Redox and Electrode Potentials Flashcards
1
Q
What is a loss of electrons called?
A
Oxidation
2
Q
What is a gain of electrons called?
A
Reduction
3
Q
What is a redox reaction?
A
A reaction where oxidation and reduction happen simultaneously
4
Q
What is an oxidising agent?
A
A substance that accepts electrons and gets reduced
5
Q
What is a reducing agent?
A
A substance that donates electrons and gets oxidised
6
Q
What is a redox reaction made up of, and how can you represent this?
A
Made up of an oxidation half-reaction and a reduction half-reaction, which you can write both as ionic half-equations
7
Q
Write an ionic half-equation for the oxidation of iron
A
Fe —> Fe 3+ + 3e-
8
Q
Write an ionic equation for the reduction of oxygen
A
O2 + 4e- —> 2O 2-
9
Q
How do you work out the overall redox equation?
A
Write out the 2 half-equations, then make sure they have the same number of electrons, and combine and cancel out
10
Q
How can you balance out half-equations if the oxidising/reduction agent contains oxygen or hydrogen?
A
You can add water, H+ ions (if acidic conditions) or OH- ions (if alkaline conditions)
11
Q
What is the oxidation number of an element?
A
It tells you the total number of electrons it has donated or accepted
12
Q
What does it mean if a chemical name has roman numerals after it?
A
It tells you the oxidation number of the element e.g. dichromate (VI) has oxidation number +6
13
Q
What happens to the oxidation number for each electron lost?
A
It increases by 1
14
Q
What happens to the oxidation number for each electron gained?
A
It decreases by 1
15
Q
If the oxidation number is reduced, it’s…?
A
Reduction
16
Q
What are concordant results?
A
Readings that are within a small range of each other
17
Q
Why are transition metal ions good oxidising or reducing agents?
A
Because of their ability to gain or lose electrons easily
18
Q
Why is it easy to spot when a transition element changes oxidation number?
A
Because a colour change occurs
19
Q
What is the purpose of doing titrations using transitions elements irons?
A
You can find out how much oxidising agent is needed to exactly react with a quantity of reducing agent
20
Q
What are redox titrations?
A
A titration that can be done to find out how much oxidising agent is needed to exactly react with a quantity of reducing agent, or vice versa
21
Q
When doing redox titrations, why do you add an excess amount of dilute sulfuric acid to the conical flask containing a known amount of reducing agent?
A
To make sure there are enough H+ ions to allow the oxidising agent to be reduced
22
Q
What do you do at the very start of a redox titration?
A
Measure out a quantity of reducing agent, using a pipette, then place in a conical flask
23
Q
For a redox titration, what do you do once you’ve added the excess amount of dilute sulfuric acid to the conical flask?
A
You add some oxidising agent to a burette and take an initial reading of the volume, then gradually add the oxidising agent to the reducing agent
24
Q
For a redox titration, what happens when you’re adding the oxidising agent to the reducing agent?
A
The oxidising agent you’re adding will react with the reducing agent, this reaction will continue until all of the reducing agent is used up
25
How do you know when the end point is reached for a redox titration?
The next drop of oxidising agent you add will result in a colour change to the colour of the oxidising agent
26
What do you do in a redox titration when a colour change in noticed?
Record the value for the volume of the oxidising agent added
27
For a redox reaction, what do you do differently between the rough redox titration and the accurate redox titration?
During the rough titration, you add the oxidising agent to the flask until the solution becomes a very faint colour of the oxidising agent. For the accurate titration, you add the oxidising agent to within 2cm^3 of the rough titration, then add drop by drop until the colour changes
28
How many times should you repeat doing an accurate redox titration?
Until you get 3 readings which are within 0.10cm^3 of each other
29
For a redox titration, how many decimal places should you record the volume added (titre)?
2 decimal places ending with either a 0 or a 5
30
For any titration, how do you measure the titre using a burette?
Measure from the bottom of the meniscus, and at eye-level
31
When doing the redox titration with KMnO4, how do you know when the endpoint is reached?
MnO4- ions in aqueous KMnO4 are purple, so the endpoint is when the solution has a colour change from colourless to purple
32
Give an example of a way you can see whether a colour change occurs for a redox titration?
You could hold a piece of paper behind the flask or put the flask on a white tile to see easier when the colour starts to change
33
What are the 4 steps to calculate the concentration of one of the reagents in a redox titration?
1- Write out a balanced redox equation for the reaction in the flask
2- For the reagent you know both the concentration ad volume for, calculate the number of moles
3- Use the molar ratios in the balanced equation to work out the number of moles of the reagent you want to find the concentration of
4- Calculate the unknown concentration using equation conc = (moles x 1000) /volume (cm^3)
34
What are iodine-sodium thiosulfate titrations used to find?
The amount of iodine in solution
35
What are the 4 steps for the iodine-sodium thiosulfate titration?
1- Oxidise the iodine ions to iodine
2- Titrate the iodine solution with sodium thiosulfate
3- Calculate the number of iodine moles present
4-Calculate the concentration of the oxidising agent
36
For the iodine-sodium thiosulfate titration, what chemical do you use as the oxidising agent?
Potassium iodate (V)
37
For the iodine-sodium thiosulfate titration, how do you oxidise the iodine ions to iodine?
Measure a certain volume of oxidising agent (Potassium iodate (V)). Add this to an excess amount of acidified potassium iodide solution. The iodate ions oxidise some of the iodide ions to iodine
38
What is the ionic equation for the oxidising of iodide ions to iodine?
IO3 - + 5I- + 6H+ ---> 3I2 + S4O6 2-
39
What are the 5 steps for the titration method for the iodine-sodium thiosulfate titration?
- Take a flask containing the solution made when oxidising the iodide ions
- From a burette, add sodium thiosulfate solution to the flask drop by drop
- When the iodide colour fades away, add 2cm^3 of starch to detect the iodine. The solution will go dark blue showing there is some iodine still left
- Add sodium thiosulfate one drop at a time until the blue colour disappears
- Record the volume of sodium thiosulfate added to the solution
40
What is the colour of the solution in the conical flask at the start of an iodine-sodium thiosulfate titration?
Red-brown
41
What is the equation for the iodine-sodium thiosulfate titration?
I2 + 2S203 2- ---> 2I- + S4O6 2-
42
What is an electrochemical cell?
An electrical circuit made from 2 metal electrodes (connected by a wire) which are dipped in salt solution (connected by a salt bridge)
43
What is an electrochemical series?
A list of electrode potentials written from most negative to most positive
44
Why is the process in electrochemical cells called a redox process?
Because oxidation and reduction happens simultaneously
45
How do you make an electrochemical cell?
Can be made from 2 different metals dipped in a salt solution of their own ions and connected by a wire (external circuit)
46
What are half-cells?
The 2 different sides of an electrochemical cell
47
Why are the solutions connected by a salt bridge?
To allow ions to flow through and balance out the charges, thus completing the circuit
48
In an electrochemical cell, what is the salt bridge made from?
A piece of filter paper soaked with KNO3 (aq)
49
What is the cell potential?
The voltage between 2 half-cells in an electrochemical cell
50
Describe the flow of electrons in an electrochemical cell
They flow from the more reactive metal to the less reactive metal
51
Describe the properties of the electrode needed for an electrochemical cell
Needs to be a solid that conducts electricity
52
How do you create a standard hydrogen electrode?
You can use non-metals as reactants in electrochemical cells. The gas can be bubbled over an inert electrode sitting in a solution of its aqueous ions
53
What are the 5 steps to set up an electrochemical cell and then take measurements of voltage?
1: Get a strip of each of the metals you're investigating
2: Clean the strips of metal using propanone
3: Place each electrode into a beaker filled with a solution of ions of the metal
4: Create a salt bridge to link the 2 beakers together by soaking a piece of filter paper with salt solution
5: Connect the electrodes to a voltmeter using wires and crocodile clips
54
If you perform the experiment to take measurements of voltage from an electrochemical cell in standard conditions, what is the voltage equal to?
Standard cell potential for that cell
55
Describe the reactions at the electrodes in an electrochemical cell
They are reversible reactions e.g. Zn2+ + 2e- ⇌ Zn
56
What are electrode potentials?
The voltage measured when a half-cell is connected to a standard Hydrogen electrode
57
Describe the electrode potential value for a metal electrode that is easily oxidised
Very negative electrode potential
58
For a zinc/copper cell, which direction does the reaction go?
Electrode potential values:
```
Zn2+/Zn = -0.76V
Cu2+/Cu = 0.34V
```
The zinc electrode has a more negative electrode potential so it's oxidised easier, so the reaction is backwards. Meaning Zinc is oxidised and the Copper is reduced
59
Name a substance that could be used for the electrode in a half-cell involving solutions of 2 aqueous ions of the same element?
Platinum
60
Why is Platinum suitable for use for the electrode in a half-cell involving solutions of 2 aqueous ions of the same element?
It's a solid that conducts electricity and is inert
61
What are half-cell reactions described as?
Reversible reactions
62
What 3 factors affect the equilibrium position in a half-cell?
Temperature
Pressure
Concentration
63
What changing the equilibrium in a half-cell affect?
The cell potential
64
Why are standard conditions used for measuring electrode potentials?
So you always get the same value for the electrode potential because the equilibrium will be the same for each measurement, meaning you can compare different cells
65
What is the standard electrode potential of a half-cell?
The voltage measured under standard conditions when the half-cell is connected to a standard hydrogen electrode
66
What is a standard hydrogen electrode?
An electrode where Hydrogen gas is bubbled through a solution of aqueous H+ ions under standard conditions
67
What is the value given to the standard electrode potential of a hydrogen cell?
0V
68
How can you use a standard hydrogen electrode to measure electrode potentials?
Because a hydrogen electrode has a potential of 0V, the voltage reading is equal to the standard electrode potential of whatever you've attached to the hydrogen half-cell
69
What are the 3 conditions needed to measure electrode potentials under standard conditions?
1. Any solutions of ions have concentration 1.00mol dm^-3
2. Temperature = 298K (25°C)
3. Pressure = 100kPa
70
What 4 things are important when drawing a diagram to show how the standard electrode potential for a particular half-cell is measured?
- Always put hydrogen electrode on left
- Make sure to complete circuit (salt bridge, wires between electrode and voltmeter)
- Label any solutions as being 1.00mol dm^-3
- If half-cell contains aqueous ions of same element but different oxidation numbers, include a platinum electrode
71
Why is a platinum electrode used in a standard hydrogen half-cell?
It is inert so is corrosion-resistant and catalyzes the proton reduction reaction
72
When 2 half-equations are put together in an electrochemical cell, which one goes in the direction of oxidation (backwards)?
The half-equation with the most negative electrode potential
73
When 2 half-equations are put together in an electrochemical cell, which one goes in the direction of reduction (forwards)?
The half-equation with the most positive electrode potential
74
Describe the electrode potential of a more reactive metal
The more reactive a metal, the more it wants to lose electrons to form a positive ions, the more negative the standard electrode potential is
75
Describe the electrode potential of a more reactive non-metal
The more reactive a non-metal, the more it wants to gain electrons to form a negative ion, so the more positive the standard electrode potential is
76
What is the equation to work out the standard cell potential or e.m.f when 2 half-cells are connected together?
EΦcell = EΦreduced - EΦoxidised
EΦcell: Standard cell potential or e.m.f
EΦreduced: Standard electrode potential of half-cell which is reduced (has the more positive electrode potential)
EΦoxidised: Standard electrode potential of half-cell which is oxidised (has the more negative electrode potential)
77
What is cell potential?
Voltage between 2 half-cells
78
What are the 4 steps to predict whether a redox reaction will occur and to show which direction it'll go?
1: Find the 2 half-equations and write them both as reduction reactions
2: Use electrochemical series to work out which half-equation has the more negative electrode potential
3: Write out half-equation of with more negative electrode potential as going in backwards direction and vice versa for more positive electrode potential
4: Combine both half-equations to form a redox equation
79
Give 2 problems when predicting redox reactions causing you to potentially get the reaction wrong
- The conditions are not standard
| - The reaction kinetics are not favourable
80
Why does not having standard conditions give a problem when predicting reactions?
It can cause the electrode potentials to change because the equilibrium position would be different
81
Why does not having reaction kinetics that are favourable give a problem when predicting reactions?
The rate of reaction may be so slow that the reaction seems not to happen. Also, if a reaction has a high activation energy this would cause it to not happen
82
What are energy storage cells?
A (rechargeable) battery that stores chemical energy to be converted into electricity
83
How do you calculate the voltage of an energy storage cell?
Use same equation to work out the cell potential:
EΦcell = EΦreduced - EΦoxidised
84
Why can some batteries be recharged?
Because the reactions that occur in them are reversible
85
How do you recharge a battery?
A current is supplied to force electrons to flow in the opposite direction around the circuit and reverse the reactions. This is possible because none of the substances in a battery are used up or can escape
86
How do you write the equations for recharging a battery?
Just reverse the equation for the storage cell
87
Give 2 advantages of electrochemical cells
Cheap to make
| Relatively high power densities
88
Give a disadvantage of electrochemical cells
The production involves using toxic chemicals, which have to be disposed of when the battery has reached the end of its lifespan
89
What is a fuel cell?
A cell that produces electricity by reacting a fuel with oxygen
90
Give 3 advantages of using a fuel cell over a combustion engine
- Fuel cells are more efficient at producing energy because energy is wasted during combustion as heat
- Fuel cells produce a lot less pollution
- For Hydrogen fuel cells, the only waste product is water
91
What is the fuel in a hydrogen-oxygen fuel cell?
Hydrogen
92
What are the 4 stages for how a Hydrogen fuel cell works?
1: At the anode, the platinum catalyst splits the H2 into protons and electrons
2: The polymer electrolyte membrane (PEM) only allows H+ across, forcing the electrons to travel around the circuit to get to the cathode
3: An electric current is created in the circuit from the flow of electrons
4: At the cathode, O2 combines with the H+ from the anode and the electron to form water, the only waste product
93
What is the overall redox reaction for a hydrogen fuel cell in acidic or alkaline conditions?
1/2 O2 + H2 ---> H2O
