Redox reactions at electrodes

redox electrolysis

1. Redox reactions at electrodes

2. Redox (oxidation and reduction)reactions take place during electrolysis of chemical compounds. One element gains electrons whilst one element loses electrons

3. What is electrolysis? An ionic compound conducts electricity when it is molten or in solution. The current causes the ionic compound to split up and form new substances. Electrolysis has many uses, including: purifying copper plating metals with silver and gold extracting reactive metals, such as aluminium making chlorine, hydrogen and sodium hydroxide. This process is called electrolysis , a word which comes from Greek and means “splitting by electricity”.

4. heat What happens during electrolysis? In electrolysis, the substance that the current passes through and splits up is called the electrolyte . Positive ions move to the negative electrode (cathode) and gain electrons. This is reduction. Negative ions move to the positive electrode (anode) and lose electrons . This is oxidation. The electrolyte contains positive and negative ions . What happens to these ions during electrolysis?

5. Dilute sulphuric acid When a dilute solution of sulfuric acid is  electrolysed , gases are produced at both the anode and the cathode. The gas produced at the cathode burns with a 'pop' when a sample is lit with a lighted splint. This shows that the gas is hydrogen. The gas produced at the anode relights a glowing splint dipped into a sample of the gas. This shows that the gas is   oxygen. At cathode, 2H ⁺ + 2e −→H 2 ​ At anode, 4OH−→2H 2 ​ O +O 2 ​ +4e−

6. What happens at the electrodes during the electrolysis of dilute sulfuric acid? Electrolysis of dilute H 2 SO 4 – redox equations What is the overall equation for the electrolysis of dilute sulfuric acid? At the negative electrode: 2H + + 2e -  H 2 (reduction) 2H 2 O (l)  2H 2 (g) + O 2 (g) Twice as much hydrogen forms as oxygen. Why is this? At the positive electrode: 4OH -  2H 2 O + O 2 + 4e - (oxidation) In water, there are 2 hydrogen atoms for every oxygen atom, so the ratio by volume, of H 2 to O 2 , is 2:1.

7. Concentrated Aqueous Sodium Chloride Electrolysis of aqueous NaCl results in hydrogen and chloride gas. When concentrated NaCl solution undergoes electrolysis, sodium hydroxide and hydrogen are produced at the cathode while chlorine is produced at the anode. Cathode : 2H 2 O + 2e- H 2 + 2OH ⁻ Anode : 2Cl ⁻ Cl 2 + 2e-

8. Products of electrolysis of NaCl solution The electrolysis of sodium chloride solution produces three very useful products: Chlorine used for killing bacteria in water, for bleach and making plastics like PVC. Hydrogen used for making margarine and fertilizers, and for rocket fuel. Sodium hydroxide used in many chemical reactions, such as making soap, neutralizing acids and making paper. Chlorine is expected as a product of this process but hydrogen and sodium hydroxide are surprising products.

9. Dilute aqueous Sodium Chloride During the electrolysis of dilute NaCl solution, Na+, Cl- and water are present . At the cathode Cathode : 2H 2 O + 2e- H 2 + 2OH(-) At the anode Anode : 2H 2 O O 2 + 4H ⁺ + 4e-

10. Copper (ii) sulphate (inert electrodes) The negative cathode electrode attracts Cu2+ ions (from copper sulfate ) and H+ ions (from water). Only the copper ion is discharged, being reduced to copper metal. A  brown copper deposit forms as the positive copper ions are attracted to the negative electrode (cathode) Cu 2 ⁺ ( aq )  +  2e–  Cu(s ) The blue colour fades as more and more copper is deposited, depleting the concentration of the blue copper ion Cu2+ ions in solution. Oxygen gas  is formed at the positive electrode, an oxidation reaction (electron loss). The negative sulfate ions (SO 4 2- ) or the traces of hydroxide ions ( OH –  from water) are attracted to the positive electrode. Hydroxide ions or water molecules are discharged and oxidised to form oxygen. ( i ) 4OH – ( aq )   4e –   2H 2 O (l)  + O 2(g) The negative hydroxide ion is oxidised by electron loss or  written as:  4OH – ( aq ) 2H 2 O (l)  + O 2(g)  + 4e –

11. Copper (ii) sulphate (copper electrodes ) The negative cathode electrode attracts  Cu2+ ions (from copper sulphate ) and H+ ions (from water ). Only the copper ion is discharged, being reduced to copper metal. A  reduction electrode reaction at the negative cathode Cu2+( aq ) + 2e–  Cu(s ) A copper deposit forms, reduction of the copper ions, each gains 2 electrons.   (ii) The positive anode reaction with a copper electrode Its the copper anode that is the crucial difference than electrolysing copper sulphate solution with a inert carbon/graphite/platinum electrode . An oxidation electrode reaction at the positive anode The copper dissolves, oxidation of the copper atoms,  each losing 2 electrons to form blue Cu2+ ions or written as:  Cu(s)  Cu2 +( aq )  +  2e–

12. What redox processes occur at the electrodes during the electrolysis of molten lead bromide ( PbBr 2 )? Electrolysis of molten PbBr 2 – redox equations What is the overall equation for the electrolysis of molten lead bromide ? At the negative electrode: Pb 2+ + 2e -  Pb (reduction) lead bromide  lead + bromine PbBr 2 (l)  Pb (l) + Br 2 (g) At the positive electrode: 2Br -  Br 2 + 2e - (oxidation)

13. References https ://www.middlesbrough.gov.uk › default › files. Chapter 6 Electrochemistry https:// www.docbrown.info/page01/ExIndChem/electrochemistry04.htm Tindale, Ritchie et al, 2014, Chemistry for CSEC 2nd Edition , Nelson Thornes. Chapter 10, p182-187