U3AOS1 - Galvanic Cells as a source of energy Flashcards
Redox Reactions
Transfer of electrons, from one species to another, in two half reactions (oxidation and reduction)
Oxidation Number
number of electrons lost or gained by an atom to form a chemical bond with another species
Examples of Redox Reactions
Combustion, Photosynthesis
How to determine if a redox reaction has occured
If the Oxidation Number of the molecules has changed
What is the oxidation number of a free element (e.g. O2)?
Zero
What is the Oxidation of a simple ion (e.g. Na+)?
Equal to the charge of the ion
(so Na+ is +1)
What is the sum of the oxidation numbers of a compound (e.g. KCl or SO42-)
KCl -> ON of 0
SO42- -> ON of -2
What is Hydrogens Oxidation Number (in a compound - including special rule)
Normally +1
Except in metal hydrides, where its -1 (e.g. CaH2)
What is Oxygens Oxidation Number (in a compound - including special rule)
Normally -2
Except in peroxides where its -1 (e.g. H2O2)
Conjugate Reducing & Oxidizing Agents
An electron donator and its corresponding electron reciever
Spectator Ions
Ions that are not involved in the redox reaction - will have no change in oxidation number
Do Noble Gases react in a redox reaction
No - they can have no change in electrons - as they have a full shell
Oxidation (electron, ON, sample reaction)
Reactant loses an Electron
Increase of ON
Reactant = Product + e-
Reduction (electron, ON, sample reaction)
Reactant gains an electron
Decrease in ON
Reactant + e- = Product
Reduction Agent (reductants)
substance that causes the other substance present to undergo reduction
it will itself, undergo oxidation
Oxidation Agents (oxidants)
substance that causes the other substance present to undergo oxidation
it will itself, undergo reduction
How to balance a half equation
K - key elements
O - balance oxygen by adding H2O
H - balance hydrogens by adding H+
E - balance electrons
S - include states
How to balance a basic (alkaline) half equation
use KOHES,
Then, for every H+ present, add OH- to BOTH sides (should cancel out, creating waters instead)
What is a Galvanic Cell (+basic structural features)
Electrochemical Cell in which chemical energy from a - spontaneous redox reaction - is converted into - electrical energy -
Each cell contains two half cells and a salt bridge
Common Design Feature of Galvanic Cells - Electrodes
material that is able to conduct electricity
Often a metal (which can react) or a pure carbon rod (inert)
Common Design Feature of Galvanic Cells - Anode
Negatively Charged Electrode
Oxidation occurs at the anode (will lose electrons)
Label as ‘Anode (-ve)’
Common Design Feature of Galvanic Cells - Cathode
Positively Charged Electrode
Reduction occurs at the cathode (gains electrons)
Label as ‘Cathode (+ve)’
Common Design Feature of Galvanic Cells - Salt Bridge
connection between two half cells - allows the flow of ions into the two half cells
purpose is to maintain electrical neutrality, and complete the circuit (allowing electrons to flow through the connecting wire) by preventing polarity
Common Design Feature of Galvanic Cells - Salt Bridge - how it balances the anode half cell
Over time, this cell becomes positively charged (as there is a loss of electrons - oxidation)
therefore, the salt bridge will provide a flow of negatively charged substances to maintain electrical neutrality
Common Design Feature of Galvanic Cells - Salt Bridge - how it balances the cathode half cell
Over time, this cell becomes negatively charged (as there is a gain of electrons - reduction)
therefore, the salt bridge will provide a flow of positively charged substances to maintain electrical neutrality
Common Design Feature of Galvanic Cells - Salt Bridge - composition requirements
Must be:
- soluble in water (doesn’t form an insoluble compound - precipitate - as this will prevent a circuit from flowing as electrons cannot flow through solid ionic substances)
- nonreactive
Chemical Processes of Galvanic Cells - Electron Ion Flows
Electrons will flow form the anode to the cathode
and charged substances will flow from the salt bridge
Common Design Feature of Galvanic Cells - Separation of Half-Cells
allows for electrical current to be captured through the connecting wire
Common Design Feature of Galvanic Cells - Potential Difference
Ecell = Ereduction - Eoxidation
What is a Spontaneous Redox Reaction
reactions that can occur naturally (spontaneous), and do not need to be forced
What is a direct, spontaneous redox reaction
Occurs when the reacting species are in the same vessel - with chemical energy being released as heat
e.g. combustion
What is an indirect, spontaneous redox reaction
Occurs when the reacting species are kept separately in different vessels
chemical energy is released predominantly as electrical energy (although not 100% efficient, some energy will be released as heat)
e.g. galvanic cells
Types of Half Cells
Solid/Aqueous Cell: The electrode will participate in the reaction
Double Aqueous Cell: Inert Electrode will be used
Gas/Aqueous Cell: Inert Electrode will be used - which is often covered in a fine powdery - which increases the surface area of electrode - increasing reaction rate (for platinum electrodes - its called platinum black)
Designing Galvanic Cells - how to influence voltage and current
Voltage - depends on how much energy is being released in the reaction - can be determined by looking at the electrochemical series
Current - depends on the rate of reaction - with a higher rate of reaction yielding a higher current because the transfer of electrons is occurring at a faster rate
