Chapter 9: Redox reactions Flashcards
What is oxidation and reduction?
Oxygen is the loss of hydrogen or the gain of oxygen
Reduction is the gain of hydrogen or the loss of oxygen
Also OIL RIG
Oxidation - loss of electrons
Reduction - gain of electrons
What does [H] and [O] mean in an reaction equation?
[O] indicates oxygen added in an oxidation reaction
[H] indicates hydrogen added in a reduction reaction
Why must oxidation and reduction always occur together?
If something loses electrons, some else must gain them
If something is oxidised, something else must be reduced
They are called redox reaction, indicating that both reduction and oxidation occur
When do you wrote the signs first (-2) and when do you write it after (2-)?
Oxidation number is written with the sign first, e.g. -2, but a charge is written with the number first e.g. 2-.
What are the general rules for working out oxidation numbers?
- Treat the compound as totally ionic (if the compound is ionic then the charges on the ions are the oxidation numbers)
- The most electronegative atom in a molecule is assigned a negative oxidation number according to how many electrons it needs to gain to have a full outer shell.
- Assign oxidation numbers accordingly to give the overall charge on the molecule/ion - the sum of the oxidation numbers, taking into account signs and coefficients, is equal to the overall charge on the molecule/ion
- The oxidation number of atoms in an element is zero - so the oxidation number of oxygen in O2 is 0
- The elements in groups 1 and 2 virtually always have the group number as their oxidation number
- The maximum possible oxidation number for an element will be its group number for elements in group 1, and 2 and the group number -10 for elements in groups 13-17. It is not possible to lose more electrons than there are in the outer shell - the maximum possible oxidation number for a group 16 element is +6 because there are six electrons in the outer shell
What is the oxidation number of O and H?
O: -2
H: +1
Learn these (not the numbers but just their general names)
What is oxidation and reduction in terms of oxidation numbers?
Reduction - decrease in oxidation number
Oxidation - increase in oxidation number
For a reaction to be a redox reaction, what must it involve in terms of oxidation number?
It must involve a change in oxidation number
What is a oxidising or reducing agent?
An oxidising agent is a substance that oxidises something else.
- Oxidising agents (oxidants) oxidise other species and, in the process, are themselves reduced. An oxidising agent takes electrons away from something.
A reducing agent is a substance that reduces something else.
- Reducing agents (reductants) reduce other species and, in the process, are themselves oxidised. A reducing agent gives electrons to something.
How to quickly identified whether the half equation is oxidation or reduction?
If the e- is on the right side = oxidation
If the e- is on the left side = reduction
In half equations, what must balance?
in terms of number of atoms on both sides and the total charge on both sides
electrons lost in the oxidation reaction is equal to the number of electrons gained in the reduction reaction
What is the procedure that should be followed for balancing equations in acidic solution?
- Balance all atoms except H and O
- Add H2O to the side deficient in O to balance O
- Add H+ to the side deficient in H to balance H
- Add e- to the side deficient in negative charge to balance charge
What must you do before combining half equations to produce an overall redox equation?
Balance the electrons then cancel them out
Then subtract any duplications on left and right hand side
Overall redox equations never contain electrons - only half equations have electrons
How are metals in activity series arranged?
In order of how easily they are oxidised to form positive ions
Why are metals high up in the activity series and why are they strong reducing agents?
Metals form positive ions more easily and so are more reactive.
They lose electrons more readily and stronger reducing agents
They are strong reducing agents because they lose electrons easily for others to gain them
What do oxidising and reducing agents do?
Oxidising → takes electrons from the other reactant, usually a non-metal, fluorine is the most powerful oxidising element (powerful attraction)
Reducing → gives electrons to other reactants, usually metal, lithium is the most powerful reducing element
How is the strength of reducing and oxidising agents determined?
depending on their relative tendencies to lose or gain electrons
What determines whether there will be an reaction or not?
The reducing agent but be strong enough to reduce the other element
What is the biochemical oxygen demand aka biological oxygen demand?
BOD: is used as a measure of the quality of water. It is a measure of the amount of oxygen used by microorganisms to oxidise the organic matter in the water.
Any organic pollutants in the river water will be decomposed (oxidised) by microorganisms (aerobic bacteria) in the water and this process uses up dissolved oxygen.
The higher the BOD, the more organic waste there is in water.
What might increase the BOD?
Sewage is released into a river or lake
Tree leaves falling, animal manure, dead plants and animals, effluent from water treatment plants also contain organic matter
What does good quality river water have a BOD of?
Less than 1ppm
Water is generally regarded as unpolluted if it has a BOD lower than 5 ppm.
What is the basic principle in measuring BOD?
The basic principle in measuring BOD is to compare the initial amount of dissolved oxygen in a sample of water with the amount present when the sample has been incubated for 5 days at 20 °C.
What do voltaic cells provide us with?
a way of harnessing redox reactions to generate electricity
this is the basis of cells (batteries)
What happens when a piece of zinc is put into a solution of copper (II) sulfate?
an exothermic reaction occurs and the zinc becomes coated with copper and the blue colour of the copper sulfate solution fades
What happens when zinc metal is added to a solution containing Cu2+ ions? (in terms of electrons)
electrons are transferred from the zinc to the Cu2+ - the Cu2+ is reduced and the zinc is oxidised
What is this
a voltaic cell
What is happening in the left-hand beaker?
- zinc atoms are oxidised to Zn2+ ions
- electrons that are lost flow around the circuit to the other beaker where they are gained by Cu2+ ions to form copper
If the reaction was allowed to keep going, what would we see?
- zinc electrode getting smaller (as the zinc goes into solution as Zn2+)
- Cu eletrode getting larger (as it is coated with copper)
- the colour of the solution in the right-hand beaker becoming paler (as copper ions are converted to copper atoms)
How does the reaction differ between zinc and copper (II) ions together and when it is separated in a voltaic cell?
Zinc and copper (II) ions
* exothermic
* chemical energy (internal energy) converted to heat
In voltaic cell
* converted to electrical energy
Why is a salt bridge necessary?
- current will not flow
- flow of electrons from the positively charged half-cell to the negatively charged half-cell would not occur
- it contains ions that can flow out of the salt bridge into the individual half-cells to prevent any build up of charge
- any exces ions in the individual half-cells can flow into the salt bridge to prevent any build up of charge
What does the salt bridge provide?
an electrical connection between the two half-cells to complete the circuit. it allows ions to flow into or out of the half-cells to balance out the charges in the half-cells
What does a salt bridge usually contain?
a concentrated solution of an ionic salt such as KCL
What would the reaction of the voltaic cell be like if the salt bridge were not present?
- there would be a build up of Zn2+ ions in the left-hand beaker
- solution would become positively charged overall
- any further oxidation of zinc atoms to Zn2+ would be opposed
- decrease in the concentratino of Cu2+ ions in the right-hand beaker
- solution would have a negative charge
- any further reduction of Cu2+ would be opposed
What is the beaker on the right blue due to?
the blue colour of the solution is due to the presence of Cu2+ (aq) ions
How does negative charge always flow?
in the same continuous direction around a complete circuit
Which way is are the electrons and the negative ions travelling?
clockwise around the circuit
In a voltaic cell, what is an anode?
the electrode at which oxidation occurs
is the negative electrode
Which is the anode?
the zinc electrode
because electrons are released at the zinc electrode, this is the negative electrode
What factors affect the tendency of a metal to be oxidised and be the negative electrode (anode) in a cell?
the more reactive metal
In a voltaic cell, what is the cathode?
the electrode at which reduction occurs
is the positie electrode
What is the cathode in this cell?
the copper electrode
because electrons flow through the external circuit towards this electrode and are used up here
is the positive electrode
What does a single vertical line represent
a phase boundary (between solid and aqueous solution)
What does a double line indicate in cell notation for voltaic cell?
the salt bridge
What can be read when reading from the left to right?
- we can see that Zn is oxidised to Zn2+
- Cu2+ is reduced to Cu
By convention, which side is the anode and cathode?
- Anode on the left (negative electrode)
- Cathode on the right (positive electrode)
When some cells are set up using gases or metal ions but no solid metal, what can we use instead?
platinum (a very inert metal) as the electrode
since we dont have anything that we can physically connect a wire to
In cell notation for voltaic cell, which reaction is on the left and right? (oxidation and reduction)
Oxidation reaction on the left
reduction on the right
Why is this cell notation written with a comma that separates Fe3+ (aq) and Fe2+ (aq)?
they are in the same phase (both aqueous solutions)
In voltaic cell notation, where are the electrode always written?
at the extremes on both sides
Why does the reaction of zinc and copper read a lesser voltage than the reaction of magnesium and copper?
Because there is a bigger difference in reactivity between magnesium and copper than between zince and copper
magnesium and copper are further apart in the activity series than magnesium and zinc
the magnesium has a greater tendency than zinc to donate electrons to copper ions
How does the difference in reactivity betwen the metal electrodes affect the voltage of the cell?
bigger the difference in reactivity = higher the voltage
What is electrolysis?
the breaking down of a substance (in molten state or solution) by the passage of electricity through it
What is happening here?
It is the experimental set-up for the electrolysis of molten lead bromide
the lead bromide is broken down into its elements, lead and bromine, by the passage of the electricity
How to determine which electrode is the anode or cathode?
It depends on whether it is oxidatin or reduction occuring at that electrode
Red Cat - reduction + cathode (always together)
An Ox - Anode + oxidation (always together)
What is happening at the positive electrode?
Bromide ions are oxidised and lose electrons to form bromine
oxidation = annode
Why is the positive electrode the anode in an electrolytic cell while it is the cathode in a voltaic cell?
beause oxidation occurs at this electrode, the positive electrode is the anode in an electrolytic cell
What is happening at the negative electrode?
lead ions are reduced as they gain electrons to form lead
reduction = cathode
Why are electodes usually made of graphite?
because it is a fairly inert (chemically inactive) non-metal which conducts electricity
How is the current/conduction occuring by in the external circuit and in the molten salt of an electrolytic cell?
External circuit
* current is carried by electrons (delocalised electrons in the metal wire)
Moleten salt (electrolyte)
* conduction involves the moveoment of ions
Which pole do electrons travel from and to?
Electrons travel from the negative pole of the battery to the negative electrode
Where do the positive ions in the elctrolyte move towards?
The negative electrode
Where do the negative ions in the electrolyte move towards?
towards the positive electrode
What travels to the positive electrode?
- the extra electron(s) from the negative ion is/are transferred to the electrode
- the electrons released from the negative ions travel through the external circuit to the positive pole of the battery
How are electrons transfered from one electrode to the other thourgh the external circuit?
electrons are taken from the external circuit at the negative electrode (by the positive ions) and given back to the external circuit at the positive electrode (by the negative ions)
Where do electrons not travel through?
the electrolyte
What is an electrolyte?
A solution or a molten compound that will conduct electricity, with decomposition at the electrodes as it does so.
Electrolytes contain ions that are free to move towards the electrodes
when do ionic salts not conduct electricity?
when solid
Why do ionic salts not conduct electricity when solid?
because the ions are held tightly in the lattice structure and therefore not free to move
What is the products at the electrodes when a molten salt is electrolysed?
products at the electrodes are the elements of which that salt is made up
What is the product formed at the negative electrode? (electrolytic cell)
a metal is formed at the negative electrode because metals form positive ions
What is the product formed at the positive electrode? (electrolytic cell)
a non-metal because non-metals form negative ions
What is the product of electrolysis of molten alumnium oxide (AI₂O₃)?
Negative electrode (cathode) -> a metal: aluminium
Positive electrode (anode) -> non-metal: oxygen
What are the products of electrolysis of molten potassium chloride?
Negative electrode (cathode) -> a metal: potassium
Positive electrode (anode) -> non-metal: chorine
What isare 3 differences between an electrolytic cell and a voltaic cell?
