7. Electrochemical Cells Flashcards
What do electrochemical cells make
electricity
What are the 2 reactions always present within an electrochemical cell
oxidation
reduction
(it is a redox reaction)
Ways in which Electrochemical cells can be made
involving 2 different metals
half cells containing
2 different metals
dipped in salt solutions of their own ions
and connected by a wire (the external circuit)
What is salt bridge made of and what is it soaked in
filter paper
KNO3(aq)
What is the purpose of the salt bridge?
allows ions to flow through it
balances out charges
What happens in a zinc / copper electrochemical cell
Zinc looses electrons more easily than copper.
- Zinc from zinc electrode is oxidised to form Zn2+(aq) ions.
this releases electrons into the external circuit. - in other half-cell same number of electrons are taken from the external circuit
reducing the Cu2+ ions to copper atoms
What direction do the electrons flow through the wire?
from most reactive metal to least reactive metal
What does the volt metre in the external circuit tell us
voltage between the two half cells
the cell potential or e.m.f or Eθcell
Half cells involving solutions of 2 aq ions of the same element
What electrode is used and why ?
Platinum electrode Its inert (chemically inactive) and conducts electricity
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?
On surface of the platinum electrode
Reactions at each electrode are _____
e.g. zinc / copper
Reversible
e.g.
Zn2+(aq) + 2e- Zn(s)
Cu2+(aq) + 2e- Cu(s)
What does the direction of the reaction depend on
how easily each metal looses electrons
how easily its oxidised
How easily a metal is oxidised is measured using _____
Electrode potentials
Eθ (V)
a metal that is easily oxidised has a
very negative electrode potential
a metal that is harder to oxidise has a
less negative or positive electrode potential
2 conventions when drawing cells
- half cell with more negative potential goes on the left
2. oxidised forms go in the centre of the cell diagram
the cell potential or e.m.f or Eθcell
formula
Eθcell = Eθ right hand side - Eθ left hand side
What are electrode potentials measured against?
Standard hydrogen electrodes
definition
The standard electrode potential Eθ of a half cell is
the voltage measured under standard conditions when the half-cell is connected to a standard hydrogen electrode
What 3 things are needed for standard conditions?
- any solutions must have concentration of 1.00 moldm-3 or be equimolar (i.e. contain same number of moles of ions)
- temperature must be 298 k (25C)
- pressure must be 100kPa
the standard hydrogen electrode is always shown on the ____
left
the standard hydrogen electrode half cell has a value of
0.00 V
standard hydrogen electrodes
the whole cell potential =
Eθ right side - Eθ left side
Eθ left side = 0
so whole cell potential/ voltage reading = Eθ right side
why are standard conditions used to measure electrode potentials
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.
ways in which Electrochemical cells can be made
involving same element
half cells containing
solutions of 2 aqueous ions of the same element
e.g. Fe 2+(aq) / Fe 3+(aq)
ways in which Electrochemical cells can be made
involving non metals
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)
