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School Chemistry notes: Describing & explaining the electrolysis of acidified water
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The electrolysis of acidified water
(Suitable for AQA, Edexcel and OCR GCSE chemistry students)
2. Using an electrolysis cell - investigating the electrolysis of acidified water (actually dilute sulfuric acid)
AND the aqueous solutions of certain sulfate salts of reactive metals e.g. sodium sulfate, magnesium sulfate and strong alkalis like sodium hydroxide.
The electrolysis of dilute sulfuric acid is described and explained. This is the classic 'electrolysis of water' experiment and good introduction to electrolysis experiment. The electrode products and electrode equations for the electrolysis of water are quoted. What are the products of the electrolysis of water acidified with dil. sulfuric acid? All the electrode equations for the electrolysis of water are explained and diagrams of the apparatus.
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Electrolysis of dilute sulfuric acid - the products of electrolysing water acidified with sulfuric acid are hydrogen gas and oxygen gas
Two experimental setups are described, the Hofmann voltammeter demonstration (left diagram) and a simple cell (right diagram) for use in schools and colleges for pupils to use. Dilute sulfuric acid is used as the electrolyte in this investigation. The Hofmann voltammeter is filled with the electrolyte (dilute sulfuric acid) by opening the taps at the top of the outer tubes to allow any gas to escape. The gases formed on the electrolysis of the dilute sulfuric acid can be collected via the same taps. The platinum or carbon electrodes are inert.
You need inert (nonreactive) electrodes like platinum (left) and much cheaper carbon (graphite electrodes, right). In the simple electrolysis cell, the graphite (carbon) electrodes are inserted through a large rubber bung, 'upwardly' dipped into an solution of acidified water. In this cheap and simple apparatus the gaseous products (hydrogen and oxygen) are collected in small test tubes inverted over the carbon electrodes. You have to fill the little test tubes with the electrolyte (dil. sulfuric acid), hold the liquid in with your finger and carefully invert them over the nearly full electrolysis cell. The gases can be collected and tested.
On the right is another simple apparatus for doing experiments like the electrolysis of water. The electrodes must be made of an inert wire.
Students should note that the electrolysis will only take place when electricity is passed through the dilute sulfuric acid solution.
The electrolyte is dilute sulfuric acid (= acidified water) which, during electrolysis is split into hydrogen and oxygen gases. Theoretically into a 2 : 1 ratio by gas volume - which you can measure with the Hofmann Voltammeter.
This is one experimental method of showing water is a compound composed of the elements hydrogen and oxygen atoms i.e. by splitting liquid water into two gaseous element molecules of hydrogen (H2) and oxygen (O2).
Water only ionises to a tiny extent giving minute concentrations of hydrogen ions and hydroxide ions, so the presence of high concentrations of hydrogen ions (H+ or H3O+) and sulfate ions (SO42) from the acid, makes water a much better electrical conductor (a much better electrolyte solution).
Diagram of the electrolysis of water.
The electrode reactions and products of the electrolysis of acidified water are illustrated by the theory diagram above
The half-equations for the electrolysis of water electrode equations (electrolyte of acidified with dilute sulfuric acid) are described and explained below.
(a) The negative cathode electrode reaction for the electrolysis of water
(b) The positive anode electrode reaction for the electrolysis of water
Extra COMMENTS on the electrolysis of water
Electrolysis Quiz (GCSE 9-1 HT Level (harder)
Electrolysis Quiz (GCSE 9-1 FT Level (easier)
keywords and phrases:revision study notes for AQA Edexcel OCR IGCSE/GCSE chemistry topics modules on electrolysis of water acidified dilute sulfuric acid, the electrolyte needed, apparatus needed for investigating the electrolysis of water, the formation of products from the electrolysis of dilute sulphuric acid, the anode and cathode electrode equations for the electrolysis of water, a labelled and explained diagram for the electrolysis of water, the products of the electrolysis of sulfates, magnesium sulfate, sodium sulfate and sodium hydroxide
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