hey it's professor Dave, let's talk about redox chemistry. an oxidation-reduction reaction or a redox reaction is one in which electron transfer occurs. take for example this single replacement reaction.
watch what happens if we write out the net ionic equation. the sulfates cancel and we see that the only chemistry that has occurred is that two electrons are transferred from iron to copper. iron has lost electrons so it has been oxidized.
copper has gained electrons so it has been reduced. it may seem confusing to associate a reduction with a gain of electrons so instead associate reduction with a reducing of charge. copper 2+ had its charge reduced to zero when it gained two electrons.
more electrons means a more negative or lower charge. the substance that is oxidized is the reducing agent because it's the thing that caused the reduction in the other substance. likewise the substance that is reduced is the oxidizing agent because it caused the oxidation in the other substance.
we will use oxidation numbers to keep track of electron flow. these are like hypothetical charge values for each atom in a molecule but they are not actual electrical charges. here are some rules for a signing oxidation numbers.
for an element the oxidation number is always zero. if it's a monatomic ion the oxidation number is the same as the charge on the ion. oxygen is almost always -2 except for in peroxides so we can usually assume -2.
for a neutral molecule oxidation numbers must add up to zero so if the oxygens in carbon dioxide are each -2 the carbon must be +4. for polyatomic ions the oxidation numbers must add up to the charge on the ion and hydrogen is almost almost always +1. using these numbers for oxygen and hydrogen we can usually assign all the other numbers accordingly.
let's take a look at a reaction, on the left we have elements so those have oxidation numbers of zero. on the right we know oxygen will be -2 and there's three of them so we need iron to be + 3 for everything to add to zero. this means the iron was oxidized since its oxidation number increased.
oxygen was the oxidizing agent. oxygen was also reduced since its oxidation number decreased, that makes iron the reducing agent.
