Balancing Chemical Equations Using Oxidation Numbers

• carlodelmundo
In summary, the conversation is discussing how to balance a chemical equation involving Fe(OH)2 and Fe2O3. The solution involves multiplying all components by 3 and then using intuition and trial and error to balance the remaining elements. The use of the half reactions method is suggested as a more effective approach. The ON method is also mentioned but not required.
carlodelmundo

Homework Statement

http://carlodm.com/pictures/problem.png

The Attempt at a Solution

I'm stuck. Usually I balance the equation by multiplying the Fe(OH)2 and Fe2O3 by 3. However, I can no longer do this because Fe2O3 has two iron's per one molecule.

Do I multiply one by 6 and the other by 3?

Last edited by a moderator:
Assume 2Fe(OH)2 is your reagent (why? because Fe2O3 contains two Fe atoms, just keep iron atoms forming one molecule of product togehter) - then you will have to multiply everything by 3. Basically that's the same as multiplitcation by 3 & 6, but it is easier to follow the reasoning.

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I see. So I should balance the reactant iron first, to 2... then do the multiplications of irons by 3.

I've arrived at that step but the products side still looks like a mess. Is this where I stop all "techniques" and rely on intution and trial and error to figure out the balanced equation?

Once you have Fe and Cr balanced rest you have to "balance by inspection" - which, as you have correctly pointed out, is a fancy name for "intuition and trial and error"

Do you have to use ON method?

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No we're not required to use the ON method... but it is a tool that we are able to utilize for ugly equations such as this. Thanks for your help! This'll come handy for reviewing my Chem exam.

1. What are oxidation numbers and why are they important in balancing chemical equations?

Oxidation numbers are a way to keep track of electron transfer in a chemical reaction. They represent the charge an atom would have if all its shared electrons were completely transferred to the more electronegative atom. Balancing equations using oxidation numbers is important because it ensures that the number of electrons gained by one atom is equal to the number of electrons lost by another, maintaining overall charge neutrality.

2. How do I assign oxidation numbers to elements in a chemical equation?

The general rules for assigning oxidation numbers are:

• Elements in their elemental form have an oxidation number of 0.
• Monoatomic ions have an oxidation number equal to their charge.
• The sum of the oxidation numbers in a neutral compound is 0.
• The sum of the oxidation numbers in a polyatomic ion is equal to the ion's charge.
• Fluorine has an oxidation number of -1 in all compounds.
• Oxygen has an oxidation number of -2 in most compounds, but can have a different number in peroxides and compounds with more electronegative elements.
• Hydrogen has an oxidation number of +1 in most compounds, but can have a different number in metal hydrides and compounds with more electronegative elements.

3. What steps should I follow to balance a chemical equation using oxidation numbers?

The steps for balancing a chemical equation using oxidation numbers are:

1. Determine the oxidation number for each element in the equation.
2. Identify the element that is undergoing oxidation and the one undergoing reduction.
3. Write out the half reactions for each element, showing the change in oxidation number.
4. Balance the number of atoms for each element in the half reactions.
5. Balance the charges in the half reactions by adding electrons.
6. Combine the half reactions and cancel out any common terms.
7. Check that the number of atoms and charges are balanced on both sides of the equation.

4. Can oxidation numbers be fractional or negative?

Yes, oxidation numbers can be fractional or negative in some cases. For example, in compounds with multiple covalent bonds, the oxidation number can be a fraction of the difference between the oxidation numbers of the two atoms. In polyatomic ions, the central atom may have a negative oxidation number if it is more electronegative than the outer atoms.

5. Are there any exceptions to the rules for assigning oxidation numbers?

Yes, there are some exceptions to the general rules for assigning oxidation numbers. These include:

• In peroxides, oxygen has an oxidation number of -1.
• In compounds with more electronegative elements, oxygen and hydrogen may have different oxidation numbers than usual.
• The oxidation number of an element in a compound may change if it is in a different oxidation state in another compound.

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