redox (2)

Question 1

wwhat oxidation number to uncombined elements have

Answer 1

0

Question 2

what oxidation number is aluminium

Answer 2

+3

Question 3

what oxidation number does hydrogen have

Answer 3

+1
-1 in metal hydrides eg NaH

Question 4

what oxidation number does chlorine have

Answer 4

-1
in a compound with F or O it has positive value eg ClF3 it is +3

Question 5

what oxidation number does oxygen have

Answer 5

-2
in peroxides it is -1 eg H2O2
+2 in OF2

Question 6

what is an oxidising agent

Answer 6

accepts electrons from a species to oxidise it

Question 7

what is a reducing agent

Answer 7

donates electrons to species to reduce it

Question 8

rules for ionic half equations

Answer 8

1) balance all species
2) balance oxygen using H2O
3) balance hydrogen using H+
4) balance charges on species being oxidised using e-

Question 9

what is a half cell/electrode

Answer 9

an equilibrium system consisting of a solid metal in a solution of its own ions

Question 10

what is electrode potential

Answer 10

a measure of the equilibrium position for a half-cell / electrode

Question 11

what is a salt bridge made up of usually

Answer 11

KNO3

Question 12

why use a salt bridge in measuring electrode potential

Answer 12

maintains electrical neutrality within the cell by providing ions to either side depending on the charge built up in each half cell

solution they are soaked in should not react with either of the solutions in the half cells

Question 13

why not use a wire instead of salt bridge

Answer 13

to avoid further metal/ion potentials in the circuit and also to provide/remove ions to balance charge on either side

Question 14

describe a standard hydrogen electrode

Answer 14

-hydrogen gas at 100KPa
-hydrogen ions in solution at 1moldm-3
-platinum electrode for electron exchange with hydrogen ions
-298K temperature

Question 15

what is the potential of a hydrogen electrode

Answer 15

0

Question 16

which electrode is always the hydrogen electrode

Answer 16

left hand

Question 17

what does the voltmeter measure in a half cell

Answer 17

the difference in electron build up on the surface of the metals involved in the 2 half cells

Question 18

rules for drawing cell notation

Answer 18

-more positive cell on right except when using hydrogen
-double line = salt bridge
-single line = state change
-most oxidised species written closest to salt bridge

Question 19

why is a reference electrode necessary (hydrogen electrode)

Answer 19

it provides a stable, known potential that allows changes in a cell to be attributed to the working electrode

Question 19

how to calculate emf values

Answer 19

positive (runs right) - negative value (runs left)
emf always positive unless hydrogen electrode is in it as it is always on the left

Question 20

how do you use emf to tell if reaction is feasible

Answer 20

if its positive the reaction is feasible

Question 21

what is meant by the term oxidation number

Answer 21

the number of electrons lost/gained by an atom relative to its atomic state

Question 22

what is a disproportionation reaction

Answer 22

where an element in a single species is being simultaneously oxidised and reduced

Question 23

what is E cell directly proportional to

Answer 23

total entropy change and to lnK

Question 24

limitations of predictions made using standard electrode potentials

Answer 24

departure from standard conditions- temp or conc may change slightly from standard conditions reaction may have slow rate due to high Ea so it is unfeasible

Question 25

describe an acid fuel cell

Answer 25

hydrogen gas is fed into the anode, where it is split into protons and electrons. The protons pass through a semi-permeable membrane to the cathode, where they combine with oxygen and electrons to create water

Question 26

describe alkaline fuel cell

Answer 26

oxygen enters the cell from the cathode side and hydrogen enters from the anode side. oxygen receives the electrons coming through the external circuit, forming hydroxide ions in the electrolyte

Question 27

equations in acid fuel cell

Answer 27

H2 -> 2H+ + 2e- 1/2 O2 + 2H+ + 2e- -> H2O

Question 28

equations in alkaline fuel cell

Answer 28

H2 + 2OH- -> 2H2O + 2e- 1/2 O2 + H2O + 4e- -> 4OH-

Question 29

why are fuel cells good

Answer 29

only waste product is water very efficient compared to combustion engines lightweight don't require fossil fuels

Question 30

why are fuel cells not good

Answer 30

not widely available hydrogen often made from methane (non renewable) explosive when stored badly

Question 31

why can you not add starch indicator too early to a redox titration

Answer 31

it forms complexes with the iodine and prevents all of it from reacting

Question 32

if E cell is positive then what is lnK and s total

Answer 32

positive

Question 33

how to set up an electrochemical cell

Answer 33

1) obtain metals under investigation and clean with sandpaper to remove surface impurities 2) some metals have grease on surface from handling, wash with propanone and wear gloves 3) place each metal in solution of ions of the same metal 4) make salt bridge with filter paper soaked in KNO3

Question 34

why use platinum metal in your half cell

Answer 34

inert but electrically conductive

Question 35

why can't hydrochloric acid be used in KMnO4 titration

Answer 35

MnO4- would oxidise Cl- to Cl2 so affect the volume of KMnO4 required for titration

Question 36

why should dilute sulfuric acid be used and not conc acid in KMnO4 titration

Answer 36

conc acids are oxidising agents themselves so affect volume of KMnO4 required

Question 37

equation for KMnO4 titration

Answer 37

MnO4- + 8H+ + 5e- -> Mn2+ + 4H2O

Question 38

colour change for KMnO4 titration

Answer 38

purple-> pale pink

Question 39

equation for Fe2+ and MnO4- titration

Answer 39

5Fe2+ + MnO4- + 8H+ -> 5Fe3+ + Mn2+ + 4H2O

Question 40

reacting ratio of Fe2+ to MnO4-

Answer 40

5:1

Question 41

equation for C2O42- and MnO4- titration

Answer 41

5C2O42– + 2MnO4+ 16H+ -> 10CO2 + 2Mn2+ + 8H2O

Question 42

reacting ratio of C2042- to MnO4-

Answer 42

2.5 : 1

Question 43

what is electromotive force

Answer 43

a measure of the voltage of an electrochemical cell

Question 44

what is a salt bridge

Answer 44

an ionic connection between the solutions of two half-cells

Question 45

what is a storage cell

Answer 45

a rechargeable cell

Question 46

what is a fuel cell

Answer 46

uses a constant supply of fuel and oxidising agent to produce a voltage via a chemical reaction (without burning the fuel)

Question 47

what is the anode

Answer 47

the electrode where oxidation occurs

Question 48

what is the cathode

Answer 48

the electrode where reduction occurs