This practical is like being a chemical detective! You use "test-tube" reactions to identify unknown substances.

1. Identifying Cations (Positive Ions)

  • Group 2 (\(Mg^{2+}, Ca^{2+}, Sr^{2+}, Ba^{2+}\)): Add sodium hydroxide (\(NaOH\)).
    Trend: Solubility of hydroxides increases down the group. \(Mg(OH)_2\) is a white precipitate; \(Ba(OH)_2\) is soluble.
    Alternatively, add sulfuric acid (\(H_2SO_4\)).
    Trend: Solubility of sulfates decreases down the group. \(BaSO_4\) is a thick white precipitate.
  • Ammonium (\(NH_4^+\)): Add \(NaOH\) and warm gently. Ammonia gas is produced. Test with damp red litmus paper—it will turn blue.

2. Identifying Anions (Negative Ions)

  • Halides (\(Cl^-, Br^-, I^-\)): Add acidified silver nitrate (\(AgNO_3\)).
    \(Cl^-\) = White ppt; \(Br^-\) = Cream ppt; \(I^-\) = Yellow ppt.
    (Note: Nitric acid is added first to remove any carbonates that might interfere!)
  • Sulfate (\(SO_4^{2-}\)): Add acidified barium chloride (\(BaCl_2\)). A white precipitate (\(BaSO_4\)) forms.
  • Carbonate (\(CO_3^{2-}\)): Add any dilute acid. It will fizz (effervescence) as \(CO_2\) is released. Bubble the gas through limewater; it will turn cloudy.
  • Hydroxide (\(OH^-\)): These are alkaline, so they will turn red litmus paper blue or universal indicator purple.

Required Practical 4: Distillation of a Product

Distillation is used to separate a liquid from a mixture based on its boiling point. A classic AS example is the partial oxidation of a primary alcohol to an aldehyde.

The Setup

  • The reaction mixture is heated in a round-bottomed flask.
  • A Liebig condenser is attached horizontally or sloped downwards.
  • Crucial: Water must enter the condenser at the bottom and leave at the top. This ensures the condenser is always full and cools the vapours efficiently.
  • A thermometer is placed at the "T-junction" to monitor the boiling point of the vapour being collected.

Did you know? If we wanted to make a carboxylic acid instead of an aldehyde, we would use reflux (vertical condenser) to keep the vapours in the flask until they are fully oxidized!

Required Practical 5: Organic Functional Group Tests

You need to be able to distinguish between different organic families using quick chemical tests.

1. Alkenes (\(C=C\))

  • Test: Add bromine water.
  • Result: Colour change from orange to colourless (decolourises).

2. Aldehydes vs. Ketones

  • Fehling’s Solution: Heat gently. Aldehydes turn the blue solution into a brick-red precipitate. Ketones show no change.
  • Tollens’ Reagent: Heat gently. Aldehydes form a silver mirror on the inside of the test tube. Ketones show no change. (Tollens' contains the complex ion \([Ag(NH_3)_2]^+\)).

3. Alcohols

  • Test: Add acidified potassium dichromate(VI) (\(K_2Cr_2O_7\)).
  • Result: Primary and secondary alcohols turn the solution from orange to green. Tertiary alcohols stay orange.

4. Carboxylic Acids

  • Test: Add sodium hydrogencarbonate (\(NaHCO_3\)) or sodium carbonate.
  • Result: Effervescence (fizzing) as \(CO_2\) gas is produced.

Key Takeaway: For Unit 5, you don't just need to know the results; you need to be able to describe how to do the test (e.g., "warm gently in a water bath" for Tollens' because organic liquids are flammable!).

Note: For more details on handling the data from these experiments, such as calculating uncertainties or drawing lines of best fit, see the chapter on "Data handling, graphs and uncertainties". For the A2 level practicals, see "Required practicals 6-10".