- Oxidation–reduction (redox) reactions
- Worked example: Using oxidation numbers to identify oxidation and reduction
- Balancing redox equations
- Dissolution and precipitation
- Precipitation reactions
- Double replacement reactions
- Single replacement reactions
- Molecular, complete ionic, and net ionic equations
- Molecular, complete ionic, and net ionic equations
- 2015 AP Chemistry free response 3a
What is an oxidation–reduction reaction?
An oxidation–reduction or redox reaction is a reaction that involves the transfer of electrons between chemical species (the atoms, ions, or molecules involved in the reaction). Redox reactions are all around us: the burning of fuels, the corrosion of metals, and even the processes of photosynthesis and cellular respiration involve oxidation and reduction. Some examples of common redox reactions are shown below.
During a redox reaction, some species undergo oxidation, or the loss of electrons, while others undergo reduction, or the gain of electrons. For example, consider the reaction between iron and oxygen to form rust:
In this reaction, neutral
loses electrons to form ions and neutral gains electrons to form ions. In other words, iron is oxidized and oxygen is reduced. Importantly, oxidation and reduction don’t occur only between metals and nonmetals. Electrons can also move between nonmetals, as indicated by the combustion and photosynthesis examples above.
How can we determine if a particular reaction is a redox reaction? In some cases, it is possible to tell by visual inspection. For example, we could have determined that the rusting of iron is a redox process by simply noting that it involves the formation of ions (
and ) from free elements ( and ). In other cases, however, it is not as obvious, particularly when the reaction in question involves only nonmetal substances.
To help identify these less obvious redox reactions, chemists have developed the concept of oxidation numbers, which provides a way to track electrons before and after a reaction. An atom’s oxidation number (or oxidation state) is the imaginary charge that the atom would have if all of the bonds to the atom were completely ionic. Oxidation numbers can be assigned to the atoms in a reaction using the following guidelines:
- An atom of a free element has an oxidation number of
. For example, each atom in has an oxidation number of . The same is true for each atom in , each atom in , and so on.
- A monatomic ion has an oxidation number equal to its charge. For example, the oxidation number of
is , and the oxidation number of is .
- When combined with other elements, alkali metals (Group
) always have an oxidation number of , while alkaline earth metals (Group ) always have an oxidation number of .
- Fluorine has an oxidation number of
in all compounds.
- Hydrogen has an oxidation number of
in most compounds. The major exception is when hydrogen is combined with metals, as in or . In these cases, the oxidation number of hydrogen is .
- Oxygen has an oxidation number of
in most compounds. The major exception is in peroxides (compounds containing ), where oxygen has an oxidation number of . Examples of common peroxides include and .
- The other halogens (
, , and ) have an oxidation number of in compounds, unless combined with oxygen or fluorine. For example, the oxidation number of in the ion is (since has an oxidation number of and the overall charge on the ion is ).
- The sum of the oxidation numbers for all atoms in a neutral compound is equal to zero, while the sum for all atoms in a polyatomic ion is equal to the charge on the ion. Consider the polyatomic ion
. Each atom has an oxidation number of (for a total of ). Since the overall charge on the ion is , the oxidation number of the atom must be .
One thing to note is that oxidation numbers are written with the sign (
or ) before the number. This is in contrast to the charges on ions, which are written with the sign after the number. Now, let’s see some examples of assigning oxidation numbers!
Example 1: Assigning oxidation numbers
What is the oxidation number of each atom in (a)
, (b) and (c) ?
To assign the oxidation numbers to the atoms in each compound, let’s follow the guidelines outlined above.
(a) We know that the oxidation number of
is (guideline 4). Because the sum of the oxidation numbers of the six atoms is and is a neutral compound, the oxidation number of must be :
(b) The oxidation number of
is (guideline 5) and the oxidation number of is (guideline 6). The sum of these oxidation numbers is . Since has no net charge, the oxidation number of must be :
(c) The oxidation number of
is (guideline 6), so the sum of the oxidation numbers of the three atoms is . Since the net charge on is , the oxidation number of must be :
Concept check: What is the oxidation number of the carbon atom in
Recognizing redox reactions
How do we actually use oxidation numbers to identify redox reactions? To find out, let’s revisit the reaction between iron and oxygen, this time assigning oxidation numbers to each atom in the equation:
Notice how iron (which we already know is oxidized in this reaction) changes from an oxidation number of
to an oxidation number of . Similarly, oxygen (which we know is reduced) changes from an oxidation number of to an oxidation number of . From this, we can conclude that oxidation involves an increase in oxidation number, while reduction involves a decrease in oxidation number.
So, we can identify redox reactions by looking for changes in oxidation numbers over the course of a reaction. Let’s explore this idea more in the next example.
Example 2: Using oxidation numbers to identify oxidation and reduction
Consider the following reaction:
Is this reaction a redox reaction? If so, which element in the reaction is oxidized and which element is reduced?
Considering this is an article about redox reactions, the reaction probably is a redox reaction! However, let’s prove it by assigning oxidation numbers to the atoms of each element in the equation:
The oxidation numbers of
and are different on either side of the equation, so this is definitely a redox reaction! The oxidation number of increases from to , which means that N loses electrons and is oxidized during the reaction. The oxidation number of decreases from to , which means that gains electrons and is reduced during the reaction.
Oxidation–reduction reactions, commonly known as redox reactions, are reactions that involve the transfer of electrons from one species to another. The species that loses electrons is said to be oxidized, while the species that gains electrons is said to be reduced. We can identify redox reactions using oxidation numbers, which are assigned to atoms in molecules by assuming that all bonds to the atoms are ionic. An increase in oxidation number during a reaction corresponds to oxidation, while a decreases corresponds to reduction.
Want to join the conversation?
- Shouldn’t equation H2 + O2 -> 2 H2O be balanced to 2 H2 + O2 -> 2 H2O?(65 votes)
- Is it possible to have reaction where only oxidation or reduction happens, or does the occurrence of one result in the other? Thank you.(22 votes)
- They must both occur. The sum of all the oxidation states cannot change unless there is a change in the overall charge of the ion/molecule. Even with a change in the charge, there must be somewhere else that the change in charge (and thus oxidation states) is exactly offset. That is because of conservation of electrical charge.(17 votes)
- In the last paragraph, it states that there is a transfer of electrons. Is there not always a transfer of electrons?(29 votes)
- There is not always a transfer of electrons (not all reactions are redox reactions). An example of a reaction that is not a redox reaction might be a neutralization reaction:
H3O+ + OH- ---> 2 H2O
The oxidation number of H is +1 and the oxidation number of O is -2 for for both the reactants and products, so it is not a redox reaction.(47 votes)
- What is the difference between a monatomic ion and an atom in its elemental state?(11 votes)
- An ion is an atom that has gained or lost electrons. Atoms in their elemental state are not ionised.
Also, atoms in their elemental state can be joined to other atoms. For example, H2 is the elemental state for hydrogen.(22 votes)
- In the example of determining the oxidation state in H2 and H2O, it reads:
Rule 4 tells us that all the oxidation numbers in a compound have to add up to charge on the compound, and rule 3 says that oxygen usually has an oxidation number of +2.
The "+2" should be "-2".(14 votes)
- What is the use of knowing about oxidation numbers ?(4 votes)
- Knowing oxidation numbers allows you to predict what compounds or reactions will form when different elements mix together. This is really important, as you will need to be able to write compounds and reactions to do everything else you will learn in chemistry.
I hope this helps!(12 votes)
- do we have to memorize these rules?(7 votes)
- Above it says, "oxygen is usually assigned a −2 oxidation number (except in peroxide compounds where it is −1, and in binary compounds with fluorine where it is positive);"
The part where it says "...with fluorine where it is positive", did they mean +1 or +2? Or did they mean, it varies depending on the binary compound with fluorine but it'll always be positive?(6 votes)
- With Florine it forms OF2 where it shows +2 oxidation state otherwise it shows a -1 or -2 oxidation state(4 votes)
- In the section "Determining the oxidation state in H2 and H2O"
There is a line (oxidation # of O×# of O atoms)+(oxidation # of H×# of H atoms)=(−2×1)+(−1×2)=−3
But the result should be -4(6 votes)
- Does anyone know what the OH radical is? It is in one of my compounds for my experiment(3 votes)