Electrolysis 2
Sodium hydroxide and chlorine are used to make bleach.

Electrolysis 2

Electrolysis is a process you need to understand for GCSE Chemistry. This is the second of two quizzes on the subject but what exactly is electrolysis and how was it discovered? Electrolysis is a method of using a direct electrical current to cause a chemical reaction. It was discovered by accident in 1800 when the scientist William Nicholson and surgeon Anthony Carlisle attempted to reproduce the experiment in which Italian scientist Alessandro Volta created the world's first battery. Instead of using frog's legs to test whether electricity was produced, they were using a device developed by Nicholson, similar to the gold leaf electroscope. To get a better contact between the wires from their battery and the measuring device, they used a drop of water. When they noticed that a gas was bubbling off from this water, they investigated further and were astonished to find that when they passed the electric current from their battery through river water there was a reaction - a gas bubbled off from both wires they had dipped into their sample. That was the first time electrolysis was seen.

A few years later in 1807, potassium was discovered by Sir Humphry Davy whilst using electrolysis on molten potassium hydroxide. Later in the same year, he discovered sodium using the same method. The following year, he discovered barium, calcium, boron, strontium, and magnesium too. But it was the assistant of Davy who worked out the laws of electrolysis - Michael Faraday. These laws enabled scientists to predict and plan how much of each substance could be produced from electrolysis. He also introduced the terms used in electrolysis that you are familiar with such as anode, cathode, electrode, and ion (although they were actually invented by the Master of Trinity College, Cambridge, William Whewell).

Once electricity generating equipment had improved and electrolysis was better understood, the commercial value of the process was realised and it was taken up by industry. The first large scale electrolytic cells were developed in the 1880s for the electrolysis of brine which gave birth to the chlor-alkali industry. Also during the same decade, the Hall-Héroult process for the extraction of aluminium was developed which transformed aluminium from a rarely seen and extremely expensive metal into the cheap and widely used metal of today. There were no health and safety at work regulations at the time and workers in both industries could expect short lives due to the toxic and corrosive nature of the chemicals involved. For the GCSE, you should know how electrolysis works for both processes and the main uses of the products.

Have a go at this quiz and test your knowledge of electrolysis, the chemical reaction caused by direct electrical current.

Choose the correct half equations for the electrolysis of brine.
Anode 2Cl- → Cl2 + 2e-
Cathode 2H+ + 2e- → H2
Anode Cl- → Cl2 + e-
Cathode 2H+ + e- → H2
Anode 2Cl- → Cl2 + 2e-
Cathode Na+ + e- → Na
Anode Cl- → Cl + 2e-
Cathode H+ + e- → H2
Both chlorine and hydrogen are diatomic molecules
What gas forms at the positive electrode during the extraction of aluminium?
This oxygen reacts with the carbon of the positive graphite electrodes forming carbon dioxide and as a result, they have to be replaced frequently
Copper can be purified using electrolysis. Pick the correct combination of components for the electrolytic cell.
Cathode: Impure copper
Anode: Pure copper
Electrolyte: Copper sulfate
Cathode: Pure copper
Anode: Impure copper
Electrolyte: Copper sulfate
Cathode: Pure copper
Anode: Pure copper
Electrolyte: Copper sulfate
Cathode: Impure copper
Anode: Pure copper
Electrolyte: Water
Copper ions in solution are attracted to the cathode, where they are deposited as copper atoms
The hydrogen produced by the electrolysis of brine is NOT used to...
make margarine
make bleach
make hydrochloric acid
make hydrogen chloride gas
Sodium hydroxide and chlorine are used to make bleach
If graphite electrodes are used instead of copper in the electrolysis of copper sulfate, the equation that shows what happens at the anode is...
H2O → H+ + O2 + e-
Cu2+ + 2e- → Cu
2H2O → 4H+ + O2+ 4e-
Cu → Cu 2+ +2e-
Graphite is carbon and is a conductor of electricity and can therefore be used for making electrodes
Which of the following is NOT a use for chlorine?
Kill bacteria
Make bleach
Make PVC
Control pH
The use of chlorine to disinfect drinking water in Victorian Britain had a profound effect on health, reducing the number of people catching water borne bacterial diseases like cholera
Aluminium ore (bauxite) must be dissolved in molten cryolite before it can be electrolysed. Why?
Raise the melting point
Lower the melting point
Increase its viscosity
Reduce its viscosity
Cryolite melts at less than 1000°C, bauxite melts at over 2000°C
Pick the correct half equations for the electrolysis of copper sulfate using copper electrodes.
Cathode: Cu(s) → Cu2+(aq) + 2e-
Anode: Cu2+(aq) + 2e- → Cu(s)
Cathode: Cu2+(aq) + 2e- → Cu(s)
Anode: Cu(s)→ Cu2+(aq) + 2e-
Cathode: Cu(s) → Cu+(aq) + e-
Anode: Cu2+(aq) + 2e- → Cu(s)
Cathode: Cu(s) → Cu2+(aq) + 2e-
Anode: Cu+(aq) + e- → Cu(s)
Remember, in a pair of half equations, everything needs to be balanced, just like in any symbol equation
When an electrical current is passed through brine (sodium chloride solution), three products are formed. They are...
chlorine gas, hydrogen gas, sodium chloride solution
chloride ions, hydrogen gas, sodium hydroxide solution
chlorine gas, hydrogen gas, sodium hydroxide solution
chlorine gas, hydrogen ions, sodium hydroxide solution
All three have important uses in the chemical industry
The third product from the electrolysis of brine is sodium hydroxide. This is NOT used to...
make bleach
make paper
make soap
make margarine
Hydrogen is used for hydrogenating oils to produce margarine
You can find more about this topic by visiting BBC Bitesize - Electrolysis

Author:  Kate Gardiner

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