in today’s lesson we will be discussing precipitation reactions remember the title of our chapter aqueous solutions and all of the chemistry that occurs in water well one of our reactions is indeed the formation of a precipitate I wouldn’t mind just taking a moment and reminding us a little bit of the chemistry we learned from our first year together one of the patterns of change we studied in chemistry was called a double replacement or a double displacement double exchange whatever you’d like to call it double replacement the general formula of compound a B reacting with compound c d remember we have two ionic compounds two ionic salts if you will compound a b c d and we talked about outer and inner x’ the new products that form our compound a d and compound c be in a double displacement we have learned that there were three possible driving forces three observations or pieces of evidence that let you know a chemical change indeed took place one of them was making a water molecule very specific to an acid neutralizing a base making salt and water when an acid neutralizes a base water is the driving force a second observation we studied in first-year chemistry was the formation of a gas typically we saw that carbonates co 3 the polyatomic ion when they are placed into solution and start to bubble we see evidence of change in the formation of gas and the third driving force was the formation of a precipitate and we use something called a solubility table it was a chart given to us in chemistry it hangs above my door in my classroom where we hooked together the positive and negative ion and identify the insoluble product as the precipitate when we make those solid fine particles that fall out of solution two aqueous solutions when poured together and form a solid we call that a precipitate so one of the driving forces of a double replacement is the formation of a precipitate today’s lesson and the entire lesson number two in our note pack is related to the formation of precipitates so the pattern of change is very familiar to us compound a be reacting with compound c d keep in mind that these would be aqueous two aqueous solutions coming together one of our two products ends up to be solid and the other remains aqueous one of the two compounds in the solubility table allows us to determine which of the two compounds indeed is insoluble so a little bit of background walking into our precipitation reactions lesson when we take two aqueous solutions of ionic compounds and we pour them together in a solid forms that solid is known as the precipitate the solid forms when mixed solutions come together and form a precipitate so there’s just the definition the solid chunky particle that falls out of solution is called a precipitate if you’re not part of the solution you’re part of a precipitate the little saying that helps us remember in solution little adjective is aqueous out of solution the solid little s for solid let’s take a peaking at an example follow this demonstration our note pack will say let me just put on a new slide here let’s take aqueous solutions of sodium hydroxide an aqueous solution of sodium hydroxide and react that with iron 3 chloride iron 3 chloride so there is the beginning when we have an aqueous solution of let me make that full page we’ll go there so here we have NaOH a queue with fecl3 a queue now keeping in mind we have na going to CL and SE going to O H I always find it helpful especially as the beginning chemistry student to keep in mind the charges as we hook together and form new compounds because as we get new partners they hook together by charges do not carry the subscript with you but hook together by the new charge

so let’s put our products down below so I ran out of room over here we would form na going to CL and Fe going to O H remember that iron is a plus 3 charge so Fe o H taken 3 times will need some help in balancing we have three hydroxides three chlorines wanting to end up needing some coefficients where 3 1 3 1 balance is our equation I’m going to call this for the first time we’re going to hear a term this is called the molecular equation and again I’ll be writing this multiple times but the first exposure here a molecular equation shows the entire reaction let me just clean it up and kind of write it all together we have 3 units of NaOH aqueous reacting with one unit of iron 3 chloride aqueous forming 3 units of sodium chloride aqueous and iron 3 hydroxide which is our precipitate now how did I know that well a couple of different ways I recognized ordinary table salt know that it indeed dissolves in water just from experience but also I used my solubility table and I’ll show you a little trick here a little bit later on in our notes about not even needing that solubility table I’m going to end up remembering that hydroxides are typically insoluble so there is our driving force it is the solid precipitate that I always abbreviated PPT use your solubility table and verify that for me you hook together iron you’re gonna find hydroxide when you hook together and find that box in your solubility table it definitely says insoluble therefore it is the solid precipitate the driving force of our double displacement reaction now in this Molecular equation we have aqueous sodium hydroxide aqueous iron chloride forming aqueous sodium chloride but what we’ve learned is that these strong electrolytes in which all of these are strong electrolytes all water soluble salts are strong electrolytes this is a non electrolyte because it did not break apart it stayed together iron hydroxide or iron 3 hydroxide is a non electrolyte because it does not dissociate in water I want to turn this into something called a complete ionic equation in this complete ionic equation we’re going to rewrite in ionic form including all adjectives just as I had done up above but instead of writing NaOH put together an A and O H we understand that when that’s actually in aqueous form those ions are separate from one another they have dissociated so taking this first reactant sodium hydroxide and really showing what’s swimming around in that water we have three aqueous ions of sodium and three aqueous ions of hydroxide three aqueous ions na+ three aqueous ions o h- now do you notice any o h in a molecular equation is written in our ordinary fashion but in the ionic form we separate because it’s a strong electrolyte let’s do the same with iron three chloride we recognize it’s a strong electrolyte which now we understand from lesson 1 means that it’s in associated form the ions are separate from one another in solution we’d have an aqueous ion of iron +3 and we’d have three units of chloride ion in aqueous form Fe plus three aqueous and three units of chloride ion aqueous we’ve just separated all of the ions from the reactant side let’s do the same for the product side Fe o h3 remember is a non electrolyte it will stay together but sodium chloride is a strong electrolyte so it indeed separates we’d have on the product side three aqueous ions of sodium we’d have three aqueous ions of chloride notice I’m just distributing that coefficient counting three sodium’s and three chlorides however for our precipitate the driving force it stays together

it is solid a non electrolyte the complete ionic equation dissociates the strong and leaves together everything else boy you will hear me say that a thousand times break apart the strong leave together everyone else do you notice anything that appears the same on both sides of our equation let me illustrate what I mean the same simply means notice how on the left side I have three positive aqueous sodium ions that appear the same identical way on the product side I also notice that I have three aqueous ions of chloride appearing the left and again on the right the vocabulary term that expresses ions that do not chemically change through the course of our reactions as mere spectator ions they did not undergo a chemical they’re only in there watching the others participate now iron plus three three units of hydroxide come together to form our driving force precipitate called iron 3 hydroxide FeO H taken three times the spectators are the sodium ion and the chloride ion do you have your charges on your ions as I remember learning in lesson one I get marked down if aqueous ions are not representing with their correct charge let’s turn this into something now called a net ionic equation in a net ionic equation we get rid of the spectators and simply show the chemistry I want to know just wonder went the chemical change what’s left after we eliminate the spectators well the formation of our precipitate now friends even though hydroxide is written first by convention you’ll see that the positive ion is always placed in the first position we have an aqueous ion of iron +3 combining with three aqueous ions of hydroxide forming the solid precipitate our driving force of our double replacement reaction Fe o H they connect way ssin comes out balanced we have moved from and it was our first time of many practices to come writing out what I would call our very familiar a traditional form of equations Molecular equations might be a brand new word but we’re used to this format from first-year chemistry we did a double displacement followed by the identity of our precipitate once we’ve identified the precipitate in step 2 we broke apart all the strong electrolytes li up left together the solid precipitate since it is indeed a non electrolyte only the strong break apart we identified what we called spectator ion’s and eliminated them to show the net ionic equation in the formation of our precipitate the driving force is always the insoluble product using our solubility table so there’s our notes trying to catch that up in precipitation reactions sometimes you’ll hear a Greek word called metathesis now you really won’t hear that often I’ll probably just say it once you can put it into your note pack at least you’ve heard it but I tend to just call them precipitation or double replacements the metathesis is simply a Greek word to represent the term transpose which means the two positive ions exchanged places we predict the products but however to be certain we really should experiment and that just simply reminds us that even though we can go a goes to D and C goes to B’s outer enters in order to really predict you should see or or take a good observation by experimentation so we can predict the products by following a general pattern however as any chemist knows can only be certain by experimenting the anions and cations the negative and positive simply switch partners you should pause the video here pause it and try just writing out

complete molecular equations just as we did in first-year chemistry take the outers and inners and switch their places and when you have your answers ready to those three you try eight equations turn the video back on and simply check your work well let’s see how you did the first example had an aqueous solution of silver nitrate agno3 is aqueous you reacted that with an aqueous solution of potassium chloride not that it’s necessary but sometimes it’s helpful AG is a plus no.3 is a negative potassium chloride I don’t need those parentheses or I didn’t need to write those but they’re alright we will form AG CL positive 1 negative 1 and potassium nitrate k + o3 now I’m not done yet because I have to include the adjectives what is the identity of our driving force checking your solubility table did you notice AG CL is your insoluble product it then is correctly identified as a solid C the adjective going there a solid precipitate leaving in solution potassium nitrate this equation ends up to be balanced it did not ask us to do this but let’s add it on because we’re going to be asked eventually to identify the spectator ions remember those spectators they didn’t do a darn thing they’re just in there observing who are the ions that are not part of the solid precipitate that’s over here isn’t it these ions the potassium aqueous and the nitrate aqueous are the spectator ions those ions that were not involved in the formation of our solid precipitate are called spectators let’s see what you did with the equation 2 here we had zinc nitrate aqueous looks I messed that up tonight Oh start that over in my bed let me try that again Zn no.3 taken twice and now my aqueous sign it’s being placed into an aqueous solution of barium dichromate be a CR 2 O 7 aqueous not necessary but sometimes it’s just helpful to remind ourselves charges zinc carries a plus 2 nitrates a minus 1 barium is a plus 2 and the polyatomic dichromate is also a negative 2 we go back to switching partners when zinc hooks to dichromate ZN CR 2 O 7 and so we have a plus 2 and a negative 2 so we’re good with charges and barium will go to nitrate so we end up with ba and O 3 taken twice this one will involve some coefficients let’s see what we have one zinc looks fine two nitrates two nitrates one barium on barium ago this comes out balanced sorry they’re all ones good to check though how about the driving force is it zinc dichromate or barium nitrate if you properly identified your driving force as the dichromate put an adjective there of solid and therefore we now know this has to be aqueous spectator ions what are those ions that were not involved in the formation of our precipitate well that would be the aqueous barium ion and the aqueous nitrate ion even though there’s two units there I’ll just show that the spectator ions are identified as the barium ion and the nitrate ion so our equation came out balance with all coefficients of 1 we identified zinc dichromate with a solid an aqueous barium nitrate is the other product you had one more to try let’s see how it went

you had aqueous cadmium chloride CD CL 2 is an aqueous being placed into a solution of sodium sulfide na 2’s cadmium goes to sulfide based on charge you get CD s and we’ll end up with two units of sodium chloride how do we properly identify the driving force we have two aqueous solutions the driving forces of precipitate is that cadmium sulfide or sodium chloride will right off the bat you’re probably familiar sodium chloride is table salt we clearly know that’s water soluble so here is the solid the driving force of our equation cadmium sulfide you’ll correctly label that as your precipitate it’s adjective is a little less solid spectator ions well who is not involved in the formation of your precipitate spectator ions here are the aqueous sodium ion and the aqueous chloride ion so two spectators in our double replacement patterns I hope those went well for you good review in terms of remembering double displacements remember precipitation reactions really only happen if I can find a product one product has to be insoluble so if both of my products come out to be soluble really there is no driving force and no reaction would occur so if all of the ions stay in solution nothing has really happened in the AP curriculum you are expected to memorize the solubility rules the solubility rules are found on page 127 of your textbook I gave you a cell you table that’s probably the most handy tip to work with however if you’re planning on taking the AP exam for college credit or potential college credit you’re going to be expected to memorize the solubility rules you will not be given a table so in other words you’re gonna have to just know this is what you find on your in your chemistry book talking about solubility rules and I would like to just read through them with you alrighty rule number one all nitrates are soluble so put yourself a little note if you have a polyatomic ion whose last name is nitrate no.3 negative 1 it is never the driving force do not pick it it is always soluble not the driving force so right away if somebody’s last name is nitrate toss it out as your choice rule to all acetates are soluble if you see someone’s last name C 2 H 3 o 2 negative it is not the precipitate it is not the 1/2 circle as your driving force nitrates always soluble acetates are always soluble rule 3 alkali metals remember what alkali metals are they’re the metals found in column 1 a they’re the metals that make up lithium sodium potassium rubidium cesium do you see them in that first column of group 1a if your first name comes from a metal found in the first family it is not the driving force they are always soluble the polyatomic ion ammonium is always soluble so if the first name is a metal from column 1 or if the first name is ammonium polyatomic ion do not pick it they are always soluble toss that out as your choice rule for halides are soluble except for silver mercury and lead now what are halides please tell yourself in a note that’s the name given to group 7a now that we hear the name called halogens halogens are fluorine chlorine bromine and iodine and if they’re going to carry a negative charge they end with IDE so what we’re talking about here is fluoride chloride bromide and iodide

those are known as the halides group 7a and a generic name for all of those negative ions fluoride chloride bromine iodine so here’s what rule forcing most of the time fluoride chloride bromides and iodides are soluble do not pick them the only time they happen to fall out of solution and become your precipitate is when they’re hooked to a heavy metal silver mercury or lead silver fluoride that’s a precipitate silver chloride silver bromide silver iodide all would be the driving force however if I had sodium fluoride sodium chloride sodium bromide sodium iodide they would not be my driving force as sodium ions are always soluble so the moral of this story rule number 4 rule number 4 suggests fluorides chlorides bromides and iodides they’re worth checking on your solubility table rule 5 talks about the polyatomic ion sulfate polyatomic ion sulfate so4 negative 2 generally these guys are always soluble except there’s always exceptions if you see the first name of lead barium mercury or calcium lead barium mercury and calcium sulfates would be insoluble so if you see the sulfate check it out on the solubility table it might be your driving force this particular slide talked about those ions that are generally soluble let’s discuss in the second set of rules those particular ions that tend to be insoluble oops rule six sulfides sulfides s- two are generally insoluble except if they’re hooked to a first family because remember ammonium and alkali metals from rule number three well they’re always soluble aren’t they and the heavy metals of group two a calcium strontium and barium will be soluble carbonates are generally insoluble unless they’re hooked to a first family metal phosphate so remember let’s put these here carbonate highly suspicious if someone’s last name is carbonate check it out if someone’s last name is sulfide check it out same is true for phosphates generally that’s going to be the one you select and hydroxides very suspicious if the last name is hydroxide most likely it’s going to be your solid precipitate sulfides carbonates phosphates and hydroxides all fall under the mostly insoluble I have a little saying that I’d like you to write down with me you have a little room under your rule number nine rule number nine let’s put this here I’m gonna make a funny saying that has served students well through the years in helping them memorize the solubility table the term chops na lots of eyes chops na as silly as it sounds it goes a long way in helping you memorize the solubility rules the first word chops these tend to be the in soluble driving force those that stay together as non-electrolytes instead of dissociate chops stands for the following right this with me the see of the word chops are carbonates carbonates tend to be insoluble H in the word chops represents the term hydroxide carbonates have a/c hydroxides is the H the term for O are oxides oxides is the ion that comes from the element oxygen most often it’s insoluble P from the word chops comes from the word phosphate the polyatomic ion that tends to be insoluble and the S from the word chops

is sulfide carbonates hydroxides oxides phosphates and sulfides put together in a word to help me remember their names chops are insoluble most likely to be your driving force when given a choice how about the NA what might that stand for this part of our little word stands for always soluble the polyatomic ion nitrate tends to stay in solution nitrates if your last name is a nitrate you are a strong electrolyte you will not be the driving force the same is true for acetate if the last name is the polyatomic ion acetate do not pick it as your solid precipitate another one of our eighth alkali metals but those were the lithium sodium potassium cesium rubidium if your first name comes from the first column it is not your driving force it is a strong electrolyte and will completely dissolve acids dissolve in water by the total definition of what an acid is it must be water soluble remember how acids start with an H so H a for acid HCl hydrochloric h2so4 sulphuric we talked about those in lesson 1 acids are soluble the ammonium is another a isn’t it ammonium polyatomic ion that could be your first name ammonium Tsar always soluble and the last a in my chops na is always soluble do not pick any of these as the driving force on your product side of your double replacement chops na is a funny little saying but it has really helped students memorize the solubility rules and that’s your goal is to work towards memorizing them you will not be provided a solubility table on your AP exam using these solubility tables chops na we have some general you’ll remember those exceptions to the rules you can go back and highlight these if you’d like from rules one through nine and you’re expected to memorize what are those exceptions we talked about the halides halide this group 7a that would represent fluoride chloride bromide and iodide remember these are mostly soluble unless the hook to silver mercury or lead that’s a heavy metal that pulls them out of solution and forces them to precipitate remember sulfates had some general exceptions as well sulfates stay in solution unless the first name in front of the sulfate is either lead barium mercury or calcium they tend to fall out of solution and be your driving force those are highlighted up there in rule number four and rule number five so repeating in a different way I cannot stress enough how important it is to memorize the solubility rules because identifying the driving force identifying that solid precipitate really is key when we go to start writing our molecular and ionic equations when we write out molecular equations we have to identify correctly the driving force now let me make this slide I’ll work it with you I’m going to extend the page get rid of it for a minute we realize that there are three different categories of equations and we’ve written them already I kind of seated this part of the lesson the first type of equation is called a molecular equation very familiar from our last year when we would simply write out the whole formulas in an example I gave you for this first Molecular equation we have a compound of aqueous potassium chromate k2 cro4 in an aqueous form we’re going to place that into a solution of aqueous barium nitrate two aqueous solutions coming together now in a molecular equation we just simply write out the formulas potassium goes to nitrate barium goes to chromate remember to consider charges so when we hook together we’d get two units of potassium

nitrate and one unit of barium chromate so here’s our Molecular equation we need some adjectives over here we have to properly identify the driving force now I remember part of our chops naaah chops talked about insoluble nah talks about the soluble one of those solubility rules talked about alkali metals of which potassium is an alkali metal do you remember that being as part of the NA it’s always soluble and the end in chops na stood for nitrates once I spelled there are nitrates so here is two components of the second category in our little funny word nah nitrates and alkali metals are always soluble I now know with great certainty that that’s aqueous barium chromate must by elimination be a solid we’ve recognized the driving force as a solid precipitate of barium chromate and you could check your solubility table that’s fine I’m not taking it down from on top of the doorway but work to memorize barium chromate is your driving force now I’ll consider what we called the complete ionic equation in a complete ionic equation we show all the strong electrolytes in the dissociated form we break apart the strong remember that saying break apart the strong strong electrolytes leave together everyone else all aqueous salts are strong electrolytes all precipitates are non electrolytes leave them together break apart the aqueous leave together the solid and let’s do that very thing we have to break apart our first reactant out would come to aqueous ions of potassium please remember your charges two units of K plus one aqueous we’d have a chromate ion carries a negative two aqueous barium nitrate is strong so we dissociate it we break it apart separate the ions aqueous ion a barium and notice that we have two nitrate aqueous ions there is the dissociated form the dissociated form of the reactants dissociate means to separate the dissociated form let’s do the product side potassium nitrate is a strong electrolyte we break it apart dissociate or separate into the ionic aqueous ions we leave together the solid barium chromate I have completely separated all aqueous salts forming their ions I left together the barium chromate thus the non electrolyte or the solid driving force now in the net ionic equation the third type and really what our ultimate goal will be in writing these out the net ionic equation eliminates the spectators and simply shows the chemistry what is the chemical change going on in our equation so if we just practice identifying what are those ions that remained unchanged as reactants turn to products the potassium sure did and the nitrates did the aqueous ions of potassium and nitrate were not part of the solid precipitate so if we eliminate the spectators we come up with a net ionic equation and even though up here chromate is written first by convention you’ll always see the positive ion written first an aqueous ion of barium combines with an aqueous ion of chromate to form lose the arrow the solid

precipitate of barium chromate I’m going to write something I want you to write it on your page ions carry charges because that way I’ll know you heard me say a million times that without my charges on my aqueous ions and I mean each and every time if I don’t see those charges I have to mark it wrong there is a big difference between an atom and an ion in their chemical and physical properties we have to indicate that ions in an aqueous solution carry charges you should try it you should pause the video where it says you try it and write the three types of reactions for the for the following examples so I’m on the bottom of page five it’s the last two examples before our lesson will be final you have iron 3 sulfate and potassium sulfide three equations and then the final one led to nitrate with sulfuric acid right all three equations pause the video work ahead and when you’re ready to check your work turn the video back on let’s see how you did you’ve turned the video back on and now I we’re gonna check those out iron 3 sulfate feso4 it told me iron was a +3 sulfates a minus 2 so we had to crisscross fe2 so4 taken three times aqueous reacting with potassium sulfide potassium with its plus 1 sulfide with its minus 2 so you get k 2 s this should be second nature if you’re stumbling over this that’s a summer assignment I’m just go back and study how to write out chemical formulas it needs to be just like writing the alphabet for us at this point iron 3 hooks to sulfide we’re going to form a product Fe 2 s 3 and potassium goes to sulfate we get k2so4 let’s work to balance this equation first and then we’ll identify our precipitate 2 irons balanced on both sides we have 3 sulfates on the left 3 on the right to potassium so we’re going to need a 3 here 1 3 1 3 balances our equation here I notice an alkali metal part of our na always always soluble so since the first name is potassium it’s an alkali metal it is always aqueous here I have a sulfide and I notice in the word chops the S is sulfide sulfides are generally insoluble therefore I know this is my solid this is my driving force the precipitate we broke apart in ionic form to come up with what’s called d complete ionic equation two units of aqueous ion iron three and three units of the aqueous ion of sulfate – to react with two times three is six aqueous ions of potassium and carry that three through you get three aqueous ions of sulphide here’s the left side of our complete ionic equation here’s the solid leave it together Fe 2 s 3 a non electrolyte in solid form we only break apart the strong over on the other product three times two gives me six aqueous ions of potassium and three units of aqueous sulfate carrying a – to identify those spectator ions what remain unchanged from the left to the right your aqueous ions of potassium is a spectator and the aqueous ions of sulfate are a spectator when we eliminate the spectator we come up with

what’s called the net ionic equation what’s left we have two aqueous ions of iron three reacting with three aqueous ions of sulphide carrying a negative two they’re forming our solid precipitate Fe 2 s 3 here are all three equations required for the first now you try it example we’ve identified the molecular complete and net ionic equations let’s check that last one we’ll go a little bit quicker you had led to nitrate PB no3 taken twice we reacted that with sulfuric acid h2so4 acids are always aqueous PB goes to sulfate we get pbso4 and H goes to no.3 and will balance at the same time H and O 3 well a couple of things I noticed led the heavy metal lead when it’s attached to nitrate remember that’s part of the knob chops nah nitrates are always soluble also na acids acids are always soluble so right away the acids that I formed here are not my precipitate but sulfates especially when hooked to a heavy metals such as lead fall out a solution and our solid so the correctly identified solid precipitate is lead sulfate let’s break them apart into a complete ionic equation we’d have one unit of an aqueous ion of lead reacting with two aqueous ions of nitrate dissociated form of lead ii nitrate here’s hydrogen sulfate or sulfuric acid we get two ions of hydrogen aqueous and a sulfate ion aqueous forming keep together the solid it’s a non electrolyte leave it together break apart only the strong here we’d have two hydrogen ions and the two nitrates break apart the strong leave together the precipitate did you correctly identify hydrogen and nitrate those are your spectators they appear unchanged from left to right so eliminate those spectators and we’ll come up with our net ionic equation we have one aqueous ion of lead carries a +21 aqueous ion of sulphate – to forming our solid precipitate of lead – sulfate the net ionic equation we have practiced practiced so many important skills in precipitation reaction your end of lesson number two prepares you to do well for your assignment number two off your assignment sheet remember the criteria for credit write out the problem and write out your complete solution under the problem working down the page write the problem out first then your answer