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Salts - Concept

Teacher/Instructor Jacqueline Spivey
Jacqueline Spivey

Ph.D.,U.C.Santa Cruz
Teaching at a top-ranked high school in SF

She teaches general and chemistry at a top-ranked high school in San Francisco. Prior to that, she lead and published a number of research studies and lectured at SF State University.

Salts are ionic compounds whose cations comes from bases and whose anions come from an acids. They are formed through neutralization reactions in which the reactants are an acid and a base and the products are salt and water.

So this segment let's go ahead and talk about salts. So basicaly this is going to come up when you're thinking about acid base reactions. So remember that the most basic thought about an acid base reaction is that the driving force forward is the production of water. So here just very simply I've written out that when you have an acid and a base, what you get is a salt and water. So a salt meaning an ionic species, so something similar to table salt with which you are very familiar which is sodium chloride which when you put into a solution, in an aqueous solution, you get sodium ions and chloride ions. So when acid and a base forming salt and water is what's called a neutralization reaction.

So depending on the solubility of the salt, it may either remain in its ionic form. so in plus or cations or minus or anions or precipitate oput of solution. So you can also have the reverse of this reaction occuring so where the salt nd the water go backwards to form back the acid and the base. And so we call that a hydrolysis reaction. Hydro meaning water.

So let's do four different scenarios of the types of reactions that you normally encounter. And so the first one is kind of the easiest one is the reaction of a strong acid and a strong base. So remember that strong acids and strong bases are strong electrolytes and when something is an electrolyte it means that when you put it in an aqueous solution, it completely ionises to make its cationic states and its anionic states. Okay, so you don't have any of the actual acid or base left lying around. They're both completely dissociated. So here strong acid HCl aqueous, sodium hydroxide, our strong base aqueous, will go to form the salt sodium chloride and water. And so basically two, something to think about like going back in your mind is when you learned things about other chemical reactions. This is kind of an acid base displacement reaction. So here because you have a strong acid and a strong base, it's going to be completely neutral. so it will have a ph of seven. And then again your products will be salt and water.

So let's do another scenario where you can have a strong acid and a weak base. So here the strong acid I'll use again one of the most common, hydrochloric acid, HCl, and a common weak base ammonia. So I've put the unpaired electrons here on top of ammonia because that just reminds you that it's acting as a weak base which means that it can remove the protons from HCl to to make the ammonium, basically it makes ammonium chloride. So NH4Cl. Here I've written it in its ionic species just to kind of jog our memories. The NH4 is a plus and Cl is a minus. Okay? So you can also this will go further because remember the strong acid is going to completely dissociate but the weak base means that we'll have some of this ammonia lying around NH3 and some of this NH4Cl lying around. So then this ammonium ion can react with water to then recreate the weak base ammonia. And so since the strong acid is here, it's going to dominate. So your ph is going to be less than seven. So your since you have a strong acid your solution will still be fairly acidic, okay?

So then let's do a weak acid and a strong base. So the opposite basically of the previous scenario. So here we have hydrochlorous acid as our weak acid and sodium hydroxide as our strong base to form sodium hypochlorite and water. So remember again water being formed because it's an acid base reaction. Okay. So since we have the strong base here, that means our ph is going to be greater than seven. Greater than seven. So upwards of 14, right? Because its basic and since we have a strong base, that means that our ph is going to remain higher. And so here similar to the previous situation, when you have the strong base, that means the strong base is going to completely dissociate into its ionic species. But with the weak acid, you're still going to have some of the acidic species kind of lying around. So that means that here, your sodium hypochloride can react with water to reform your weak acid, your hydrochlorous acid.

Okay. And then the last scenario is when you have a weak acid and a weak base. And so in this case I have again hypochlorous acid plus ammonia, I shoved a lone unpaired electrons here to remind us that it's acting as a weak base ajnd so this can remove the proton from the hydrochlorous acid to form this ammonium chloride product and I didn't write it in its ionic species but this will be NH4 plus 1 and ClO minus 1. So here the ph situation is going to be a little bit different. So since you have both a weak acid and a weak base, the ph of your overall solution is going to depend on the relative strength of your reactants, okay.

So then that brings in this concept something called ka which is the acid dissociation constant. So the higher the ka, the likelier it is that one of these guys is going to give away its proton. So for this particular scenario, it would end up being basic because the ka of the ammonia is higher than that of the hydrochlorous a- the hypochlorous acid. Okay, so you would need to have an idea about what the ka's were for each of your reactants in order to know what the overall ph of this situation would turn out to be. And that is salts in a nutshell.