Redox Flashcards
1
Q
Define redox
A
A reaction in which electron transfer occurs from the reducing agent to the oxidising agent.
2
Q
What occurs in a redox reaction
A
the oxidation number of one element will increased (oxidised) and the oxidation number of another element will decrease (be reduced)
3
Q
Define Oxidation numbers
A
used to determine the change in ‘oxidation’ (change in e-)
4
Q
What is the oxidation numbers for uncombined elements?
A
0
5
Q
Give 2 examples of uncombined elements and their oxidation numbers. Mg and O₂
A
Mg = 0 O2 = 0
6
Q
What is the oxidation number for a simple ion?
A
the charge on that ion
7
Q
Give 2 examples of the oxidation number of a simple ion. Cl- and Mg2+
A
Cl- = -1 Mg2+ = +2
8
Q
What is the oxidation number of O in a combined molecule?
A
O = -2
9
Q
What is the oxidation number for main group metals?
A
an oxidation number equal to the charge on their ions
10
Q
What are two exceptions for the oxidation numbers of Oxygen?
A
Exceptions:
In compounds with fluorine oxygen has a positive oxidation number (because fluorine is more electronegative than oxygen)
In peroxides, O= -1
11
Q
What is the oxidation number for hydrogen?
A
H=+1 when it forms compounds with non-metals.
12
Q
Give 2 examples of the oxidation number for peroxides. H₂O₂ and BaO₂
A
H₂O₂: ON=-2
BaO₂: ON=-2
13
Q
What is the exception for the oxidation number for hydrogen?
A
In metal hydrides, H=-1
14
Q
Give an example of the oxidation number for hydrogen in compounds with non-metals. H2O
A
H2O: ON=+1
15
Q
Give an example of the oxidation number for hydrogen in metal hydrides. NaH and CaH₂
A
NaH: ON=-1, CaH₂: ON=-1
16
Q
Give an example of the oxidation number for main group metals
A
KCl: ON=+1, MgSO₄: ON=+2
17
Q
What is the sum of the oxidation number in a neutral compound?
A
0
18
Q
Give an example of the sum of the oxidation number in a neutral compound. CO₂
A
+4 -2
CO₂: +4 + 2(-2) =0
19
Q
What is the sum of the oxidation numbers in a polyatomic ion?
A
equal to the charge on the ion
20
Q
Give two examples of the sum of the oxidation numbers in a polyatomic ion.
A
+6, -2
SO₄⁻²
-3 +1
NH₄⁺
21
Q
What is the sum of the oxidation number for the most electronegative element?
A
assigned the negative oxidation number
22
Q
Give an example of the sum of the oxidation number for the most electronegative element.
A
+2 -1
OF₂
23
Q
What are 5 examples of redox reactions
A
bleaching hair
corrosion of metals
extraction of metals from their ores
combustion of fuels and reactions of batteries that produce electrical energy as well as respiration and photosynthesis
24
Q
Does oxidation reacts gain or lose oxygen?
A
Gain
25
Does reduction reacts gain or lose oxygen?
lose
26
Does oxidation reacts gain or lose electrons?
lose
27
Does reduction reacts gain or lose electrons?
gain
28
Does oxidation reacts gain or lose hydrogen?
lose
29
Does reduction reacts gain or lose hydrogen?
gain
30
Does oxidation increase or decrease the oxidation number?
increase
31
Does reduction increase or decrease the oxidation number?
decrease
32
Does oxidation use a reducing or oxidising agent?
reducing agents
33
Does reduction use a reducing or oxidising agent?
oxidising agent
34
Define Electron transfer process
oxidation and reduction reactions
35
What are redox reactions?
involve the transfer of electrons from one chemical species to another.
36
What are the two reactions that are happening simultaneously in a redox reaction?
One of the reactants loses electrons in a process called oxidation
One of the reactants gains electrons in a press called reduction
37
What is an oxidising agent? (4)
- enables or causes another chemical to be oxidised
- accepts electrons
- oxidises the reducing agent
- undergoes reduction
38
What is a reducing agent? (4)
- enables or causes another chemical to be reduce.
- loses electrons
- reduces the oxidising agent
- undergoes oxidation
39
What is more likely to act as a reducing agent, metal or non-metal?
metals as they tend to lose electrons. , donating electrons to the substance being reduced.
40
Is the energy required to remove valence electrons alot?
It is relatively small amount of energy
41
What type of metals act as reducing agents? How do they increase in strength?
Metals with their smaller number of valence electrons. They increase in reducing agent strength as it becomes more reactive
42
What type of metals act as oxidising agents? How do they increase in strength?
Metals with a larger number of valence electrons. They increase in oxidising agent strength as metals becomes less reactive
43
The lower the amount of energy required to remove the valence electrons
the more readily a metal will act as a reducing agent
44
As you go down the reactivity series of metals the metal solids
become more reactive, meaning metals lower in the series are easier to oxidise and therefore stronger reducing agents
45
As you go down the reactivity series of metals, metal cations....
Become increasingly harder to reduce and are therefore less reactive. Cations higher in the series have a greater attraction for electrons, so they are easier to reduce and are therefore relatively strong oxidising agents
46
What are electrochemical cells?
device in which chemical energy is converted into electrical energy and vice versa
47
What are galvanic cells?
type of electrochemical cell in which chemical energy is converted into electrical energy
48
What is a battery?
combination of cells to obtain a higher potential difference or voltage
49
What is an external circuit?
electrical current that flows through the wire and light globe
50
What is a spontaneous reaction?
What is a spontaneous reaction?
51
Describe the observations in a galvanic cell (5)
| Zinc and Copper(II)
- Redox reactions are occurring
- Zinc electrode (reducing agent) corrodes as zinc metal forms zin ions in solution
Zn(s) → Zn²⁺(aq) +2e⁻
- oxidation of Zn metal releases electrons, which flow through the wire to the copper electrode
- electrons are accepted by Cu²⁺ ions (oxidising agent) in the solution where ions collide with the copper electrode
Cu²⁺(aq) + 2e⁻→Cu(s)
- Cu metal that is formed deposits on the electrode as a dark brown coating
52
The oxidising agent is also the...
| The reducing agent is also the...
reaction being reduced
| reaction being oxidised
53
What happens if the reactions come into direct contact with each other?
their chemical energy is transformed directly into thermal energy
54
Why in galvanic cells do half-reactions occur in separate containers?
as electrons are transferred through the external circuit so that chemical energy is transformed into electrical energy
55
How does a galvanic cell operate?
designed so that half-reactions occur in two separate compartments,so the oxidising agent and reducing agent do not come into direct contact, electrons can only be transferred through an external circuit connective the negative and positive electrodes.
56
What are half-cells?
| contains? species present form?
a half cell contains an electrode in contact with a solution, species present in each half-cell forms a conjugate redox pair (oxidising agent and its corresponding reduced form)
57
If no metal is present what can be used?
inert electrode such as platinum or graphite or a gas electrode, H⁺/H₂ half-cell
58
Where does oxidation occur in a galvanic half-cell?
anode where electrons are released, negative terminal
59
Where does reduction occur in a galvanic half-cell?
cathode where electrons are gained, positive terminal
60
What is the purpose of the salt bridge? (2)
- contains ions that are free to move so that they can balance charges formed in the two compartments
- cations move towards cathode and anions move towards anode
61
What occurs without a salt bridge? (2)
- the solution in one compartment in the galvanic cell would accumulate negative charge and the solution in the other compartment would accumulate positive charge.
- Accumulation of charge would stop reaction very quickly, hence prevent further reaction
62
Where is the stronger reducing agent held in?
negative electrode (anode)
63
Where is the stronger oxidising agent held in?
postive electrode (cathode)
64
How does the current flow within the half cells?
as one half-cell has a greater tendency to push electrons into the external circuit than the other half-cell
65
What is potential difference (E)?
electromotive force/emf referred to voltage (V), which exists between two half-cells
66
What standard conditions are cells usually measured under?
| pressure, concentration and symbol and temp
- a pressure of 1 bar (100kPa)
- 1 mol/L concentration of solution
- E°
- 25°C
67
Why is it impossible to measure the potential difference of an isolated half-cell?
both oxidation and reduction must take place for potential difference to exist, but you can assign the standard half-cell potential (E°) by connecting it to a standard reference cell (H⁺/H₂) and measuring the voltage produced.
68
What is the standard reference half-cell used?
hydrogen half-cell/standard hydrogen electrode, under standard conditions its E° Value is assigned as 0
69
Summarise the energy transformation in a secondary cell . (2)
- When a secondary cell discharges it acts as a galvanic cell, converting chemical energy into electrical energy
- When the cell is recharged, it acts as a electrolytic cell. Electrical energy is transformed into chemical energy in an electrolytic cell.
70
What happens when the battery is discharging? (3)
- Chemical energy is cinverted into electrical energy
- acts as a galvanic cell
- spotaneous reaction
71
What happens when the cell is recharging? (3)
- Electrical energy is converted into chemical energy
- acts as electrolytic cell
- non-sponatneous reaction
72
Why are car batteries more widely used type of secondary cell?
Lead acid batteries as they are cheap and reliable, providing high currents and have a long lifetime.
73
What recharges the car battery?
Once the engine starts, an alternator which is run by the engine, provides electrical energy to operate the car's electrical system and recharges the battery
74
When is a car battery used?
Start the cars' ongoing and operate the cars' electrical accessories when the engine is not running
75
What potential difference does each cell in a car battery have? And overall with all 6 cells connected in a series?
2V and 12V
76
What enables the battery to be recharged?
Produce of both electrode regions lead (II) sulphate forms as a solid of the surface
77
What are fuel cells?
continuously produce electricity for as long as is fuel is fed into the cells
78
What is the one limitation of primary/secondary cells from fuel cells?
Limitation of other cells is that they contain relatively small amounts of reactants. But reactants are supplied continuously allowing constant production of electrical energy in fuel cells
79
What happens when E° value is negative when connected to hydrogen half-cell?
it indicates that the electrode is negative and oxidation is occurring
80
What is the E° value known as when it is connected a hydrogen half-cell?
the standard electrode potential and the standard reduction potential
81
What does the standard electrode potential give a numerical measure of?
It gives a numerical measure of the tendency of a half-cell reaction to occur as a reduction reaction
82
In a galvanic cell, the stronger reducing agent is oxidised
so it is in the half-cell with the negative electrode (anode)
83
The stronger oxidising agent is reduced
so it is in the half-cell with the positive electrode (cathode)
84
Strong reducing and oxidising agents donate electrons....
more readily than weak ones
85
Strong reducing agents have...
weak conjugate oxidising agents
86
Strong oxidising agents have....
weak conjugate reducing agents
87
What type of reactions occur in cells and batteries?
spontaneous redox reactions that give something a source of energy
88
What are the three basic types of galvanic cells?
Primary cells
Secondary cells
Fuel cells
89
What are primary cells?
cells that disposable and designed not to be recharged
90
What are some types of primary cells?
common commercial alkaline cells, such as batteries in a torch or remote control are non rechargeable cells
91
When does a primary cell get used up?
When most of the reactants are used up, the cell reaction reaches equilibrium
92
What are secondary cells?
which are rechargeable and designed to be reused many times
93
What are some types of secondary cells? (3 types)
- lithium-ion cells and nickel-metal hydride cells (known as accumulators)
- found in laptops, mobile phones, cameras and portable power tools
- fundamental to the operation of electric vehicles and solar power energy storage system
94
What reaction occurs when a secondary cell is recharged? (2)
the cell reaction must occur in reverse: the products of the reaction are converted back to the original reactants
Electrical energy supplied by the charger is converted into chemical energy in the cell.
95
What is the secondary cell attached to? (2)
by connecting the cell to a charger, source of electrical energy which has a potential difference a little greater than the potential difference of the cell.
positive terminal of charger is connected to the cell's positive electrode and the negative electrode of the charger to the cell's negative electrode
96
What must be possible for reactants to regenerate?
the products formed in the cell during discharge must remain in contact with the electrodes in convertible form
97
What is a key difference between a fuel cell and a primary or secondary cell?
reactants are not stored in the fuel cell. They must be continuously supplied from an external source
98
What chemical energy does fuel cells use?
chemical energy of hydrogen or other fuels to cleanly and efficiently generate electricity.
99
What are fuel cells used for?
a source of power for transport and for emergency back-up power applications
100
What makes fuel cells efficient?
fuels cells transform chemical energy directly into electrical energy, enabling efficient use of the energy released by spontaneous redox reactions
101
When does energy loss occur for fuel cells when applied?
occur in coal-fired power stations and combustion engines are avoided with a consequential reduction in the volume of greenhouse gases produced.
102
Why do fuel cells use hydrogen?
a fuel produces electricity, water, heat and very small amounts of nitrogen dioxide and other emissions
103
What are the two compartments in a fuel cells?
one for hydrogen gas and the other for oxygen gas
104
Describe the fuel cell design. (6)
- gas compartments are separated form each other by two porous electrodes and usually a KOH electrolyte solution
- electrode at the hydrogen compartment is the anode
- electrode at the oxygen compartment is the cathode
- oxygen gas inlet on the right
- hydrogen gas inlet on left
- water outlet on the bottom where the electrolyte solution is.
105
Explain the 3 step reaction that occurs within a fuel cell.
At the anode, H₂ (the fuel) is oxidised by reacting with hydroxide ions :
H₂(g)+2OH⁻(aq)→2H₂O(l) +2e⁻
At the cathode, oxygen gas is reduced:
O₂(g) + 2H₂O(l) + 4e⁻→4OH⁻(aq)
The overall reaction for the reaction is:
2H₂(g)+O₂(g) →2H₂O(l)
106
How much voltage does each fuel cell produce?
1V
107
How are higher voltages obtained for a fuel cell? And what is the byproduct?
higher voltages are obtained by connecting a number of fuel cells in series to form a battery or fuel cell stack.
byproduct is water and heat is given off
108
At what nature of the electrodes is best for efficiency of a fuel cell?
electrodes must both conducting and porous to allow the hydrogen and oxygen to diffuse through them to come into contact with the ions in the electrolyte and to allow the redox half-reactions to occur at their surface
109
What does the size of the current drawn from a fuel cell depend on?
size of current that can be drawn from a fuel cell depends on the surface are of the electrodes
110
What do catalysts help with in a fuel cell?
| Normally
enhancing the rate of reaction and the current that can be produced from a cell
111
What do catalysts help with in a fuel cell? and what catalyst is used?
(anode)
in an anode increase the rate of oxidation of fuel gas
| platinum metal is used as a catalyst at this electrode
112
What do catalysts help with in a fuel cell? and what catalyst is used?
(cathode)
the cathode catalyst, which increases the rate of reduction half-reaction
can be made from a different material, such as nickel power or nanomaterial
113
Why is producing hydrogen not great?
if the production of hydrogen is produced using renewable energy the production of hydrogen can result in significant levels of greenhouse gases and other pollutants
114
What are two practical methods of generating hydrogen without producing CO2?
- using electrical energy to convert water to hydrogen, electricity can be generated from renewable sources such as solar-power farms and wind farms
- collecting biogas from landfill sites and converting the methane in the gas to hydrogen
115
What are is 5 advantage of fuel cells? (convert)
convert chemical energy directly to electrical energy.
- it is more efficient than the series of energy conversions that takes place in power stations that burn fossil fuels: chemical energy→mechanical energy→heat energy→mechanical energy→electrical energy
116
What are 5 advantages of fuel cells?
Fuel cells convert chemical energy directly to electrical energy more efficiently
Hydrogen fuel cells produce water and heat as byproducts
no greenhouse gases, such as CO2 are released fuel cells can use a variety of fuels
electrodes do not get depleted during the process of generating electricity
no memory effect (as with secondary) efficiency doesn't reduce over time
fuel cells will generate electricity for as long as the fuel supplied.
Conventional batteries need to be recharged or replaced
117
What are 6 disadvantages of fuel cells?
fuel cells require a constant fuel supply
fuel cells are expensive. Still developing, and are not made in large numbers
some types of fuel cells use expensive electrolytes and catalysts
hydrogen used in many fuel cells is mainly sourced from fossil fuels, the process involves energy losses and generates greenhouse gases
The use of fuel cells in transport will require an extensive network of hydrogen filling stations before it can become widespread
significant issues associated with the storage of hydrogen fuel
118
What is dry corrosion? (direct corrosion)
Direct reaction with oxygen in the air to form a metal oxide
119
What are some examples of metals that dry corrode?
Na is so reactive that it must be stored under oil to prevent contact with oxygen
4Na(s) + O₂(g) → 2Na₂O(s)
While Al forms a tough, impervious coating of aluminium (Al₂O₃), which protects the metal underneath from further contact with oxygen
120
What is wet corrosion?
The presence of moisture accelerates the corrosion of iron, formation of rust as flaky, brown-red coating
121
What are two ways wet corrosion is accelerated?
the presence of water
impurities, such as salt and acidic pollutants that dissolve in the water
122
When can wet corrosion be reduced?
corrosion can be reduced when iron is alloyed with certain other materials or when it has a protective coating
123
What is rust?
hydrate oxide of iron Fe₂O₃·xH₂O.
with 1-3 water molecules associated with the iron oxide
124
What are the 4 steps of wet corrosion? (step 1)
Iron is oxidised to form Fe²⁺ ions at one region on the iron surface.
Fe(s)→Fe²⁺(aq) + 2e⁻
at another region on the surface, using the electrons produced by the oxidation process, oxygen is reduced in the presence of water to hydroxide ions.
Overall: O₂(aq) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq)
125
What are the 4 steps of wet corrosion? (step 2)
Formation of precipitate of iron(II) hydroxide
| Fe²⁺(aq)+2OH⁻(aq)→Fe(OH)₂(s)
126
What are the 4 steps of wet corrosion? (step 3)
Further oxidation of iron(II) hydroxide occurs in the presence of oxygen and water to produce iron(III) hydroxide, a red-brown precipitate:
4Fe(OH)₂(s) + O₂(aq) + 2H₂O(l) → 4Fe(OH)₃(s)
127
What are the 4 steps of wet corrosion? (step 4)
In the air, the iron(III) hydroxide loses water to form hydrated iron(III) oxide (Fe₂O₃∙xH₂O), rust
128
What are the 4 steps of wet corrosion?
1. Iron is oxidised to form Fe²⁺ ions at one region on the iron surface.
Fe(s)→Fe²⁺(aq) + 2e⁻
at another region on the surface, using the electrons produced by the oxidation process, oxygen is reduced in the presence of water to hydroxide ions.
Overall: O₂(aq) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq)
2. Formation of precipitate of iron(II) hydroxide
Fe²⁺(aq)+2OH⁻(aq)→Fe(OH)₂(s)
3.Further oxidation of iron(II) hydroxide occurs in the presence of oxygen and water to produce iron(III) hydroxide, a red-brown precipitate:
4Fe(OH)₂(s) + O₂(aq) + 2H₂O(l) → 4Fe(OH)₃(s)
4. In the air, the iron(III) hydroxide loses water to form hydrated iron(III) oxide (Fe₂O₃∙xH₂O), rust
129
How does the presence of dissolved ions/salts in water increase rate of wet corrosion?
Salts are good electrolytes and accelerate the rusting process because they increase the conductivity of moisture on the surface of the iron.
130
What are 2 ways to prevent corrosion?
Surface protection
| Electrochemical protection
131
What is surface protection?
involves covering the surface of the iron to prevent contact with oxygen and moisture.
materials such as paint and plastic can be used for this purpose
132
What can be used for surface protection? Including moving parts(3)
- alloying it with small quantities of metals, such as chromium, nickel, manganese or molybdenum to produce stainless steel. Atoms of the metals used to make the alloy are all bonded into the metallic lattice. It may slightly oxidise in air, but the oxide coating produced is continuous and unreactive. Oxide layer protects the metal from further oxidation.
- materials such as paint and plastic can be used for this purpose
- moving parts, such as a bicycle chain, can be coated with oil or grass to both reduce friction and prevent contact with oxygen and moisture
133
What can iron and steel be coated in? (and tin cans)
coated with thin layers of less reactive metal in a process known as electroplating.
tin cans are steel plated with tin, tin is much less reactive metal than iron and does not corrode greatly in the atmosphere
134
What are two types of electrochemical protection?
Cathodic protection
| Sacrificial protection
135
What is cathodic protection?
involves the use of low-voltage, direct current power supply to give the iron being protected a negative charge.
136
How does cathodic protection reduce corrosion? (2)
for corrosion to occur, iron loses electrons (is oxidised)
act of making iron negative pushes electrons towards the iron, thus reducing the chance of oxidation occurring. Instead the iron becomes the site of reduction reactions and forms the cathode.
137
What is sacrificial protection?
involves iron acting as the cathode
A more easily oxidised metal, such as zinc forms the anode and is sacrificed in order to protect the iron from corrosion. More reactive metal loses electrons and forms metal cations in preference to the iron
138
What is galvanised iron? How does it help with corrosion?
Iron coated in zinc
When the zinc coating is scratched, the iron is still protected because the zinc loses electrons more readily than the iron. Zinc slowly corrodes, but the iron is protected for years
139
Give an example of how sacrificial protection is used in underground steel pipelines, bridge pillars and steel hulls of ships
can be protected by connecting them to blocks or plates of zinc or other more reactive metals such as magnesium.
Because these metals are more easily oxidise than iron, they lose electrons which are transferred to the iron where reduction of oxygen and water occurs
reactive metals, which are called sacrificial anodes are eventually consumed and must therefore be replaced, but this is less expensive than replacing the steel structures that are being protected.
140
What is oxidation?
Loss of electrons
141
What is reduction?
Gain of electrons
142
What is a disproportionation reaction?
Process where a substance undergoes oxidation and reduction simultaneously. (Undergoes self-redox)
143
Describe the structure of a battery (7)
```
Metal cap (+)
Cathode: outer steel case
Powered zince
Mixture of mangahese dioxide and carbon
Potassium hydroxide electrolyte
Anode: steel or brass
Metal base (-)
```
144
Describe the construction of a lead-acid battery
Lead plates (-) anode and lead plates impregnated with lead(IV) oxide (+) cathode are seperated by a porous seperator and about 4mol/L of sulfuric acid
145
What is the chemical equations for the corrosion on iron?
2Fe(s) + O2(aq) + 2H2O(l) -> Fe2+(aq) + 4OH-(aq)
| Fe2+(aq) + 2OH-(aq) -> Fe(OH)2(aq)
146
How is corrosion of iron a redox reaction?
As the oxidation number increases from 0 to 2 for the oxidation reaction
And the oxidation number increases from +2 to +3 for the reduction reaction
147
Underwater steel pillars often corrode more rapidly just beneath the surface of the water than above. Suggest a reason.
Reduction of oxygen occurs when there is plenty of moisture and the oxygen concentration is high; that is, at or just above the surface of the water. The cathodic region forms here. So the anodic region is close to the cathode and where the oxygen concentration is less; that is, just below the surface. Oxidation or corrosion therefore occurs more rapidly below the surface.
148
What are 6 common oxidising agents?
O₂, Cl₂, MnO₄⁻, Cr₂O₇²⁻, ClO⁻, H⁺
149
What are 5 common reducing agents?
Zn, C, H₂, Fe²⁺, C₂O₄²⁻
150
Why can hydrogen peroxide solution spontaneously decompose to form water and oxygen?
Hydrogen peroxide is both a strong oxidising agent and as a weak reducing agent (it appears in electrochemical series on the left side of one-half reaction and the right side of another ) hydrogen peroxide therefore reactive itself. The reaction is very slow unless a catalyst such as manganese dioxide is used.
151
Why does tin metal added to a solution of tin(II) chloride prevent oxidation of tin(II) ions by oxygen air?
Tin(II) ions can be oxidised to Tin(IV) ions by a suitable oxidising agent. Tin metal can reduce Sn(IV) ions so that they re-form Sn(II) ions.
152
What are 5 differences between a fuel cell and a primary cell?
Fuel cell produces electrical energy continuously when reactants are provided; primary cells can produce power for only a limited time until their reactants are depleted
Fuel cell uses combustioe fuel and air or oxygen; primary cells can be made from different cimbinarions of two conjugate redox pairs
The reactants in fuel cells are usually gaseous; a range of solids, liquids and gases can be used as the reactants in the primary cells
Electrodes in fuel cells must be porous; electrodes in primary cells need not be porous
Catalysts are used in fuel cells to increase cell efficiency; catalyst are not required in primary cells
153
What are 5 similarities between fuel cells and primary cells?
Both cells are designed to convert chemical energy into electrical energy at relatively high efficiencies.
Oxidation and reduction reactions take place in different
Both have an anode, which is negative, at which an oxidation half-reactions occurs
Both have a cathode, which is positive, at which a reduction half-reaction occurs
Both have an electrolyte, which provides ions to balance charges formed at the electrodes; cations flow towards the cathode and anions flow towards the anode.
154
What is electrolysis?
Involves passage of electrical energy from, a power supply (such as a battery) through a conducting liquids
155
What is electroplating of copper?
a thin surface coating of metal only a fraction of a milllimetre thick, is applied over another metal surface, perform in electrolytic cells.
156
What is the process for electroplating of copper? (4)
- object to be plated, like key, is connected by a wire to the negative terminal of a power supply. Object becomes the negative electrode
- Rod or sheet of copper metal is connected to positive terminal of the power supply. Metal becomes s the positive electrode in cell
- Two electrodes are immersed in aqueous solution, such as copper(II) sulphate solution (which contains ions of the metal to be plated). Copper sulphate is an ionic solid composed of Cu2+ and SO42- ions, which dissociated when dissolved in water. Describe as an electrolyte as it conducts electricity
- During electrolysis, reactions occur at the surface of both electrodes
157
What occurs in the first step in the electroplating of copper?
object to be plated, like key, is connected by a wire to the negative terminal of a power supply. Object becomes the negative electrode
158
What occurs in the second step in the process of electroplating of copper?
Rod or sheet of copper metal is connected to positive terminal of the power supply. Metal becomes s the positive electrode in cell
159
What occurs the third step of the process of electroplating of copper?
Two electrodes are immersed in aqueous solution, such as copper(II) sulphate solution (which contains ions of the metal to be plated). Copper sulphate is an ionic solid composed of Cu2+ and SO42- ions, which dissociated when dissolved in water. Describe as an electrolyte as it conducts electricity
160
What happens in the fourth step in the process of electroplating of copper?
During electrolysis, reactions occur at the surface of both electrodes
161
What happens at the negative electrode during the process of electroplating of copper?
copper ions are attracted to negative electrode (object to be plated), where they accept electrons and converted too copper metal:
Cu2+ + 2e- -> Cu(s)
Coating of copper forms on object, as reduction of copper ions occurs
Cathode
162
What happens at the positive electrode in the process of electroplating of copper? Is it the cathode or anode?
positive terminal of power supply withdraws electrons from copper electrode, causing oxidation to occur
Copper metal slowly dissolves as Cu2+ ions are formed
Cu(s) -> Cu2+(aq) + 2e-
Reaction replaces Cu2+ ions in solutions that were consumed by the reaction at the negative electrode
Concentration of copper ions remains constant
Anode
163
What type of electrodes are used in electrolysis of molten sodium chloride?
Platinum metal or graphite is used for the electrodes as there are inert
164
What occurs at the negative electrode for the electrolysis of molten sodium chloride?
Na+ ions in electrolyte are attracted to negative electrode, accepting electrons and becoming sodium atoms
Na+(l) + e- -> Na(I)
Sodium is less dense than molten sodium chloride therefore floats to the top of cell
165
What occurs at the positive electrode for the electrolysis of molten sodium chloride?
Cl ions move to the positive electrode to give up electrons to form Cl atoms
Bubbles of Cl2 gas appear at the electrode
2Cl-(l) -> Cl2(g) + 2e-
Electrons from chloride ions move through electrode to the power supply
166
Why in electrolysis do products need to be kept apart although it occurs in a single container?
They will react with each other to reform
167
Are electrolysis reactions spontaneous or non spontaneous?
Non spontaneous reaction as electrical energy must be supplied to convert it into chemical energy
168
What are 4 differences between galvanic cells and electrolytic cells? (Electricity)
Galvanic cells produce electricity
| Electrolytic cells consumes electricity
169
What are 4 differences between galvanic cells and electrolytic cells? (Spontaneous)
Galvanic cells are spontaneous reactions
| Electrolytic cells are non-spontaneous
170
What are 4 differences between galvanic cells and electrolytic cells? (Conversion)
Convert chemical to electrical
| Convert electrical energy to chemical
171
What are 4 differences between galvanic cells and electrolytic cells? (Anode and cathode )
Anode is negative and cathode is positive
| Anode is positive and cathode is negative
172
What are two similarities between galvanic and electrolytic cells?
Oxidation occurs at anode and reduction at cathode
| Anions flow towards anode and cations flow towards cathode
173
What are two applications of electrolysis?
Electroplating and electrorefining
174
What is electroplating?
Plating a thin surface coatings of one metal over the surface of another metal
175
Where is the object to be plated connected to in electroplating of tin cans? (Positive or negative terminal)
The negative terminal of power supply
176
Where is the electrode of tin metal connected to in electroplating tin cans? (Positive or negative terminal of power)
positive terminal of power supply
177
What electrolyte solution is the object immersed in (tin)?
Solution containing ions of the metal to be plated
| Tin(II) nitrate solution
178
What occurs at the negative electrode in electroplating of tin cans?
Tin ions are attracted to cathode, undergo reduction and are converted to tin metal
Sn2+(aq) + 2e- -> Sn(s)
Coating of tin is formed on object
179
What occurs at the positive electrode in electroplating of tin cans?
Tin undergoes oxidation
Sn(s) -> Sn2+(aq) + 2e-
As Sn2+ ions are consumed by the reaction at the cathode and produce at the anode, the overall concentration of tin ions remains constant.
180
Object to be plated is at...
Cathode (negative)
181
Electrode of metal is at....
Anode (positive)
182
What are 6 factors that are critical for achieving a smooth, tightly bonded metal coating that prevents oxygen and water from reaching the base metal:
```
Voltage
Current
Temperature
Electrode positions
Concentration
Identity of electrolyte
```
183
What is blister copper in electrorefining of copper?
Copper metal being extracted from its ores by smelting, where copper ore is heated strongly in air to produce impure molten copper metal. As metal solidifies hot gases escape and the surface becomes blistered.
Containign 2% of impurities
184
What is electorefining of copper?
Blistered copper is purified through a process called electrorefining, uses a large amount amount of electrical energy, so refinery must be located close to a source of abundant cheap power
185
What happens at the positive electrode during electrorefining of copper? (3)
anode
Electrons are drawn away from the blister copper anode to positive terminal of power source, copper and impurities (such as nickel and zinc) that are more reactive (stronger reducing agents) than copper, are oxidised
Cu(s() -> Cu2+(aq) + 2e-
Impurities less reactive than copper like silver, gold and platnum are not oxidised and simply fall from the anode, collecting at the bottom
186
What happens at the negative electrode in electrofining of copper? (3)
cathode
Copper metal is deposited as electrons from the power source and are accepted by metal ions from solution
Copper metal is the strongest oxidising agent present in solution therefore it is formed
Cu2+(aq) +2e- -> Cu(s)
In copper electrorefining cell, copper metal undergoes oxidation at the anode and copper ions undergo reduction at cathode, yields copper metal of high purity
187
Why do iron objects found at the bottom of the ocean have very few signs of corrosion?
As there is very little dissolved O2 at the bottom of the ocean. For corrosion to occur to an appreciable extent-both H2O and oxygen must be present.
188
Why is the anode negative in a galvanic cell but positive in an electrolytic cell?
The process of oxidation involves the loss of electrons, and oxidation occurs at the anode that is why the galvanic cell is negative. But in a electrolytic cell, since the positive terminal of power is connected to it the electrode it draws away the electrons from the anode therefore oxidation occurs.
189
Why are two half cells usually used in a galvanic cell whereas the reactants of an electrolysis cell are often placed in a single container?
As reactions in galvanic cells are spontaneous, in reactants were in one container and in contact with one another they would be releasing heat energy instead of electrical energy.
While since electrolytic cells are non-spontaneous, both reactions can occur in the same container, however the products should not come into contact or a reaction will occur.
190
The initial voltage found measured in a cell was higher than expected. Give one possible reason for this observation.
not measure at 25C
| not a 1 mol L solution
191
What does scratched tin cause iron to corrode more rapidly than if the tin were not there?
As iron is more reactive than tin. if it is scratched the iron will become the anode and will oxidise quicker than a piece of uncoated iron. The increase in corrosion rate occurs in any situation where iron is in contact with any metal less active than itself
192
Why does the rate of production of electrical current from an electrochemical cell decreases as it operates?
rate of reaction decreases as reactants are consumed. Therefore the rate of production of electrical current decreases.
193
During the operation of an electrochemical cell, why is it important that the anode and cathode do not come into contact with each other?
Short circuits and so current will not flow. Current will be forced through an external circuit.
194
Why is sulfuric acid and not hydrochloric acid used in the acidification of potassium permanganate solution prior to it being used in redox titration?
Chloride ions in HCl will react with the permanganate ions, since the permanganate ions oxidise the chloride ions, there are less (or no) MnO4-(aq) to react with the reducing agent
195
Explain the function of the electrolyte.
allow for the movement of ions
maintains electrical neutrality of ions
completes the circuit
196
State 2 reasons why the theoretical (calculated) value was not the same as the actual EMF generate by the fuel cells.
fuel cell is not at standard conditions, eg. not at 25ºC
| resistance in the wire
197
Explain the chemical principles of an electrolytic cell
electrolytic cell utilises an external applied voltage this allows a non-spontaneous redox reaction to occur.
198
What must happen for a cell to go flat (stop providing electrons)? (2)
- Reactants are used up
| - chemical equilibrium is reached
199
State the reason for the reactants being kept in seperate half cells.
The electrons which are transferred can then pass through an external circuit (rather than being transferred through direct contact)
- create a current
- create a potential difference across half cells
200
What is the reaction at the anode for 1.0M solution of NaCl?
Possible anode reactions:
- 2Cl- -> Cl2 + 2e- E=-1.36V
- 2H2O -> O2 + 4H+ + 4e-
2H2O -> O2 + 4H+ + 4e-
201
Why is phenolphthalein a good indicator for NaCl?
- NaCl ions are neutral as they do not undergo hydrolysis
202
What is the reaction at the anode for concentrated solution of NaCl?
Possible anode reactions:
- 2Cl- -> Cl2 + 2e- E=-1.36V
- 2H2O -> O2 + 4H+ + 4e- E=-1.23V (E=-0.82V at pH 7)
2Cl- -> Cl2 + 2e-
203
What is the reaction at the anode for 1.0M solution of NaCl?
Possible cathode reactions:
- Na* + e- -> Na E= -2.71V
- 2H2O + 2e- -> H2 + OH- E= -0.83V (E= -0.41V at pH 7)
2H2O + 2e- -> H2 + OH- E= -0.83V
204
What is the reaction at the anode for concentrated solution of NaCl?
Possible cathode reactions:
- Na* + e- -> Na E= -2.71V
- 2H2O + 2e- -> H2 + OH- E= -0.83V (E= -0.41V at pH 7)
2H2O + 2e- -> H2 + OH- E= -0.83V
