Lesson 9: Redox Reactions
Electrochemical reactions are also called oxidation-reduction reactions or redox reactions for short.
Oxidation
The term oxidation itself now has a slightly different meaning than what it originally started out with. The term oxidation originally meant that something was combining with oxygen. If potassium combined with oxygen, the potassium was oxidized or went through the process of oxidation. So, oxidation meant the process of becoming combined with oxygen. When chemists started taking a look at what was going on, they looked at what oxygen did to these chemicals that were being oxidized. They found that the oxygen was pulling electrons away from the elements. Those elements were losing electrons to oxygen. In time the term oxidation became associated with loss of electrons instead of only combination with oxygen. So oxidation can occur with or without oxygen, but it must involve the loss of electrons.
Reduction
The flip side of oxidation is reduction. If one chemical is losing electrons, another is gaining them. Gaining electrons is called reduction. That sounds contradictory, but it should make sense when we consider the oxidation states of the elements involved in the reaction, which we will do in a moment.
In order to recognize a redox reaction you have to look for evidence that electrons have moved from one chemical to another. You have to look for changes in the oxidation states of the elements involved. This requires that you be able to figure out the oxidation states of elements from formulas. The section on oxidation states gives you guidelines on how to do that if you are not already familiar with it. The section on terminology gives you some information on the meanings of various terms if you are not familiar with them.
Oxidation States | Terminology
Oxidation States
Remember that elements can have various oxidation states depending on the number of electrons that they have compared to the number of protons that are in the nucleus.
- If the number of electrons is the same as the number of protons, then we say that the atom is neutral and that it has an oxidation state of 0.
- If the element has reacted in a way to have gained electrons, then it has a negative oxidation state, because the gained electrons have negative charges.
- If the element has reacted in a way that has caused it to lose electrons, then it has a positive oxidation state, because it has lost negative charges.
Guidelines
Here are some guidelines that you can use to figure oxidation states of chemicals.
If you have a pure element, the oxidation state is 0. |
For example, K is a pure element in its 0 state. |
If you have an isolated ion that consists of one atom that has either gained or lost electrons, the oxidation state is equal to the charge on that ion. |
Na+ is in a +1 state. Chloride ion (Cl-) has a -1 state for the chlorine. |
In a compound more figuring has to be done. Remember that all of the oxidation states have to add up to zero. First find the most electronegative element and give it the negative state that it wants. That usually works. Then figure out what the other element has to be to bring the total to zero. |
In Cl2O the oxygen gets to be -2, then the chlorine must be +1 in order for the oxidation states of all the atoms to add up to zero. In Fe2O3 the oxygen gets to be -2, then the iron must be +3 in order for the oxidation states of all the atoms to add up to zero. |
If there is more than one other element, start with the ones you can count on being +1 because they are in group IA or +2 because they are in group IIA. Then figure out the others. |
In HNO3 the oxygen gets to be -2, and we can count on the hydrogen to be +1, then the nitrogen must be +5 in order for the oxidation states of all the atoms to add up to zero. |
If you are dealing with a polyatomic ion by itself, the only change is that all of the oxidation states have to add up to the charge on the ion. |
In PO43- the oxygen gets to be -2, then the phosphorus has to be +5 so that the oxidation states of all the atoms adds up to -3. |
If you recognize a polyatomic ion in the formula, it is often helpful to note it and its charge because that can help you figure out the oxidation state of the element it is combined with. Then continue with the rest as before. |
In FeSO4 the sulfate ion has a -2 charge, therefore the iron has to be +2 in this compound. The oxygen gets to be -2, then the sulfur has to be +6 in order for the oxidation states of all the atoms to add up to zero. CoCO3, because carbonate is -2 the cobalt has to be +2. The oxygen is -2 and the carbon is +4. |
Practice
Now practice by determining the oxidation states of the elements in these chemicals (which are also listed in exercise 11 in your workbook). Check your answers below.
| Au3+ | N2 | FeCl3 | N2O3 | NO3- | PCl5 | OF2 | H2SO4 | HNO2 | MnO4- |
Answers
| Au3+ | N2 | FeCl3 | N2O3 | NO3- | PCl5 | OF2 | H2SO4 | HNO2 | MnO4- |
| Au is +3 | N is 0 |
Cl is -1 Fe is +3 |
O is -2 N is +3 |
O is -2 N is +5 |
Cl is -1 P is +5 |
F is -1 O is +2 |
O is -2 H is +1 S is +6 |
O is -2 H is +1 N is +3 |
O is -2 Mn is +7 |
Terminology
In this lesson we will use a number of terms with which you need to be very careful to make sure that you use and interpret them correctly. Sometimes you need to use the context to get the proper meaning. For example, the phrase "he burns" doesn't mean quite the same thing in these two sentences: "He burns the trash." "He burns at the stake."
Four terms in particular will come up again and again and you must be able to define and identify those four terms.
Oxidation is a process and it means the loss of electrons.
Reduction is also a process and it means the gain of electrons.
Oxidizing agents are those chemicals which cause oxidation to occur. That does not mean that they become oxidized; it means they cause oxidation.
Reducing agents cause reduction to occur. A reducing agent causes something else to gain electrons.
Along with those four terms you will hear lots of other terms associated with oxidation and reduction. Although you may not be called upon to define those terms specifically, you will need to understand what they mean in order to understand questions and be able to relay your information to other people. To assist you in this, there is a long list of terms which are used with oxidation in example 12 in your workbook. What follows is an expansion on that list, if you need or want more explanation than what is given in the workbook.
The first one is simple. Oxidation is the process of losing electrons. |
The word oxidize, however, does create some problems in the way that our English language works. Some verbs are transitive verbs; some are intransitive. If you are not familiar with those terms, you can check with an English instructor; but essentially they are the active and passive forms of the verbs.
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The word oxidized has the same problem. It can be the past tense of the verb oxidize. "The tin can oxidized" or "oxygen oxidized the tin can." Whether you are talking about the passive or active form makes a big difference in terms of what is gaining electrons and what is losing electrons. The subject of the sentence may be the cause of oxidation or the object of oxidation, so again the context is very important. The word can also be used as an adjective. "Rust is one form of oxidized iron." |
Next, the terms oxidized form and oxidized state refer to the condition of having fewer electrons than some other form or state of the same element. For example, sodium can exist either as Na or as Na+. Na+ is the oxidized form or oxidized state of sodium. Some chemicals have three or more different oxidation states, so when you say the oxidized state of something like iron, you have some choices to make. Do you mean the +2 or the +3 state? It's obvious that you don't mean the 0 state. If you say the reduced form, you might mean the 0 or the +2 state (but not the +3). So these phrases can be somewhat ambiguous. |
The oxidation state is a numerical value associated with the condition of having lost or gained a certain number of electrons. Unlike most of these terms, this one does not have a corresponding reduction term. The oxidation state simply means how many electrons has it lost or gained. If the element has lost electrons, then it will have a positive state. If it has gained electrons, that is, if it has been reduced, then it will have a negative oxidation state. If it has neither lost nor gained electrons, then it will have a 0 state. |
Oxidation number is a phrase that can be used interchangeably with oxidation state. The only real distinction is that it refers specifically to the number rather than the condition. |
Oxidizing agent is a chemical that causes oxidation to occur. |
A couple of terms that you won't run into are oxidizer and oxidizee, although in some ways they would be a very logical set of terms.
There are parallel terms for reduction, which are not shown here. Reduction is the process of gaining electrons. Reduce means both to go through the process of reduction and to cause the process of reduction to occur, and so on. There are parallel terms for reduction for all of these terms except oxidation state and oxidation number (we don't ever refer to an element's "reduction state" or its "reduction number").