9.2

Question 1

electrochemical cells are applications of

Answer 1

the fact that redox reactions involve transfers of electrons that immediately suggests a link between this type of chemical reactivity and eelectricity

Question 2

types of electrochemical cells

Answer 2

Voltaic (galvanic) cells - generate electricity from chemical reactions
Electrolytic cells - drive chemical reactions using chemical energy

Question 3

Difference between carrying out a redox reaction normally and in a voltaic cell

Answer 3

When the reaction is carried out in a single test tube the electrons flow spontaneously from the reducing agent to the oxidizing agent in the solution and energy is released in the form of heat. (specific case of Zn and Cu)
There is however a different way of organizing this reaction so that the energy released in the redox reaction instead of being lost as heat is available as electrical energy. It is really just a case of separating the two half equations into so called half-cells and allowing the electrons to flow between them only through an external circuit.

Question 4

SImplest type of cell

Answer 4

Putting a strip of metal into a solution of its ions

Question 5

What happens in the half-cell of a reducing agent

Answer 5

reducing agent’s atoms will form ions by releasing electrons that will make the surface of the metal negatively charged with respect to the solution causing a charge separation, known as an electrode potential, between the metal and its ions in solution. At the same time, ions in the solution gain electrons to form reducing agent’s atoms, so an equilibrium exists.
position of this equilibrium determines the size of the electrode potential in the half cell and depends on the reactivity of the metal

Question 6

in general how are metal reactivity and electrode potential in its half cell linked

Answer 6

The more reactive a metal, the more negative its electrode potential in its half-cell

Question 7

electrodes in Voltaic cell

Answer 7

Cathode - positive; Reduction
Anode - negative; oxidation
*metals higher on activity series at anode ( better reducing agent is oxidized)
electrons flow from the anode to cathode

Question 8

external electronic circuit in voltaic cell

Answer 8

electrons flow from the anode to cathode through the external electronic circuit connected to the metal electrodes - a voltmeter can also be attached to record the voltage generated

Question 9

salt bridge in voltaic cell

Answer 9

ion movement through salt bridge - glass tube or strip of absorptive paper that contains an aqueous solutions of ions
movement of ions neutralises any build up charge and maintains the potential difference
anions move from the cathode to the anode
cations move from the anode to the cathode

Question 10

solution chosen in the voltaic cell is often

Answer 10

aqueous NaNO3 or KNO3 as these ions will not interfere with the reactions at the electrodes

Question 11

no salt bridge =

Answer 11

no voltage generated

Question 12

cell diagram convention

Answer 12

*a single vertical line represents a phase boundary such as that between a solid electrode and an aqueous solution within a half-cell
*a double vertical line represents the salt bridge
*the aqueous solutions of each electrode are placed next to the salt bridge
*the anode is generally put on the left and the cathode on the right, so electrons flow from left to right
*spectator ions are usually omitted from the diagram
*If a half cell includes two ions, they are separated by a comma because they are in the same phase
E.g.
ZN(s) | Zn2+ (aq) ||Cu2+ (aq) | Cu(s)
anode cathode
oxidation reduction
———————>
electrons flow this way via external circuit
Zn(s) –> Zn2+ (aq) + 2e-
Cu2+ (aq) + 2e- –> Cu(s)

Question 13

Electromotive force (EMF)

Answer 13

a potential difference generated by a voltaic cell
magnitude of this voltage depends on the difference in the tendencies of these two half-cells to undergo reduction

Question 14

why do we need standard hydrogen electrode (SHE)

Answer 14

clearly, the electrode potential of a single half-cell cannot be measured in isolation, but only when electrons flow as it is linked in this way to another half-cell. Therefore, in order to draw up a list of the relative reducing power of different half-cells, it is necessary to compare them all with some fixed reference point that acts as a standard for measurement.
In electrochemistry, the reference standard is the standard hydrogen electrode

Question 15

The standard hydrogen electrode

Answer 15

sometimes called the standard hydrogen half-cell
half cell that has an electrode potential value of 0 V
therefore while connecting it to another half cell it will give us a single electrode potential of that half cell
platinum electrode is put into a glass tube with holes (to allow bubbles of H2(g) to escape) containing acid solution with 1.0 mol dm-3 H+(aq)
platinum is used as the conducting metal in the electrode because it is fairly inert and will not ionize - and it also acts as a catalyst for the reaction of proton reduction
concentration of H+ (aq) is 1.0 mol dm-3
pH=0
Pressure of H2(g) is 100kPa
298K
2H+ (aq) + 2e- ⇌ H2 (g)
reaction is reversible occurring as reduction of H+ ( forward reaction) or as oxidation of H2 ( backward), depending on the electrode potential of the half cell to which it is linked

Question 16

standard half-cells

Answer 16

half cells under standard conditions

Question 17

standard electrode potential

Answer 17

EMF generated when it is connected to standard hydrogen electrode by an external circuit and a salt bridge measured under standard conditions

Question 18

standard electrode potentials are given for

Answer 18

the reduction reaction

Question 19

points regarding standard electrode potentials

Answer 19

*all E values refer to reduction reaction
*the E values do not depend on the total number of electrons, so do not have to be scaled up or down according to the stoichiometry of the equation
*the more positive the E value for a half cell, the more readily it is reduced

Question 20

In a voltaic cell the half-cell with the higher (more positive) electrode potential is the

Answer 20

cathode +

Question 21

In a voltaic cell the half cell with the lower (more negative) electrode potential is the

Answer 21

anode -

Question 22

electrons always flow towards the half-cell with

Answer 22

the highest E value

Question 23

calculating the cell potential E cell

Answer 23

E cell = E half cell where reduction occurs - E half-cell where oxidation occurs

Note:
*The E values used in this expression must be the reduction potentials as supplied in data tables. Do not invert them before substituting into equation.
*The E values do not have to be multiplied according to the stoichiometry of the redox equation. This is because they are intensive quantities and do not depend on the total number of electrons shown in the equation

Question 24

How to determine the spontaneity of a reaction

Answer 24

If E cell is positive, the reaction is spontaneous as written

Question 25

Electrode potential and free energy change ( E cell and Delta G)

Answer 25

Delta G = -nFE
n = number of moles of electrons transferred in the reaction
F = the charge carried by 1 mole of electrons, known as the Faraday constant ( value of approximately 96500)
when E = 0 and delta G = 0 –> reaction is at equilibrium

Question 26

trends of strength and E values

Answer 26

In the table of standard electrode potentials
up to down –> increasing strength as oxidizing agent
down to up –> increasing strength as reducing agent

Question 27

The higher the value of E of a half cell

Answer 27

the stronger the oxidizing agent

Question 28

The lower the value of E

Answer 28

the stronger the reducing agent

Question 29

A little caution about interpreting E data

Answer 29

E data does not give any info about the rate
so a reaction that is predicted to be spontaneous may give no observable sign of reaction because the activation energy may be to high for the reaction to occur at any appreciable rate
+ E values discussed relate to only standard conditions and any changes in temp concentration or pressure will alter the values, possibly even changing whether a reaction is spontaneous or not

Question 30

Electrolytic cell

Answer 30

*uses external source of electrical energy to bring about a redox reaction that would otherwise be non-spontaneous
*source of electric power is a battery or a DC power source
*the electrodes are immersed in the electrolyte and connected to the power supply; they must not touch each other; Electrodes are made from a conducting substance - generally metal or graphite; They are described as inert when they do not take part in the redox reactions
*Electric wires connect the electrodes to the power supply

Question 31

electrolyte

Answer 31

where the reactant is present in the process of electrolysis
liquid, usually molten ionic compound or solution of an ionic compound
as the electric current passes through the electrolyte, redox reactions occur at the electrodes, removing the charges on the ions and forming products that are electrically neutral
the ions are therefore said to be discharged during this process

Question 32

difference between electrodes and electrolytes

Answer 32

electrodes are electronically conducting
electrolytes are ionically conducting

Question 33

electrodes in electrolytic cells

Answer 33

Cathode - negative; reduction; ions gain electrons
Anode - positive; oxidation; ions lose electrons

Question 34

determining the products in electrolytic cells

Answer 34

1. Identify all the ions present in the electrolyte and determine which will migrate to which electrode : anions to anode and cations to cathode
2. Where there is more than one possible reaction at each electrode, determine which will occur. Write the half equation for the reaction at each electrode, showing electrons released at the anode in oxidation and taken up at the cathode in reduction.
3. Balance the electrons lost and gained at the anode and the cathode, then add the two half-equations to write the equation for the net reaction
4. Consider what changes would be observed in the cell as a result of the redox processes occurring. These may include colour changes in the electrolyte, precipitation of solid, gas discharge, or pH changes.

Question 35

The electrolysis of molten salts

Answer 35

When the electrolyte is a molten salt, the only ions present are those from the compound itself as there is no solvent. So, usually, only one ion migrates to each electrode, and it is straightforward to predict the reactions that will occur

Question 36

Electrolysis of Aqueous solutions

Answer 36

• when electrolysis is carried out on an aqueous solution, predicting the products at the electrodes is more difficult because water itself can be oxidized or reduced
• So when a solute M+A- is in an aqueous solution, there is more than one redox reaction possible at each electrode ——> specifically at the anode: either A- or H2O can be oxidized; At the cathode: either M+ or H2O can be reduced.
  *the discharge of an ion at the electrode in these cases is known as selective discharge.
  *The outcome is determined by the following factors:
  1. relative E values of the ions
  2. the relative concentrations of the ions in the electrolyte
  3. the nature of the electrode

Question 37

Charge =

Answer 37

current x time

Question 38

number of faradays (F) =

Answer 38

moles of electrons

Question 39

The amount of product formed in electrolysis is determined by

Answer 39

the current, duration, and the charge on the ion.

Question 40

equations that link amount of product and charge

Answer 40

Q= It
C= A x s [current - A; Time - s]
F = ~Q/96500 = number of moles
use mole ratio in equation
m = n x M

Question 41

electroplating

Answer 41

the process of using electrolysis to deposit a layer of a metal on top of another metal or other conductive object

Question 42

an electrolytic cell used for electroplating has the following features:

Answer 42

• an electrolyte containing the metal ions which are to be deposited
• the cathode made of the object to be plated
• sometimes the anode is made of the same metal which is to be coated because it may be oxidized to replenish the supply of ions in the electrolyte

Question 43

Voltaic cell vs electrolytic cell

Answer 43

types of cell - Ecell - delta G - type of reaction
voltaic - >0 - <0 - spontaneous
electrolytic - <0 - >0 - non-spontaneous
equilibirum - 0 - 0 - dead battery