A Level Chemistry Ocr > Module 5 Chapter 23 > Flashcards

Module 5 Chapter 23 Flashcards

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1
Q

How do you balance complex half equations

A

Using KOHES

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2
Q

What is KOHES

A

Key elements
Oxygen
Hydrogen
Electrons
States

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3
Q

What occurs in alkaline conditions too do with KOHES

A

OH- is added instead of H+ when doing KOHES

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4
Q

What is potassium manganate

A

It is an oxidising agent under acidic conditions and is reduced to form Mn2+

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5
Q

What is required for MnO4- to be reduced

A

H+ ions

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6
Q

What is potassium manganate commonly used for

A

The analysis of reducing agents that are oxidised

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7
Q

What are common reducing agents that potassium manganate is used to analyse

A

Iron (II) Fe 2+
Ethanedioic acid (COOH)2

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8
Q

What are the differences in a redox titration and an acid titration

A

It does not need na indicator as MnO4- is purple and Mn2+ is colourless

Due to the dark colour of KMnO4, readings are taken from the top of the meniscus instead of

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9
Q

What is the endpoint

A

It is the first permanent pink colour

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10
Q

What is the method for determining the concentration of a solution of potassium manganate (1)

A
  1. Pipette a known volume of the standard solution into a conical flask
  2. Add an excess of an acid to the flask
  3. Rinse the burette with a small sample of potassium manganate solution
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11
Q

What is the method for determining the concentration of a solution of potassium manganate (2)

A
  1. Refill the burette with the
  2. Complete the rough titration until you achieve a persistent pink colour, record reading
  3. Complete more accurate titrations until you obtain concordant sets of results
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12
Q

What can iodine thiosulphate titrations be used for

A

To determine the content of different oxidising agents

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13
Q

What are some oxidising agents that iodine thiosulphate titrations can be used to determine the content of

A

ClO- in bleaches
Cu 2+ compounds
Cu in alloys

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14
Q

What is reduced in the reaction and give the half equation

A

I2 + 2e- —> 2I-

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15
Q

What is oxidised in the reaction and give the half equation

A

S2O3 2-

2S2O3 2- —> S4O6 2- + 2e-

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16
Q

How is a sodium thiosulphate titration done (1)

A

You add thiosulphate solution dropwise to a small sample of aqueous iodine until pale yellow.

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17
Q

Why does the solution turn pale yellow when the thiosulphate is added

A

As the iodine is in excess, most of the iodine is reduced to I- however some iodine remains causing it to have a change in colour from brown to pale yellow.

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18
Q

How is a sodium thiosulphate titration done (2)

A

Add one drop of starch solution
Then continue to add thiosulphate solution dropwise

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19
Q

Why does the solution turn blue when the starch is added

A

As the iodine forms a complex with the starch causing it to turn blue-black

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20
Q

Why does the solution turn colourless after the remaining thiosulphate is added drop by drop

A

It reduces the remaining I2 to I- and when there are no more I2 molecules the starch can’t form the complex causing a change in colour from blue to clear.

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21
Q

How do you modify the method to analyse an oxidant

A
  1. Pipette/weigh a sample of oxidant into a conical flask
  2. Add excess of potassium iodide solution
    Oxidant + iodide —> iodine + reduced form of oxidant
  3. Carry out the titration normally
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22
Q

What can you use this method to analyse

A

Copper

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23
Q

What is an extra step that is required if the copper salt is insoluble in water

A

You don’t need to react the salt with acid to form aqueous ions.

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24
Q

What occurs in a electrochemical cell

A

Two redox half equations take place separately , with electron transfer caused by by external circuits.

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25
What is the energy released by the transfer in a electrochemical cell measured with
A voltmeter
26
What is the circuit in an electrochemical cell composed of
Two half cells which each represent a half equation of a redox reaction
27
What is a feature of an electrochemical cell
It is a spontaneous reaction —> energy transferred as electricity (opposite of electrolysis)
28
What are the electrons transferred via in an electrochemical cell and what is this value called
via the circuit and the voltmeter reading corresponds to the energy transfer This value is an electrode potential
29
Draw an electrode potential diagram
DRAW IT
30
What is a metal ion half cell
It contains a piece of metal dipped into a solution containing its ions
31
What is the phase boundary
It is where the metal is in contact with its ions
32
What occurs at the phase boundary
Equilibrium can occur
33
What is required for there to be electron transfer and why
It must be connected in order for there to be an electron transfer as the flow direction depends on the electrode’s tendency to release electrons
34
What is a feature of the two half cells in an electrode potential (separation)
The two half cells must be kept apart, if allowed to mix the electrons would flow uncontrollably and heat energy would be released rather than the desired electricity.
35
Draw the metal/ion half cell
DRAW IT
36
What does an ion/ion half cell contain
It contains ions of the same element in different oxidation states
37
What are the electrodes in an ion/ion half cells
They require two inert electrodes There is no metal to transport electrons in and out of the half cell- hence an inert metal electrode is used
38
Draw an ion/ion half cell
DRAW IT
39
What is standard electrode potential
The tendency to be reduced and gain electrons
40
How is standard electrode potential potential determined
From hydrogen gas and a solution of H+ ions as a reference in a hydrogen cell
41
What is a hydrogen cell
DRAW IT
42
What do the different parts of a the electrochemical cell and conditions required to determine standard electrode potential potential
Platinum electrode - allows electrons into an out of the half cell Standard conditions You need to complete the circuit in order to measure the voltage so it must be connected to a metal/ion half cell
43
What is the value for a standard hydrogen cell
0V
44
What does the sign of the standard electrode potential show
The sign of the half cell connected to the standard hydrogen electrode and shows the relative tendency to gain electrons in comparison to the hydrogen cell
45
How is standard electrode potential for a half cell circuit determined
Electrodes with wires Half cells with a salt bridge to allow ion movement
46
What is a salt bridge normally
A concentrated solution of KNO3
47
What do the different standard electrode potentials show
More negative values tend to lose electrons —> metals suggest more reactive More positive values tend to gain electrons —> non metals suggests more reactive
48
What does the salt bridge do
It connects the half cells and completes the circuit by providing ions to balance the charge in each half cell.
49
What is a Daniell cell
Where chemical energy is being transferred to electrical in a chemical reaction Where one specie sis oxidised and the other is reduced and a salt bridge is used to connect the two
50
What is the shorthand form for an electrode potential between Cu/Cu2+ Ag+/AG
DRAW IT
51
In the shorthand form what do the different lines represent
Horizontal dashes are the salt bridge The solid straight line is giving distinction to the species in the half cell
52
How are electrochemical cells formed
By connecting two half cells and corresponds to the cell potential
53
What is the formula for the cell potential
E Cell = E (positive electrode) - E (negative electrode)
54
What is a standard electrode potential
The e.m.f of a half cell compared with a hydrogen half cell measured at 298K with a solution concentration of 1 moldm^-3 and a gas pressure of 100kpa
55
How can you predict whether a reaction will occur based on electrode potentials
All reactions will occur if the oxidising agent has a more positive E than the redox system of the reducing agent
56
How are redox equilibria arranged in a table
From the most negative to most positive
57
Due to how the redox equilibria are arranged in the table what does this mean for whether a reaction would retake place
Reducing agents react with the oxidising agents that are below them Oxidising agents react with all reducing agents above them
58
When will a reaction take place (specific difference in E)
The reactions will take place if the difference between E values is large but if less than 0.4V it is unlikely
59
What are limits to prediction made from standard electrode potentials (1)
Non-standard conditions after the value for E Half-equations are equilibria so changes in concentration will shift the position, which alters the electron transfer
60
What are limits to prediction made from standard electrode potentials (2)
Reaction rates may be very slow due to high activation energies. E may indicate thermodynamic feasibility but no indication of rate Not all reactions occur in aqueous solutions
61
What is another limitations of using E to predict reactions
Standard electrode potentials use 1M concentrations Many reactions use concentrated, diluted solutions, therefore the EP will be different from the standard value as equilibrium position can shift
62
How do primary and secondary cells work
They work by using two half cells and causing electrons to move through an external circuit , electricity is generated by the cell. The chemical energy of the reactants is transferred into electrical energy.
63
What are the two different types of cells
Those which can’t be recharged, primary cells Cells which can be recharged, secondary cells
64
What are most primary cells based on
Zinc and manganese dioxide
65
What are secondary cells (1)
They are rechargeable Originally containing the products of a spontaneous redox reaction Electricity is used to convert these products to reactants which then react back to original materials to produce electricity on demand
66
What are secondary cells (2)
Reaction and electricity generation continue until the reaction reaches equilibrium when the charging process can be repeated The process will eventually deteriorate
67
What are examples of secondary cells
Lead-acid batteries in cars Nickel-cadmium cells and nickel metal hydride NiMH used in cylindrical batteries Lithium-ion and lithium-ion polymer cells used in laptops, tablets and phones
68
What are fuel cells
Cells in which the reactants are continuously supplied and products continuously removed They produce a voltage directly from the reaction of a fuel with oxygen
69
What are the most common type of fuel cell
Hydrogen-oxygen fuel cells
70
What is a feature of fuel cells
They do not need re-charging if reactants are supplied continuously They are much more efficient than burning the fuel using the heat to generate steam and then using that to turn a generator
71
What are the two different types of fuel cells
Acid hydrogen fuel cell and alkali hydrogen fuel cell
72
Draw the two different fuel cells
73
What is the redox system in the alkali hydrogen fuel cell and the overall equation
H2 + 2OH- <-> 2H2O + 2e- (anode) (oxidation) 1/2 O2 + H2O + 2e- —> 2OH- (cathode) (reduction) H2 + 1/2 O2 —> H2O
74
What is the redox system in the acid hydrogen fuel cell and the overall equation
H2 <-> 2H+ + 2e- (oxidation) (anode) 1/2 O2 + 2H+ + 2e- <-> H2O (reduction) (cathode) H2 + 1/2 O2 —> H2O
75
What is a similarity between both of the different fuel cells
The voltage is 1.23V
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