Balancing Redox ReactionsBalancing Redox Reactions to Show Electron Transfer
Balancing redox reactions is an easy process once we have a clear understanding of oxidation number rules. All we need is the reactants which are the starting chemicals and the products, the chemicals that result from the reaction. It is worth noting that using oxidation numbers to balance chemical equations gives us exactly the same result as balancing equations using other methods.We need to keep in mind several things regarding these redox reactions: * there is no nett gain or loss of electrons * the numbers of each type of atom in the equation must be exactly the same in the reactants and the products * the change in the arrangement of the atoms in the chemicals represents either gain or loss of energy, which is stored in or lost from the bonds between the atoms in the chemicals * reduction and oxidation must occur together; one cannot occur without the other. The rules for balancing redox reactions are simple and, as with oxidation numbers, need to be followed in order. Steps that are not relevant can be skipped, but the sequence must not be altered. Rule 1: Write the reactants, or starting chemicals, on the left side of the equation and the products on the right. Put an arrow between them pointing toward the products. Rule 2: Assign each atom in each chemical its correct oxidation number. Rule 3: SPLIT the reactants into two half equations where the reactant goes to its exact product. Do NOT worry about how many of each atom type there is in the products at this stage Rule 4: Balance the numbers of atoms on both sides by adding numbers in front of the ingredient. This is done only on the product side Rule 5: Balance any oxygen atoms in each half equation by adding water molecules to the OTHER side of the arrow Rule 6: Balance the number of Hydrogens on each side by Rule 7: Add electrons (each one represents a -1 charge) to the appropriate side of the equation to make the charges on each side equal Rule 8: Take both half equations and multiply one or both so that they both have the same number of electrons in each Rule 9: Join both half reactions together again Rule 10: Cancel out any parts that are the same on each side of the equation. The electrons are always the first to go, since we will have made them equal before putting the half equations together. Other items that are usually canceled are H+ units and water molecules, either partially or completely. An Example of a Redox Reaction The process of balancing redox reactions and therefore showing electron transfer can best be demonstrated through a clear and simple example. The simplest example of all chemical reactions is the burning of Methane gas (CH4) with Oxygen gas (O2) to give water, H2O and Carbon Dioxide, CO2. The steps for this process are described and shown visually below. Rule 1: We are told that the reaction is between Methane gas and Oxygen gas, and that the products are water and CO2. The starting chemicals (reactants) go on the left and the final products go on the right.
Rule 2: We follow the rules for assigning Oxidation Numbers here; in the Methane Carbon is at -4 with Hydrogen at +1; the oxygen atoms in the gas have oxidation numbers of zero. Water on the right is a compound so we follow the rules to assign each Hydrogen the number +1 and the Oxygen atom the number -2. The Carbon Dioxide has Carbon at +4 and Oxygen at -2. We can see straight away that the Carbon is being oxidized as its oxidation number is becoming more positive, going from -4 to +4. Incidentally, this gives us a good guess at the number of electrons that will be transferred in this reaction: 8. The Oxygen is being reduced since its oxidation number is becoming more negative, and the Hydrogen has been unaffected in the reaction.
Rule 3 + 4: Now we split the equation. Each reactant gets its own line and its own product which we assign to it based on the major atom in the reactant. In the Methane, that is the Carbon, so the Carbon Dioxide is its product. The Oxygen gets the water as its product since the major atom in water is the oxygen. The numbers in front of the atoms on the right hand, product side are obtained purely from the number on the left side; these must balance. The Carbon in Methane and in CO2 are already numerically balanced. This is not the case for the Oxygen atoms, so we need to double the number of water molecules to account for the two oxygen atoms in oxygen gas.
Rule 5: We balance the number of oxygen atoms on each side of each half equation by adding water molecules in the correct place:
Rule 6: Now we balance the H atoms that are in the equation already by adding protons (H+ units) to the appropriate places for both half equations:
Rule 7: Now we add electrons to make the overall charges in each half reaction balance. Since all the products and reactants are uncharged, we need to add electrons to get our overall charges to equal zero in both cases:
Rule 8: Since the top half equation has 8 electrons and the bottom one has 4, we can easily make the numbers of electrons the same in both by multiplying the whole bottom half equation by two.
After expanding the bracket, we get the same number of electrons in each half equation, as follows:
Rule 9: In this step of balancing redox reactions, all the parts of both half equations that are on the left of their arrows are joined together on the left of the overall equation. The same goes for the components on the right. Now we cancel out everything that is the SAME on both sides of the arrow. All the electrons and H+ units go. Also, two waters can be removed from each side, leaving only two on the right side of the completed reaction:
Balancing Redox Reactions: Why Bother With This? At first this seems like a cumbersome method for balancing redox reactions. If we have studied Chemistry in middle school we can balance that Methane and Oxygen giving water and CO2 reaction without too much difficulty. What we learn from this HALF EQUATION method is the amount of electron transfer that is going on in the reaction. Since the aim of this particular section is to understand electron flow from batteries, having an idea of where the electrons are coming from is surely a good thing. It also lets us be 100% correct in our equation. Balancing redox reactions does not require any more information than the chemical formulae of the reactants and products, and a correctly balanced equation can be achieved every time.
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