SPM Chemistry

Transfer of Electrons from One Point to Another

spmchemistry
  1. A redox reaction occurs when a solution of an oxidising agent is mixed with a solution of a reducing agent.
  2. If the solution of the oxidising agent and the solution of the reducing agent are separated by an electrolyte in a U-tube, the redox reaction will still take place but the transfer of electrons will occur through the connecting wire.
  3. The reducing agent undergoes oxidation with the loss of electrons. So, the carbon electrode that is immersed in the solution of the reducing agent becomes the negative cell terminal.
  4. Electrons flow through the connecting wire to the carbon electrode that is immersed in the oxidising agent solution. The carbon electrode acts as the positive cell terminal.
  5. The oxidising agent undergoes reduction with the acceptance of electrons.

Guide to Solve Problems Related to the Transfer of Electrons Through a Distance

  1. Oxidation always happens at the anode.
  2. Reduction always happens at the cathode.
  3. The anode is the negative terminal.
  4. The cathode is the positive terminal.

Reaction Between Potassium Dichromate(VI) and Potassium Iodide

Step 1: Identifying the Oxidising Agent and Reducing Agent
Oxidising Agent: Potassium dichromate(VI)
Reducing Agent: Potassium Iodide

Step 2: Determining the Oxidation and Reduction Process and Predicting the Observation
Oxidation
The reducing agent (Potassium Iodide) undergoes oxidation

2I– –> I2 + 2e

Observation: The colourless solution turn yellow/orange.

Note: Potassium iodide is colourless whereas iodine is yellow or orange in colour when dissolve in water.

Reduction
The oxidising agent undergoes reduction

Cr2O72- +   14H+ +  6e   →      2Cr3+ +     7H2O

Observation: The orange colour of the solution turn green.

Note: Dichromate(VI) ion is orange in colour whereas chromium(III) ion is green in colour

Step 3: Identifying the Anode and Cathode
Electrode P: Anode

Electrode Q: Cathode

Note: Oxidation occurs at anode whereas reduction occurs at cathode.

Step 4: Determine the positive and Negative Terminal
Positive Terminal: Electrode Q

Negative Terminal: Electrode P

Note: Anode is the negative terminal whereas cathode is the positive terminal.

Step 5: Determine the Direction of Flow of Electrons.
From electrode P to electrode Q.

Note: Electrons flow from the negative terminal to the positive terminal through the wire.

Reaction Between Iron(II) Sulphate and Bromine Water

Step 1: Identifying the Oxidising Agent and Reducing Agent
Oxidising Agent: Bromine water
Reducing Agent: Iron(II) sulphate

Step 2: Determining the Oxidation and Reduction Process and Predicting the Observation
Oxidation
The reducing agent undergoes oxidation

Fe2+ + 2e –> Fe3+

Observation: The green colour of iron(II) sulphate solution turn brown.

Note: Iron(II) ion is green in colour whereas iron(III) ion is brown in colour.

Reduction
The oxidising agent undergoes reduction

Br2 + 2e –> 2Br–

Observation: The yellow/orange colour of bromine water become colourless.

Note: Bromine is yellow/orange in water whereas bromide is colourless.

Step 3: Identifying the Anode and Cathode
Electrode P: Anode

Electrode Q: Cathode

Note: Oxidation occurs at anode whereas reduction occurs at cathode.

Step 4: Determine the positive and Negative Terminal
Positive Terminal: Electrode Q

Negative Terminal: Electrode P

Note: Anode is the negative terminal whereas cathode is the positive terminal.

Step 5: Determine the Direction of Flow of Electrons.
From electrode P to electrode Q.

Note: Electrons flow from the negative terminal to the positive terminal through the wire.