Redox Flashcards
Voltaic cell
-electrochemical cell that produces electrical currents from a spontaneous chemical reaction
Electrochemical cell
-called an electrolytic cell and it consumes electrical current to drive a non-spontaneous chemical reaction
A voltaic cell
-2 half cells physically separated from each other
-in each half cell oxidation or reduction occurs
-each half cell requires a conductive solid electrode to allow the transfer of electrons
-and electrolyte solution for ion exchange between the 2 half cells
-the two metal electrodes are connected by a wire
-contains a salt bridge: composed of saturated KNO3 or KCL
-anions (NO3^-) float to neutralize accumulation of positive charge at the anode
-Cations (k+) flow to neutralize accumulation of negative charge at the cathode
-Salt bridge complete the circuit
Anode and cathode
-oxidation occurs at the anode (-)
AN OX
-reduction occurs on the cathode (+) REDCAT
Cell notation
-Electrode I ELECTROLYTE II ELECTROLYTE I ELECTRODE
Redox reaction
-reactions where electrons are transferred from one atom to another
-atoms that lose electrons are being oxidized (LEO)
-oxidation and reduction must occur simultaneously
-atoms that gain electrons are being reduced (GER)
Reduction
-gaining electrons (charge becomes more negative)
-the reactant that is reduced is called the oxidizing agent
-reduction occur occurs when an atom is oxidation state decreases during a reaction (0 to -2)
-Reduction half reaction has electrons as reactants
-Oxidation
-losing electrons (becomes more positive)
-the reactant is oxidized is called the reducing agent
-oxidation occurs when atoms oxidation number increases during a reaction Ex: -4 to +4
-oxidation half reaction has electrons as products
Covalent compounds
-In these reactions, no electrons are actually lost or gained instead of sharing of electrons between two atoms and a covalent bond changes
-this change is represented by oxidation states
Oxidation state rules
- Any element alone = 0 (Na,Hg,O2)
- Single atom ions have oxidation states equal to their charge Na+ = 1 Cl- =-1
-in a neutral compound all oxidation numbers must add to zero
-in a polyatomic ion, they must add to the ions charge - Group 1 metals = +1
-Group 2 metals = +2
-Group 15 = usually -3
-Group 16 = usually -2
-Fluorine = -1
-Oxygen = usually -2
- Hydrogen = +1 with nonmetals and -1 with metals
-other halogens usually -1
-Lower number rules always win
Half reaction steps
-assign oxidation numbers and find what’s being reduced and what’s being oxidized
-write two separate half reactions
-Balance half reactions
-Balance O with H2O
-Balance H with H+
-If basic, neutralize H+ with OH-
-multiply to make electrons equal
-add the half reactions together
-count Atoms and charge
Inert lectrodes
-material that does not react with a redox reaction that provides a surface for electron transfer. It allows oxidation and reduction to occur without participating in the reaction itself.
Common ones: Pt Au C
When do we use Inert electrodes
-when there is oxidation or reduction of an ion without a solid metal
-when a gas is involved in their reaction
-One redox reaction involves only ions or gases an inert electrode is required to provide a surface or electron transfer
How do we determine when a voltaic cell is spontaneous
- From standard reduction potential of two half reactions
- From standard cell potential
- From delta G
Standard reduction potential
E^0 red: tendency for a chemical species to be reduced, and is measured in volts at standard conditions (data sheet)
-the higher of substance is on the standard reduction potential table, the stronger tendency has to be reduced (gain electrons)
-stronger oxidizing agent
-more positive reduction potential
Ex: silver ions have a strong tendency to gain electrons and become solid silver
-Silver ions are a stronger oxidizing agent
-the lower substance is on the standard reduction potential table the weaker tendency it has to be reduced
-stronger tendency to be oxidized (lose electrons)
-stronger reducing agent
-less positive reduction potential
Ex: zinc ions have a weak tendency to gain electrons
-zinc metal is a strong red reducing agent because it prefers to lose electrons and become zinc2+
How to determine people take cell spontaneous
-forever will take so to be spontaneous the overall cell potential (Ecell) must be positive
- Identify 2 half reactions from the standard reduction potential table.
- The one with higher E value will occur as production at the cathode (RED CAT)
- The one with the lower E value will be flipped (oxidized) at the anode
- Calculate E cell (cathode - anode)
-If Ecell = positive voltaic cell is spontaneous and will produce electricity
understanding reduction potential E
-A higher more positive (E) means the species really wants to be reduced (gains electrons) “ good at reduction”
-stronger oxidizing agent
-a lower more negative (E) means the species does not want to be reduced and instead prefer to lose electrons (be oxidized)
-stronger reducing agent
Delta G, E, K
-If E is positive the reaction is spontaneous because delta G is negative reaction favours product products
-If E is negative the reaction is non-spontaneous because delta G is positive reaction favours reactants and does not occur under standard conditions unless energy is added
-If K is greater than 1 reaction favours products
-if K is less than 1 reaction favours reactants
Volta cell at non-standard condition
-Voltaic cell is at non-standard conditions when the concentrations of the reactants and products are not 1M
From the Nernst equation
- Calculate cell potential at any given concentration.
- Calculate the concentration of region at any non-standard cell potential
- Cell potential and concentration in a concentration cell.
- Calculate solubility product (Ksp)
Application of Nernst equation
- Calculate self potential at any given concentration.
Step 1: calculate E cell
(cathode-anode)
Step 2. Determine Q, redox reaction must be balanced.
Step 3. Find the value of N
Step 4. Calculate each cell from the Nernst equation.
Application of Nernst equation
- Calculate the concentration of reagent at any measured self potential
- Determine cathode and anor reactions.
- Write overall reaction.
- Calculate E cell (cathode-anode)
- Calculate Q and solve for X.
Concentration cell
-2 half cells with the same electrodes, but the solutions they are in have different ion concentrations
-the driving force is a concentration difference between the 2 half cells
-electrons will flow from the electrode in the less concentrated solution to the electrode in the more concentrated solution
(anode to cathode)
-Since the 2 half reactions are just the reverse of each other. The standard cell potential is 0.00V
Calculate cell potential in a concentration cell
Step 1: calculate E cell
(cathode-anode)
Step 2: determine Q, redox reaction must be balanced
Step 3: find N
Step 4: calculate E cell from the Nernst equation or it will be given
Step 5: plug into equation and solve for x
Calculate solubility product (Ksp) from non-standard self potential
Voltaic cell
-electrical current is generated, a spontaneous redox reaction occurs
-Cathode gains electrons and has (+) charge (electrons are drawn to cathode Cu)
-Anode loses electrons and has a (-) charge
-electrons flow out of anode Zn
