Topic 9: Redox Processes Flashcards
What is a redox reaction?
Involves reduction and oxidation.
List three ways of describing redox reactions.
1) in terms of specific elements - hydrogen and oxygen
2) in terms of electron transfer
3) in terms of oxidation number
What is an antioxidant?
Substance that delays the onset of oxidation or slows down the rate at which oxidation occurs.
List three main types of reaction that occurs in chemistry.
1) acid-base reactions
2) precipitation reactions
3) redox reactions
OILRIG
Oxidation Is Loss of electrons
Reduction Is Gain of electrons
Describe why combustion reaction cannot be described as redox reaction in terms of electron transfer.
For example, complete combustion of solid carbon (eg in the form of coal) in oxygen yields carbon dioxide:
C(s) + O2 (g) → CO2 (g)
However, carbon dioxide is molecular, with covalent bonds, so no ionic bonds are formed. this combustion reaction as a redox process in terms of electron transfer as in theory no electrons are lost or gained and carbon dioxide is a neutral species.
It’s oxidation by adding of oxygen.
Define oxidation state.
An apparent charge of an atoms in a free element, a molecule or an ion.
Define oxidation and reduction in terms of oxidation states.
Oxidation describes a process in which the oxidation state increases and reduction describes a process in which the oxidation state decreases
What is the oxidation state of hydrogen?
The oxidation state of hydrogen is +1 when hydrogen
is bonded to a non-metal, such as in HCl and HNO3.
However, when hydrogen is bonded to a metal, for example in a metal hydride such as NaH, the oxidation number of hydrogen is 1.
What is the oxidation state of oxygen?
The oxidation state of oxygen is usually -2, such as in H2O and H2SO4 . The main exception is in a peroxide
(a species with an -O-O- linkage) ; here the oxidation state of oxygen is -1. A typical example of such a compound is H2O2, hydrogen peroxide
What is the oxidation state of group 17 elements?
The oxidation state of fluorine is -1 in all its compounds, for example HF, OF2, and LiF.
For the other
group 17 halogen elements the oxidation state is usually -1 in binary compounds (HI, NaCl, KBr) but in combination with oxygen in oxoanions and oxoacids the oxidation state is positive (for example, in HClO4 chlorine has a +7 oxidation state) .
What are the activity series?
The activity series ranks metals according to the ease with which they undergo oxidation
List three formulae used in volumetric analysis (titration).
1) Amount of substance in mol = n
n = m/M
2) n = volume (in dm^3) x concentration (in mol dm^(-3)
(If in cm^3 then divide all by 1000)
3)1/Va(na) = 1/Vb(nb) and hence
1/Va(Va x ca) = 1/Vb(vb x cb)
Va = volume of reactant A (in dm^-3) cv = concentration of reactant A (in mol dm^(-3))
What is the formula to calculate concentrations in parts per million?
concentration in ppm = (mass of component in solution/total mass of solution) x 10^6
= mass of solute in mg/volume of solution in dm^3
What is the Winkler method?
Technique based on redox reactions used to measure the amount of dissolved oxygen in water.
High [dissolved oxygen] = low level of pollution.
Define BOD.
BOD - biochemical oxygen demand.
BOD is the amount of oxygen required to oxidize organic matter in a sample of water at a definite temperature over a period of 5 days.
Measured in ppm.
<1 = pure water 100-400 = untreated domestic sewage
What is the substance produced from carbon in aerobic and anaerobic conditions?
Aerobic: CO2
Anaerobic: CH4
What is the substance produced from hydrogen in aerobic and anaerobic conditions?
Aerobic: H20
Anaerobic: CH4, NH2, H2S and H20
What is the substance produced from nitrogen in aerobic and anaerobic conditions?
Aerobic: NO3^(-)
Anaerobic: NH3, amines
What is the substance produced from sulfur in aerobic and anaerobic conditions?
Aerobic: SO4^(2-)
Anaerobic: H2S
What is the substance produced from phosphorous in aerobic and anaerobic conditions?
Aerobic: PO4^(3-)
Anaerobic: PH3
Define energy.
The capacity to d work.
Unit: joule (J)
What is the law of conservation of energy?
It states that energy cannot be created or destroyed, but is converted from one form to another.
List two main types of electrochemical cells.
1) Voltaic (or galvanic) cell - converts chemical energy (from spontaneous exothermic processes) to electrical energy
2) Electrolytic cells - convert electrical energy to chemical energy bringing about a non-spontaneous process.
Study tip: EE
Electrical cell - Electrical to chemical
Study tip: CROA
Cathode - Reduction
Oxidation - Anode
Study tip: CNAP
Cathode - Negative
Anode - Positive
this is for electrolytic cell, the opposite is true for the voltaic cell.
List three types of electrodes used in voltaic cell.
1) metal/metal-ion electrode
2) metal ions in two different oxidation states
3) gas-ion electrode
In a voltaic cell, how are the two electrons connected?
via liquid junction called a salt bridge, which contains a strong electrolyte.
What are three main functions of a salt bridge?
1) allows for a physical separation of cathode and anode, and therefore the oxidation and reduction processes
2) provides electrical continuity
3) reduces the liquid junction potential - a voltage generated when two different solutions come into contact
How can you determine which metal will be oxidized and which metal will be reduced in a voltaic cell?
By looking at the activity series, for the Daniell cell, zinc (anode) is higher up in the series than copper (cathode), so it’s more easily oxidized.
Write down the cell diagram of the Daniell cell.
Zn(s) |Zn2+
aq) || Cu2+ (aq) |Cu(s
Define a fuel cell and give an example.
Fuel cell is a voltaic cell (therefore based on redox processes).
Common type is the hydrogen-oxygen fuel cell, which uses the reaction between hydrogen and oxygen.
How much energy in fuel can a fuel cell convert into electrical energy?
about 70%
Name two advantages of a hydrogen-oxygen fuel cell and three disadvantages.
1) Water is the only component so it’s non-polluting
2) does not need recharging
Disadvantages
1) they are expensive to produce.
2) they are prone to poisoning by impurities in the fuel, which reduces their lifetime or requires expensive purification of the fuel.
3) storage of hydrogen is a problem.
What is electrolysis?
Process by which electrical energy is used to drive a non-spontaneous chemical reaction.
Describe an electrolytic cell
Consists of a single container, two electrodes (cathode and anode), a solution (electrolyte) and a battery, which can be considered as an electron pump.
What are the 5 steps in working method for the electrolysis of a molten salt?
Step 1: Identify all species present.
Step 2: Identify which species are attracted to the cathode (negative electrode) and which species are attracted to the anode (positive electrode) .
Step 3: Deduce the two half-equations taking place at the cathode and anode and the overall cell reaction.
Step 4: Draw and annotate the electrolytic cell and show the direction of the movement of electrons and the direction of ion flow.
Step 5: State what would be observed at each electrode.
