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redox ii Flashcards

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1
Q

what is oxidation

A

loss of electrons
when oxidation number increases
loss of hydrogen
gain of oxygen

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2
Q

what is reduction

A

gain in electrons
decrease in oxidation number
gain in hydrogen
loss of oxygen

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3
Q

s block metals tend to react by being …

A

oxidised (lose electrons to form positive ions)

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4
Q

p block metals can react by being …

A

oxidised (losing electrons to form positive ions)

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5
Q

p block non metals can react by being …

A

reduced (gaining electrons to form negative ions)

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6
Q

d block metals tend to get …

A

oxidised (losing electrons to form positive ions)

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7
Q

electrochemical cells make …

A

electricity

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8
Q

how can electrochemical cells be made ?

A

made by two different metals dipped in salt solutions of their own ions connected by the wire

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9
Q

what are the two reactions that always take place in a electrochemical cell ?

A

oxidation and reduction so its a redox process

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10
Q

oxidation always happens at the …

A

anode(positive electrode)

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11
Q

reduction always happens at the …

A

cathode(negative electrode)

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12
Q

which metal becomes the anode

A

the more reactive metal becomes the anode as it is easily oxidised because it gives up its electrons more easily

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13
Q

which metal becomes the cathode

A

less reactive metal

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14
Q

where do the electrons flow from ?

A

the anode

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15
Q

in zinc/copper electrochemical cell which half cell do the electrons flow from

A

zinc as it loses electrons more easily (so in copper half cell it receives the same amount of electrons that left the zinc half cell)

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16
Q

in zinc/copper electrochemical cell which which metal is oxidised which metal is reduced

A

zinc is oxidised as it loses electrons more easily copper is reduced as it gains electrons more easily

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17
Q

which way do the electrons flow in electrochemical cell

A

from more reactive metal to least reactive metal

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18
Q

what does the voltmeter do in the circuit

A

shows the voltage between two half cells which is called cell potential or EMF or E(cell)

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19
Q

what is salt bridge made from

A

filter paper soaked in salt solution

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20
Q

what is the salt bridge for

A

salt ions flow through the cell to complete the circuit and balance out the charges in the beaker

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21
Q

if neither products or reactants are solid what can you use for electrodes

A

has to conduct electricity
has to be inert
eg pt but its expensive so graphite is used instead

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22
Q

when drawing electrochemical cells where should you draw the anode and the cathode

A

anode(where oxidation happens) on the left and cathode (where reduction happens) on the left

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23
Q

the reactions at both electrodes are …

A

reversible

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24
Q

method to construct electrochemical cell

A

.get strip of each metal and clean the surfaces using piece of emery paper/sandpaper

.clean grease or oil from electrodes using propane don’t touch cells with your hands after this or you could transfer back the grease

.place each electrode into a beaker containing solution of ions of that metal

.create salt bridge to link solutions together by dipping filter paper in salt solution ends of filter paper should be immersed in the solution

.connect electrodes to voltmeter using crocodile clips and wires and wires

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25
Q

the electrode potential is a measure of ?

A

how easily the substance is oxidised in the half cell

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26
Q

the half reaction with the more positive electrode potential value goes in what direction ?

A

forward

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27
Q

the half reaction with the less positive electrode goes?

A

backwards

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28
Q

if half cell Zn^2+ + 2e^- —>Zn has electrode potential of -0.76v
and half cell Cu^2+ + 2e^- —> Cu has electrode potential of +0.34
write the electrode potential equation

A

Cu^2+ + Zn —> Cu + Zn^2+

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29
Q

you measure the electrode potential of a half cell against what ?

A

standard hydrogen electrode

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30
Q

what is the standard electrode potential ?

A

voltage measured under standard conditions when the half cell is connected to a standard hydrogen electrode

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31
Q

what are the standard conditions when measuring standard electrode potential ?

A

*solution of the ions you are interested in must have a concentration of 1.00 mol dm⁻³
*the temperature must be 289 K (25 C)
*the pressure must be 100 kPa

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32
Q

what is the equation for the reaction at the hydrogen electrode ?

A

2H⁺(aq) + 2e⁻ ⇌ H₂(g)

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33
Q

when drawing standard hydrogen electrode where is it positioned?

A

on the left

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34
Q

the standard hydrogen electrode is a reference electrode which means

A

it has a value of 0.00V meaning the voltage reading will be equal to the E of the other half cell

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35
Q

E(cell) =

A

E(reduction)-E(oxidation)

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36
Q

cell potential will always be what charge?

A

positive as the more negative E is being taken away from the more positive E

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37
Q

what conditions change the equilibrium position in a half cell?

A

temperature pressure and concentration

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38
Q

changing the equilibrium position in half cell will affect the ?

A

cell potential

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39
Q

how can you get around, equilibrium position affecting the cell potential?

A

by using standard conditions which means u always get the same value for electrode potential and you can compare the values for different cells

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40
Q

more reactive metals have a more negative or positive standard electrode potential ?

A

more positive

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41
Q

if a reaction is thermodynamically feasible then then overall potential will be ?

A

positive if its negative then it isn’t feasible

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42
Q

electrode potentials can predict wether disproportionation reactions will happen, if the overall cell potential is negative will the element disproportionate in solution

A

no overall cell potential is negative and so element will not disproportionate in solution

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43
Q

the half reaction with the more negative electrode potential value E goes

A

backward

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44
Q

Zn²⁺(aq) 2e⁻ ⟶ Zn(s) E=-0.76V
Cu²⁺(aq) + 2e⁻ ⟶Cu(s) E=+0.34
what is the full equation

A

Cu²⁺(aq) + Zn(s) ⇌ Cu(s)+ Zn²⁺

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45
Q

in Cu²⁺(aq) + Zn(s) ⇌ Cu(s)+ Zn²⁺
what is the oxidising agent and what is the reducing agent

A

zinc is reducing agent and copper as the oxidising agent

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46
Q

how do you measure the electrode potential of a half cell ?

A

measure it against standard hydrogen electrode

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47
Q

what is the standard electrode potential

A

E of a half cell is the voltage measured under standard conditions when the half cell is connected to a standard hydrogen electrode

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48
Q

what are the standard conditions when measuring electrode potential of a cell

A

*the solutions of the ions you’re interested in must have a concentration of 1.00 mol dm⁻³
*the temperature must be 298 K (25°c)
*the pressure must be 100 kPa

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49
Q

what is the equation for reaction at the hydrogen electrode

A

2H⁺(aq) 2e⁻ ⇌ H₂(g)

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50
Q

when drawing the standard hydrogen electrode where is it positioned

A

on the left

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51
Q

standard hydrogen electrode is a reference electrode what does this mean?

A

the hydrogen half cell has a value of 0.00V which means voltage reading will be equal to E of the other half cell

52
Q

E(cell)=

A

E(reduction - oxidation)

53
Q

cell potential will alway be positive or negative ?

A

positive because the negative E is always being taken from bigger E

54
Q

what conditions affect equilibrium position in a half cell

A

temperature pressure concentration

55
Q

what does changing the equilibrium position In a half cell do ?

A

changes the cell potential

56
Q

to get around equilibrium position changing cell potential what can you do ?

A

use standard conditions to measure electrode potentials, using these conditions means you always get the same value for the electrode potential and you can compare values for different cells

57
Q

more reactive metals have more ?

A

negative standard electrode potentials

58
Q

more reactive non metals have more?

A

positive standard electrode potentials

59
Q

if a reaction is thermodynamically feasible the overall potential will be?

A

more positive. a reaction isn’t feasible if E is negative

60
Q

for a disproportionation reaction to happen the cell potential has to be ?

A

positive

61
Q

a prediction using E only states if a reaction is possible under standard conditions the prediction might be wrong if ?

A

*the conditions are not standard
*the reaction kinetics are not favourable.(rate of reaction may be slow that the reaction might not appear or reaction has a high activation energy which stops it happening)

62
Q

entropy change is bigger the ?

A

the bigger the cell potential

63
Q

What are energy storage cells

A

Batteries

64
Q

How can you work out the voltage of the energy storage cells

A

Using electrode potentials of the substances in the cell

65
Q

when drawing conventional representation which half cell goes on the left

A

the one with the more negative potential

66
Q

when drawing conventional representation what goes in the middle of the centre of the diagrams

not the salt bridge

A

the oxidised forms

67
Q

when drawing conventional representation what goes on the outside of the diagrams

A

the reduced forms

68
Q

when drawing conventional representation what do double vertical lines represent

A

salt bridges

69
Q

when drawing conventional representation what do single lines represent

A

different physical states

70
Q

when drawing conventional representation when do you use commas

A

when species in the same half cell are in the same physical state

71
Q

when drawing conventional representation if you have standard hydrogen electrode where should it go

A

on the left

72
Q

when drawing conventional representation what should you do if you use platinum lead or other inert electrode

A

put it on the outside

73
Q

when should you use inert or platinum electrode

A

when a solution is fully aqueous

74
Q

Zn²⁺(aq) + 2e⁻ ⟶ Zn(s) E=-0.76
Cu²⁺(aq) + 2e⁻ ⟶ Cu(s) E=0.34
draw the conventional representation

A

Zn┆Zn²⁺(aq)┆┆Cu²⁺(aq)┆Cu(s)

75
Q

in fuel cells where are the chemicals that generate electricity

A

in the electrodes and electrolyte

76
Q

cells are non rechargeable when

A

reactions that occur within them are non reversible

77
Q

a fuel cell uses

A

the energy from the reaction of a fuel with oxygen to create a voltage

78
Q

hydrogen oxygen fuel cells will maintain a constant voltage over time if

A

they are fed fresh O₂ and H₂

79
Q

advantages of fuel cell

A

*less pollution and less CO₂
greater efficiency

80
Q

limitations of hydrogen fuel cells

A

*expensive
*has a limited lifetime requiring regular replacement
*high production cost
*uses toxic chemicals in their production

81
Q

hydrogen can be stored in fuel cells

A

*as a liquid
*adsorbed on the surface of a solid material
*absorbed within a solid material

82
Q

what advantages do ethanol fuel cells have over hydrogen fuel cells

A

*ethanol made from renewable sources in carbon neutral way
*materials used to produce ethanol by fermentation are abundant
*ethanol less explosive and easier to store than hydrogen
*new petrol stations would not be needed as ethanol is a liquid fuel

83
Q

where are chemicals stored in a fuel cell

A

separately outside the cell

84
Q

when are the chemicals fed into a fuel cell

A

when electricity is required

85
Q

what can a alkaline hydrogen oxygen fuel cell power

A

used to power electric veichles

86
Q

in a alkaline hydrogen oxygen fuel cell what are the chemicals used

A

hydrogen and oxygen

87
Q

in a alkaline hydrogen oxygen fuel cell where are the hydrogen and oxygen fed into

A

two separate platinum containing electrodes

88
Q

what does a anion exchange membrane allow

A

allows an ions and water to pass through it but not the chemicals

89
Q

in hydrogen oxygen fuel cell what does the platinum catalyst do

A

splits the H₂ into protons and electrons

90
Q

in hydrogen oxygen fuel cell what odes polymer electrolyte membrane do

A

only allows H⁺ across forcing electrons to travel around the circuit

91
Q

in hydrogen oxygen fuel cell what does the electrons travelling around do

A

creates electric current which can power things

92
Q

in hydrogen oxygen fuel cells what happens at cathode

A

O₂ combines with H⁺ and electrons making water which is the only waste product

93
Q

iodine sodium thiosulfate titrations are a way of finding

A

the concentration of oxidising agent

94
Q

CH₃OH + H₂O→

A

CO₂ + 6e– + 6H⁺

95
Q

6H⁺ + 6e– + 1·5O₂ →

A

3H₂O

96
Q

what do titrations allow you to do

A

find out how much acid is needed to neutralise a quantity of alkali

97
Q

a known volume of alkali is titrated with an

A

acid of known concentration

98
Q

volume of acid of known concentration needed to neutralise alkali can be used to work out

A

concentration of the alkali

99
Q

what are pipette used for

A

to measure only one volume of. solution

100
Q

how do you use a pipette

A

fill pipette just above the line then take pipette out of solution and drop the level down carefully to the line

101
Q

what are burette used for

A

to measure different volumes of solutions and let you drop the solution drop by drop

102
Q

An oxidising agent

A

accepts electrons and gets reduced.

103
Q

A reducing agent

A

donates electrons and gets oxidised.

104
Q

Transition (d-block) elements are good at changing

A

oxidation number

105
Q

because transition elements good at changing oxidation number they are good as oxidising and reducing agents

A

as they readily give out and receive electrons

106
Q

how do you work out concentration of reducing agent

A

you titrate it against a known volume of a oxidising agent of known concentration

107
Q

how to find out how many manganate(VII) ions (MnO₄–) are needed to react with a reducing agent

A

*measure quantity of reducing agent using pipette
*add some dilute sulphuric acid
*do rough titration adding MnO₄–using burette swirling conical flask
*stop when mixture in flask becomes tainted with purple colour
*record volume added repeat and run few accurate titrations and calculate mean volume of MnO₄–

108
Q

when doing titrations you need to watch out for

A

sharp colour changes

109
Q

what can make a colour change easier to spot

A

doing reaction in front of white surface

110
Q

the more concentrated an oxidising agent the more

A

ions will be oxidised by a certain volume of it

111
Q

iodine sodium thiosulfate titration with potassium iodate (V)

A

*measure out volume potassium iodate(V) solution
*add this to excess of acidified potassium iodide iodate(V) ions in potassium iodate(v) solution oxidise some of the iodide ions to iodine
*find out how many moles of iodine have been produced by titrating resulting solution with sodium thiosulfate
*work out concentration of potassium iodate(v)

112
Q

step 1 in iodine sulphate thiosulfate

A

use sample of oxidising agent to oxidise as much iodide as possible

113
Q

what is step 2 in iodine sodium thiosulfate titration

A

find out how many of moles of iodine have been produced

114
Q

I₂(aq) + 2S₂O₃²⁻(aq)⟶

A

2I⁻ (aq) + S₄O₆²⁻ (aq)

115
Q

what is stage 3 in odine sodium thiosulfate titration

A

calculate the concentration of oxidising agent

116
Q

can use titration to find percentage of

A

copper in an alloy

117
Q

copper(II) ions will

A

oxidise iodide ions to iodine

118
Q

stage 1 in finding percentage of copper in an alloy

A

use sample of oxidising agent to oxidise as much iodide as possible

119
Q

stage 2 in finding percentage of copper in an alloy

A

find out how many moles of iodine have been produced

120
Q

stage 3 in finding percentage of copper in an alloy

A

calculate the concentrations of the oxidising agent

121
Q

how finding percentage of copper in an alloy

A

*dissolve weighed amount of alloy in conc nitric acid
*pour in 250 cm³ volumetric flask and make up to 250 with deionised water
*pipette 25cm³ of solution to flask
*slowly add sodium carbonate solution to neutralise any nitric acid
*keep going until slight precipitate forms
*removed if you add few drops of ethanoic acid
*add excess of potassium iodide solution
*white precipitate of copper(I) iodide forms
*titrate product mixture against sodium thiosulfate solution to find number of moles of iodine present

122
Q

what are the sources of error in finding the percentage of copper in alloy

A

*starch indicator needs to be added at right time when most of iodine has reacted or else blue colour will be slow to disappear
*starch solution needs to be freshly made or it won’t behave as expected
*percipitate of copper (I) iodide makes seeing colour of solution quite hard
*iodine produced in reaction can evaporate from solution giving false titration reading so final percentage of copper would be too low so solution should be kept cool

123
Q

when does starch indicator need to be added and why

A

when most of the iodine has reacted or else the blue colour will be slow to disappear

124
Q

why does starch solution need to be freshly made

A

or else it won’t behave as expected

125
Q

iodine produced in reaction can evaporate what does this mean and what should be done

A

false titration reading is given meaning final figure of percentage of copper would be too low solution should be kept cool

chem (16 decks)

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