Electrochem Flashcards
What increases emf in a galvanic cell
Increase in concentration of reactants
Decrease in concentration of products
LCP: Forward - Increases Voltage
Reverse - Decreases Voltage
How to increase current without affecting emf
Larger surface area of the electrode (Increases reaction rate)
A wider, shorter and more conductive salt bridge(Lowers internal resistance in cell)
Redox reaction
A reaction involving the transfer of electrons.
Oxidation
The loss of electrons.
Reduction
The gain of electrons.
Oxidising agents
A substance that accepts electrons.
Reducing agent
A substance that donates electrons.
Anode
The electrode where oxidation takes place.
Cathode
The electrode where reduction takes place.
Electrolyte
A substance that can conduct electricity by forming free ions when molten or dissolved in solution.
Brine
A highly concentrated solution of sodium chloride.
Galvanic cell
self-sustaining electrode reactions
conversion of chemical energy to electrical energy
Current is produced to the occurrence of spontaneous redox reactions taking place.
Has the capacity to produce current unless it reaches equilibrium or reaction runs to completion (Voltage = 0V when this happens)
Function of Salt bridge
To complete the circuit
- maintaining electrical neutrality in the half-cells
Why is a platinum (Inert metal used)
Metal is completely unreactive thus it provides a metal surface for redox reactions to take place
Standard Hydrogen Electrode
The standard hydrogen electrode consists of hydrogen gas (H2) bubbled through an electrolyte containing hydrogen ions (H+). Thus it is a redox electrode which forms the basis of the scale of oxidation-reduction potentials.
Standard electrode potential conditions
pressure = 101,3 kPa (1 atm) temperature = 298 K (25 ℃) concentration = 1 mol.dm−3
Ecell = Ecat - Eano
E>0 - Spontneous
E < 0 - Non-spontaneous
Electrolytic cells
electrode reactions that are sustained by a supply of electrical energy
conversion of electrical energy into chemical energy
Mercury cell
Adv: A relatively high percentage purity of sodium hydroxide is obtained (50%) because the NaOH is separated from the brine.
It also allows Cl2 and H2 to be formed in separate areas. Cl2 and H2 can combine explosively if they met.
Disadv: The mercury cell is more expensive to build, run and maintain than the other cells.
• Mercury is highly toxic causing the NaOH and H2 (High contamination) to be purified before they can be used (which is expensive).
• The toxicity of mercury causes environmental concerns as the disposal of waste mercury is extremely hazardous.
Diaphragm cell
Ad: Large quantities of sodium hydroxide and chlorine can be produced at reasonable costs and at quite acceptable levels of purity.
Dis: There is always a small amount of chloride in the NaOH produced.
• There are health and environmental problems with small losses of asbestos in making and using diaphragms. Asbestos is carcinogenic.
• Diaphragm is non-ion selective, so the control of ion movements is limited.
Membrane cell
Ad: There is no by-product of sodium chloride due to the membrane preventing the chloride and hydroxide ions to pass through.
• The use of this process results in virtually pure sodium hydroxide being produced as there is no contamination by chloride ions.
• There is very little harmful impact on the environment as the membrane is environmentally friendly.
• It is cheaper to run the membrane cell compared to the other cells.
How does a salt bridge maintain neutrality
Anions of the salt bridge move the anode to balance the increase in positive ions, cations of the salt bridge move the cathode to balance the decrease in the positive ions there
Observations at electrolysis of salt
Coating at the cathode due reduction of cations into metal
Bubbles at the anode due to oxidation of anions into a gas
Electroplating
If thing being coated with a metal like silver use an electrolyte with that cation (AgNO3)
Hall Heroult process
Aqueous Al(OH)3 formed by treating the bauxite/impure Al2O3 with NaOH. It is then heated to 1000C in order to convert it back to pure Al2O3 (Alumina)
Alumina then is melted to release Al3+ ions for extraction however it is dissolved in cryolite in order to lower its melting point
It is then put in a reaction vessel with carbon electrodes forming the anode while the graphite lining is the cathode
Cathode - Al3+ is reduced into Al
Anode - Oxide ions are oxidised to form Oxygen gas, react with carbon electrodes to form CO2
Potential risks of Hall Heroult process
Requires large amounts of electrical energy = Expensive
Green house emissions from CO2s
Fluoride wastes and emissions are dangerous to environment
Why is Na+ reduced in a mercury cell instead of water
Know that in the mercury cell, the nature of the mercury cathode adjusts the electrode potentials of the Na+ ions and water, so that the reduction of Na+ is now more likely than that of water
