Chapter 10 - Electrolysis Flashcards
What is Electrolysis?
Describe what in general occurs with this process.
The PROCESS of electrolysis is carried in a type of ELECTROCHEMICAL CELL known as ELECTROLYTIC CELL (or electrolysis cell)
The process involves EVENTS that is the REVERSE of what occurs in a GALVANIC CELL.
This Process is the CONVERSION OF ELECTRICAL ENERGY INTO CHEMICAL ENERGY.
THE NON-SPONTANEOUS REDOX REACTIONS OCCUR
it involves the passage of electrical energy from a power supply (battery etc.) through a conducting liquid.
Occurs in a SINGLE CONTAINER
- No need to locate the electrodes in separate containers because a non-spontaneous reaction is involved
- However, a PRODUCTS NEED TO BE KEPT APART
- otherwise, they would spontaneously react with each other to reform the original reactants.
What is an electrolytic cell?
The PROCESS of electrolysis is carried in a type of ELECTROCHEMICAL CELL known as ELECTROLYTIC CELL (or electrolysis cell)
electrolytic cells use an external electrical potential difference to provide the energy to allow a non-spontaneous redox reaction to occur;
electrolytic cells are used in a range of industrial situations, including metal plating and the purification of copper
The components of an electrolytic cell include:
DC POWER SUPPLY
ANODE
CATHODE
ELECTROLYTE
- DC POWER SUPPLY:
- DRAWS ELECTRONS AWAY FROM ANODE (+)
- FORCES ELECTRONS ONTO CATHODE (-)
-** Applied DC voltage PROVIDES the ENERGY needed to CAUSE OXIDATION TO OCCUR AT THE ANODE, and REDUCTION AT THE CATHODE
- ANODE:
- OXIDATION OCCURS HERE
- electrode is ATTACHED to the POSITIVE TERMINAL OF THE DC POWER SUPPLY - CATHODE:
- REDUCTION OCCURS HERE
- electrode ATTACHED to the NEGATIVE TERMINAL OF THE DC POWER SUPPLY
- **CONDUCTS ELECTRONS FROM THE POWER SUPPLY INTO THE ELECTROLYTE
ELECTROLYTE:
- ELECTRODES are IMMERSED in a MOLTEN Electrolyr or Aqueous electrolytes, ie one that contains water.
- Anions (-) in the electrolyte are attracted to the anode (+) while Cations (+) move to the cathode (-)
What occurs at the negative electrode of an electrolytic cell?
What occurs at the positive electrode of an electrolytic cell?
Negative electrode:
- ions are attracted to the negative electrode where they accept electrons and are converted
- because REDUCTION OCCURS of the IONS occurs, by definition the NEGATIVE ELECTRODE is known as the CATHODE
Positive electrode:
- the positive terminal of the power supply withdraws electrons from the solid electrode, Causing OXIDATION TO OCCUR.
- the IONS in the SOLUTION that were consumed by the reaction at the negative electrode.
- Overall CONCENTRATION of IONS in the electrolyte REMAINS THE SAME
- because oxidation reaction is involved, by definition the POSITIVE ELECTRODE is Acting as an ANODE.
Oxidation occurs at the anode, which is the positive electrode
Reduction occurs at the cathode, which is the negative electrode
What is the process of electrolysis of a molten salt?
- This process uses an Electrolytic cell consisting of a pair of INERT ELECTRODES
- (platinum electrodes OR graphite)
- They allow the passage of electrons to and from the power supply
- they are inert (unreactive), the electrodes do not react with the contents of the cell. - They DIPPED INTO MOLTEN SALT and Connected to a DC electric power supply.
- Applying a DC voltage to the electrodes causes these to become OPPOSITELY CHARGED
- if the Applied voltage is sufficiently high then negative ions (anions) from the molten salt migrate to the positive electrode (anode) whereupon contact with the electrode they lose electrons to become oxidized
- Simultaneously, positive ions (CATIONS) from the salt migrate to the negative electrode (CATHODE) where they GAIN ELECTRONS to become REDUCED.
Explain Non-Spontaneous Reaction in an electrolytic cell?
How should it be organised?
Non-Spontaneous Reaction - a reaction that could not occur naturally
- in such reactions, the supply of electrical energy from a power supply is converted into CHEMICAL ENERGY in the products of electrolysis
The Reverse Reaction is a Spontaneous Reaction
Electrolytic - Occurs in a SINGLE CONTAINER
- No need to locate the electrodes in separate containers because a non-spontaneous reaction is involved
- However, a PRODUCTS NEED TO BE KEPT APART
- otherwise, they would spontaneously react with each other to reform the original reactants.
Compare and contrast electrolytic and galvanic cells
Both are types of electrochemical cells, involve the conversion between electrical and chemical energy.
GALVANIC CELLS
- Produces electricity
- Have SPONTANEOUS REACTIONS
- Converts chemical energy to electrical energy
- Oxidation occurs at the Anode and Reduction occurs at the Cathode
- Anode is Negative and Cathode is Positive
- Anions flow toward the anode and cations flow towards the cathode.
ELECTROLYTIC CELLS
- Consume electricity
- Have Non-Spontaneous reactions
- Convert electrical energy to chemical energy
- Oxidation occurs at the Anode and Reduction occurs at the Cathode
- Anode is Positive and Cathode is Negative
- Anions flow towards the Anode and Cations flow towards the Cathode.
What is the process of electrolysis of an aqueous salt?
- More complex problem to predict the electrode reactions that occur for electrolytic cells containing an aqueous solution or having electrodes that are potentially REACTIVE.
EG. CONTAIN WATER - In these cells WATER is often OXIDISED OR REDUCED or the ANODE ITSELF may be OXIDISED.
- Changes taking place in these cells can be INFERRED from OBSERVING the cell as it OPERATES.
- 1.* if WATER IS OXIDISED
- COLOURLESS GAS is FORMED at the ANODE
- Solution pH falls in the VICINITY of the ANODE
2H2O(L) —> O2(G) + 4H^+(aq) +4e^- (ANODE REACTION) - 2.* If WATER is REDUCED
- COLOURLESS GAS is FORMED at the CATHODE
- pH RISES in the vicinity of the CATHODE
- 2H2O(L) + 2e^- –> H2(g) +2OH^-(aq)
-3* If a METALLIC ELECTRODE is OXIDISED then the ANODE is SEEN TO DISSOLVE and LOSE MASS.
How can reactions during the electrolysis of an aqueous salt solution be predicted?
reactions during the electrolysis of an aqueous salt solution be predicted FROM STANDARD REDUCTION POTENTIAL DATA.
Most probable electrode reaction in electrolysis cell will be the most positive E.
*Note that these predictions are limited to situations involving standard conditions of TEMPERATURE, PRESSURE AND CONCENTRATION.
What is Electroplating?
A process in which a thin surface coating of metal, only a fraction of a millimetre thick, is applied over another metal surface.
Performed in Electrolytic cells
used to improve the appearance of a metal or its resistance to corrosion
How is the setup of an electrolytic cell for electroplating summarised
The setup of an electrolytic cell summarised
- The OBJECT BEING PLATED is at the CATHODE (NEGATIVE)
- An Electrode Of the METAL is at the ANODE (POSITIVE)
- Salt solution of the same element is the ELECTROLYTE.
- Applying a small DC VOLTAGE TO THE ELCTRIDES causes the METAL IONS FROM ELECTROLYTE TO BECOME REDUCED ONTO THE CATHODE.
- As the cell operates, OXIDATION OF THE ANODE REPLACES THE REDUCED METAL IONS.
- Maintains the concentration of the ions within the electrolyte.
Techniques and examples in which electrolysis is used in
- EXTRACTION of Reactive metals like Na, Mg, Al
- PURIFICATION of metals like Au, and Cu
- PRODUCTION of substances like Cl, NaOH
- applications such as Silver and Chrome PLATING METALS
Explain the process of ELECTROPLATING COPPER
- The OBJECT to be PLATED is CONNECTED BY A WIRE TO THE NEGATIVE TERMINAL OF A POWER SUPPLY.
* **the object becomes the NEGATIVE ELECTRODE IN THE CELL - A ROD or a SHEET of COPPER METAL is CONNECTED TO THE POSITIVE TERMINAL OF THE POWER SUPPLY.
* **the METAL becomes the POSITIVE ELECTRODE - 2 ELECTRODES ARE IMMERSED IN AN AQUEOUS SOLUTION WHICH CONTAINS IONS OF THE METAL TO BE PLATED.
- eg Copper(II) Sulphate - ELECTROLYTE - CONDUCTING LIQUID
- cooper 2 sulphate is an IONIC SOILD which dissociates into it’s ions when it DISSOLVES IN WATER. - During electrolysis, REACTIONS OCCUR AT THE SURFACE OF BOTH ELECTRODES.
THE ELECTRICAL ENERGY REQUIRED FOR THESE REACTIONS ARE PROVIDED BY THE POWER SUPPLY
- “electron pump”
POWER SUPPLY = pushes the electrons ONTO AN electrode and WITHDRAWS it from another.
- The ELECTRODE THAT NEGATIVE ELECTRONS ARE PUSHED TO (BY THE EXTERNAL POWER SOURCE) IS THE NEGATIVE ELECTRODE.
THE Cu2+ ions in the solution move towards the negative electrode (the objects to be plated) and SO4 ^2- ions move towards the POSITIVE ELECTRODE, ALLOWING THE CURRENT TO PASS THROUGH THE CELL.
POSITIVE AND NEGATIVE ELECTRODES
- CATHODE (NEGATIVE ELECTRODE)
Copper ions are attracted to the negative electrode, they accept electrons and are converted to copper metal.
- ANODE (POSITIVE ELECTRODE)
- Positive terminal of the power supply withdraws electrons from the copper electrode, causing an oxidation reaction to occur.
- Reaction replaces Copper ions in the Solution that were consumed by the reaction at the negative electrode.
- Overall concentration of the copper ions in the electrolyte remains constant.
Explain the process of electroplating Tin.
WHY IS TIN USED?
- Tin cans are mainly composed of STEEL which is an alloy of CARBON AND IRON.
Only a thin layer of Tin plated over the surface of the steel can.
- Tin corrodes very slowly and prevents contact between the IRON, MOISTURE AND AIR.
- Tin prevents the iron from corroding. - The OBJECT TO PLATED IS CONNECTED TO THE NEGATIVE TERMINAL OF THE POWER SUPPLY.
- ELECTRODE OF TIN METAL IS CONNECTED TO THE POSITIVE TERMINAL OF THE POWER SUPPLY.
- THE OBJECT IS IMMERSED INTO AN ELECTROLYTE SOLUTION WHICH CONTAINS THE IONS OF THE METAL TO BE PLATED.
- eg Tin(II)Nitrate - CATHODE
- THE NEGATIVE ELECTRODE
- tin ions are attracted to the CATHODE, where they undergo REDUCTION and are converted to TIN METAL.
- A COATING OF TIN IS FORMED OVER THE METAL - ANODE
- THE POSITIVE ELECTRODE
- The Tin at the Anode UNDERGOES OXIDATION.
- Tin metal DISSOLVES as Sn+2 ions
- because the tin ions are consumed by the reaction at the cathode and produced by the reaction at the anode, the overall concentration of tin ions in the electrolyte remains the same.
Why does the concentration of ions remain the same?
The ions are consumed by the reaction at the cathode
AND produced by the reaction at the anode.
HENCE, the overall concentration of tin ions in the electrolyte remains the same.
