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Edexcel A Level Chemistry:復習筆記2.1.1 Oxidation Number

Category: A-level課程, 教材筆記, 福利干貨 Date: 2022年9月13日上午10:46

There are three definitions of?oxidation?and?reduction? used in different branches of chemistry
Oxidation?and?reduction?can be used to describe any of the following processes

Definitions and Examples of Oxidation & Reduction

1.6-Electrochemistry-OIL-RIG-Diagram

Use the acronym "Oil Rig" to help you remember the definitions of oxidation and reduction

The?oxidation number?of an atom is the charge that would exist on an individual atom if the bonding were completely ionic
It is like the electronic ‘status’ of an element
Oxidation numbers are used to
- Tell if oxidation or reduction has taken place
- Work out what has been oxidised and/or reduced
- Construct half equations and balance redox equations

Oxidation Numbers of Simple Ions

9.1.1-Oxidation-number-of-simple-ions-table

What are the oxidation numbers of the elements in the following species?

a) C ?????????????? b)? Fe3+?? ??????????????????? c)? Fe2+

d) O2-???????????? e)? He ???????????????????????? f)? Al3+

Answers:

a) 0? ? ?b) +3? ? c) +2

d) -2? ? e) 0 ? ? ?f) +3

So, in simple ions, the oxidation numbers of the atom is the charge on the ion:
- Na+, K+, H+?all have an oxidation number of +1
- Mg2+, Ca2+, Pb2+?all have an oxidation number of +2
- Cl–, Br–, I–?all have an oxidation number of -1
- O2-, S2-?all have an oxidation number of -2

A few simple rules help guide you through the process of determining the oxidation number of any element
Remember, you are determining the oxidation number of a?single?atom
The oxidation number (ox.no.) refers to a?single?atom in a compound

Oxidation Number Rules Table

1.6-Electrochemistry-Table-1_Oxidation-Numbers

In molecules or compounds, the sum of the oxidation numbers on the atoms is zero

Oxidation Number in Molecules or Compounds

9.1.1-Oxidation-Number-of-Molecules-Compounds-table

Because CO2?is a neutral molecule, the sum of the oxidation numbers must be zero
For this, one element must have a positive oxidation number and the other must be negative

How do you determine which is the positive one?

The more electronegative species will have the negative value
Electronegativity increases across a period and decreases down a group
O is further to the right than C in the periodic table so it has the negative value

How do you determine the value of an element’s oxidation number?

From its position in the periodic table and / or
The other element(s) present in the formula
The oxidation numbers of all other atoms in their compounds can vary
By following the oxidation number rules, the oxidation number of any atom in a compound or ion can be deduced
The position of an element in the periodic table can act as a guide to the oxidation number

Oxidation Numbers & the Periodic Table

9.1.2-Oxidation-numbers-and-the-Periodic-Table-1