Oxidation and reduction
A redox reaction is one in which oxidation and reduction are taking place.
Oxidation refers to the addition of oxygen, the loss of electrons from a substance and an increase in oxidation number.
Reduction refers to the addition of hydrogen, the gain of electrons and a decrease in oxidation number of a substance.
Oxidation number or oxidation state refers to the charge on the ion or species.
Common rules for determining oxidation numbers.
1. Metals – usually based on the charge on the ion or the group number. E.g., Mg +2 (in group 2) and Al +3 (in group 3). The number of electrons lost.
2. Non-metals – usually based on the electrons to be added for ion tobe stable.E.g. Cl -1,O -2, P -3.
3. In compounds, the sum of the oxidation states must add up to zero.
4. In complex ions, the sum of the oxidation states is equal to the charge on the ion.
5. Oxidation state of an uncombined element or one in its natural state is zero.
6. Some species have the oxidation state in the name of the species. E.g., Copper I oxide (Cu2O) and copper II oxide (CuO)
Oxidation and reduction usually take place in a reaction through the action of oxidizing and reducing agents. In some cases, these redox reactions are accompanied by colour changes.The oxidizing agent is usually reduced in the reaction. The reducing agent is usually oxidized in the reaction
You can determine which substance is oxidized or reduced by tracking changes in the oxidation number.
Common oxidizing agents include concentrated sulphuric acid, oxygen, nitric acid, chlorine, potassium manganate VII and potassium dichromate solutions.
Examples of reducing agents include carbon monoxide, hydrogen, hydrogen sulphide, metals, potassium iodide and sulphur dioxide.
Manganese forms a number of compounds in which it exhibits different oxidation numbers.
(a) Potassium manganate VII (KMnO4) is a common reagent used in laboratories.
(i) Describe one use of potassium manganate as a laboratory reagent (include the test it is used for and any colour changes during the reaction).
(ii) Calculate the oxidation number of manganese in KMnO2 and in MnO2.
(iii) Based on the answer given above, deduce the type of reaction which occurs when KMnO4 is converted to MnO2. (b) Explain the main principles involved in the breathalyser test (include colour changes etc).
(a) Potassium manganate (VII) is used extensively in school laboratories.
(i) KMnO4 is used as an oxidizing agent. It can be used as a test for alkenes, in which the KMnO4 is acidified and added to the alkene. If the purple colour of the KMnO4 disappears, this confirms the presence of alkenes or KMnO4 can be used to test for reducing agents. The acidified KMnO4 is added to the unknown reagent. If the KMnO4 is decolourized, then the reagent is a reducing agent. (ii) KMnO4 - if K = +1; O= -2; Mn = x; +1+x+4(-2) =0; x = +7 The oxidation number of Mn in KMnO4 = +7 MnO2 - if O= -2; Mn = x; x+2(-2) =0; x = +4 The oxidation number of Mn in MnO2 = +4
(iii) In the reaction, Mn changes oxidation number from +7 to +4. The oxidation number has decreased, which indicates a reduction; the KMnO4 has been reduced to MnO2.
(b) The breathalyser test is used to test the alcohol levels of drivers. In this test, the person’s breath passes over potassium dichromate which changes colour from orange to green if alcohol is present. In this case, the alcohol is oxidized while the potassium dichromate is reduced. Cr2O7 (aq) + 14H+ ----> 2 Cr3+ (aq) + 7H2O (l) orange green
Students of The Queen’s School give a dramatic presentation on the topic ‘The Relevance and Importance of Nutrition to Maintaining Good Health’ at the National Child Month Committee’s Youth Forum, held recently at the St Andrew Parish Church Hall.