Electrochemistry Flashcards
(25 cards)
What is an Electrochemical Cell?
Generate electric current from a spontanoues chemical reaction (Galvanic)
Use electric Current, to make a non spontaneous reaction go. (Electrolytic)
What is a galvanic cell?
A galvanic cell is an electrochemical cell that is able to generate electricity from a spontaneous chemical reaction.
How do Galvanic Cells Work:
Galvanic cells use redox reactions to produce voltage without direct contact between the oxidant and the reductant.
They Consist of Two Electrodes:
- Anode: Where the oxidation occurs (In galvanic it is the negative electrode)
-Cathode: Where reduction occurs (In galvanic cells this is the positive electrode)
Electrons travel from the anode to cathode.
Daniell Cell:
- The Anode is made up of the more reactive material, which is oxidised
- The cathode is made up of the less reactive material which is reduced
What is a primary cell?
A primary cell are single use batteries that cannot be recharged.
An example of the primary cell is the dry cell.
Another type are alkaline batteries.
What is a secondary cell?
Secondary cells/batteries are rechargeable.
If an electric current is applied to the cell, it will regenerate the reactants.
Fuel Cells:
These are galvanic cells that need to be continuously supplied with reactants.
What is an electrolytic cell?
An electrolytic cell is a type of electrochemical cell that employs electric current to make a non spontaneous reaction occur.
An electrolytic Cell:
There is a power supply, to provide electricity to the cell.
The anode is still the site of oxidation however it is the positive electrode.
The cathode is the site of reduction and is the negative electrode.
Does not require a salt bridge.
Electrolysis of Molten Salt:
To perform this technique, inert electrodes (graphite/platinum) are immersed into a vessel of molten salt.
The power supply is connected causes the electrodes to be oppositely charged, and allows the reaction to begin.
The negative ions migrate to the positive anode and oxidation occurs.
The positive ions migrate to the negative cathode and reduction occurs.
Electrolysis of Aqueous Electrolyte:
An electrolytic cell that contains an aqueous salt is a more complex situation as the water the salt is dissolved it can be preferentially oxidised or reduced.
In a situation where inert electrodes are not used, there is also the potential that the anode itself could be oxidised.
If water is oxidised at the anode, then colourless oxygen gas is produced.
2H2O(l) O2(g) + 4H+(aq) + 4e-
If water is reduced at the cathode, then colourless hydrogen gas is produced.
2H2O(l) + 2e- H2(g) + 2OH-(aq)
Electro refining Copper:
Impure(Blister) copper is cast into slabs and made the ande of the electrolytic cell.
The cathode is a sheet of pure copper.
The electrolyte copper sulphate Solution acidified with H2SO4.
The copper is a reactive metal impurities are oxidised to ions in the solution.
H2SO4 prevents the oxidation of water. A small voltage is applied to allow Cu to deposite at the cathode but not impurities.
Extraction of Gold:
Anode: Rod of stainless steel
Cathode: Steel wool
Electrolyte: Au(CN)2^-
Oxygen gas forms at the anode according to the reaction:
4OH-(aq) 2H2O(l) + O2(g) + 4e- Gold deposits on the steel wool according to the reaction: [Au(CN)2]- (aq) + e- Au(s) + 2CN-(aq) Once the gold has deposited, it is treated with HCl and all of the steel wool dissolves.
Electroplating:
The anode is a pure rod of the metal you want to plate on.
The cathode is the object you want to have metal plated onto.
The electrolyte is a solution of the plating ions.
The reaction at the anode is the metal oxidising into ions, such as:
Ag(s) Ag+(aq) + e-
The reaction at the cathode is ions in solution forming metal:
Ag+(aq) + e- Ag(s)
The object is then plated with the me
Electroplating is the process of coating one metal object with another metal.
Corrosion?
Corrosion is the surface degradation of metallic substances by their oxidation in the presence of oxygen.
Corrosion Reactions Are spontaneous
Oxidation-reductions but take a long time to react.
Water vapour in the air acts as the electrolyte through which charged particles (electrons) are carried.
Factors that accelerate corrosion
Factors that accelerate corrosion.
Presence of Water: Water is essential for rusting as it facilitates the electrochemical reactions involved.
Oxygen: Increased exposure to air, especially in environments with high oxygen levels, accelerates rust formation.
Salt: Salt, particularly sodium chloride, accelerates rusting by increasing the conductivity of water, which enhances the electrochemical reactions.
Acids: Acidic environments, such as those with acid rain or industrial pollutants, can increase the rate of rusting. Acids provide hydrogen ions that facilitate the oxidation of iron.
Temperature: Higher temperatures generally increase the rate of chemical reactions, including rusting.
Impurities in the Metal: Impurities or variations in the composition of iron can create anodic and cathodic regions, which accelerate the electrochemical reactions involved in rusting.
Mechanical Stress: Mechanical stress or damage to the protective oxide layer on iron can expose fresh metal to the environment, accelerating rusting.
Barrier Methods of Controlling Corrosion
Barrier methods of controlling corrosion.
A barrier method is on where a coating is applied to the iron to prevent oxygen and water from contacting the iron.
Adding paint, oil/grease or an inert coating such as tin can act as a physical barrier between the metal and corrosive agents such as water, oxygen, and salts. This prevents the electrochemical reactions that lead to corrosion.
This property is crucial in preventing the formation of anodic and cathodic sites on the metal surface, which are necessary for corrosion to occur.
Sacrifical Method Of controlling Corrosion:
Sacrificial methods of controlling corrosion involve using a more reactive metal to protect a less reactive metal from corroding.
The sacrificial anode oxidises, supplying electrons to the protected metal.
Oxygen and water use these surplus electrons to undergo reduction (instead of iron breaking down to provide them).
The sacrificial anodes need to be replaced over time to ensure there is sufficient material to be oxidised.
Common sacrificial anodes include zinc, magnesium, and aluminium.
Cathodic Protection:
The use of a sacrificial anode is known as cathodic protection, because it forces the iron to become the cathode, protecting it from corrosion.
Cathodic protection involves the supply of electrons to the iron.
Another method of cathodic protection is impressed current cathodic protection, where an external voltage supply is connected to the iron, providing it with electrons and thus protecting it from corrosion.
The negative terminal is connected to the protected iron and the positive terminal is connected to a piece of scrap iron, which will be preferentially oxidised.
Galvanising: Using both barrier and Sacrifical Method:
This involves coating the iron with a more reactive metal such as zinc.
This limits the interaction of oxygen and water with the surface (barrier method).
If the surface is damaged, and the iron is exposed, the zinc is more likely to be oxidised (sacrificial method).
The zinc doesn’t oxidise because it forms a layer of ZnCO3.Zn(OH)2 on the surface which protects it from corrosion.
Fuel Cells
These are galvanic cells that need to be continually supplied with reactants.
An example of a fuel cell is the alkaline hydrogen fuel cell.
H2 and O2 are constantly supplied to the cell.
The OH- electrolyte interacts with the reactants and H2O is released.
(ANODE): 2H2(g) + 4OH-(aq) ⟶ 4H2O(l) + 4e-
(CATHODE): O2(g) + 2H2O(l) + 4e− ⟶ 4OH-(aq)
Primary Cell:
Primary cells are single-use batteries as they cannot be recharged.
dry cell:
An example of a primary cell is the dry cell.
The zinc coating serves as a container and the anode.
The cathode is a rod made of carbon surrounded by a paste of MnO2, ZnCl2, NH4Cl, carbon powder, and a small amount of water.
Alkaline Battery - Another type of dry cell
(ANODE): Zn(s) + 2OH−(aq) ⟶ ZnO(s) + H2O(l) + 2e−
(CATHODE): 2MnO2(s) + H2O(l) + 2e− ⟶ Mn2O3(s) + 2OH−(aq)
