Flashcards in Redox 2 Deck (32)
What is oxidation?
Loss of electrons, increase in oxidation number
What is reduction?
Gain of electrons, decrease in oxidation number
What is oxidation number?
Shows the total number of electrons which have been removed or added to an element
What are standard conditions?
298K, 100 kPa, 1.0 moldm^-3
What is a standard hydrogen electrode?
A redox electrode which all other electrode potentials are measured relative to at 298K. It contains HCl at 1.0 moldm^-3, hydrogen gas at 100kPa and a solid platinum electrode
What is an electrochemical cell?
A device that that creates an electrical current from a reaction. Consists of 2 half cells; electrons flow from the negative one to the positive one through an external circuit
What is a cell diagram?
A shorthand way of representing an electrochemical cell. The parts of each half cell are separated by solid lines and the half cells are separated by 2 dotted lines
What is a half cell?
One half of an electrochemical cell. Either negative (oxidation takes place) or positive (reduction takes place). The two half cells are linked by a salt bridge
What is a salt bridge?
A piece of filter paper, soaked in a soluble salt (like potassium nitrate) that is used to complete the circuit in an electrochemical cell
What is standard electrode potential?
Potential of a cell acting as a cathode and the standard hydrogen electrode acting as an anode. Reduction always takes place at the cathode, and oxidation at the anode
What is cell potential?
The cell potential is the way in which we can measure how much voltage exists between the two half cells - the emf that electrons feel
What is thermodynamic feasibility?
A reaction that is thermodynamically favourable. A feasible reaction will occur spontaneously - without continuous input of energy
What is kinetic stability?
Reactants are kinetically stable if there is a high activation energy preventing them from reacting i.e. reactions can be thermodynamically feasible but not actually ever happen
What is disproportionation?
A reaction in which an element in a species is simultaneously oxidised and reduced in the same reaction
What is a fuel cell?
The energy released on reaction of a fuel with oxygen is utilised in a fuel cell to generate a voltage (e.g. a hydrogen-oxygen fuel cell (2H2 + O2 -> 2H2O))
What is a redox titration?
The concentration of a reaction can be calculated from the results of a redox titration. The number of moles of electrons involved must be taken into account
What does a more positive value for standard electrode potential indicate?
Reduction is more likely to happen
What does a more negative value for standard electrode potential indicate?
Oxidation is more likely to happen
Why is a platinum electrode used in the standard hydrogen electrode?
It is unreactive and it acts as a connection to complete the circuit
What are the 3 possibilities for electrode setups?
- Metals in contact with their ions in aqueous solution (the metal itself is a solid so it can be used as a connection to the external circuit)
- Non-metals in contact with their ions in aqueous solution (no solids present so platinum must be used as a connection to the external circuit)
- Ions of the same element with different oxidation numbers (no solids present so platinum must be used as a connection to the external circuit)
Why is a high resistance voltmeter used in electrochemical cells?
It stops the flow of current. If no electrons can flow between the electrodes, no reaction can happen but we measure the emf that is trying to push the electrons
Why is a salt bridge used in electrochemical cells?
Ions move through the paper to complete the circuit and it is usually filter paper soaked in potassium or sodium nitrate
What are the rules for half cell notation?
- Two dashed vertical lines are used to represent the salt bridge/external circuit between the electrodes
- Single vertical lines represent phase boundaries
- The most reduced species are placed on the outside, furthest from the 2 dashed lines
What important things is Ecell directly proportional to?
ΔStotal and lnK
Why might a reaction have a positive Ecell value but not actually happen?
- Reactants could be kinetically stable, so a high activation energy prevents the reaction from happening
- The standard electrode potentials used to calculate Ecell are measured under standard conditions, so a reaction may not happen under standard conditions which would make the prediction inaccurate
Why are hydrogen-oxygen fuel cells not used a lot?
They can be hard to store and are flammable
What are the reactions of an acidic hydrogen-oxygen fuel cell?
2H^+ (aq) + 2e^- -> H2 (g)
4H^+ (aq) + O2 (g) + 4e^- -> 2H2O (l)
Overall reaction becomes O2 + 2H2 -> 2H2O
What are the reactions of an alkaline hydrogen-oxygen fuel cell?
4H2O (l) + 4e^- -> 4OH^- (aq) + 2H2 (g)
2H2O (l) + O2 (g) + 4e^- -> 4 Oh^-
Overall reaction becomes O2 + 2H2 -> 2H2O
Why can methanol fuel cells be preferable to hydrogen-oxygen fuel cells?
Methanol is liquid at room temperature so it's easier to store