Learn about Junior Chemistry, Balancing 4, in this comprehensive video by bannanaiscool.
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Rob Lederer: Now here is a double replacement reaction, now simply figure this out it will come here it comes by a tension Barium nitrate Ba (NO3)2 were it has a two positive charge and nitrate is negative one. We'll say that, that's the solution of barium nitrate being mixed with a solution of sodium sulphate which is Na2So4 So4 is two negative and Na is positive one so question could be these two chemicals are mixed together write the reaction and balance it. Well what are they going to form on the other side look at double replacement now. The metal or the cation is going to go with the other anion and the cation from here it goes with the cation from here and remember now I didn't mention this before would you got to write this cation first and the anion second always it's always positive before negative always. So in this double replacement reaction but barium can replace here It's going to go with the sulphate but it got to use the proper charges so what's the charge of Ba its two positive to check on your chart what's the charge of So4 its two negative so Ba2 positive So4 two negative makes for its simply BaSo4. Now what else is produced here sodium goes with the nitrate its Na positive No3 negative, so its NaNo3 write Na first because it's the positive ions so it's NaNo3 but what are the states of matter here. When you look this up on your chart that Ba2 positive with So4 two negative goes under that, that solid called which means that if the concentrations are sufficient this is going to precipitate out as a solid chemical and may be in class you are going to be with the mixed chemicals together and form precipitates. Well this is just the way of predicting when the precipitation occurs in ion. This is a solvent but nothing precipitates with nitrate and you know what nothing one precipitates with sodium ions either I mean generally in normal concentrations and so this one would be aq that should be there that's it be there so that's the solid that's aq. In my class if you get this whole thing right you forget that solid I mark the whole thing wrong it's tough but its vary now and look how do you balance it, Ba (No3) +Na2So4-->BaSo4+NaNo3 One Ba, One Ba we get the two, one nitrate that here and wouldn't be having there we have a one the nitrate one that's can guys have the talent use a lot to do that so look, two nitrate is here one nitrate here put that 2 in front but that means now lets go back 2 sodiums here what we got 2 sodiums that's all good so therefore look So4 is one here and one here we do not balance it one, one, one, two and that's how you do double replacement that's not so bad. I'm going to sneak another single replacement reaction here because this one is a little bit different than the other one and what catches my students all the time. So I thought that you know you could learn from the student if you take Iodine and reacted with the sodium bromine solution what you get is products and balance the equation of course. Well, here is the thing a lot of people I use to just taking this and substituting this Na goes up itself and hydrogen with Br no, don't do that because here is the thing you know what iodine which is I2 group 17 in periodic table when it born there is something it is a negative charge you go look at some negative one charge on your periodic table so it doesn't actually go with the bromine, which is also a negative but the sodium goes with iodine and the bromine goes off by itself. So in this case this thing is going to replace the negative ion and then go with the positive and actually it doesn't really bond sodium and iodide either when we get to the solutions you know which is on the other disk you'll be able to see that in solution lot of these compounds stay floating around and don't bond to each other at all just get way to let lets go in. But right now all the rules still apply the rule what we were doing I2 with NaBr so that Na goes with the I because that's a negative that's