Topic A8: Redox II
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What does Standard Electrode Potential mean?
the tendency of a species to be reduced and gain electrons
What are the conditions for the standard hydrogen electrode?
-298K -100kPa -solution conc of 1moldm-3
What is a voltaic cell?
connecting 2 different half-cells together allowing electron flow between half cells
What solution is used for standard hydrogen electrode?
strong acid (e.g. sulfuric acid) contains H+ ions
What does it mean if the standard electrode value is positive?
-forwards reaction is easiest -equilibrium is further right -species has a greater tendency to gain electrons (reduced easily) -fewer electrons present (on electrode = cathode)
What does it mean if the standard electrode value is negative?
-backwards reaction is easiest -equilibrium is further left -greater tendency to lose electrons (oxidised easily) -more electrons present (on electrode = anode)
Why is a salt bridge used when measuring standard electrode potential?
allows ions to flow uses a concentrated solution of an electrolyte that doesn’t react with either solution (e.g. potassium nitrate)
What does a high-resistance voltmeter do in measuring standard electrode potential?
measures potential difference between terminals -prevents electrons moving across it -keeps electrode potentials constant
When measuring the standard electrode potential of an electrode relative to the standard hydrogen electrode, what side must the hydrogen be on?
left side - ensures correct sign (right - left = right - 0)
What is the movement of electrons in a voltaic cell?
electrons move away from electrode with lower potential towards electrode with higher potential (as they are negative = repulsion)
How does the movement of electrons in a voltaic cell impact equilbirium?
-lower electrode potential equilibrium shifts left (as conc of electrons decreases) -higher electrode potential equilibrium shifts right (as conc of electrons increases)
How does a hydrogen fuel cell generate energy?
by oxidising hydrogen
What is an anode?
electrode where oxidation happens (negative electrode)
What is a cathode?
electrode where reduction happens (positive electrode)
What would the impact be of adding water to a Cu2+/Cu half cell?
-Cu2+ concentration would decrease -hence, position of equilibrium would shift left -more elecrtons would be released by Cu -hence, standard electrode value would decrease
What is the equation for the reaction of Fe2+ and acidified MnO4-?
MnO4- + 8H+ + 5Fe2+ —> Mn2+ + 5Fe3+ +4H2O
What ions are reduced in manganate (VII) titrations?
MnO4- (aq) ions
What is used to provide H+ ions in manganate (VII) titrations?
excess dilute sulfuric acid
What is the end point of manganate (VII) titration?
permanent pink colour - excess of MnO4- ions present
Give 2 reducing agents that manganate (VII) titrations are used to analyse
• iron (II) ions • ethanedioic acid
What is reduced and what is oxidised in iodine-thiosulfate titrations?
• thiosulfate ions (S2O32-) are oxidised • iodine (I2) is reduced
How is starch used to identify the end point of iodine-thiosulfate titrations?
• iodine colour has faded to become a pale straw colour • small amount of starch indicator is added - deep blue-black colour forms • at end point, all iodine will have reacted and blue-black colour disappears
Give 2 examples of oxidising agents that are analysed using iodine-thiosulfate titrations?
• chlorate (I) ions (ClO-) • copper (II) ions (Cu2+)
What are the general equations for an iodine-thiosulfate titration?
• Oxidising agent (e.g. Cu2+) + Iodide ions (I-) —> … + Iodine (I2) • Iodine (I2) + 2S2O32- —> 2I- + S4O62-