1
Q
What is electrochemistry?
A
Chemistry relating to the conversion of chemical energy <—–> electrical energy
2
Q
What is oxidation?
A
the process in which electrons are lost
3
Q
What is Reduction?
A
the process in which electrons are gained
4
Q
What is a redox reaction?
A
Reaction in which oxidation and reduction reactions occur. transfer of electrons from one entity to another
5
Q
What is an oxidizing and reducing agent?
A
- oxidizing agents: cause other substances to be oxidized, but are themselves reduced
- reducing agents: cause other substances to be reduced, but are themselves oxidized
6
Q
What are half reactions used for?
A
Used to monitor the transfer of electrons for oxidation and reduction separately
7
Q
What is an oxidation number?
A
positive or negative number corresponding to the apparent charge that an atom in a molecule or ion would have if the electron pairs in covalent bonds belonged entirely to the more electronegative atom
- Written as + or - then the number
8
Q
How does determining oxidation numbers help chemists?
A
Allows chemists to track the quantity and movement of electrons, which is important in the development of batteries and consumer materials
9
Q
What are the rules for assigning oxidation numbers?
A
- Pure elements -> oxidation # of 0
- Mon-atomic ion -> the charge of the ion
- Hydrogen in compounds has a # of +1 expect when in a metal hydride (than it’s -1)
- Oxygen in compounds usually have -2 except peroxides, superoxides, and OF2(g)
- In molecular compounds (w/o oxygen or hydrogen) # is based on the charge of the more electronegative element
- Sum of oxidation # of atoms in a neutral compound is 0
- Sum of oxidation # of atoms in a polyatomic ion = the charge on the ion
10
Q
What does oxidation number tell you about redox reactions?
A
- Substance is oxidized if oxidation # increases
- Substance is reduced if oxidation # decreases
- Assigning oxidation numbers in a chemical reaction indicates if electron transfer has occurred (if yes then it’s a redox reaction)
11
Q
What do electrochemical cells do and types?
A
- They involve the transfer of electrons from one species to another
- Types include Galvanic and Electrolytic Cells
12
Q
What takes place in a galvanic cell?
A
- involves spontaneous redox reactions that produce electrical energy
- Half reactions are separated into half-cells
- Each electrode is in contact with an electrolyte in a separate container
- Electrons are routed through a wire
- The anode side gets smaller (oxidizes and loses electrons) cathode side gets larger (reduces and gains electrons)
13
Q
What is a porous boundary?
A
separates electrolytes at least over a short time while still permitting ions to move through tiny openings between two solutions. Ex. salt bridge
14
Q
What does a salt bridge do?
A
- Maintains electrical neutrality (positive ions migrate to cathodes, negative ions migrate to anode)
- salt bridge permits the redistribution of charge that is needed to maintain electrical neutrality
15
Q
How do you figure out which substance is the anode and cathode?
A
- The half reaction that is HIGHER in the reduction potential table stays as the reduction reaction
- Flip the lower half reaction to make it an oxidation reaction
16
Q
Where do these half reactions for galvanic cells take place?
A
- electrodes are usually metals so the half reactions for galvanic cells usually take place on the surface of the metals
17
Q
What happens if an oxidizing/reducing agent other than metals are used?
A
- an inert electrode is needed (solid conductor that will not react in the cell)
- Examples of inert electrodes: carbon (graphite) or platinum metal foil
18
Q
What does an inert electrode do?
A
- Provides a location to connect a wire and a surface on which a half reaction can occur
19
Q
What is standard cell potential?
A
- represents the energy difference (per unit of charge) between the cathode and the anode
- the degree sign indicates that standard 1.0 mol/L and SATP conditions
- a positive cell potential indicates that the net reaction is spontaneous
20
Q
What idea is standard cell potential based on?
A
- Based on the idea of competition for electrons
- The half-cell with the greater attraction for electrons (the one with the more positive reduction potential—gains electrons from the half cell with the lower reduction potential
21
Q
Do LCM used to balance electrons affect reduction potentials?
A
No, reduction potentials are not altered by the factors used to balance the electrons
22
Q
What is corrosion?
A
an electrochemical process in which a metal reacts with substances in the environment (oxygen), returning the metal to an ore-like state
23
Q
How do you tell if a metal will easily oxidize?
A
if the metal appears below the oxygen half reaction then it will oxidize easily
24
Q
Why does iron corrode easier than aluminum?
A
- When aluminum is oxidized, the aluminum oxide that is formed adheres to the surface of the metal which prevents the surface from being exposed and corroded
- Iron oxides do not adhere to the surface and flake off, causing more surface area to be exposed and corroded
25
How does Iron rust?
- water and oxygen are required (or any other oxidizing agent) to convert iron into iron hydroxide and iron oxides
- Iron (II) ions diffuse through the water on the iron surface while the electrons easily travel through the iron metal (a conductor)
- The electrons are picked up by oxygen molecules dissolved in the water on the surface
- The red-brown rust is formed by the dehydration of iron (III) hydroxide to form hydrated iron (III) oxide
26
How do electrolytes affect rusting?
- Electrolytes accelerate rusting bu increasing conductivity of aqueous solutions, (ex. ships rust more rapidly in sea water than fresh water)
- Electrolytes, like sodium chloride,
27
What accelerates the rusting of iron?
the presence of acidic solutions, electrolytes, mechanical stresses, and contact with less active metals.
28
What are some ways corrosion can be prevented?
- Barrier methods that employ protective coatings (Paint, Galvanizing)
- Cathodic protection
- Other methods (e.g. oil sprays)
29
How does Protective coating work?
- Both tin and zinc are used as metallic coatings
- When the tin coating oxidizes, the oxide layer adheres tightly to the tin and protects the tin/iron
- Zinc plating (galvanizing) of steel or iron provides double protection: a protective layer (zinc oxide) and a preferential oxidizing of zinc
30
What is the issue with protective coating?
- If a crack or break occurs, moisture collects in the crack and a spontaneous electrochemical cell with the coating and the metal electrodes is established
- Example: since iron (metal) is more easily oxidized than tin (coating), iron becomes the anode in this cell. The electrons released by the oxidation of iron flow to the tin and corrosion are accelerated.
31
How does cathodic protection work?
- metal being protected is forced to become the cathode of the cell using either an impressed current or a sacrificial anode
Impressed Current:
- impressed current supplies electrons to the metal being protected so that it cannot be oxidized, but instead it is reduced
(used for steel in buried fuel tanks)
Sacrificial Anode:
- metal more easily oxidized than iron and connected to the iron object to be protected
- The more active metal (appearing below iron in a half-reaction table) is slowly consumed or sacrificed at the anode, forcing the iron object to be the cathode of the cell
32
How does oil spray under vehicles work?
- Completely covers the underside of the car with a light coating of oil
- Acts as a barrier, reduces exposure to water, moisture and road salt
- Less expensive than traditional rust proofing
33
What is an electrolytic cell?
cell that uses energy to move electrons from high to low potential energy (forcing a redox reaction/ reverse of a galvanic cell)
- has the possibility for two half reactions to take place in the same container
34
What is a power source?
provides energy needed to push electrons from anode to cathode (needed for electrolytic cells)
- Anode becomes positively charged, cathode becomes negatively charged
- positive ions -> cathode (reduction)
- negative ions -> anode (oxidation)
35
What is electrolysis?
converts electrical energy to chemical energy (not a spontaneous reaction, negative cell potential value)
36
What are the parts included in an electrolytic cell?
- electrodes
- electrolyte(s)
- external source of electricity
- external circuit
37
What are the differences between galvanic and electrolytic cells?
Galvanic:
- converts chemical energy to electrical
- spontaneous reactions
- positive standard potential energy value
- Cathode is positive, anode is negative
- salt bridge is needed to prevent charge build up
– energy flows from one electrode to the other, driven by the reduction potential of
each metal
Electrolytic:
- not spontaneous
- negative standard potential energy value
- cathode is negative, anode is positive
- Salt bridge may or may not be present
- energy flows into the cell from an outside source, then from one electrode to the other
**cathode of galvanic cell is the anode of electrolytic cell (visa versa)
38
What are the similarities between galvanic and electrolytic cells?
- cathode is site for reduction, anode is the site for oxidation
- direction of electrons movement is from the anode to the cathode
39
What is electroplating?
depositing a layer of metal onto another object at the cathode of an electrolytic cell using an electric current (the metal being deposited is either the anode or supplied as metal ions in a solution)
- a solution is needed including the metal being used and a spectator ion (like SO4) that will balance charges
40
What metals are resistant to corrosion?
Ag, Au, Zn, and Cr
41
How do secondary cells function?
- as cell discharges energy -> electrical energy is spontaneously produced and the cell functions as an galvanic cell
- When the cell recharges, electrical energy forces the products to react to re-form the reactants
- during this process the cell functions like an electrolytic cell
**primary cells cannot be recharged
42
What is a car alternator and how does it relate to electrolytic cells?
- Car battery charger
- when the lead acid car battery is recharged by the car alternator, it acts as an electrolytic cell
43
What are the advantages and disadvantages of electroplating?
Advantages:
- improves corrosion resistance, durability and appearance
- Used in electronics, tools
Disadvantages:
- relies on aqueous solutions containing metal ions and reactive chemicals
- Wastewater often contains heavy metals and strong acids which can contaminate soil and groundwater (heavy metals aren't biodegradable)
- Exposure to electroplating waste can cause neurological damage, kidney and liver damage etc. (ventilation, protective equipment needed)
44
Where does electronic waste come from and disadvantages?
- Comes from discarded phones, laptops, batteries, etc. which rely heavily on electrochemical components like batteries, circuits, etc.
Disadvantages:
- E-waste contains toxic substances including lead, mercury, lithium, etc., inproper recycling methods causes these substances to escape into the air, soil and water
- health impacts include damage to nervous system, developmental delays, kidney and lung damage, etc.
45
What are ways to reduce electrochemical health risks?
- electroplating facilities treat wastewater to remove metal ions before release
- worker safety protocols including protective clothing, fume hoods, and monitoring exposure levels
- Government regulations limiting the export and improper disposal of hazardous e-waste
46
What is an electrolyte?
An electrolyte is a substance filled with the cations of the metal ions allowing electric current to flow.
47
How do you find which metal would be better in protection against corrosion?
whichever metal has a lower Er value (weaker oxidizing agent) will be better for protection
48
What are the similarities and differences between sacrificial anodes and impressed current?
Sacrificial anodes:
- stronger reducing metal is attached to the protected metal
- more active metal is more readily oxidized, providing electrons to the protected metal
- more active metal gets used up and must be replaced
Impressed current
- inert electrode is attached to the positive terminal of the DC power source.
- a direct current is applied to the protected metal providing electrons directly to the metal
- very expensive and have to continuously supply direct current
Similarities
- both supply the protected metal with electrons
- protected metal acts like a cathode
- for buried steel pipes, the electrolyte is moist soil
49
Why is it necessary to separate the oxidizing agent from the reducing agent in a galvanic cell?
- the chemical energy of the transferred electrons would be released into the
environment as thermal energy and could not be harnessed to do useful work.
- When the oxidizing agent and reducing agent are separated, electron transfer can occur through a wire and produce electric current, which can be put to use operating various devices.
50
What characteristics should the solution in a salt bridge have?
- solution should contain a soluble electrolyte
- not react with the electrodes or electrolyte solutions in the cell
12U Chemistry
flashcards
Decks in class (6)
# Cards
