大学化学第16章

With the knowledge that good oxidizing agents react with good reducing agents, explain why each of the following compounds does not exist: (a) F Oxidation-Reduction Equations
Oxidation is an increase in oxidation number as a result of the loss of electrons or of ―control‖ of electrons. Reduction is a decrease in oxidation number due to the gain of electrons or of ―control‖ of electrons. A chemical species serves as an oxidizing agent by causing the oxidation of another species. When it acts as an oxidizing agent, one (or more) of its elements is reduced. A chemical species serves as a reducing agent by causing the reduction of another species. When it acts as a reducing agent, one (or more) of its elements is oxidized.
Determine the oxidation numbers of phosphorus and chlorine in phosphorus pentachloride, PCl5.
What is the oxidation number of Mg?
What is the oxidation number of nitrogen in the nitrite ion NO2 –
Oxidation Numbers
16.1 Assigning Oxidation Numbers
• The oxidation number氧化数, also known as the oxidation state, of an atom in a compound (or in a free element or polyatomic ion) is defined as the number of electrons • possessed by a free (an uncombined) atom of the element— Nfree—minus the number that is assigned to the atom in the compound (or element or ion)—Ncombined:
Rules for Assigning Oxidation Numbers
1. For any compound, the sum of the oxidation numbers totals zero because the net charge on any compound is zero. The sum of the oxidation numbers of the atoms of any polyatomic ion is equal to the charge on that ion. 2. The oxidation number of any uncombined element is zero 3. The oxidation number of any alkali metal atom in any of its compounds is always equal to +1 4. The oxidation number of any alkaline earth metal atom in any of its compounds is always equal tom+2 5. The oxidation number of hydrogen in its compounds is equal to +1 except in its binary compounds with an alkali metal, an alkaline earth metal, or aluminum, when it is -1 6. The oxidation number of oxygen in its compounds is generally -2 (except in peroxides -1, superoxides, -1/2, and its two compounds with fluorine). 7. The oxidation number of a halogen atom in its compounds is -1 except for chlorine, bromine, or iodine when combined with oxygen or another halogen that is above it in the periodic table.
Oxidation number= N free - Ncombined
Alternatively, we can often use the numbers of valence electrons in the free atom and in the atom in the compound to calculate the oxidation number.
What is the oxidation number of sulfur in the sulfite ion SO3 2–
Calculate the oxidation number of sulfur in the thiosulfate ion S2O3 2–
16.2 Using Oxidation Numbers in Naming Compounds
For compounds of monatomic ions:
the oxidation number turns out to be equal to the charge on the ion. Example: magnesium chloride MgCl2 Each chlorine atom started out with 17 electrons and has gained 1 electron to form the chloride ion in the compound, so the oxidation number of chlorine is: 17-18=-1 the number of valence electrons in magnesium goes from 2 to 0, yielding+2
For example, the ending -ous corresponds to the oxoacid in which the central atom has the lower oxidation state in each case, and the ending -ic corresponds to the higher oxidation state. For example phosphorous acid has phosphorus in the +3 oxidation state; phosphoric acid has phosphorus in the +5 oxidation state. Phosphorus in hypophosphorous acid has an oxidation number +1
16.3 Periodic Variation of Oxidation Numbers
The maximum oxidation number for most elements is equal to the classical group number of the element, especially for the elements in the main groups Every element has an oxidation number of zero when it is uncombined. Some compounds have atoms with oxidation numbers of zero. For example, the carbon atom in formaldehyde CH2O has an oxidation number of zero The minimum oxidation number of the metallic elements and the noble gases is generally zero and that for hydrogen is The minimum oxidation number of other nonmetallic elements is equal to the classical group number of the element minus 8.
What is the minimum oxidation number of nitrogen in any of its compounds?
5-8=-3 What are three possible oxidation states of sulfur? 1s22s22p62s23p4 +6, +4, 6-8=-2, What are three possible oxidation states of copper? 1s22s22p63s23p63d104s1 Ar 3d104s1 or Ar 3d94s2 +1, +2 Ne3s23p4
Example: Nitrogen fluoride: NF3 Each chlorine atom started out with 17 electrons and has gained 1 electron to form the fluoride ion in the compound, so the oxidation number of fluorine is: 17-18=-1 the number of valence electrons in nitrogen goes from 2 to 0, yielding+2
The active metals listed in Table 8.2 are good reducing agents; the more active, the better the reducing agent. Similarly, F2 is a very powerful oxidizing agent, and Cl2 and O2 are slightly lower in oxidizing ability.
A reaction in which an oxidation and a reduction occur is called an oxidation reduction reaction, or redox reaction for short. Every redox reaction must have an oxidation and a reduction.