3/14 Redox

Question 1

Oxidation

Answer 1

loss of e-

Question 2

reduction

Answer 2

gain of e-

Question 3

O.N rules

Answer 3

1) uncombined elements have an O.N of 0
2) overall OS of compounds is 0
3) elemental ions ahve an O.S equal to charge
4) overall OS of moelcular ion equal to charge
5) group 1: always +1
6) group 2: always +2
7) aluminium: always +3
8) hydrogen: always +1 (except in metal hydrides, eg NaH where it is -1)
9) oxygen: always -2 (except in peroxides, eg H202 where it is -1)
10) group 7: always -1 (except when combined with a more electronegative element)

Question 4

disproportionation

Answer 4

element simultaneously oxidised and reduced

Question 5

oxidising agent

Answer 5

what has been reduced, electron acceptor

Question 6

reducing agent

Answer 6

what has been oxidised, electron donor

Question 7

redox in acidic conditions

Answer 7

any oxygen lost is H20 in products, add H+ ions to balance the H in H20

Question 8

half cell

Answer 8

1) made up of a solid metal electrode and 1.0 mol dm-3 solution of X ions
2) an eq is set up between X(s) and X ions (aq)
3) eq can shift: forward reaction is reduction, reverse reaction is oxidation

Question 9

combining half cells

Answer 9

1) any two half cells can be combined to create a potential difference (voltage)
2) put most negative E0 value at top
3) apply anticlockwise rule
4) high resistance voltmeter
5) salt bridge (AgNO3) completes circuit

Question 10

LOAN

Answer 10

LHS
oxidation
anode
negative

Question 11

RRCP

Answer 11

RHS
reduction
cathode
positive

Question 12

convention

Answer 12

oxidised state on left and reduced state on right
double line to represent salt bridge
single line to represent different phase

Question 13

half cells of ions in solution

Answer 13

Fe3+(aq) + e- –>

Question 14

standard hydrogen electrode

Answer 14

1) the redox power of half cells is relative
2) redox potential of all half cells measured against the SHE
3) platinum:unreactive and porous: large SA fro e- transfer
4) SHE connected to another half cell and potential difference measured
5) 2H+(aq) + 2e- –> H2(g) 0.00v
6) 298K, 1.0mol dm-3 H+ ions eg HCL, H2 gas pumped in at 100kPa

Question 15

down the electrochemical series

Answer 15

1) increase in oxidising power
2) eq lies to right
3) ions readily gain electrons
4) easily reduced
5) last one is strongest O.A Ag+ (aq)

Question 16

up the electrochemcial series

Answer 16

1) increase in reducing power
2) eq lies to left
3) atoms readily lose e-
4) easily oxidised
5) first one is strongest R.A Li (s)

Question 17

calculating emf

Answer 17

most positive - most negative
oxidised cell value - reduced cell value
emf always postivie

Question 18

greatest EMF value

Answer 18

largest difference in E0 values

Question 19

smallest EMF value

Answer 19

smallest difference in E0 values

Question 20

Li(s)

Answer 20

stronger reducing agent
most easily oxidised
most negative E0 value

Question 21

Ag+ (aq)

Answer 21

strongest oxidising agent
moest easily reduced
most postive E0 value

Question 22

Predicting feasibility of reactions

Answer 22

predictin if reaction will occur (spontaneous)

only happens if reactants are top right and bottom left

Question 23

limitations to predicting feasibility using E0 values

Answer 23

all E0 values are based on standard conditions: if any of these change then the Ecell values will also change
eg Cu will react with conc h2so4 (10moldm-3) but not with 1 moldm-3
kinetics (rate) of reaction may be so slow that no real reaction is seen due to high AE

Question 24

effect of changing concentration

Answer 24

1) chateliers principle: when conc or temperature changed
2) increase conc of reactant and decrease conc of product: Ecell increases
3) decrease conc of reactant and increase conc of product: Ecell decreases
4) no E0 value because deviated from standard conditions

Question 25

effect of changing temperature

Answer 25

1) vast majoity of cells with a postiive E0 cell value (spontaneous) will have a negative deltaH (exothermic)
2) increase in temp shifts eq to left favouring endothermic reaction (against the expected reaction) decreasing Ecell
3) decrease in temp shifts eq to right favouring the exothermic reaction (with the expected reaction) increasing Ecell

Question 26

disproportionation and E0 values

Answer 26

Cu2+(aq) + Cu+(aq)–> Cu2+(aq) + Cu(s)
cu+ oxidises and reduces itself
of the reaction is feasible then the reaction can occur

Question 27

storage cells (Zn, C batteries)

Answer 27

1) at the anode (LHS), zinc oxidises to Zn2+ and releases e-
2) e- flow through deice and through to the carbon cathode
3) from the carbon cathode flow through the graphite rod and leave into the electrolyte
4)electrolyte: NH4Cl
5) non rechargeable: zinc used up walls get thinner over time and may leak
6) battery runs out when NH4+ runs out
7)Zn2+ + 2e- –<>— Zn
2NH4+ + 2e- —> 2NH3 + H2

Question 28

Relationship between Ecell and entropy

Answer 28

if E0 cell is positive, it is considered a spontaneous/feasible reaction - delta G must be negative
the more positive E0 cell the more negative delta G
the more positive delta S, the more negative delta G
the more negative delta G, the more positive E0 cell
therefore delta S and E cell are directionally proportional

Question 29

relationship between E cell and lnK

Answer 29

lnK and E0 cella are directionally proprotional

the larger the lnK the more positive the E0 cell

Question 30

the lithium ion battery: during use

Answer 30

1) copper anode on left (not involved in redox), next to it is graphite to allow flow of e- (store of delocalised e-)
2) aluminium cathode on right(not involved in redox), next to it is lithium cobalt oxide Li+(CoO2)-
3) lithium oxidises to lithium ions and loses e- which flow through graphite and cu anode through device
4) lithium ions move through electrolyte to the cobalt oxide near the right and gain e’s that flow to the aluminium cathode to form Li+(CoO2)-
recharge: opposite reactions

Question 31

advantages of lithium ion battery

Answer 31

1) store lots of energy
2) small
3) high voltage - powerful
4) fast recharge

Question 32

disadvantages of lithium ion battery

Answer 32

1) lithium very reactive
2) lithium very toxic
3) high production cost
4) becomes less efficient over time

Question 33

fuel cells

Answer 33

reaction between hydrogen rich fuels such as ethanol and oxygen can be utlisied to generate a voltage
1)liquid ethanol and liquid water enter left into a redox reaction releasing hydrogen ions and e- and co2 emitted as a waste product
2)e- build up inside anode causing an electrode potential thus e- flow through device
3) hydrogen ions flow through proton membrane in the middle of the fuel cell
4) oxygen flows thorugh inlet on the rHS where it reacts with the hydrogen ions that have come through and the e- that have come through the cathode which creates water as a by product
5)oxidation at the anode
reduction at the cathode
6)C2H5OH + 3H2O –>12H+ + 12e- + 2CO2 Oxidation
3O2 + 12H+ + 12e- –>6H2O reduction
C2H5OH + 3O2 –> 3H20 + 2CO2

Question 34

hyrdogen fuel cell alkaline electrolyte

Answer 34

1)H2(g) goes in at the anode
2)O2 gas goes in the cathode
3) at the anode, the hydrogen combines with hydoxide ions to form water and electrons (oxidation)
2H2+40H- –> 4h2O + 4e-
4) water is expelled as a by product whilst the e-‘s flow through anode through the device
the e-‘s come through the cathode and combine with oxygen and water to form hydoxide ions (reduction)
o2 + 2H2O + 4e- –> 4OH-
5) this is how OH- ions are produced for the reaction at the anode, these OH- ions act as an alkaline electrolyte which can move through the membrane
6) these OH- ions dont actually get used up as 2H2 + 02 –> 2H2O

Question 35

hydrogen fuel cell for acidic electrolyte

Answer 35

1) anode: 4H+ +4e- —> 2H2O
Hydrogen ions move from left and right with acidic electrolyte compared to alkaline electrolyte where OH- ions move from right to left

Question 36

Pros of hydrogen fuel cell

Answer 36

1) 0 emissions just water and no greenhouse gases
2)highly efficient
3)h2 + o2 readily available
4)maintain a constant voltage as voltage in traditional cells tend to decrease near end of life
5)number of options for hydrogen storage
liquefied under pressure
adsorbed on a solid medium
absorbed in a solid medium

Question 37

Cons of a hydrogen fuel cell

Answer 37

1) very expensive to produce the fuel cells
2) toxic chemicals used in production
3) h2 comes from h2o via electrolysis which is expensive and energy used may come from fossil fuels or h2 may come from hydrocarbons
4) storage of h2 is dangerous
5) limited life span