electro Flashcards
membrane cell(electrolysis)
These gases must be kept separate to prevent spontaneous reactions.
Electrode Reactions
Anode (+) Reaction: 2Cl- → Cl2 + 2e-
Cathode (–) Reaction: 2H2O + 2e- → H2 + 2OH-
Overall Reaction: 2NaCl + 2H2O → Cl2 + H2 + 2NaOH.
Electrode Materials:
Anode (+): Carbon
Cathode (–): Steel mesh
Operating Conditions
Membrane Function:
Allows Na + ions to migrate
Blocks Cl and O ions
downs cell (aluminium production )
Construction of the Downs Cell:
Electrolyte: Fused (molten) NaCl mixed with CaCl₂ (to lower the melting point from 801°C to around 600°C).
Electrodes:
Anode: Graphite (carbon)
Cathode: Iron
At the Cathode (-) Reduction of Na⁺ ions:
2Na⁺(l) + 2e⁻ → 2Na(l)
Sodium metal collects at the top and is removed.
At the Anode (+) Oxidation of Cl⁻ ions:
2Cl⁻(l) → Cl₂(g) + 2e⁻.
Chlorine gas is collected separately.
Overall Cell Reaction:
2NaCl(l) → 2Na(l) + Cl₂(g)
Applications of Electrolysis
Electrolysis of water:
Produces oxygen and hydrogen gases
Production of reactive metals & gases:
i.e. sodium and potassium production, hydrogen gas and chlorine gas production, aluminium production
Electroplating:
Coating (plating) of materials in a metal i.e., silver plating cutlery
Gold plating of tropies/awards
Electrorefining:
Purification process taking impure metals and using electrolysis to produce pure metals
electrolysis of water
Process Overview:
Pure water does not conduct electricity efficiently, so an electrolyte (e.g., HSO or KNO) is added.
This enables the breakdown of water (HO) into hydrogen (H) and oxygen (O) gases.
Reactions at Electrodes:
At the Cathode (Reduction Reaction):
H₂O + 2e⁻ → H₂(g) + 2OH⁻(aq)
At the Anode (Oxidation Reaction):
2H₂O(l) → O₂(g) + 4H⁺(aq) + 4e⁻
Overall Reaction:
The H and OH ions cancel out, leading to:
2H₂O → 2H₂ + O₂
electroplating
Process of electroplating:
It typically involves metal ions being reduced to form a solid metal coating on an object.
At the cathode:
Is where the object to be plated is connected so that the metal desired can be deposited à reduction reaction
Ag+(aq) + e- → Ag(s)
At the anode:
Is a piece of metal of what is being plated onto the object chosen
Ag(s) → Ag+(aq) + e-
Overall equation:
Ag(s) + Ag+(aq) + e-→Ag+(aq) + e- + Ag(s)
redox flow battery
A redox flow battery is an electrochemical energy storage device that converts chemical energy into electrical energy through the reversible oxidation and reduction of working fluids.
It provides an efficient, reliable, and cost-effective energy storage system.
Redox flow batteries are often paired with renewable energy technologies such as wind and solar power, storing the generated energy in chemical form for later conversion into electrical energy.
When receiving electrical energy from wind or solar sources, redox flow batteries recharge by acting as electrolytic cells, converting electrical energy into chemical energy.
Conversely, when discharging, they act as galvanic cells, converting the stored chemical energy back into electrical energy.
Lithium-Ion Cells
Li-ion batteries are commonly used to power devices including mobile phones, laptops and tablet devices because of their high electric potential and low mass.
During discharge, the following reaction occurs at the anode:
Li → (Li+)+ e-
The Li+ ions migrate to the electrolyte, an organic solvent, which contains LiPF6. Once formed
During discharge, the cathode reaction is:
CoO2 + (Li+) + e- → LiCoO2
The overall discharge reaction for a lithium-ion cell is:
Li + CoO2 →C6 + LiCoO2.
Alkaline Fuel Cell
(AFC)
Fuel is oxidised at the anode (H2)
Oxygen is reduced at the cathode
Alkaline fuel cell means the electrolyte is a hydroxide based solution (OH- in equations)
Key Characteristics of Fuel cell
Fuel is oxidised at the anode
Oxygen is reduced at the cathode
Acidic fuel cell means the electrolyte is a hydrogen ion (H+) based solution
Methanol Fuel Cell - DMFC
Anode (oxidation):
2CH3OH(aq)+2H2O(l)→2CO2(g)+12H+(aq)+12e−
Cathode (reduction):
12H+(aq)+12e−+3O2(g)→6H2O(l)
Overall:
2CH3OH(aq)+3O2(g)→2CO2(g)+4H2O(l)
Solid Oxide Fuel Cell
(SOFC)
Fuel is oxidised at the anode
Oxygen is reduced at the cathode
Solid electrolyte allows for the movement of the products of the reaction at the cathode to move through the cell and react with the fuel at the anode
Alkaline fuel cell means the electrolyte is a hydroxide based solution
secondary cell
Secondary Cells (Rechargeable)
These cells are rechargeable (reactants can be reformed from the products of the reactions)
Include Lead-acid accumulator, NiCad cell, NiMH cells
Common examples in modern society are Li-ion batteries
Redox flow batteries also offer potential in combination with renewable resources
Factors Affecting Battery Selection
Cost: Depends on materials and technology used in production.
Size & Shape: Common types include cylindrical, coin, button, pouch, and prismatic.
Mass: Lightweight batteries are preferred for portable devices, while heavier ones suit transport vehicles.
Memory Effect: Some rechargeable batteries lose capacity if not fully discharged before recharging.
Voltage: Aqueous electrolyte cells typically provide no more than 2V. Discharge Curve: Voltage may decrease steadily or remain stable until most charge is consumed.
Current: Larger electrode surface area increases current but limits electrolyte capacity.
Shelf Life: Some batteries lose charge over time, even when unused.
Environmental Impact & Disposal: Mercury button cells are phased out; NiCd batteries raise toxicity concerns. Rechargeable batteries should be recycled.
key characteristic of fuel cell
A fuel cell is a type of galvanic cell that converts chemical energy from a fuel into electrical energy.
Unlike conventional galvanic cells, which have a fixed amount of reactants and produce a limited amount of electricity, fuel cells operate continuously as long as fuel and oxygen are supplied.
The two main distinguishing components that govern a fuel cell’s function are the electrolyte and the electrodes.
In a fuel cell:
The fuel always reacts at the anode.
Oxygen (O₂(g)) always reacts at the cathode.
primary cells
Primary Cells (Leclanché, Alkaline, Button)
Non-rechargeable; energy supply is limited by available reactants.
Reductant: A metal (often zinc), which can act as the anode or be in contact with an inert electrode (e.g., brass rod in alkaline cells).
Oxidant: Usually a transition metal oxide (MnO₂, AgO, ZnO), in contact with an inert cathode (e.g., carbon rod or steel can).
Electrolyte: Typically an alkaline paste or gel.
Porous separators function as salt bridges.
uses of secondary cell
Secondary cells, also known as rechargeable batteries or accumulators, can be used multiple times by storing and releasing energy through reversible chemical reactions.
Commonly used in:
Phones, laptops, and tablets (Lithium-ion)
Hybrid and electric cars (Nickel-metal hydride, Lithium-ion)
Power tools and medical devices (Nickel-cadmium)
Car batteries and backup power supplies (Lead-acid)
Battery Type Common Uses:
Lithium-ion (Li-ion)- Phones, laptops, electric vehicles
Nickel-metal hydride (NiMH)- Hybrid cars, rechargeable AA/AAA batteries
Nickel-cadmium (NiCd)- Power tools, medical devices
Lead-acid- Car batteries, backup power
condition for recharge
For a battery to be rechargeable, these conditions must be met:
Reversible chemical reactions – The reactions must be capable of being undone.
Stable reaction products – The products of discharge must remain at the electrodes.
Durable electrodes – Electrodes must not degrade significantly over time.
To recharge a secondary cell, the applied voltage must be greater than the natural potential difference of the discharge reaction.
Steps for Recharging:
Connect the positive terminal of the power source to the positive battery terminal.
Connect the negative terminal of the power source to the negative battery terminal.
Ensure the applied voltage is slightly higher than the discharge voltage, but not too high (to prevent damage).
Role of the Electrolyte in a Fuel Cell
The electrolyte is a crucial component of a fuel cell, as it facilitates ion movement while preventing direct mixing of the reactants.
It allows only the movement of charged ions, enabling the redox reaction to proceed efficiently.
For the reaction to occur, the reactant species must have access to the electrolyte, as it provides the necessary ions.
The type of ions transferred depends on the operating conditions:
Acidic conditions → H⁺ ions move through the electrolyte.
Alkaline conditions → OH⁻ ions move through the electrolyte.
Operation of polymer electrolyte membrane (PEM) electrolysis
The polymer electrolyte membrane (PEM) is made of a plastic polymer that separates electrons and gases.
The electrodes are usually covered with a platinum catalyst to increase the rate
of production of gases.
Gas diffusion layers comprised of gold-lined, titanium and carbon paper help to distribute the reactant gases evenly across the electrode surfaces.
Bipolar plates prevent the build-up of current and provide mechanical support for the electrolyser.
