Electrode potentials Flashcards
what is a voltaic cell?
type of electrochemical cell which coverts chemical energy into electrical energy
how can a voltaic cell be made?
-two different half cells are connected allowing electrons to flow.
-ion half-cell contains ions of same element in different oxidation state in solution
why should chemicals of the two half-cells not be mixed?
if mixed, electrons would flow in an uncontrolled way and heat energy would be released rather than electrical energy
In what direction do the electrons flow in a voltaic cell?
depends upon the relative tendency of each electrode to release electrons
if we have a half cell with 2 aqueous ions, we use what electrode?
Platinum electrode as it is inert and electrically conductive
which metals in electrodes loses and gains electrons?
-the electrode with a more reactive metal loses electrons and is oxidised-negative electrode
-the electrode with less reactive metal gains electrons and is reduced- positive electrode
Describe standard electrode potential of hydrogen
-standard chosen is a half-cell containing hydrogen gas and a solution containing H+(aq) ions
-platinium electrode used
-standard conditions
-electrode potential of 0V as it is the standard
what are the standard conditions?
-soutions of 1mol/dm cubed
-298K (25 degrees)
-100kPa
standard electrode potential definition?
the e.m.f of a half cell connected to a standard hydrogen half cell under standard conditions
the electrodes connected by wire allow the flow of ?
electrons
salt bridge allows the flow of?
ions
what does the salt bridge contain?
concentrated solution of an electrolyte that does not react with either solution
the more negative the electrode potential…
-the greater the tendency to lose electrons and undergo oxidation
-the less the tendency to gain electrons and undergo reduction
-the greater the reactivity of a metal in losing electrons
the more positive the electrode potential…
-the greater the tendency to gain electrons and undergo reduction
-the less the tendency to lose electrons and undergo oxidation
-the greater the reactivity of a non-metal in gaining electrons
Metal and non-metals tend to have what electrode potential?
-metals tend to have negative electrode potentials
-non-metals tend to have positive electrode potentials
how do you calculate standard cell potential?
electrode potential of positive electrode - electrode potential of negative electrode
how do you predict redox reactions using electrode potentials?
a reaction should take place between an oxidising agent on the left and a reducing agent on the right, provided that the redox system of the oxidising agent has a more positive electrode potential than the redox system of the reducing agent
how do you work out the overall equation?
-balance the electrons
-a more redox system with a more positive electrode potential will reverse the redox system of a less positive electrode potential
-combine equations
-cancel electrons
what are the limitations to predicting reactions using electrode potentials?
-very large activation energy which can result in a slow rate
-concentration of solutions should be 1mol/dm cubed
-conditions may not be standard conditions
what happens if the concentration of solution is higher than 1mol/dm cubed?
-equilibrium will shift to the right, removing electrons from the system and making the electrode potential more negative
what happens if the concentration of solution is less than 1mol/dm cubed?
the equilibrium will shift to the left increasing electrons in the system and making the electrode more negative
what are the three main types of cells?
primary, secondary and fuel cells
Describe primary cells?
- non-rechargeable and single use
-when chemicals used up, voltage will fall, the battery will go flat and the cells will be discarded or recycled
-used in low current, long storage devices e.g clocks and smoke detectors
How is electrical energy produced in a primary cell?
-produced by oxidation and reduction at the electrodes
-reactions cannot be reversed
Describe most modern primary cells?
based on zinc and manganese dioxide, Zn/MnO4, and a potassium hydroxide alkaline electrolyte
What are the systems (equations) and their electrode potential in most modern primary cells? (Zn/MnO4)
1) ZnO(s) + H2O(l) + 2e- (reversible reaction) Zn(s) + 2OH-(aq)
electrode potential: -1.28V
2) 2MnO2(s) + H2O(l) + 2e- (reversible reaction) Mn2O3(s) + 2OH-
What is the oxidation, reduction equations and their overall cell reaction in most modern primary cells? (Zn/MnO4)
oxidation: Zn(s) +2OH-(aq) -> ZnO(s) + H2O(l) + 2e-
reduction: 2MnO2(s) + H2O(l) + 2e- -> Mn2O3(s) +2OH-(aq)
cell reaction: Zn(s) + 2MnO2(s) -> ZnO(s) + Mn2O3(s) electrode potential: 1.43V
Describe secondary cells?
-rechargeable, reaction producing electrical energy is reversible, chemicals in cells are regenerated
what are common examples of secondary cells?
- lead-acid batteries used in car batteries
- nickel-radium, NiCd, cells and nickel-metal hydride, NiMH- the cylindrical batteries used in radios, torches
-lithium-ion and lithium ion polymer cells used in laptops, cameras and cars
describe fuel cells?
-fuel cell uses the energy from the reaction of a fuel with oxygen to create a voltage
-fuel and oxygen flow into fuel cell and the products flow out. The electrolyte remains in the cell
-Fuel cells can operate continuously provided that the fuel and oxygen are supplied into the cell
-don’t have to be recharged
Describe hydrogen fuel cells?
-water is the only combustion product
-alkali or acid electrolyte
what are the redox systems with electrode potentials found in an alkali hydrogen fuel cell?
2H2O(l) +2e- (reversible reaction) H2(g) + 2OH-(aq) electrode potential: -0.83V
1/2O2(g) + H2O(l) + 2e- (reversible reaction) 2OH-(aq) electrode potential: 0.40V
What is the oxidation, reduction and overall equation in an alkali hydrogen fuel cell?
oxidation: H2(g) + 2OH-(aq) -> 2H2O(l) +2e-
reduction: 1/2O2(g) +H2O(l) + 2e- -> 2OH-(aq)
overall: H2(g) + 1/2O2(g) -> H2O(l)
electrode potential: 1.23V
What is the oxidation, reduction and overall equation in an acid hydrogen fuel cell?
oxidation: H2(g) -> 2H+(aq) + 2e-
reduction: 1/2O2(g) + 2H+(aq) + 2e- -> H2O(l)
overall: H2(g) + 1/2O2(g) -> H2O(l)
electrode potential: 1.23V
what are the redox systems with electrode potentials found in an acid hydrogen fuel cell?
2H+(aq) + 2e- (reversible reaction) H2(g)
electrode potential: 0.00V
1/2O2(g) + 2H+(aq) + 2e- (reversible reaction) H2O(l)
electrode potential: 1.23V
Describe a lead-acid battery
negative electrode: lead (Pb)
positive electrode: lead with lead oxide coating (PbO2)
electrolyte: dilute sulfuric acid
-rechargeable battery used in cars
what are the common rechargeable cells?
lead-acid batteries, lithium ion cells
How is a lead-acid battery used in cars
-When the car is in motion, the generator provides a push of electrons that reverses the reaction and regenerates lead and lead(IV) oxide
-Lead-acid batteries are designed to produce a high current for a short period of time, hence their use in powering a starter motor in car engines
What are the disadvantages of lead-acid batteries
-They are very heavy
-They contain toxic materials: lead and lead(IV) oxide
-The sulfuric acid electrolyte is very corrosive
-challenges of disposal
Describe lithium ion cells
-powers mobile devices
-At the negative electrode (graphite, carbon), Lithium is discharging
Li -> Li+ + e-
-Graphite acts as a support medium for the Li+ ions
-Li+ ions move to the positive electrode, Cobalt (IV) oxide, across the electrolyte
-during charging Li+ ions move to positive electrode, during discharging Li+ ions move to negative electrode
Advantages of lithium ion cells
-Lithium is the lightest metal.
-Lithium is one of the strongest reducing agents (meaning that it can create a relatively high voltage).
-Lithium ion batteries can be constructed without any moisture whatsoever (saving weight and avoiding water-related damage over time).
disadvantages of lithium ion battery?
-cost
-cell temperature is monitored to prevent temperature extremes
Describe fuel cells
generate electricity from electrochemical reaction in which oxygen (from air) and a fuel (e.g hydrogen) combine to form water. Electrolyte can be acidic or alkali.
what is the overall equation in a hydrogen fuel cell?
2H2 + O2 -> 2H2O
Disadvantages of hydrogen fuel cells?
-producing hydrogen gas from fossil fuels releases greenhouse gases. Therefore hydrogen should be produced by electrolysis.
-hydrogen is highly flammable
-hydrogen is difficult to store
Describe what happens in a hydrogen fuel cell with acidic electrolyte?
-hydrogen gas is oxidised at anode
2H2 -> 4H+ + 4e-
-electrons travel via external circuit
-H+ ions travel through electrolyte and membrane
-oxygen enters and is reduced at the cathode.
O2 + 4H+ + 4e- -> 2H2O
- A catalyst speeds up the reaction with oxygen
Describe what happens in a hydrogen fuel cell with alkaline electrolyte?
-At the cathode, platinum (catalyst) causes hydrogen to split into H+ ions and electrons
-electrons travel via external circuit
-In electrolyte, H+ ions and OH- ions react to form water
-reaction at the negative electrode: 2H2 + OH- -> 4H2O + 4e-
-oxygen enters the cell
-reaction at the positive electrode: O2 + 2H2O + 4e- -> 4OH-
what are the limitations for using electrode potentials to predict reactions?
-activation energy may be too high to overcome
-rate of reaction is too slow
