Chapter 20 - Electrochemistry Flashcards
A species is oxidized when
It loses electrons
What is transferred in electrochemical reactions?
Electrons from one species to another
A species is reduced
When it gains electrons
Oxidizing agent
What is reduced
Reducing agent
What is oxidized
Elements in their elemental form have an oxidation number of
Zero
The oxidation number of a mono atomic ion is
The same as its charge
Oxidation numbers of non metals
Tend to be negative
Oxygen is -2
Hydrogen is -1 when bonded to metal and +1 when bonded to nonmetal
Oxidation number of halogens
-1 usually
Sum of oxidation numbers in neutral compound
Equals 0
Sum of oxidation numbers in a polytonal ion
Is the charge on the ion
Half reaction method
- Assign oxidation numbers
- Write half reactions
- Balance each half
- balance elements other than O and H, balance O by adding water, balance H by adding H+, and balance charge by adding electrons - Multiply reactions by integers so tht the electron gained and lost is equal
- Add reactions
- Make sure balance in mass and charge
Balancing in basic solution
Once eqn is balanced add OH to each side to neutralize the hydrogen ion and create water
Voltaic cell
Setup where an external device is used to make electrons flow
Oxidation in voltaic cells occur at
The anode (-)
Reduction in voltaic cells occur at
The cathode (+)
Salt bridge and movement of ions in voltaic cell
Salt bridge is used to keep the charges balanced
Cations move toward cathode
Anions move toward anode
Electron flow in voltaic cell
Anode to cathode
Electron flow in a redox reaction
Electrons only spontaneously flow from higher to lower potential energy
Electromotive force (emf)
The potential difference between the anode and cathode in a cell
Also called cell potential Ecell
Cell potential
Measured in volts = J/C
The reduction potential for hydrogen
0 V
Cell potential at standard conditions can be found with this eqn
Ecell = Ered (cathode) - Ered (anode)
Relationship between oxidizers/reducers and reduction potential
The strongest oxidizers have the most positive reduction potentials
The strongest reducers have the most negative reduction potentials
The greater the difference between the reduction potentials
The greater the voltage of the cell
Free energy for a redox reaction
Delta G = -nFE
N is moles of electrons transferred
F is 96485 C/mol
Also applies under standard condition if E is standard
Nernst equation
-nFE = -nFEstandard + RTlnQ
Nernst equation at room temperature
E = Estandard - 0.0592/n logQ
As long as the concentrations are different
E will not be 0
