GCSE OCR Gateway Chemistry C3.4c-e Electrolysis (2)

New OCR Gateway specification from September 2016 Higher tier: grades 9 to 4: 

In this and subsequent posts I’m simply going to explain and illustrate each learning objective as they come up in the topics in the new GCSE specification.
 
I’m giving you my notes from each lesson.

You can really get ahead of your class if you follow this blog and all the posts that will appear here about the new GCSEs over the coming months. 

This rejigging of the specification is just that: there is nothing really new here it has all been with us for the past half century at least. 

That written in italics is for the higher tier paper only.

C3 Chemical Reactions

C3.4 Electrolysis(2)

C3.4d describe electrolysis in terms of the ions present and reactions at the electrodes

The electrolysis of molten lead bromide (PbBr₂(l))

The lead bromide must be molten liquid.

Solid lead bromide does not conduct electricity.

Ionic compounds conduct electricity using their ions, not using electrons.

Ionic compounds do not contain any free or delocalized electrons.

The ions only conduct electricity when they are free to move because they must be free to move to either the cathode or the anode

In a solid ionic compound, the ions are not free to move.

Only when the compound is heated to a temperature above its melting point will the ions be free to move. 

For sodium chloride that temperature is 801^oC

Then diagram below illustrates these points:

When the ions reach the oppositely charged electrode a reaction takes place:

At the positive anode, the negative ion loses its additional electrons i.e. the ion is oxidized. 

At the negative cathode, the positive ion gains enough electrons to fill its outer shell i.e. the ion is reduced.

For both these processes to happen electrons have to be conducted through the external circuit but not through the electrolyte.

In the electrolyte, it is the ions that are moving, not electrons.

The diagram below shows what happens with hot molten lead bromide:

As you can see in the diagram the elements of lead bromide are formed at the electrodes: lead at the cathode and bromine at the anode because positive lead ions are electrostatically attracted to the negative cathode and negative bromide ions are attracted to the positive anode.

Here are the key points:

• The electrodes themselves play no part in the redox reactions at anode and cathode. 

• They are inert electrodes.

• Bromide ions lose electrons at the anode (they are oxidized) to form bromine molecules.

• Lead ions are reduced at the cathode where they gain two electrons to form lead atoms.

• Electrons flow though the external circuit released at the anode and collecting on the cathode.

• Molten lead sinks to the bottom of the crucible since it is denser than molten lead bromide.

• A red vapour hangs over the anode because bubbles of pure bromine are given off here.

• Bubbles of bromine are visible in the molten lead bromide.

• The power pack pushes the electrons round the external circuit.

A video of the electrolysis of molten lead bromide is available here on Youtube.

C3.4c describe competing reactions in the electrolysis of aqueous solutions of ionic compounds in terms of the different species present

i)           the electrolysis of aqueous NaCl using inert electrodes

Here’s a way of listing what happens at the electrodes in electrolysis of concentrated sodium chloride solution:

Inert Electrodes	Graphite Anode (+)	Graphite Cathode (—)
Ions migrating	Cl— , OH—	H+,  Na+
Electrode reactions	Cl— → Cl   +   e— Each chloride ion loses an electron (oxidised) to form a chlorine atom. Cl   +   Cl    →      Cl2 Two chlorine atoms pair up to form a chlorine molecule: Overall: 2Cl—  →  Cl2   +  2e— OH— ions are not discharged because their concentration is too low.	H+  +   e—   →  H Each hydrogen ion gains an electron (reduced) to form a hydrogen atom. H   +    H   →    H2 Two hydrogen atoms pair up to form a hydrogen molecule. Overall: 2H+   + 2e—   →   H2 Na+ ions are not discharged because the metal is much more reactive than hydrogen.
Products formed	Chlorine gas	Hydrogen gas
Identifying tests	Chlorine gas bleaches damp indicator paper.	Hydrogen gas gives a squeaky pop with a lighted spill.
Remaining solution	Hydroxide ions	Sodium ions

This electrolysis is the basis for the production of sodium hydroxide, chlorine and hydrogen using the Diaphragm Cell.

Here is a lab you can perform to study the electrolysis of sodium chloride solution.

ii)         the electrolysis of aqueous CuSO4 using inert electrodes

Inert Electrodes	Graphite Anode (+)	Graphite Cathode (—)
Ions migrating	SO42— , OH—	H+,  Cu2+
Electrode reactions	4OH— → 2H2O  + O2 + 4e— Hydroxide ions lose electrons (they are oxidized) to form oxygen and water. SO42— sulfate ions are not discharged because their concentration is too low and the energy cost is too high.	Cu2+  +   2e—   →  Cu Each copper ion gain two electrons (they are reduced) to form a copper atom. H+ ions are not discharged because the gas is much more reactive than copper.
Products formed	Oxygen gas	Solid copper metal plates the cathode.
Identifying tests	Oxygen relights a glowing spill	Copper metal plates the cathode with a pink colored metal.
Remaining solution	Sulfate ions	Hydrogen ions

The final solution is sulfuric acid formed from the hydrogen ions and the sulfate ions that do not discharge.

The blue solution gradually fades a way as the copper ions are discharged.

You can find the experiment here.

C3.4e describe the technique of electrolysis using non-inert electrodes

Electrolysis of copper sulphate, CuSO_4, solution using copper electrodes.

Active Electrodes	Copper Anode (+)	Copper Cathode (—)
Ions migrating	SO42—, OH—	H+,  Cu2+
Electrode reactions	Neither anion is discharged instead the copper metal of the electrode loses electrons and copper ions are discharged into the solution.  So the electrode itself is oxidized.  Cu(s)  →  Cu 2+(aq)   +   2e—	Cu2+  +   2e—   →  Cu Each copper ion gain two electrons (they are reduced) to form a copper atom. The copper ions that are reduced are the same ions as are drawn off the anode.  H+ ions are not discharged because the gas is much more reactive than copper.
Products formed	No products formed but the anode loses mass	Solid copper metal plates the cathode. The cathode increases in mass but by less than the anode loses mass.
Identifying tests		Copper metal plates the cathode with a pink colored metal.
Remaining solution	Sulfate ions	Hydrogen ions Copper ions

The effect of the involvement of the electrodes in the reactions at the electrodes is for the copper to transfer from the anode to the cathode.

So if the anode is made of impure copper and only copper ions transfer then pure copper is plated onto the cathode.

This is an effective and profitable way of purifying copper for electrical cabling and circuit boards.

This is the basis of the industrial purification of copper. 

This process shows how electroplating takes place.

The object to be electroplated is made the cathode and the electroplating metal is made the anode and a suitable salt like a sulphate of the metal to be electroplated is used as the electrolyte.