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Course: MCAT>Unit 9

Lesson 27: Redox reactions

Worked example: Balancing a redox equation in basic solution

When balancing equations for redox reactions occurring in basic solution, it is often necessary to add OH⁻ ions or the OH⁻/H₂O pair to fully balance the equation. In this video, we'll walk through this process for the reaction between ClO⁻ and Cr(OH)₄⁻ in basic solution. Created by Jay.

Want to join the conversation?

• Does someone know where I can find some good questions of balancing redox reactions to practice?
• Why do we use water to balance Oxygen, why not something else, like Oxygen itself?
• Because water already exist in the solution, if the molecules were in a medium other than water and it doesn't affect the reaction and that medium contains one oxygen molecule then we will use that medium's atoms to balance the oxygen atoms.
• If the solution doesn't have a reactant with OH-, do we assume that the solution is acidic?

Do we only use OH- to neutralize the H+ if its a basic solution?
• Usually a question should tell you whether or not the solution is acidic or basic. If it is basic, then the extra two steps are always necessary (unless there is no H's) . For your second question, yes.
• At , Jay says that the oxidation state of oxygen is 2-, and since the net charge on the the molecule is 1-, the charge on chlorine has to be 1+. Can I not consider the charge on chlorine to be 1- as it usually is, and calculate the charge on oxygen accordingly?
• When you see a molecule like H2O, ClO, etc, always do Oxygen first. Refer to the Electronegativity chart, and find the 2 most electronegative atoms you can find.

https://en.wikipedia.org/wiki/Electronegativity#Electronegativities_of_the_elements

If you look, it'll be first, Fluoride, then Oxygen. Since Oxygen is so electronegative, it will alway be considered first when calculating charge because it'll be more powerful at attracting electrons than the other molecule...

...Except in the case of Fluoride and Oxygen, such as chemical OF2 (Oxygen difluoride). The Fluorides will be more electronegative than oxygen and steal the electrons first. When OF2 is neutral, the charge will change for Oxygen to being 2+, and the Fluorides 1-.
• How to find an oxidation number for a substance or an elemental entity in a chemical reaction?
• You look at each chemical in the reaction individually, not the reaction as a whole, to assign oxidation states. It is quite likely, and in redox reactions a certainty, that some of the atoms will have have a change in their oxidation states as a consequence of the reaction.

The oxidation state is an assigned value based upon am agreed-upon set of rules. You should be taught the basics of these rules in class (there are many details in the rules that we don't typically cover at the introductory level because some of these are for special situations). The oxidation state is a method of summarizing a large number of chemical properties.

Here are some of the main rules, listed in order of importance from greatest to least. If there is a contradiction in the rules, the rule listed first should be followed:
1. The oxidation of an element in its elemental form (that is, a neutral element that is bonded to nothing except other atoms of itself) is 0. Thus, H₂. F₂, Ne, S₈ all have the state 0. There are no exceptions to this rule.
2. The oxidation state of a monatomic ion is the charge on that ion. There are no exceptions to this rule.
3. Hydrogen has a state of +1 except in the hydrides of active metals in which case it has -1.
4. Fluorine has a state of −1.
5. Oxygen has a state of −2, except in peroxides in which case it has −1 or when directly bonded to fluorine in which case it may have a positive oxidation state.
(There are some other exceptions, but these are obscure).
6. Group 1 elements have a state of +1, except for hydrogen as mentioned above.
7. Group 2 elements usually have a state of +2
8. Group 17 (the halogens) usually have a state of −1 unless directly bonded to other halogens, in which case the lowest atomic number halogen has a state of −1. For example, in BrCl, Bromine has the +1 state and Cl has the −1 state. Also, halogens other than F may have different states when bonded directly to oxygen.
9. The sum of oxidation states of all atoms in a neutral compound must be zero. For ions, the sum of the oxidation states of all atoms in the ion must equal the charge of that ion.
10. Ag nearly always has the +1 state. Cd and Zn nearly always have a state of +2. Al nearly always has a state of +3.
NOTE: not an actual rule, but the oxidation state is nearly always an integer.

Atoms not explicitly mentioned in the above rules have their oxidation states determined by enforcing those rules. For example, in WCl₅, following the rules we assign −1 to each of the 5 Cl atoms. Thus, in order for the sum of the oxidation states to equal 0 (since this is a neutral compound) the state of W must be +5. However, in WCl₆, there are six atoms of Cl to assign the −1 charge to, thus W must have a state of +6.

Here is a very difficult example: NaC₅H₅ (called Sodium cyclopentadienide).
Following the rules: Na gets +1 and each of the five H gets +1
There are six +1, so in order to sum to zero, the sum of the oxidation states of the five C atoms must sum to −6. Thus, carbon has a −⁶⁄₅ oxidation state. (Again, unusual because oxidation states are nearly always integers.)
• Does it matter when you add your OH-? In my AP Chemistry class, the standard is to add the OH- in the half reactions.
• yes you can add in the half-reactions but you have to make sure you have a balanced reaction
• In the beginning when writing the oxidation states, Jay wrote -2 for Oxygen in ClO- . But isn't the oxygen bonded to only one chlorine atom? How did it acquire a -2 charge?

Thank You!
• Because one of the rules for assigning oxidation numbers is that oxygen will usually have an oxidation number of -2. Remember these are not really actual charges just how we keep track of electrons.
• in this video they add H+ IONS in both cases
hows that possible
its hould be OH-
• They're adding OH- in the end. Guess that doesn't make a difference though, 'cause I learnt another method at school in which you add twice as many OH- instead of H+. But as it turns out, that method sometimes makes things more complicated (well, dunno about others but at least for me, it most certainly does).