UNIT 9 - Redox Processes Flashcards
Define SHE and state its function.
- standard hydrogen electrode
- the reference point for measuring and comparing the electrode potentials of other half-cells with standard electron potential of zero volts.
- it performs a redox reaction of protons at a platinized platinum electrode dipped in an acidic solution and pure hydrogen gas bubbled through it.
State the connections between half-cells required for a voltaic cell.
- external electronic circuit connected to the metal electrode of each half-cell through which the electrons will flow.
- salt bridge containing an aqueous solution of ions maintaining the potential difference and neutralises any build-up of charge.
Define the standard electrode potential of a half-cell.
EMF generated when it is connected to the standard hydrogen electrode by an external circuit and a salt bridge measured under the standard conditions.
Define an oxidizing agent.
reactant accepting electrons because it brings out the oxidation in the other reactant and it itself becomes reduced in the process.
State the role of a voltaic cell.
Conversion of the energy released from a spontaneous exothermic reaction into electrical energy.
State the charge of the anode and cathode in an electrolytic cell.
Anode (+)
Cathode (-)
State the full name and define BOD.
Biological oxygen demand.
Measures the degree of pollution as it is the amount of oxygen used to decompose the organic matter in a sample of water over a specified time period (usually five day at a specified temp).
State the reason for adding the starch indicator to titrate the iodine redox reaction.
The starch serves as an indicator as it forms a deep blue colour by forming a complex with free I2 as the I3- ions embed themselves within the helix of amylose as the I2 is reduced to I- during the reaction as the blue colour disappears marking the equivalence point.
State and describe the method used to calculate BOD.
The Wrinkler method.
- The dissolved oxygen, O2 in the water is ‘fixed’ by the addition of a maganese (II) salt such as MnSO4. Reaction of this salt with O2 in basic solution causes oxidation of MN(II) to higher oxidation states, such as Mn(IV).
2Mn2+ + O2 + 4OH- -> 2MnO2 + 2H2O
- Acidified iodide ions, I- are added to the solution and are oxidized by the Mn(IV) to I2.
MnO2 + 2I- +4H+ -> Mn2+ + I2 + 2H2O
- The iodine produced is then titrated with sodium thiosulfate.
2S2O3 2- + I2 -> 2I- + S4O6 2-
Define galvanized iron and state its function.
Iron with a layer of zinc deposited on its surface will be protected from corrosion as the zinc will be preferentially oxidized.
Ratio for water and S2O3 2- during the Wrinkler method.
1 mol of O2 - 4 mol of S2O3 2-
List the components of an electrolytic cell.
- battery or a DC power source as the source of electric power
- electrodes immersed in the electrolyte and connected to power supply not touching each other
- electrodes made from a nonconducting substance - metal or graphite
- electric wires connect the electrodes to the power supply
State the relationship between the cell potential and spontaneity of a reaction.
- If E-cell is positive, the reaction is spontaneous.
- If E-cell is negative, the reaction is non-spontaneous BUT the reverse reaction is spontaneous.
Define electrode potential.
Charge separation between the metal and its ions in the solution caused by the metal half-cell in which the atoms will form ions by releasing electrons that will make the surface of the metal negatively charged with respect to the solution.
State the alternative name for NaCl (aq).
Brine
State when are the electrolytic cell electrodes described as inert.
When they do not take part in the redox reactions.
Define reduction.
Gain of electrons.
State the factor determining the direction of electron flow and voltage generated by the voltaic cell.
The difference in reducing strength of the two metals judged by their relative position in the reactivity series.
Define and state the full name of EMF.
the electromotive force
Greatest potential difference that a cell can generate and is measurable only when the cell is not supplying current due to its internal resistance measured in volts.
State the relationship between the electrode potential in a half-cell and its ability to be a cathode or anode.
The half-cell with the higher electrode potential is the cathode.
The one with the lower electrode potential is the anode.
State the relationship between the reactivity of a metal and its potency as the reducing agent.
More reactive metals loose their electrons more readily and thus are stronger reducing agents.
Define electroplating.
Using electrolysis to deposit a layer of metal on top of another metal or other conductive object.
State what process occurs at the anode and what charge does it thus have.
oxidation
negative charge
State when is a half-cell given the symbol E (with the little o) and the meaning of its value.
When the SHE is connected to another half-cell by an external circuit with a high-resistance voltmeter and a salt bridge and the EMF generated is then know as the standard electrode potential of that half cell.
POSITIVE value of E = greater tendency to be reduced than H+ (electrons flow from the hydrogen half-cell to the other one which is reduced).
NEGATIVE value of E = less tendency to be reduced than H+ (electrons flow from the other half-cell to the hydrogen half-cell which is reduced).
State how are voltaic cells constructed.
As the process relies on separating the two half reactions the process relies on half-cells allowing the electrons to flow between them only through an external circuit.
State when does reduction occur in terms of oxidation states.
When there is a decrease in oxidation state of an element.
State the reason why platinum is used as the conducting metal in the SHE.
- fairly inert and wont ionize
- catalyst for the reaction of proton reduction
State the principles of a cell diagram convention.
- single vertical line - phase boundary
- double vertical line - salt bridge
- aqueous solutions placed next to the salt bridge
- anode on the left, cathode on the right, electrons flow from left to right
List the purpose of electroplating.
- decorative purposes
- corrosion control - sacrificial protection
- improvement of function (e.g. chromium improves the wear on steel parts)
