5.2.3 redox and electrode potentials

Br2 (aq) + 2e- > +2 Br - (aq) electrons on left electron acceptor, oxidising agent

2I- (aq) > I2 (aq) + 2e- electrons on right electron donor, reducing agent

Redox titration between I2 and S2O3^2- Yellow/brown solution > colourless Starch indicator added near the end > from blue/black > colourless

Redox titration between Fe2+ and MnO4 - Purple > colourless MnO4-(aq) + 8H+ (aq) + 5Fe2+ (aq) > Mn2+ (aq) + 4H2O (l) + 5Fe3+ (aq)

more negative electron has more of a tendency to oxidise and release electrons compared to less negative e.g. Zn > Zn2+ + 2e- more electrons build up on more negative electrode potential difference created between 2 electrodes negative strip is negative terminal and less positive is positive terminal

stop the current from flowing in the circuit in this state, it is possible to measure the maximum possible potential difference (E) reactions will not be occurring as the very high resistance voltmeter stops the current from flowing

allows free moving ions to conduct the charge usually potassium nitrate wire not used as the metal wire would set up its own electrode system with the solutions

reactions will then occur separately at each electrode voltage falls to zero as reactants are used up

connect to another half-cell of known potential and PD between the 2 half-cells are measured can assign a relative potential to each electrode by linking it to a reference electrode (hydrogen electrode) which is given a potential of O V

potential difference measured when an electrode system is connected to the hydrogen electrode system, and standard conditions (1M, 100kPa, 298K) apply

oxidises go backwards Zn (s) > Zn2+ (aq) + 2e-

reduces go forwards Fe2+ (aq) + 2e- > Fe (s)

= Ered - Eox more neg - less neg

increasing concentration of reactants > increase in EMF > side with fewer moles

most Ecells are exothermic so increasing would decrease EMF if positive, reaction might occur however reaction will not occur or will occur so slowly that it effectively doesn’t happen if the reaction has a high activation energy, the reaction will not occur

repulsion between ions results in high activation energy making a very slow reaction

electrochemical cells can be used as source of electrical energy are non-rechargeable when the reactions that occur within them are non-reversible

2H2 + O2 > 2H2O maintain a constant voltage over time as they have a constant supply of O2 and H2 so maintain constant conc of reactants

less pollution and less CO2 greater efficiency

limited lifetime (regular replacement and disposal) and high production costs use of toxic chemicals in their production