REDOX REACTIONS 1: Oxidation and reaction

Oxidation and Reduction In the past oxidation and reduction were defined in simple terms. Oxidation - the gain of oxygen or the loss of hydrogen by an element or compound. Reduction - the loss of oxygen or the gain of hydrogen by an element or compound. These older definitions are still useful, especially for organic reactions

Oxidation and Reduction Gain of oxygen examples: 4Fe + 3O2 ฎ 2Fe2O3 Iron has gained oxygen to form Fe2O3, so the iron has been oxidised, forming iron(III) oxide. 2CO + O2 ฎ 2CO2 Carbon in carbon monoxide has gained oxygen ,the carbon has been oxidised to carbon dioxide. 2NO2 + H2O2 ฎ 2HNO3 Nitrogen in NO2 gains oxygen from hydrogen peroxide (H2O2), therefore the nitrogen has been oxidised. Note that we don’t need to use O2 as the source of oxygen in our reactions.

Oxidation and Reduction Combustion is another example of oxidation, where the definition by gain of oxygen is easy to see. CH3CH2CH3 + 5O2 ฎ 3CO2 + 4H2O CH3CH2OH + 3O2 ฎ 2CO2 + 3H2O 2H2S + 3O2 ฎ 2SO2 + 2H2O SiH4 + 2O2 ฎ SiO2 + 2H2O Combustion occurs in each of the above cases, elements on the left hand side of the equation gain oxygen to form the products.

Oxidation and Reduction Now let’s look at oxidation as loss of hydrogen. (All these equations are unbalanced for simplicity, and hydrogen is lost as water.) CH3CH2OH + KMnO4 ฎ CH3CHO + MnO2 (This reaction occurs when excess alcohol is present.) CH3CH2CH2OH + KMnO4 ฎ CH3CH2CO2H + MnO2 (This occurs when the KMnO4 is present in excess.) CH3CHOHCH3 + KMnO4 ฎ CH3COCH3 + MnO2 (Here the alcohol is present in excess.) In each case the alcohol has lost some hydrogen when it reacts, so oxidation is occurring. (Note that in the second example, gain of oxygen has also occurred.)

Oxidation and Reduction Here are a few cases where reduction occurs. i.e. either oxygen is lost or hydrogen is gained. In this reaction, which is the overall reaction occurring in a blast furnace, 2Fe2O3 + 3C ฎ 4Fe + 3CO2 Fe2O3 looses oxygen. This is reduction. In fact the name reduction comes from the early days of metallurgy, when chemists talked about “reducing a metal ore to the metal” . The metal was reduced from a complicated compound to the simple element.

Oxidation and Reduction Another example of reduction is: CuS + O2  Cu + SO2 Here too a metal ore is reduced to the metal. This one is a little different, CuS lost a sulphur atom to form the copper metal.

Oxidation and Reduction Finally, examples of reduction where hydrogen is gained. Many organic reactions are good examples of this type of reaction. CH3CH=CH2 + H2 ฎ CH3CH2CH3 2CH3CH2CHO + H2 ฎ 2CH3CH2CH2OH CH3CH2COCH3 + H2 ฎ CH3CH2CHOHCH3 Not exactly difficult to spot reduction, is it? What about the modern definition, what is it and why did chemists have to go and make things more complicated?

Oxidation and Reduction In the old system, only reactions where H or O are involved can be considered as reduction or oxidation There are lots of reactions where similar changes occur, but which cannot be classified by the old system We know that a lot of chemistry depends on electronic configuration, so what are the electrons up to?

Oxidation and Reduction Examine the three reactions below. In each of them, the metal undergoes an identical change if we consider the electrons, but by the older definition only one of them is called oxidation. 2Mg + O2 ฎ 2MgO Mg + S ฎ MgS Mg + Br2 ฎ MgBr2 In each case the magnesium atom loses two electrons to form an ion with a charge of +2 Mg ฎ Mg2+ + 2e- Only the first reaction would be classified as oxidation. From the magnesium’s point of view, this is just silly. It’s simply losing two electrons in every case. So the definition of oxidation and reduction might be improved.

Oxidation and Reduction So what are these more modern, more useful definitions? Oxidation - the loss of electrons by an atom. Reduction - the gain of electrons by an atom. All three of the previous reactions can be called oxidation as they all involve loss of electrons by Mg. 2Mg + O2 ฎ 2MgO Mg + S ฎ MgS Mg + Br2 ฎ MgBr2 Now many reactions without oxygen or hydrogen can be classified. Use these definitions from now on, For organic reactions the older definitions are still useful, as it can be hard to see which atoms have gained or lost electrons in many organic reactions

Oxidation and Reduction There are a couple of other definitions that you have to learn when working with these definitions of oxidation and reduction. Oxidising agents cause oxidation to occur. An oxidising agent is the substance which accepts electrons in an oxidation-reduction reaction. Reducing agents cause reduction to occur. A reducing agent is the substance which donates electrons in an oxidation-reduction reaction.

Oxidation and Reduction Some examples of oxidation based on the modern definition. Here’s one from an earlier slide, plus a new one: 4Fe + 3O2 ฎ Fe2O3 2Fe + 3Cl2 ฎ 2FeCl3 In each case, the iron loses electrons Fe ฎ Fe3+ + 3e-

Oxidation and Reduction Who said we had to start from an element? We can oxidize ions as well. Iron loses an electron in the next example. 2FeCl2 + Cl2 ฎ 2FeCl3 Fe2+ ฎ Fe3+ + e- See what happened? Iron was already missing 2 e-, then it lost another one – loosing electrons oxidises

Oxidation and Reduction Some reagents can oxidize halide ions to the halide or to an oxo acid anion. The following half equations, (where [O] represents an oxidising agent), show exactly this: 2I- + [O] ฎ I2 + 2e- I- + [O] ฎ IO3- + 6e- Notice that in the second example, although we start and finish with a negative ion, because electrons are lost in the process, it is oxidation. (we look at oxidation numbers, and how to count this kind of thing later on)

Oxidation and Reduction Where do these electrons go? The other reagent uses them, It gains electrons, and so by our definition it is reduced. 4Fe + 3O2 ฎ Fe2O3 2Fe + 3Cl2 ฎ 2FeCl3 We already know that iron is oxidised, losing electrons: Fe ฎ Fe3+ + 3e- In the first reaction, oxygen gains electrons, and in the second rxn. chlorine gets the electrons, so both are being reduced. O2 + 4e- ฎ 2O2- Cl2 + 2e- ฎ 2Cl- Hey, this means that in these examples, both reduction and oxidation are occurring in the same reaction. Redox: Reduction-Oxidation - get it?

Oxidation and Reduction Let’s look at a few more of our earlier examples of redox rxns. Mg + S ฎ MgS Magnesium loses electrons - oxidised Sulfur gains the electrons - reduced S + 2e- ฎ S2- Mg + Br2 ฎ MgBr2 Mg loses electrons - oxidised, bromine gains electrons - reduced Br2 + 2e- ฎ 2Br -

Oxidation and Reduction The older definition for oxidation and reduction only considered rxns with oxygen or hydrogen. The modern definition of electron transfer, means both reduction and oxidation are occuring The term REDOX describes reactions where both reduction and oxidation are occurring.. As something loses electrons, something else must gain them. We don’t have reactions where we end up with a bottle of sparks as one of the products! one reagent is oxidised, the other reduced

Oxidation and Reduction Disproportionation is a special type of redox reaction. The same element in the same compound is oxidised and reduced in the same reaction. This means two products are formed, one is the oxidation product, and the other is the reduction product. More about this in the next exciting tutorial on “oxidation numbers”

The End