GCSE Science/Electrolysis
Electrolysis is the decomposition of certain types of substance using electricity. The types of substance that can be split are ionic substances. This just means that they are made of charged ions rather than neutral atoms. {Remember that an ion is just an atom that has either a positive or negative charge}. An example of an ionic substance is common table salt sodium chloride. The sodium atom has a positive charge, the chlorine atom has a negative charge. It is usually written as Na+Cl-.
Q1) Check in a periodic table, what is the symbol for sodium: Na or Cl?
As you may already know if you've studied the Metals module, a salt is any substance made by combining an acid with an alkali. Acids, alkalis, and therefore all salts are ionic.
Q2) Which of the following substances can be broken up by electricity: sodium chloride, iron sulphate, copper nitrate?
Basic experimental setup
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| The liquid electrolyte is connected to a battery or power supply. The electrodes must not touch! |
Most ionic compounds are not liquid at room temperature. This is a problem because the ions need to be able to move for the electric current to be able to flow. This can be achieved by melting. Look at the electrical setup shown on the right. The electrodes are just two carbon rods connected to a battery. The one connected to the positive electrode is called the anode. The one connected to the negative electrode is called the cathode. this is due to a collision
Consider for example the compound lead bromide. This compound is a solid at room temperature but can be molten over a Bunsen flame. So what you would do is put some lead bromide into a beaker. Put the beaker on a tripod over a bunsen flame. Melt the lead bromide, then put in the electrodes and turn the power supply on at a setting of, say, 2V. What you would see happening is the cathode, is a silvery coating of pure lead forming, and bromine forming at the anode. The current would continue to flow until all the lead bromide was turned into lead and bromine.
Q3) It takes energy to split up a compound like lead bromide. Where does this energy come from?
Q4) Predict what products you would get at the anode and cathode if copper chloride was the electrolyte.
What happens at the anode
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| The bromine atoms (blue) give up 2 electrons each |
The anode is the positive electrode; it attracts negatively charged ions, because unlike charges attract. The bromine ions move through the melt until they reach the anode. Once they get there, they give up their two extra electrons to become bromine atoms.
2Br- → Br2 + 2e-
The electrons flow up the anode to the positive terminal of the battery.
What happens at the cathode
editThe cathode is the negative electrode; it attracts the positively charged ions. Metal ions are always positive and so the lead ions flow through the metal uhe negatively charged terminal of the battery and onto the lead ions.
Some trick to remember cations and anions, cathodes and anodes.
I have a cat......I call her by saying come here plussy! - cathodes attract positive ions
ca+ions has a plus in it, cations are positive ions
red cat: reduction occurs at the cathode
Pb2+ + 2e- → Pb
Q5) Solid ionic substances do not conduct electricity and are not split up by it. Why do you think that is?
Quantity calculations (higher tier only)
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In the experiment with lead bromide, you saw that lead was deposited at the cathode. If you actually do the experiment you will see that the lead coats the cathode. In this section we will look at how much metal will coat a cathode in a given time.
A scientist performed the following experiment.
- A copper cathode was carefully cleaned and accurately weighed.
- It was placed along with an anode into a solution of copper sulphate.
- It was connected via an ammeter to a variable power supply.
- A current was left to run for a given time, then the cathode was removed and weighed again.
His results were:
| Current /A | Time /s | Mass of copper deposited /g |
| 1 | 3000 | 1.0 |
| 2 | 3000 | 2.0 |
| 2 | 1500 | 1.0 |
| 1 | 1500 | 0.5 |
You can see from the results that the total amount of copper deposited depends on both the current and the time it flows. This is because the number of copper atoms that can be made from ions depends on the total amount of charge that flows. The unit of charge is the coulomb.
One coulomb is the amount of charge when one Ampere flows for one second.
Q6) Look at the results table above. How much copper is deposited when 1A flows for 3000 seconds?
Q7) How much copper do you predict would be deposited if 1A were to flow for 6000 seconds.
Q8) What about if 2A were to flow for 12000 seconds ?
Electrolysis of Aqueous solutions (Advanced)
editBefore studying this section check with your teacher to see if you need to.
Earlier on in this module you've learned that ions must be able to move in order for electrolysis to work. If the ions are held rigid {such as in a solid}, they can't move and no electricity will flow. We've looked at how the freeing up of ions can occur by melting the electrolyte. Another way to achieve this is by dissolving the electrolyte in water. The trouble with this method is, there will be more than one type of ion present.
Water partially splits up into ions {this is why it's such a good solvent for ionic compounds}. It splits into hydrogen ions and hydroxide ions.
H2O → H+ +OH-
So at the cathode there will be two ions present: the metal ion and the hydrogen ion from the water. Which element is actually produced at the cathode depends on how reactive the metal is. If the metal is very reactive, such as potassium or sodium, then it is unlikely to be discharged. Hence hydrogen will be produced. If the metal is unreactive such as silver, the metal will be produced. To work out which ion "wins", the metal or the hydrogen, compare their reactivities in a the reactivity series. The one that is most reactive, will not be produced at the cathode.
A similar situation occurs at the anode. Hydroxide ions {from the water} are usually discharged at the anode ultimately producing oxygen. However, if the concentration of the ions of Halites (group 7) are much higher than that of the hydroxide ions, then the halite ions are discharged. Sulphates are never discharged.
OH- → OH + e-
4OH → 2H2O + O2.
Q9) Sodium chloride is dissolved in water and subjected to electrolysis. Explain what you see at each of the electrodes.
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Summary
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