electrochemical cells

Question 1

what is a voltaic cell?

Answer 1

a type of electrochemical cell which converts chemical energy into electrical energy

Question 2

what is a half cell?

Answer 2

a cell which contains the chemical species present in a redox half equation

Question 3

how is a voltaic cell made?

Answer 3

the connection of two half cells, allowing electrons to flow

Question 4

why must the chemicals be kept apart in a voltaic cell?

Answer 4

if allowed to mix, electrons would flow in an uncontrolled way and heat energy would be released instead of electrical energy

Question 5

how is a metal half cell made?

Answer 5

metal rod dipped in solution of its aq metal ioon

Question 6

describe the electron flow in a half cell compared to an electrochemical cell

Answer 6

half cell = no net transfer of electrons in/out of metal
electrochemical = direction of electron flow depends upon relative tendency of each electrode to release electrons

Question 7

what is an ion half cell?

Answer 7

contains ions of same element in different oxidation state
eg iron II and iron III ions

Question 8

how is an ion half cell made?

Answer 8

as there is no metal to transport electrons, an inert metal electrode made of platinum is in solution
- allows electrons in/out of cell

Question 9

which is the negative electrode?

Answer 9

the electrode with more reactive metal
- loses electrons
- is oxidised

Question 10

which is the positive electrode?

Answer 10

the electrode with the less reactive metal
- gains electrons
- is reduced

Question 11

define ‘standard electrode potential’

Answer 11

the tendency for a species to be reduced and gain electrons

Question 12

describe what is meant by standard electrode potential?

Answer 12

electrode multi force of a half cell connected to a standard hydrogen half cell under standard conditions

Question 13

what is the standard electrode potential of hydrogen?

Answer 13

0 V

Question 14

describe how a standard electrode potential measured

Answer 14

• connect half cell to standard electrode
• connect 2 electrodes by a wire to allow flow of electrons
• connect solutions with a salt bridge (allows more ions to flow)

Question 15

describe the salt bridge

Answer 15

• allows ions to flow
• contains concentrated solution of electrolyte that doesn’t react with either solution
• eg filter paper soaked in KNO3 aq

Question 16

what does a more negative standard electrode potential tell us?

Answer 16

• greater tendency to lose electrons: undergoes oxidation
• less likely to gain electrons and undergo reduction

Question 17

what does a more positive standard electrode potential tell us?

Answer 17

• greater tendency to gain electrons: undergoes reduction
• less likely to lose electrons and undergo oxidation

Question 18

what standard electrode potential do metals usually have?

Answer 18

negative
- loss of electrons
- oxidation

Question 19

what is the relationship between reactivity and standard electrode potential?

Answer 19

more negative = more reactive in losing electrons

more positive = more reactive in gaining electrons

Question 20

describe how you would set up cells to measure cell potentials

Answer 20

1. prepare 2 standard half cells
   - metal: must be 1moldm-3
   - ion: both metal ions must have same conc, must be an inert electrode
   - gases: gas must be 100kPa, solution conc 1moldm-3, inert electrode
2. connect metal electrodes of half cells to voltmeter using wires
3. add a salt bridge & connect the solutions
4. record standard cell potential from voltmeter

Question 21

how is standard cell potential calculated from standard electrode potentials?

Answer 21

electrode potential of positive electrode - electrode potential of negative electrode

Question 22

in an electrochemical cell, what are the charge carriers in the wire & salt bridge?

Answer 22

wire = electrons
salt bridge = ions

Question 23

state the standard conditions used for electrochemical cells

Answer 23

1moldm-3
298K
1atm