Electrode potentials
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what is a voltaic cell?
type of electrochemical cell which coverts chemical energy into electrical energy
how can a voltaic cell be made?
-two different half cells are connected allowing electrons to flow. -ion half-cell contains ions of same element in different oxidation state in solution
why should chemicals of the two half-cells not be mixed?
if mixed, electrons would flow in an uncontrolled way and heat energy would be released rather than electrical energy
In what direction do the electrons flow in a voltaic cell?
depends upon the relative tendency of each electrode to release electrons
if we have a half cell with 2 aqueous ions, we use what electrode?
Platinum electrode as it is inert and electrically conductive
which metals in electrodes loses and gains electrons?
-the electrode with a more reactive metal loses electrons and is oxidised-negative electrode -the electrode with less reactive metal gains electrons and is reduced- positive electrode
Describe standard electrode potential of hydrogen
-standard chosen is a half-cell containing hydrogen gas and a solution containing H+(aq) ions -platinium electrode used -standard conditions -electrode potential of 0V as it is the standard
what are the standard conditions?
-soutions of 1mol/dm cubed -298K (25 degrees) -100kPa
standard electrode potential definition?
the e.m.f of a half cell connected to a standard hydrogen half cell under standard conditions
the electrodes connected by wire allow the flow of ?
electrons
salt bridge allows the flow of?
ions
what does the salt bridge contain?
concentrated solution of an electrolyte that does not react with either solution
the more negative the electrode potential…
-the greater the tendency to lose electrons and undergo oxidation -the less the tendency to gain electrons and undergo reduction -the greater the reactivity of a metal in losing electrons
the more positive the electrode potential…
-the greater the tendency to gain electrons and undergo reduction -the less the tendency to lose electrons and undergo oxidation -the greater the reactivity of a non-metal in gaining electrons
Metal and non-metals tend to have what electrode potential?
-metals tend to have negative electrode potentials -non-metals tend to have positive electrode potentials
how do you calculate standard cell potential?
electrode potential of positive electrode - electrode potential of negative electrode
how do you predict redox reactions using electrode potentials?
a reaction should take place between an oxidising agent on the left and a reducing agent on the right, provided that the redox system of the oxidising agent has a more positive electrode potential than the redox system of the reducing agent
how do you work out the overall equation?
-balance the electrons -a more redox system with a more positive electrode potential will reverse the redox system of a less positive electrode potential -combine equations -cancel electrons
what are the limitations to predicting reactions using electrode potentials?
-very large activation energy which can result in a slow rate -concentration of solutions should be 1mol/dm cubed -conditions may not be standard conditions
what happens if the concentration of solution is higher than 1mol/dm cubed?
-equilibrium will shift to the right, removing electrons from the system and making the electrode potential more negative
what happens if the concentration of solution is less than 1mol/dm cubed?
the equilibrium will shift to the left increasing electrons in the system and making the electrode more negative
what are the three main types of cells?
primary, secondary and fuel cells
Describe primary cells?
• non-rechargeable and single use -when chemicals used up, voltage will fall, the battery will go flat and the cells will be discarded or recycled -used in low current, long storage devices e.g clocks and smoke detectors
How is electrical energy produced in a primary cell?
-produced by oxidation and reduction at the electrodes -reactions cannot be reversed