C5.0 – Electricity and Chemistry

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1. State that electrolysis is the chemical effect of electricity on ionic compounds, causing them to break up into simpler substances, usually elements.

Learn the definition!

 

2. Use the terms electrode, electrolyte, anode and cathode.

The electrolyte is the compound that conducts electricity when molten and breaks down during electrolysis. Electrodes are the rods that carry the electric current to and from the electrolyte. These rods are usually inert, as in they do not react- usually graphite or platinum. The positive electrode is called the anode and the negative electrode is called the cathode.

 

3. Describe electrolysis in terms of the ions present and the reactions at the electrodes.

In order for a substance to be an electrolyte, it must contain an ionic compound, and the ionic compound must either be molten or dissolved in an aqueous solution. When an electric current is applied, the positive ions move towards the negative electrode, the cathode- this is why the positive ions are called cations; and the negative ions move towards positive electrode, the anode- these negative ions are then called anions. (Don’t get confused!).

When the ions reach their respective electrodes, they gain or lose electrons. A reaction where electrons are lost is called oxidation and a reaction where electrons are gained is called a reduction.

The cations move towards the cathode, where they will gain electrons from the cathode to become atoms (in a positive ion, there are more protons than electrons, so in order to become an atom, they need to gain those extra electrons to become neutral). So reduction occurs at the cathode.

The anions move towards the anode, where they will lose electrons at the anode to become atoms  (in a negative ion, there are more electrons than protons, so in order to become an atom, they need to lose those extra electrons to become neutral). So oxidation occurs at the anode.

 

4. Describe the electrode products, using inert electrodes, in the electrolysis of:
   • molten lead(II) bromide
   • aqueous copper chloride
   • dilute sulfuric acid.

• Molten lead(II) bromide

The electrolyte is molten PbBr₂. It will discharge into Pb²⁺ ions and Br^– ions.

 

The Pb²⁺ ions (cations) will move toward the cathode and gain electrons to form lead atoms.
Pb²⁺(aq) + 2e^– ————-> Pb(s)

The negative Br^– ions (anions) will move toward the anode and lose electrons to form bromide atoms.
2Br^–(aq) —————> Br₂ + 2e^–
Or this equation might make more sense:
2Br^–(aq) – 2e^– —————> Br₂

So now you have lead and bromine separately! The overall reaction can be written as:

Pb²⁺(aq) +  2Br^–(aq)  ——————> Pb(s) +  Br₂

• Aqueous copper chloride

The electrolyte is CuCl₂.  In water (it’s an aqueous solution!) this will discharge into Cu²⁺ ions and Cl^– ions as well as the water’s (H₂O) H⁺ ions and OH^– ions.

It should be noted that whenever hydroxide ions discharge at a cathode, they give off oxygen:

4OH^–(aq) –> 2H₂O(l) + O₂(g)

However, in aqueous solution, the positive and negative ions that are less reactive is discharged at the cathode and anode respectively. (You’ll learn more about the reactivity of elements here). According to the reactivity series, copper is less reactive than hydrogen and chlorine is less reactive than oxygen. As a general rule, whenever an aqueous solution is electrolysed, the hydroxide ion will be discharged at the anode, giving off oxygen, unless a halide is present – in that case, chlorine, bromine or iodine (depending on whichever one is present in the solution) will be discharged at the anode instead.

So in this case, copper will be discharged at the cathode and chlorine will be discharged at the anode. Now, it’s simple:

The Cu2+ ions will move towards the cathode and undergo reduction to gain electrons and become Cu atoms.
Cu²⁺(aq) + 2e^– —————–>  Cu(s)

The Cl^2- ions will move towards the anode and undergo oxidisation to lose electrons and become Cl₂ atoms.
Cl^2- (g) –  2e^– ——————->  Cl₂(g)

The overall reaction can now be written as:

Cu²⁺(aq) + Cl^2- (g)  ———————-> Cu(s)  +  Cl₂(g)

• dilute sulfuric acid.

Dilute sulfuric acid H₂SO₄ (aq). It contains water (H₂O) as well. The ions present are H⁺, SO₄^2-  and OH-.

There is only one positive ion that can discharge at the cathode and that is hydrogen. Hydrogen will undergo reduction (gain electrons) to become hydrogen atoms.
2H⁺  + 2e^–  ——————-> H₂At the anion, OH will be discharged, as it is less reactive than the sulfate. OH will undergo oxidisation to lose electrons and become water and oxygen.
4OH^2- –  4e^–  ————————> 2H₂O +O₂

5. State and use the general principle that metals or hydrogen are formed at the negative electrode (cathode), and that non-metals (other than hydrogen) are formed at the positive electrode (anode).

As a general rule of electrolysis, hydrogen and metals are formed at the cathode, while non-metals (other than hydrogen) are formed at the anode. This makes sense because metal ions and hydrogen ions have a  positive and will move towards the negative electrode which is the cathode; non-metal ions have a negative charge and so will move towards the positive electrode, that is the anode.

Look at the examples above. In the electrolysis of lead(II) bromide, lead (a metal) was formed at the cathode and bromide (a non-metal) was formed at the anode. In the electrolysis of aqueous copper chloride, copper (a metal) was formed at the cathode, and chlorine (a non-metal) was formed at the anode. In the electrolysis of dilute sulfuric acid, hydrogen was formed at the cathode and oxygen (a non-metal) was formed at the anode.

 

6. Relate the products of electrolysis to the electrolyte and electrodes used, exemplified by the specific examples in the Core content, together with aqueous copper(II) sulfate using carbon electrodes and using copper electrodes (as used in the refining of copper). 

Electrolysis can be used to purify metals, as it breaks apart compounds.

• One method of doing so, is using the metal we want to purify as electrodes (instead of an inert one like platinum or graphite). Let’s take the example of the purification of copper using an aqueous copper sulfate solution.

 

An impure strip of copper acts as the anode and a thin pure strip of copper acts as the cathode. The electrolyte is aqueous copper(II) sulfate solution.Because they are not inert the electrodes can react.

At the anode, the copper atoms in the impure copper strip, lose their valence electrons (oxidisation at anode) to form copper ions. These copper ions go into the electrolyte.
Cu(s) – 2e^–  —————> Cu²⁺At the cathode, the copper ions in the electrolyte gain electrons (reduction at cathode) to form copper atoms. These copper atoms are deposited on the strip of pure copper.
Cu²⁺(aq) + 2e^–   —————–> Cu(s)

As the electrolysis proceeds, the copper from the impure copper strip will slowly get transferred to the pure copper strip which will get thicker.  The anode loses mass as the copper atoms leave it, and the impurities fall to the bottom and is called ‘anode slime’.

• The other method of purifying to use normal inert electrodes.

The ions in the aqueous copper(II) sulfate solution will be Cu²⁺ (aq),  SO4^2- (aq),  H⁺ (aq) and  OH^– (aq).

At the anode, hydroxide ions from the water will get discharged instead of sulfate ions, as hydroxide is lower in the reactivity series.
4OH^– (aq) ——————-> O₂ (g) + 2H₂O(l) +4e^–At the cathode copper ions rather than hydrogen ions are discharged, as copper is lower in the reactivity series.
Cu²⁺ (aq) + 2e^–  ———————-> Cu(s)
These copper atoms get deposited on the cathode.

7. Describe the electroplating of metals, using laboratory apparatus.

Electroplating is use to put a thin layer of one metal on top of another metal. This can be done through electrolysis.

We connect the object to be electroplated to the negative pole of the power supply. It becomes the cathode. The object can be anything made of metal. The plating metal is connected to the positive pole of the power supply. It becomes the anode. Examples of metals used for plating: silver, gold, tin and chromium. The electrolyte is a solution of an ionic compound of the plating metal. (When plating an object with silver, silver nitrate can be used as the electrolyte).

 

Example:

At the anode copper atoms lose electrons. The become copper ions which go into the solution.
Cu(s) – 2e^–  ————–>Cu²⁺(aq)

The copper ions move to the cathode. At the cathode, the copper ions gain electrons to become copper atoms which form a thin layer on the surface of the object to be plated.
Cu²⁺ (aq) + 2e^– —————> Cu(s)

8. Predict the products of the electrolysis of a specified binary compound in the molten state, or in aqueous solution.

Of molten ionic compounds: a metal is formed at the cathode and a non-metal at the anode. It’s fairly simple to predict these results.

Examples:

Of aqueous solutions: H and OH ions are present in aqueous solutions along with the metal/non-metal ions. The ions with the lower reactivity will be discharged at their respective electrodes.When aqueous solutions of acids are electrolysed, oxygen is formed at the anode and hydrogen is formed at the cathode.

9. Describe, in outline, the chemistry of the manufacture of:
   • aluminium from pure aluminium oxide in molten cryolite
   • chlorine, hydrogen and sodium hydroxide from concentrated aqueous sodium chloride.

•  Aluminium from aluminium oxide in molten cryolite:

Aluminium oxide is found in the mineral ore called bauxite. Bauxite is first crushed and mixed with sodium hydroxide. The aluminium oxide reacts with the sodium hydroxide and dissolves
Al₂O₃(s) + 2NaOH(aq)  ———->   2NaAlO₂(aq)  +  H₂O
Sodium aluminate is formed (2NaAlO₂). This is then heated to extract the aluminium oxide.

Aluminium is extracted from aluminium oxide through electrolysis. The aluminium oxide needs to be molten first but it’s melting point is too high (2040°C). So, the aluminium oxide is dissolved in molten cryolite (cryolite is the term for sodium aluminium flouride- Na₃AlF₆). Now, electrolysis can be carried out.

Electrolysis is carried out using graphite (a form of carbon) electrodes. The cathode is the carbon lining on the steel tank.  The anodes are all dipped into the cryolite from above.

At the cathode, aluminium ions gain electrons (reduction) and are reduced to aluminium metal.
Al³⁺ + 3e^–  ————> Al
The liquid aluminium falls into the bottom of the cell.

At the anode, the oxide ions lose electrons (oxidisation) and become oxygen.
2O^2- – 4e^–   ————> O₂
This oxygen react with the carbon anodes to form carbon-dioxide gas. Because the carbon anodes burn away due to this reaction, they need to be replaced from time to time.

• Chlorine, hydrogen and sodium hydroxide from concentrated aqueous sodium chloride

A concentrated aqueous solution of sodium chloride is called brine.
The electrolysis of brine is used to produce chlorine, hydrogen and sodium hydroxide.

The ions in the solution are Na⁺, H⁺, Cl^– and OH^–.

At the anode, chloride ions lose electrons and is discharged as chlorine gas. (Chlorine is less reactive than hydroxide).
2Cl^– (aq) – 2e^–——–> Cl₂(g)
At the cathode, hydrogen ions gain electrons and hydrogen gas is discharged (Hydrogen is less reactive than sodium).
2H⁺(aq) + 2e^–  ————->  H₂(g)

The ions remaining in the solution are Na⁺ and OH^–. So an aqueous solution of sodium hydroxide is formed!

Notes submitted by Lintha

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