Studied by 15 people
What is oxidation
Gain of oxygen
Loss of hydrogen
Loss of electrons
What is reduction
Loss of oxygen
Gain of hydrogen
Gain of electrons
What is an oxidation state
The apparent change that an atom has in a molecule or ion
What is the oxidation state of a free, uncombined element.
Zero
What is the oxidation state of a simple ion
the same as the charge on the ion
what is the oxidation state of a neutral compound
All oxidation states add up to zero
What is the oxidation state of a polyatomic ion
All oxidation states add up to the charge on the polyatomic ion
What are the usual oxidation states
Halogens = -1
Alkali Metals = +1
Hydrogen = +1
Oxygen = -2
What does the oxidation number show
The higher the oxidation number, the more oxidised it it
The lower the oxidation number, the more reduced it is
What is an oxidising agent
The reactant which accepts electrons (reduced)
What is a reducing agent
The reactant which loses electrons (oxidation)
What does the activity series show
The strength of reducing agents. The element at the top is the strongest reducing agent.
What trends are there in strength of oxidising agents
The strength of an oxidising agent decreases down a group
What are the features of a redox titration
Redox reaction between oxidising and reducing agents
Electrons are transferred from reducing agent to oxidising agent
What is BOD
Biological oxygen demand
Defined as the amount of oxygen used to decompose organic matter in a sample of water over a certain period of time at a certain temperature
Used as a means of measuring pollution
How is the Winkler Method used
Dissolved oxygen in water is “fixed” by the addition of a manganese salt. The reaction causes manganese to be oxidised
Acidified iodine ions are added, and are oxidised by manganese to produce Iodine in a diatomic molecule
Iodine is titrated against sodium thiosulfate
For every molecule of O2 used, 4 moles of sodium thiosulfate are used.
What are the two types of electrochemical cells
Voltaic (galvanic) cells generate electricity from chemical reactions
Electrolytic cells drive chemical reactions using energy
How is the simplest half cell created
By putting a strip of metal into a solution of its ions
What will occur in a half cell (zinc as example)
zinc atoms will form ions by releasing electrons.
This will make the surface of the metal negatively charged with respect to the solution.
This causes a charge seperation (electrode potential)
What is electrode potential
When a charge seperation occurs. However it is a redox reaction, so the position of equilibrium determines the size of the electrode potential. The equilibrium position is determined by the reactivity of the metal.
How does the reactivity of the metal affect the equilibrium and how does this affect electrode potential.
Less reactive metals have their equilibrium to the right as they are less likely to lose electrons. Consequently there are less electrons on the surface of the metal, less charge difference so a less negative electrode potential
How is a voltaic cell constructed
Combining two half cells (electrodes) by an external wire.
What occurs in the anode and cathode of the voltaic cell.
Oxidation occurs at the anode (negative charge)
Reduction occurs at the cathode (positive charge)
What must the connection between the half cells be to produce a complete circuit
An external electronic circuit which connects to both electrodes.
And a salt bridge.
What is a salt bridge
A glass tube or piece of absorptive paper which contains an aqueous solution of ions. Movement of these ions neutralizes any charge buildup and maintains potential difference.
What are the conventions of drawing a cell diagram
The anode is on the left of the equation and the cathode on the right. Electrons flow left to right
A double line in the middle of the equation represents the salt bridge
A single line represents state boundaries eg. gas and liquid
What affects the potential difference of a voltaic cell
The difference in electrode potentials between the two species, which is affected by reactivity
What determines which species is the anode in a voltaic cell
The one with the lowest electrode potential (the greater reducing power). The more negative electrode potential. Oxidation occurs here.
What movement occurs through the salt bridge
Anions migrate from the cathode to the anode
Cations migrate from the anode to the cathode
What is the electromotive force (EMF)
The potential difference created by the cell. The magnitude of this is detemined by the difference in the ability to be reduced of both electrodes.
What is standard hydrogen electrode (SHE)
Used as a baseline for comparing electrode potentials of other half cells. Platinum is used as the conducting metal as it is fairy inert in the cell. This is assigned an electrode potential of 0 which allows us to compare other electrode potentials.
What are the standard conditions for measuring electrode potentials
Concentration of 1 mol/dm
All gases at 100 kPa
Temperature at 298 K
All substances must be pure
If the half cell does not contain a solid metal, platinum is used.
What does a positive electrode potential mean
The half cell has a greater tendency to be reduced than hydrogen.
What does a negative electrode potential mean
The half cell is less likely to be reduced than hydrogen.
What are some important points about standard electrode potential
All standard electrode potentials refer to the species being reduced
The standard electrode potentials do not depend on the number of electrons so are not scaled up/down according to the co-efficient
The more positive the electrode potential, the more readily it is reduced
How is EMF calculated from electrode potentials
EMF = Electrode potential of half cell which is reduced (cathode) - Electrode potential of half cell which is oxidised (anode)
How is spontaneity determined by EMF
If EMF is positive then it is spontaneous
If EMF is negative then it is non-spontaneous and the reverse reaction is spontaneous
How are Gibbs free energy and EMF linked in an equation
∆G = -nFE.
n is the number of moles of electrons transferred in the reaction.
F is the Faraday constant
What is the relationship between EMF, Gibbs free energy and spontaneity
EMF = positive and Gibbs = negative
spontaneous
EMF = negative and Gibbs = positive
non-spontaneous
When they both = 0 the reaction is at equilibrium
What is an electrolytic cell
An external source of electricity drives non-spontaneous redox reaction
What is the electrolyte
The reactant in the process is present in the electrolyte. The electrolyte is a liquid either molten/a solution of an ionic compound
What are electrodes made of
An inert, conducting substance such as graphite
What is the cathode in an electrolytic cell
the negative electrode
the power source pushes electrons towards the cathode
What is the anode in an electrolytic cell
The positive electrode
Charge flows through the ions in the electrolyte from the cathode to the anode where they return to the power source.
Which ions are attracted to which electrode in an electrolytic cell
Cations (postive ions) are attracted to the cathode (negative electrode) and are reduced
Anions (negative ions) are attracted to the anode (positive electrode) and are oxidised
What occurs in the electrolysis of molten salts
There is no solvent so only 2 ions present, cations go to the cathode and anions go to the anode.
What are the rules for electrolysis of an aqueous solution at the anode
If a halogen is present then that is oxidised
If not then OH-
What are the rules for electrolysis of an aqueous solution at the cathode
The species which is the lowest down in the reactivity series is reduced
which is the same as the species with the most positive electrode potential value
How can we calculate mass of product on a electrode in electrolysis
Calculate the charge of the circuit using Q= It meaning charge = current x time
Use Faradays constant to work out the number of Faradays which is equal to number of moles number of F = charge/faraday constant
The use the mole ratio in the half equation to determine moles of product
Mass = moles x mr
What is the Faraday Constant
The amount of charge carried by 1 mole of electrons.
What is electroplating
the process of using electrolysis to deposit a layer of a metal on top of another metal or other conductive object
How does electroplating work
The cathode is made out of the metal which is going to be electroplated
The electrolyte has the ions which are used to be plated
Anode is made of the same metal which is to be coated because it may be oxidised to replenish the supply of ions in the electrolyte.
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