Chapter 23 Flashcards
1
Q
What oxidation number is a pure element?
A
0
2
Q
What oxidation number is a simple ion?
A
ionic charge
3
Q
What must compound’s oxidation numbers add up to?
A
the compound’s charge
4
Q
What oxidation number is fluorine?
A
-1
5
Q
What oxidation number is oxygen?
A
-2 (unless bonded to F or in a peroxide)
6
Q
What oxidation number is chlorine?
A
-1 (unless bonded to F or O)
7
Q
What oxidation number is bromine?
A
-1 (unless bonded to F, O or Cl)
8
Q
What oxidation number is iodine?
A
-1 (unless bonded to F, O, Cl, Br)
9
Q
What oxidation number is hydrogen?
A
+1 (unless present as a hydride ion H-)
10
Q
What oxidation number is group 1 metals?
A
+1
11
Q
What oxidation number is group 2 metals?
A
+2
12
Q
Where does the sign go?
A
Before the oxidation number
13
Q
Give the formula and name of nitrate?
A
NO3-
Nitrate (V)
14
Q
Give the formula and name of nitrite?
A
NO2-
Nitrate (III)
15
Q
What happens to the oxidation number in oxidation?
A
INCREASE
16
Q
What happens to the oxidation number in reduction?
A
DECREASE
17
Q
What will always be present in redox reactions?
A
Are reducing and oxidising agent.
18
Q
What is an oxidising agent?
A
It takes the electrons from the species being oxidised. It therefore contains a species that is reduced.
19
Q
What is a reducing agent?
A
It gives the electrons to the species being reduced therefore it contains this basis which is oxidised.
20
Q
What side will electrons be found in an oxidation half equation?
A
RHS
21
Q
What side will electrons be found in an reduction half equation?
A
LHS
22
Q
How do you combine equations to give an overall redox reaction equation?
A
- Balance out electrons
- Add the equations and cancel out electrons
- Cancel out any species which are on both sides.
- Check atoms, charges should balance
23
Q
What type of agent is MnO4_
A
Oxidising
24
Q
Describe the manganate redox titration?
A
- A standard solution of MnO4- goes in the burette.
- Using a pipette a measured volume of the solution is added to a conical flask which is analysed. An excess of H2SO4 is added, which provides H+ required for the reduction of MnO4-
- The end point is judged by the first permanent pink colour indicating when there’s an excess of MnO4-. Near endpoint it should be added dropwise.
- Repeat the titration until you reach concordant titres (2 titres agreeing with + or - 0.01cm3)
25
Why is HCl not used?
As MnO4- would oxidise Cl- to Cl2
26
Why is no indicator needed in the titration?
As its self indicating
27
How do you read off the meniscus?
At the top of the meniscus as the deep purple colour makes it hard to see the bottom of the meniscus.
28
What are the two reducing agents in the manganate titration?
Iron (II) Fe 2+
| Ethanedoic acid
29
Why are manganate titrations used?
To analyse reducing agents which reduce MnO4- to Mn2+
30
What is oxidised and reduced in the iodine and thiosulphate redox titration?
Thiosulphate ions are oxidised.
| Iodine is reduced
31
What can the iodine and thiosulphate redox titration determine?
- the ClO- content in household bleach
- the Cu 2+ content in copper (II) compounds
- the Cu content in copper alloys.
32
Describe the thiosulfate and iodine titration?
1. Add a standard solution of sodium thiosulphate to the burette.
2. Prepare a solution of the oxidising agent to be analysed using a pipette add this to a conical flask. Add an excess of potassium iodide. The oxidising agent reacts with the iodide ions to produce iodine. Which turns a solution yellow brown.
3. Titrate the solution with sodium thiosulphate during this titration iodine is titrated back to iodide ions which makes brown colour fade making the end point less visible. Starch indicator is added to help identify the end point in which a deep blue black colour forms. As more sodium thigh sulphate is added the blue black colour phase. The end point is when the idea has fully reacted and the blue black colour disappears.
33
Why is a starch indicator added?
The endpoint is less visible due to the brown colour fading to a straw yellow it is added to help identify the end point.
34
What are the two oxidising agents in iodine thiosulphate redox titration?
```
Chlorate ions (I), ClO-
Copper (II), CU2+
```
35
What happens to copper (II) salts when dissolved in water?
Cu 2+ salts are produced
36
How are Cu2+ produced in insoluble compounds?
They react with acid. For copper alloys ie brass or bronze, the alloy reacts and dissolves in concentrated nitric acid followed by a neutralisation reaction to form Cu 2+.
37
What happens when Cu 2+ reacts with I-?
I2 is produced and a white precipitate of copper (I) iodine, CuI which appears a brown colour.
2Cu 2+ +4I- --> 2CuI + I2
38
Whats a voltaic cell?
It converts chemical energy into electrical energy, which occurs in modern cells and batteries which power devices ie mobile phones. As electrons are needed to transfer electrons from one species to another, the electrical energy results from the movement of electrons.
39
What is a half cell?
A half cell contains the chemical species which are present in the redox equation.
40
How can a voltaic cell be made?
A voltaic cell can be made by connecting different half cells allowing electrons to flow.
41
Why should the chemicals in the two half cells be kept separate?
If they mix the electrons could flow in an uncontrollable way and heat energy would be released instead of chemical.
42
what does chemical energy get converted into?
Electrical energy, electrons travel from one cell to another, so a current has been generated.
43
What does a volt meter measure?
The difference in potential between two electrodes
44
What reaction is the forward reaction?
Reduction
45
What reaction is the reverse reaction?
Oxidisation
46
Describe the metal/metal ion half cells?
In each half cell and equilibrium is set up between the metal strip and the solution (phase boundary). Electrons are deposited onto the surface of the strip. If the half cells isolated there is no net transfer of electrons in and out of the metal. When two horses are connected the electrons flow in a direction depending on the relative tendency of each electrode to release electrons.
47
What would happen between zinc and copper?
Zinc has a greater tendency to lose electrons and copper so has a more negative electrode potential the equilibrium lies further to the left. When I connected as it becomes the negative electrode zinc is oxidised. Copper becomes a positive electrode and copper is reduced.
48
Where do electrons flow from and to?
Negative electrode to the positive electrode
49
How is the negative electrode determined?
It's the electrode with the more reactive metal losing electrons and is oxidised
50
How is the positive electrode determined?
It's the electrode with the less reactive metal which games electrons and is reduced
51
What is standard electrode potential?
The tendency to be reduced and gain electrons is measured as the standard electrode potential.
52
What is the standard chosen half cell?
It contains hydrogen gas and a solution containing H plus ions. An inert platinum electrode is used to allow electrons into and out of the half cell.
53
What are the standard conditions?
298K
solution concentration - 1 moldm-3
100kPa
54
What is the standard electrode potential?
The emf of a half so connected to a standard hydrogen half cell under standard conditions.
55
What is the electrode potential of a standard hydrogen cell?
0V
56
What does a negative electrode potential indicate?
A greater tendency to lose electrons compared to the hydrogen half cell.
57
What does a positive electrode potential indicate?
A greater tendency to gain electrons compared to the hydrogen half cell.
58
What happens in ion/ion half cells?
It contains electrons of the same element in different oxidation states. In this half cell type there is not any metal to be transported via electrons into or out of the half so an inert metal electrode made from platinum is used.
59
How do you measure a standard electrode potential?
To measure a standard electrode potential the half cells connected to a standard hydrogen electrode. The two electrodes are connected by wire to control the flow of electrons. The two solutions are connected by salt bridge which allows the ions to fly, it contains a concentrated solution of an electrolyte which reacts with either solution e.g a strip of filter paper soaked in KNO3
60
Why is KNO3 used?
NO3- counter balances the extra Zn 2+
K+ replaces the Cu 2+
So the solution remains neutral
61
What does the electrode potential at the top indicate?
That's it is the best reducing agent with the highest tendency to lose electrons.
62
What does the electrode potential at the bottom indicate?
That it is the best oxidising agent with the highest tendency to gain electrons.
63
The more negative the E value?
The greater the tendency to lose electrons and undergo oxidation.
The less the tendency to gain and undergo reduction.
64
The more positive the E value?
The greater the tendency to gain electrons and undergo a reduction.
The less the tendency to lose electrons and undergo oxidation.
65
What value do metals tend to have
-ve E values and lose electrons
66
What value to nonmetals tend to have
+ve E values and gain electrons
67
The more negative the E value ....
The greater the reactivity of the metal losing electrons
68
The more positive the E value ....
The greater the reactivity of a nonmetal gaining electrons.
69
How is E cell calculated
Positive - negative
70
In terms of equilibrium if its more negative which way will it shift?
to the left
71
In terms of equilibrium if its more negative which way will it shift?
to the right
72
Why is platinum used?
As oxidised and reduced species in the half equation are ions in solution.
73
How do you measure standard cell potentials?
1. Prepare to standard half cells.
- For a metal/metal ion half cell it must have a metal ion concentration of one mole per decimetre cubed.
- For an ion/ion half sell both metal ions in solution must have the same concentration and an inert electrode. - Temperature must be 298 K.
2. Connect the metal electrodes of the half cells to a volt meter using a wire.
3. Prepare a salt bridge by soaking a strip of filter paper in concentrated solution of potassium nitrate.
4. Connected two half cell solutions with a salt bridge. 4. Record the standard cell potential from the voltmeter.
74
The redox system with the more negative electrode potential will react from....
The right to the left and lose electrons (oxidised).
75
The redox system with the more positive electrode potential will react from....
The left to the right and gain electrons (reduced)
76
Where are the oxidising agents found?
On the left
77
Where are the reducing agents found?
On the right
78
Where is the strongest reducing agent found?
The top right
79
What is the strongest of oxidising agent found?
Bottom left
80
What way around does the reduction half equation have to go?
The same way
81
What weight around does the oxidation half equation have to go?
It has to be reversed.
82
What are the three limitations of predicting electro potential values?
Reaction rate, concentration, other factors
83
How does reaction rate limit predictions?
Electric potentials indicate thermodynamic feasibility by giving no indication of the route.
Direction might have a high activation energy.
84
How does concentration limit predictions?
If the concentration of solution is not one mole per decimetre cubed the value of electric potential will be very different from the standard value.
85
What would happen if the concentration was greater than one?
The equilibrium which shift to the right removing electrons from the system so the electric potential becomes more positive.
86
What would happen if the concentration was less than one?
The equilibrium which shift to the left increasing electrons from the system so the electric potential becomes more negative.
87
What are the other factors which limit predictions?
Electric potential values apply to a crisp reactions many reactions which occur on aqueous. Temperature and pressure it might not be standard.
88
What are primary cells?
They are not good chargeable (designed to be used only once).
Electrical energy produced originates from the oxidation and reduction at the electrodes. The cell reaction can't be reversed. When the chemicals are used to the voltage falls the battery goes flat and the cell would be discharged or recycled.
89
Give examples of primary cells?
They can be found in low current strong storage devices i.e. smoke detectors. Most primary cells are alkaline based on zinc and manganese dioxide and potassium hydroxide alkaline electrolyte.
90
What are secondary cells?
These are rechargeable the cell reaction producing chemical energy can be reversed during recharging, the chemicals in the cell are regenerated and the cell can be used again
91
Give examples of secondary cells?
Lead- acid batteries
Nickel cadmium (NiCd), nickel- metal hydride (NiMH) - radios, torches
Lithium ions and lithium polymer - laptops, tablets, cameras and mobiles.
92
What are fuel cells?
If your cell uses the energy from the reaction of a fuel with oxygen to create a voltage.
H2 + 1/2 O2 --> H2O
CH3OH + 1 1/2 O2 --> CO2 + 2H2O
93
Describe a fuel cell
- The fuel and oxygen flow into the fuel cell and products flow out. The electrolyte remains the same.
- Fuel cells can operate continuously provided the oxygen and fuel are supplied into the cell.
- They don't need to be charged.
- The most common fuel cell is hydrogen which produces no CO2 in combustion with water being the only product. The electrolyte can be either acid or alkaline.
94
What are the three main types of cells?
Primary, secondary and fuel.
