7. Electrochemical Cells

Question 1

What do electrochemical cells make

Answer 1

electricity

Question 2

What are the 2 reactions always present within an electrochemical cell

Answer 2

oxidation
reduction
(it is a redox reaction)

Question 3

Ways in which Electrochemical cells can be made

involving 2 different metals

Answer 3

half cells containing
2 different metals
dipped in salt solutions of their own ions
and connected by a wire (the external circuit)

Question 4

What is salt bridge made of and what is it soaked in

Answer 4

filter paper

KNO3(aq)

Question 5

What is the purpose of the salt bridge?

Answer 5

allows ions to flow through it

balances out charges

Question 6

What happens in a zinc / copper electrochemical cell

Answer 6

Zinc looses electrons more easily than copper.

1. Zinc from zinc electrode is oxidised to form Zn2+(aq) ions.
   this releases electrons into the external circuit.
2. in other half-cell same number of electrons are taken from the external circuit
   reducing the Cu2+ ions to copper atoms

Question 7

What direction do the electrons flow through the wire?

Answer 7

from most reactive metal to least reactive metal

Question 8

What does the volt metre in the external circuit tell us

Answer 8

voltage between the two half cells

the cell potential or e.m.f or Eθcell

Question 9

Half cells involving solutions of 2 aq ions of the same element
What electrode is used and why ?

Answer 9

Platinum electrode
Its inert (chemically inactive) and conducts electricity

Question 10

Half cells involving solutions of 2 aq ions of the same element
Where does conversion of ions e.g. Fe2+(aq) / Fe3+(aq) take place?

Answer 10

On surface of the platinum electrode

Question 11

Reactions at each electrode are _____

e.g. zinc / copper

Answer 11

Reversible
e.g.
Zn2+(aq) + 2e- Zn(s)
Cu2+(aq) + 2e- Cu(s)

Question 12

What does the direction of the reaction depend on

Answer 12

how easily each metal looses electrons

how easily its oxidised

Question 13

How easily a metal is oxidised is measured using _____

Answer 13

Electrode potentials

Eθ (V)

Question 14

a metal that is easily oxidised has a

Answer 14

very negative electrode potential

Question 15

a metal that is harder to oxidise has a

Answer 15

less negative or positive electrode potential

Question 16

2 conventions when drawing cells

Answer 16

1. half cell with more negative potential goes on the left

2. oxidised forms go in the centre of the cell diagram

Question 17

the cell potential or e.m.f or Eθcell

formula

Answer 17

Eθcell = Eθ right hand side - Eθ left hand side

Question 18

What are electrode potentials measured against?

Answer 18

Standard hydrogen electrodes

Question 19

definition

The standard electrode potential Eθ of a half cell is

Answer 19

the voltage measured under standard conditions when the half-cell is connected to a standard hydrogen electrode

Question 20

What 3 things are needed for standard conditions?

Answer 20

1. any solutions must have concentration of 1.00 moldm-3 or be equimolar (i.e. contain same number of moles of ions)
2. temperature must be 298 k (25C)
3. pressure must be 100kPa

Question 21

the standard hydrogen electrode is always shown on the ____

Answer 21

left

Question 22

the standard hydrogen electrode half cell has a value of

Answer 22

0.00 V

Question 23

standard hydrogen electrodes

the whole cell potential =

Answer 23

Eθ right side - Eθ left side
Eθ left side = 0
so whole cell potential/ voltage reading = Eθ right side

Question 24

why are standard conditions used to measure electrode potentials

Answer 24

always get the same value for electrode potentials -> can compare values for different cells.

no standard conditions = equilibrium position affected by changes in temperature, pressure and concentration altering the cell potential.

Question 25

ways in which Electrochemical cells can be made

involving same element

Answer 25

half cells containing
solutions of 2 aqueous ions of the same element
e.g. Fe 2+(aq) / Fe 3+(aq)

Question 26

ways in which Electrochemical cells can be made

involving non metals

Answer 26

can be made from non-metals
systems involving a gas e.g. chlorine
the gas can be bubbled over a platinum electrode
sitting in a solution of its aq ions e.g. Cl-(aq)