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Summary Complete Redox II Revision Notes (A Level Edexcel) £2.99
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  2.  
  3. PEARSON (PEARSON)
  4.  
  5. Chemistry 2015
  6.  
  7. Unit 14 - Redox II

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Summary Complete Redox II Revision Notes (A Level Edexcel)

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Comprehensive study guide for Chemistry A Level, made by an Oxford Biochemistry student with all 9s at GCSE and 3 A*s at A Level! Information arranged by spec point. Notes written using past papers, textbooks and more.

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  • March 20, 2021
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14. REDOX II
1. understand the terms ‘oxidation’ and ‘reduction’ in terms of electron
transfer, applied to s-, p- and d-block elements
Oxidation and reduction in terms of electron transfer:
 Oxidation – loss of electrons.
 Reduction – gain of electrons.
 OIL RIG – oxidation is loss, reduction is gain.


2. understand the terms ‘oxidation’ and ‘reduction’ in terms of changes
in oxidation number, applied to s-, p- and d-block elements
Oxidation and reduction in terms of changes to oxidation number:
 Oxidation – an increase in oxidation number.
 Reduction – a decrease in oxidation number.
 Disproportionation is when an element is simultaneously oxidised and
reduced.


3. know what is meant by the term ‘standard electrode potential’, E ϴ
Standard electrode potential, Eϴ – the electron motive force (emf) of a half cell
relative to the standard hydrogen electrode, measured under standard
conditions.
 The emf tells you the potential difference between two electrodes when no
current is flowing.
 Eϴ tells us about the position of equilibrium and strength of the
reducing/oxidising agent.
o A large positive Eϴ shows that the equilibrium lies far to the right, and
that the reactant is a strong oxidising agent and is easily reduced.
 We also combine the Eϴ of two different half equations to give an Eϴcell value.
o This tells us about the direction of electron flow, the position of
equilibrium, species’ relative reducing and oxidising abilities and the
feasibility of the reaction.
Electrochemical cells:
 Using an electrochemical cell, we can harness the electron flow of a redox
reaction to convert chemical energy into electrical energy.
o We separate the oxidation and reduction reactions of a redox reaction,
allowing each to occur in isolation in a half cell.
o An electrochemical cell is made by joining these two half cells and
forming a complete circuit with a wire and salt bridge.
 The cell reaction is the sum of the reduction and oxidation half-reactions
taking place in the cell; i.e. it is the overall chemical reaction.

, 
 Anode – the (negatively charged) electrode of the oxidation half cell.
 Cathode – the (positively charged) electrode of the reduction half cell.
 High-resistance voltmeter – measures the potential difference between the
electrodes.
o Since V = IR, a high resistance means that there is minimal current,
allowing us to measure the emf and thus the E ϴ of the cell.
 Salt bridge – completes the circuit by allowing ions to flow between the
solutions, without the solutions mixing. Normally filter paper soaked in KNO 3
(aq) is used.
o KNO3 is inert and will not react with either of the solutions in the half
cells.


4. know that the standard electrode potential, Eϴ, refers to conditions
of:
i) 298 K temperature
ii) 100 kPa pressure of gases
iii) 1.00 mol dm-3 concentration of ions
The standard electrode potential is taken under these standard conditions:
 A temperature of 298K.
 A pressure of 100 kPa (1 atm) for all gases.
 A concentration of 1 mol dm-3 for all aqueous ions (1 mol dm-3 of the reacting
species).
o E.g. the standard hydrogen electrode needs 1 mol dm -3 H+ so we use
0.5 mol dm-3 H2SO4.


5. know the features of the standard hydrogen electrode and
understand why a reference electrode is necessary
Electrode potential and need for a reference electrode:
 A potential difference is set up when a metal electrode is dipped into a
solution of its ions. This can occur in two ways:
o The metal atoms form cations, leaving the metal with a surplus of
electrons that gives the electrode a net negative charge.
o The aqueous ions gain electrons from the metal. The electrode gets a
net positive charge.
 We can only measure the difference between two electrode potentials, not
the potential difference between an electrode and a solution (the absolute
electrode potential).

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