in this video we're going to talk about oxidation and reduction reactions and here's the example we're going to start with magnesium plus oxygen gas produces magnesium oxide an oxidation reduction reaction or simply a redox reaction electrons are being transferred from one element to another now perhaps you found or watched my video on oxidation numbers if you have then you know that the oxidation number of any pure element is zero now in magnesium oxide magnesium being an alkaline earth metal has a charge of plus two oxygen has a charge of minus two notice that the oxidation state of magnesium went from zero to plus two so the oxidation state increased whenever the oxidation state goes up the substance is set to be oxidized in the case of oxygen the oxidation number decreased from zero to negative two whenever the oxidation number decreases the substance is being reduced now you need to know that oxidation always occurs with a loss of electrons reduction is associated with a gain of electrons metals they like to give away electrons they like to form metal cations as they give away electrons they will acquire a positive charge non-metals like oxygen they like to acquire electrons and so they will develop a negative charge the substance that is oxidized is known as the reducing agent and the substance that is reduced is known as the oxidizing agent metals are reducing agents because they will cause the other substance to be reduced not metals like oxygen gas fluorine they're oxidizing agents because they cause another substance to be oxidized and that's where you got to see it so these are some terms that you want to be familiar with when dealing with redox reactions now let's go back to that reaction the reaction between magnesium and oxygen gas now magnesium is changing into the magnesium ion this magnesium ion has a plus two charge and in order to become a plus two cation it has to lose two electrons now oxygen i'm gonna write atomic oxygen because this reaction is not balanced individually oxygen acquires two electrons and turns into oxide now because magnesium lost electrons this reaction it's which is known as a half reaction that's the oxidation part of the reaction anytime a substance loses electrons it's being oxidized this second half reaction is the reduction part since oxygen is acquiring electrons whenever a substance gain electrons or if the oxidation number decreases its reduction now for half reactions anytime you have electrons on the right side it's going to be an oxidation half reaction whenever the electrons are on the left side it's a reduction half reaction that's another way in which you can see it too consider this reaction zinc metal reacts with hydrochloric acid to produce hydrogen gas and zinc chloride identify the substance that is oxidized and the substance that is reduced and also identify the oxidizing agent and reduce an agent now if you get a question like this on the test to find a substance that is oxidized or reduced always look at the reactants it's one of these two don't look at the products now let's find the oxidation states of everything the oxidation state of any pure element is always zero now whenever hydrogen is bonded to a non-metal it's going to have a positive one oxidation state which means chlorine has to be minus one now in this one two chlorine is still negative one which means zinc has to have a plus two charge now the oxidation of zinc changes from zero to positive two so zinc is being oxidized hydrogen changes from one to zero so hcl as a substance is being reduced even though only the hydrogen portion of that substance is being reduced so typically they actually for the substance even though it's only the hydrogen element that's being reduced you would say the whole substance is being reduced if you have to choose an answer on a multiple choice test now the substance that is oxidized is to reduce an agent and typically metals tend to be reducing agents the substance that is reduced is the oxidizing agent and this is a common question that you might see on a typical chemistry exam now let's try one more example for the sake of practice methane reacts with oxygen gas and it produces carbon dioxide and water so feel free to pause the video identify the oxidation states of every element in this reaction and then find a substance that is oxidized reduce and identify the oxidizing agent and reduce an agent so let's identify the pure elements which is only oxygen gas that's going to be zero now we said that whenever hydrogen is bonded to a non-metal it's going to have a plus one oxidation state and typically when oxygen is found in a compound the oxidation state is negative two except when it's bonded to fluorine or except when it's in the form of peroxide or superoxide it's different whenever you hear the word oxide the oxidation state of oxygen is negative two if you hear the word peroxide it's negative one if you hear the word superoxide it's negative one-half now we gotta find the oxidation state of carbon so let's start with methane so we have one carbon atom and four hydrogen atoms which has to add up to zero because methane is neutral in charge now each hydrogen atom has an oxidation state of one so therefore c plus four must equal zero therefore carbon has to have an oxidation state of negative four in methane now let's calculate the oxidation state of carbon in co2 so it's c plus two oxygen atoms which equals a net charge of zero and each oxygen has a charge of negative two so in this case carbon is going to have an oxidation state of positive four so carbon changes from negative four to positive four therefore the oxidation number of carbon is increasing which means carbon is oxidized or technically methane is oxidized now hydrogen doesn't change but oxygen changes from zero to negative two so therefore oxygen gas the oxidation number is decreasing so it's being reduced which means that methane is the reducing agent and oxygen gas it's being reduced which makes it the oxidizing agent so keep in mind even though the element carbon is being oxidized you still describe the whole substance methane as being oxidized because carbon is part of methane now the next thing that you need to be able to do is you need to take a look at a reaction and tell if it's an oxidation reduction reaction so consider this reaction aluminum metal reacts with copper chloride to produce aluminum chloride and copper metal is this reaction a redox reaction so this reaction is a single replacement reaction and if there's a transfer of electrons then it's going to be a redox reaction aluminum has an oxidation state of zero but in alcl3 it has an oxidation state of clustering so if the oxidation number changes then it's a redox reaction all single replacement reactions are redox reactions now if you don't want to look for the oxidation numbers there's a quick way to tell if something is going to be a redox reaction if you see a pure element on one side and then that element being part of a compound on the other side it's always going to be a redox reaction there's a transfer of electrons so let me give you two examples and i want you to determine if it's a redox reaction or not so both of these examples are synthesis reactions also known as combination reactions sometimes a combination reaction can be a redox reaction and sometimes it won't be now if you look at the first example there are no pure elements all we have is compounds when you see that chances are it's not a redox reaction so if you look at the second example we have a pure element zinc and then that same element is in a compound so the second example is a redox reaction and let's prove it so let's start with the first example oxygen has a negative two charge magnesium in magnesium oxide has a plus two charge hydrogen has a positive one charge now magnesium hydroxide is composed of mg plus two and two hydroxide ions but as you can see magnesium still has an oxidation state of positive two now let's focus on hydroxide hydroxide is oxide plus hydrogen if you want to solve it you know that hydrogen is going to have a positive one charge an oxygen is going to have a negative two charge when you add up negative two and one you're going to get the net charge of a hydroxyl ion which is negative one so in magnesium hydroxide oxygen is still negative two and hydrogen is plus one so notice that the oxidation state of magnesium did not change it's still positive two the oxidation state of hydrogen is still the same it's positive one and the oxidation state of oxygen has not changed if the oxidation state does not change that means there was no transfer of electrons and so this is not a redox reaction now let's analyze the second example zinc and chlorine has an oxidation state of zero in zinc chloride chlorine has a negative one oxidation state zinc has a positive two oxidation state so notice that zinc is being oxidized therefore there's a transfer of electrons which makes this reaction a redox reaction now consider these two decomposition reactions when mercury oxide is heated it decomposes into mercury and oxygen gas and if you were to heat calcium carbonate it will decompose into calcium oxide plus carbon dioxide gas so which of these decomposition reactions is the redox reaction looking at the first example we have oxygen as a pure element on the right side and then that same element is within a compound so the second example i mean the first example is a redox reaction in the second example there are no pure elements all we have are just compounds so the second example is not a redox reaction that's a quick way to tell if it's a redox or not so now i'm going to give you a list of reactions and determine which ones are redox and which ones are not so feel free to pause the video if you want to so the first example is it a redox reaction combustion reactions are always redox reactions here we have a pure element on one side and then that same element is inside a compound so the first example is a redox reaction what about the second example notice that there are no pure elements in this example so this is not a redox reaction acid-base neutralization reactions are not redox anytime you have a double replacement reaction is never going to be a redox reaction but all single replacement reactions will be a redox reaction the third example is a combination reaction and there are no pure elements so therefore this is not going to be a redox reaction the last one is a double replacement reaction specifically a precipitation reaction and as we can see all we have are just four compounds in this reaction there are no pure elements so it's not a redox reaction so to review all combustion reactions are redox reactions all single replacement reactions are redox reactions now synthesis and combination reactions sometimes the redox sometimes are not and the same is true for decomposition reactions double replacement reactions are never redox reactions so that includes acid-base reactions precipitation reactions and other types of double replacement reactions and a quick way to tell is if you see a pure element on one side and then the same element within a compound on the other side it's going to be a redox reaction that's the easiest way to tell so that's it for this video if you want to find more chemistry videos just check out my channel and you can find other videos on like physics calculus algebra trig precal and other stuff like that so thanks for watching you