C5: Oxidation, reduction and redox Flashcards
What is oxidation and reduction?
Oxidation reactions: the gain of oxygen.
Reduction: the loss of oxygen.
How was the definition of oxidation and reduction broadened?
Eg, the reaction of sodium with chlorine is very similar to the reaction of sodium with oxygen: both reactions involve sodium losing electrons which are then gained by the non-metal element. The definition of oxidation was broadened to include a loss of electrons, and reduction as a gain of electrons.
Describe electron sharing in covalent compounds
Only in reactions that involve ionic substances are electrons truly lost or gained. However in reactions that involve covalent compounds, the ‘degree of sharing’ of electrons can change.
Eg, in molecules O2 and H2 the bonding electrons are equally shared between the two atoms. However, in the molecule H2O the bonding electrons in the two O–H bonds are not equally shared – the oxygen has a greater share than the hydrogen.
the oxygen has gained an increased share in the bonding electrons and this can be thought of as reduction. The hydrogen has a lesser share in the bonding electrons and this can be thought of as oxidation.
To take this partial transfer of electrons into account, and to simplify the idea of oxidation and reduction, the concept of oxidation states/numbers is used
Give rules for assigning oxidation states
Rules for assigning oxidation states to atoms
The oxidation state of an atom in its elemental state is zero, for example the oxidation state of each oxygen atom in O2 is zero.
The oxidation state of a monatomic ion is equal to the charge on the ion.
For example, the oxidation state of chlorine in Cl− is −1, and the oxidation state of magnesium in Mg2+ is +2.
The sum of the oxidation states of all the atoms in a compound is zero.
For example, the sum of the oxidation states of the two hydrogen atoms, the one sulphur atom and the four oxygen atoms in H2SO4 is equal to zero.
The sum of the oxidation states of all the atoms in a polyatomic ion is equal to the charge on the ion.
For example, the sum of the oxidation states of the one carbon atom and the three oxygen atoms in CO32− is equal to −2.
The oxidation state of a Group 1 metal atom in a compound is +1.
For example, the oxidation state of the sodium atom in NaOH is +1.
The oxidation state of a Group 2 metal atom in a compound is +2.
For example, the oxidation state of the magnesium atom in MgCl2 is +2.
The oxidation state of an oxygen atom in a compound is almost always going to be −2.
The exceptions are: in peroxides, for example H2O2 where the oxygen atoms have an oxidation state of −1; and when combined with fluorine, for example OF2 where the oxygen atom has an oxidation state of +2.
The oxidation state of a hydrogen atom in a compound is almost always +1.
The exception is when hydrogen is part of a metal hydride, for example MgH2, and in this case the hydrogen atoms have an oxidation state of −1.
Using oxidation states to spot oxidation and reduction?
Using oxidation states to spot oxidation and reduction
If in a reaction an element increases, its oxidation state then it has been oxidised.
If in a reaction an element decreases, its oxidation state then it has been reduced.
What is disproportionation?
Disproportionation is a particular type of redox reaction: it is one in which a single species is simultaneously both oxidised and reduced.
An example is the decomposition of hydrogen peroxide:
2H2O2 → 2H2O + O2
The oxygen in H2O2 has an oxidation state of −1.
The oxygen in water has an oxidation state of −2
The oxygen in (O2) has an oxidation state of zero.
Hence two of the four oxygen atoms in 2H2O2 have had their oxidation state decreased from -1 to -2 in 2H2O. This is reduction. The other two oxygen atoms in 2H2O2 have had their oxidation state increased from −1 to 0 in O2. This is oxidation.
Give the reaction of chlorine with a cold dilute sodium hydroxide solution and explain why its disproportionation
2NaOH + Cl2 → NaCl + NaClO + H2O
The oxidation state of the chlorine atoms in Cl2 is zero, because the chlorine is in its elemental state.
The oxidation state of the chlorine in sodium chloride is −1. The oxidation state of the Na is +1, and hence to give a sum of the oxidation states equal to zero, the Cl must have an oxidation state of −1.
The oxidation state of the Cl in NaClO is +1. The oxidation state of the Na is +1 and that of the O is −2, and hence to give a sum of all the oxidation states equal to zero, the Cl must have an oxidation state of +1.
One chlorine atom from Cl2 has reduced its oxidation state from 0 to −1 in NaCl. This is reduction.
The other chlorine atom in Cl2 has increased its oxidation state from 0 to +1 in NaClO. This is oxidation.
The chlorine (Cl2) has therefore been simultaneously oxidised and reduced −undergone disproportionation.
The disproportionation of Cu+ ions in aqueous solution?
2Cu+(aq) → Cu(s) + Cu2+(aq)
The oxidation states of Cu+ and Cu2+ are +1 and +2, respectively, because the oxidation state of an ion is equal to the charge on the ion.
The oxidation state of Cu is zero.
One Cu+ ion has reduced its oxidation state from +1 to 0 (in Cu). This is reduction.
The other Cu+ ion has increased its oxidation state from +1 to +2 (in Cu2+). This is oxidation.
The Cu+ ions have therefore undergone simultaneous oxidation and reduction −disproportionation.
Show that the reaction between chlorine and a hot aqueous solution of sodium hydroxide is a disproportionation reaction.
3Cl2 + 6NaOH → 5NaCl + NaClO3 + 3H2O
The oxidation state of the elemental chlorine (Cl2) is zero.
The oxidation state of the Cl in NaCl is −1. The oxidation state of the Na is +1, and so in order for the sum of the oxidation states of the Na and Cl to equal zero, the oxidation state of the Cl must be −1.
The oxidation state of the Cl in NaClO3 is +5. The oxidation state of the Na is +1, and each of the oxygen atoms has an oxidation state of −2. So in order for the sum of the oxidation states of all the atoms in the compound to equal zero, the oxidation state of the Cl must be +5.
Five Cl atoms in 3Cl2 have had their oxidation states reduced from 0 to −1 (in 5NaCl). This is reduction.
The one remaining Cl atom in 3Cl2 has had its oxidation state increased from 0 to +5 (in NaClO3). This is oxidation.
Show that the reaction between chlorine and a hot aqueous solution of sodium hydroxide is a disproportionation reaction.
3Cl2 + 6NaOH → 5NaCl + NaClO3 + 3H2O
The oxidation state of the elemental chlorine (Cl2) is zero.
The oxidation state of the Cl in NaCl is −1. The oxidation state of the Na is +1, and so in order for the sum of the oxidation states of the Na and Cl to equal zero, the oxidation state of the Cl must be −1.
The oxidation state of the Cl in NaClO3 is +5. The oxidation state of the Na is +1, and each of the oxygen atoms has an oxidation state of −2. So in order for the sum of the oxidation states of all the atoms in the compound to equal zero, the oxidation state of the Cl must be +5.
No other atom has had its oxidation state changed.
Five Cl atoms in 3Cl2 have had their oxidation states reduced from 0 to −1 (in 5NaCl). This is reduction.
The one remaining Cl atom in 3Cl2 has had its oxidation state increased from 0 to +5 (in NaClO3). This is oxidation.
Hence this is a disproportionation reaction as the chlorine has been simultaneously oxidised and reduced.
Describe oxidising and reducing agents in the reaction between iron(III) oxide and carbon monoxide.
Oxidising agents give oxygen to another substance.
Reducing agents remove oxygen from another substance.
For example, consider the reaction between iron(III) oxide and carbon monoxide.
Fe2O3 + 3CO → 2Fe + 3CO2 The iron(III) oxide has been reduced (lost oxygen) to iron.
The carbon monoxide has been oxidised (gained oxygen) to carbon dioxide.
The oxidising agent is the substance that supplies the oxygen that is gained: in this case that is the iron(III) oxide.
The reducing agent is the substance that accepts that oxygen, i.e. ‘removes’ the oxygen: in this case that is the carbon monoxide.
the reaction between zinc and aqueous copper(II) ions.
An oxidising agent oxidises something else. Oxidation is a loss of electrons, so the oxidising agent must take the electrons that are being lost. Hence the oxidising agent gains electrons and is itself reduced.
A reducing agent reduces something else. Reduction is a gain of electrons, so the reducing agent must supply the electrons that are being gained. Hence the reducing agent loses electrons and is itself oxidised.
For example, consider the reaction between zinc and aqueous copper(II) ions.
Zn + Cu2+ → Zn2+ + Cu
Zn atoms have been oxidised (lost electrons) to form Zn2+ ions.
Cu2+ ions have been reduced (gained electrons) to form Cu atoms.
The oxidising agent is the substance that has gained the electrons: in this case that is the Cu2+ ions.
The reducing agent is the substance that has lost the electrons: in this case that is the Zn atoms.
Identify the oxidising agent and reducing agent in the following redox reaction:
Cl2 + 2I− → 2Cl− + I2
The first thing to do is to work out what has been oxidised and what has been reduced.
The two I− ions have been oxidised: they have lost one electron each to form an I2 molecule, or alternatively they have increased their oxidation state (from −1 to 0).
The Cl2 molecule has been reduced: it has gained two electrons to form two Cl− ions, or alternatively it has decreased its oxidation state (from 0 to −1).
Having established what has been oxidised and what has been reduced, the oxidising agent and the reducing agent can now be identified.
The oxidising agent is the species that is reduced: in this case the oxidising agent is therefore the Cl2 molecule.
The reducing agent is the species that is oxidised: in this case the reducing agent is therefore the I− ions.
Group 2 elements in their reactions tend to form positive ions. Group 17 elements in their reactions tend to form negative ions.
a) Elements from which of these two groups will make the better oxidising agents?
b) Elements from which of these two groups will make the better reducing agents?
Group 2 elements form positive ions, by losing electrons. Losing electrons is oxidation. So when Group 2 elements react, they are being oxidised.
Group 17 elements form negative ions, by gaining electrons. Gaining electrons is reduction. So when Group 17 elements react, they are being reduced.
Oxidising agents are reduced when they oxidise another substance. Hence the better oxidising agents will be the Group 17 elements.
Reducing agents are oxidised when they reduce another substance. Hence the better reducing agents will be the Group 2 elements.
