Practical skills Study Guide

Electrolysis is a process that uses electrical energy to drive non-spontaneous chemical reactions. An electrochemical cell, often referred to as an electrolytic cell in this context, is the apparatus used to carry out electrolysis. It typically consists of a power supply, two electrodes (an anode and a cathode), and an electrolyte.

The electrolyte is an ionic compound in molten form or dissolved in a solvent, allowing its ions to move freely and conduct electricity. When an electric current is passed through the electrolyte, positive ions (cations) move towards the negatively charged cathode, where they gain electrons and are reduced. Negative ions (anions) move towards the positively charged anode, where they lose electrons and are oxidised.

To safely perform electrolysis, appropriate safety measures are crucial. This includes wearing eye protection, ensuring good ventilation, and handling chemicals carefully, especially if corrosive or toxic. The voltage used should be appropriate, and the apparatus should be set up securely to prevent spills or short circuits.

After electrolysis, it's important to identify the products formed at each electrode. This is done using qualitative tests. For example:

• Hydrogen gas (H₂) produced at the cathode can be tested with a lit splint, which will extinguish with a 'squeaky pop' sound.
• Oxygen gas (O₂) produced at the anode can be tested with a glowing splint, which will relight.
• Chlorine gas (Cl₂) produced at the anode will turn damp blue litmus paper red, then bleach it white.
• If a metal is deposited at the cathode, its appearance (e.g., colour, lustre) can help identify it, or further chemical tests could be performed.

For example, during the electrolysis of aqueous copper chloride, copper metal (reddish-brown solid) forms at the cathode, and chlorine gas (yellow-green gas, turns damp blue litmus red then white) forms at the anode. The half-equations are: Cathode: Cu²⁺(aq) + 2e⁻ → Cu(s); Anode: 2Cl⁻(aq) → Cl₂(g) + 2e⁻.

Titration is a quantitative chemical analysis method used to determine the concentration of an unknown solution by reacting it with a solution of known concentration. In an acid-alkali titration, a measured volume of the acid is reacted with a measured volume of the alkali until neutralization occurs. The point of neutralization is called the equivalence point and is typically detected using a suitable indicator that changes colour over a narrow pH range. The known solution (titrant) is added gradually from a burette to a flask containing the unknown solution (analyte) and a few drops of indicator. The addition continues until the indicator changes colour permanently, marking the endpoint of the titration. This endpoint should be as close as possible to the equivalence point. By knowing the volume and concentration of the known solution used, and the volume of the unknown solution, the concentration of the unknown solution can be calculated. This process requires precise measurements of volumes using volumetric apparatus such as pipettes and burettes. Accuracy is critical, necessitating multiple titrations to obtain concordant results (results that are very close to each other, typically within 0.1 cm³), and careful observation of the indicator colour change. Safety precautions include wearing eye protection, handling corrosive chemicals with care, and ensuring good ventilation.