Electrochemistry

Question 1

Define electrochemistry

Answer 1

the interconversion of chemical and electrical energy
- spontaneous reactions can produce electricity and electricity can cause non-spontaneous reactions to occur

Question 2

redox reactions involve

Answer 2

the movement of electrons from one reagent to another

Question 3

oxidation

Answer 3

loss of electrons

Question 4

reduction

Answer 4

gain of electrons

Question 5

oxidising agent

Answer 5

• reduced
• takes electrons from the substance being oxidised

Question 6

reducing agent

Answer 6

• oxidised
• gives electrons away

Question 7

how to balance redox reactions

Answer 7

• write down the two half-reactions
• balance the atoms and charges in each half-reaction
• first balance atoms other than O and H, then O, then H
• charge is b balanced by adding electrons (e-) to the reactant side of the reduction half-reaction and to the product side in the oxidation half-reaction
• electrons should cancel out in net reaction. if necessary, multiply one or both half-reactions by an integer so that number of e- gained in reduction = number of electrons lost in oxidation
• add the balanced half-reactions and include states of matter

Question 8

two types of electrochemical cells

Answer 8

• voltaic cells
• electrolytic cells

Question 9

voltaic/galvanic cells

Answer 9

spontaneous chemical reaction (ΔG<0) generates an electric current
- batteries contain one or more voltaic cells
- voltaic cell does work on the surroundings, converting higher energy reactants in the cell into lower energy products

Question 10

electrolytic cells

Answer 10

non-spontaneous reactions (ΔG>0) are driven by electric current
- external power source supplies free energy to run electrolytic cells. the surroundings thus do work on the cell. lower energy reactants are converted to higher energy products in the cell
- used for electroplating, purification of metals, and more

Question 11

electric current is

Answer 11

flow of electrons

Question 12

electrons only flow if

Answer 12

the driving force (free energy change) is large enough

Question 13

electrodes

Answer 13

usually metal strips/wires connected by an electrically conducting wire

Question 14

anode

Answer 14

electrode where oxidation takes place

Question 15

cathode

Answer 15

electrode where reduction takes place

Question 16

describe salt bridge

Answer 16

U-shaped tube that contains a gel permeated with a solution of inert electrolyte (contains positive and negative spectator ions).

Question 17

function of salt bridge

Answer 17

The salt bridge keeps half cells electrically neutral because ions flow in and out of the salt bridge, counteracting charge build-up due to electron flow.

Question 18

conventions for notation for a voltaic cells

Answer 18

• anode components are written on the left
• cathode components are written on the right
• components of each half cell are written in the same order as in their half reactions
• single line shows a phase boundary between the components of a half cell
• double line shows that the half cells are physically separated

Question 19

addition notation for voltaic cell that is more complex

Answer 19

any inactive (inert) electrode is specified
a comma is used to show components that are in the same phase

Question 20

why does a voltaic cell work?

Answer 20

• differing abilities of metals to gain electrons gives rise to a voltage drop
• this is also known as electromotive force (EMF) or cell potential

Question 21

multimeter

Answer 21

can measure voltage

Question 22

cell potential (Ecell)

Answer 22

the difference in electrical potential between two electrodes

Question 23

units for Ecell

Answer 23

measured in Volts (V), where

V = J/C (Colomb is SI unit of charge)

Question 24

difference in Ecell for voltaic and electrolytic cells

Answer 24

Voltaic: Ecell>0 (spontaneous e- flow)
electrolytic: Ecell<0 (non-spontaneous e-flow)

Question 25

when Ecell = 0,

Answer 25

the redox reaction has reached equilibrium so the cell can do no more work

Question 26

define standard potential (E’cell)

Answer 26

cell potential under standard state conditions

Question 27

how to calculate Ecell from Ehalf-cell

Answer 27

the cell potential of any electrochemical cell is the sum of the half cell potentials for the oxidation and reduction half cells

Ecell = E ox + E red

Question 28

what does it mean to say that E is an intensive property?

Answer 28

Bigger cells with more moles of redox components will last longer, but will have the same standard output voltage

Question 29

which half cell will form the oxidation half cell?

Answer 29

the lower Ered (ie more negative/less positive = worse at being reduced)

Question 30

use of standard hydrogen electrode

Answer 30

• potentials are determined experimentally from the difference in potential between two electrodes
• the reference point is called the standard hydrogen electrode

Question 31

Standard Hydrogen Electrode

Answer 31

consists of a platinum electrode in contact with H2 gas (1atm) and aqueous H+ ions (1M)

Question 32

standard hydrogen electrode is assigned (arbitrarily) a value of

Answer 32

exactly 0.00 V

Question 33

how is Ecelll related to ΔG?

Answer 33

ΔG = -nFE(cell) = -RTln(Keq)

• ΔG is in J/mol
• n is in mol and is the no of moles of electrons transferred per mole of the reaction
• F is faraday’s constant

Question 34

at equilibrium, ΔG = and Ecell =

Answer 34

0

Question 35

Nernst Equation

Answer 35

E = E’ - RT/nF (lnQ)

E = E’ - 0.0592V/n logQ

E - The cell potential (electromotive force, EMF) under non-standard conditions, measured in volts (V).
The standard cell potential, which is the voltage of the electrochemical cell under standard conditions

Question 36

how does cell potential depend on the relative concentration of reactants and products?

Answer 36

when Q<1, lnQ<0 so Ecell>E’cell
when Q=1, lnQ=0 so Ecell = E’ cell
when Q>1, lnQ>1 so Ecell<E’cell

Question 37

two consequences of cell potential depending on the relative concentration of reactants and products:

Answer 37

• as a cell is operated, concentration of reactants will decrease and products will increase. thus, cell potential will decrease over time
• concentration cells - capture the electrical energy from a concentration difference

Question 38

how is a concentration cell created

Answer 38

• has the same half-reaction in both cell compartments, but with different concentrations of electrolyte
• there is a potential difference between cells, which drives current flow until both compartments have an equal concentration of ions
• Ecell>0 as long as the half-cell concentrations are different
• once the concentrations equalise, Ecell = 0 and current stops flowing

Question 39

write equations for corrosion

Question 40

how to protect against corrosion

Answer 40

Galvanisation: coating of iron with zinc.
Zinc is more easily oxidised than iron, giving up electrons to it. Iron is more easily reduced than zinc.