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You need to figure out which of these chemicals will react with water to produce H3O(+) or OH(-) [NOTE: If you've never seen "H3O(+)" before, know that it is exactly the same as "H(+)"]. If either of these chemicals is formed, the pH of the water will deviate from 7. Only one of these chemicals does not react with water at all, and that one is the correct answer; the four other chemicals all react with the water once they're added in some way.
This question draws on many different principles that you simply need to know to be able to figure this out. Here is, briefly, an explaination of the principles under each. Some are very simple, while others are a little more complex.
(A) This one might be difficult to explain, so feel free to message me with questions if you don't understand. However, read the explaination for (C) first, because it works the same way, but allows me to elborate a little more.
When you add KNO3 to water, it becomes K(+) and NO3(-), as you already know. What you'd be looking for at this point is whether or not these ions react with water. If they did, they would react as follows:
K(+) + H2O <-> KOH + H30(+)
NO3(-) + H2O <-> HNO3 + OH(-)
However, if you know your strong acids and bases, you know that KOH is a very strong base, and HNO3 is quite a strong acid. What this means is they dissociate completely in solution: If you add HNO3 to water, there is absolutely no HNO3 left inside, because all of it breaks apart into H(+) and NO3(-). Same with KOH.
But those two reactions are trying to FORM these strong acids and bases in water. This simply cannot happen: As soon as some forms, it will break apart again immediately. This means that KNO3 does not react with water once its added, and yes, (A) is the correct answer to this question.
(B) Releasing CO2 into the atmosphere causes global warming, this is something many people have heard. However, it also causes the acidification of oceans by these simplified reactions:
CO2 + H2O -> H2CO3
H2CO3 + H2O -> HCO3(-) + H3O(+)
Note that we're producing H3O(+), which of course will make the water acidic. Carbon dioxide acidifies water, so it will cause the pH to change from 7.
(C) Like I said, this one works by the same principles as (A), but the end result is a little bit different. When added to water, NH4I breaks down into NH4(+) and I(-). Just like in (A), we well now attempt to react each of these ions with water to form acids and bases:
NH4(+) + H2O <-> NH3 + H3O(+)
I(-) + H2O <-> HI + OH(-)
And we check what we form. Again, if you know your strong acids, you know that HI is a very strong acid, and just like KOH and HNO3, it will dissociate completely in water. You cannot make it in water, because it will break apart again right away if you try.
However, NH3 is NOT a strong base, it is a weak one. When you add NH3 to water, only SOME of it becomes NH4(+), and MOST of it remains NH3. The same happens if you "add" NH4+ to water like we did: a chunk of it will convert back to NH3, and give the extra hydrogen atom to the water. This means the first of those two reactions DOES occur, and we are producing H3O(+) by adding this chemical to water. Therefore, the pH will be lower than 7.
(D) This one was conviniently already explained as part of (C). Ammonia is a weak base, so when you add ammonia to water, you get this reaction:
NH3 + H2O <-> NH4(+) + OH(-)
Which is producing hydroxide and therefore making the solution basic. This is simply a property of ammonia and almost all similar nitrogen compounds: They are weak bases.
(E) This compound kind of looks like ammonia, doesn't it? Instead of 3 hydrogens, one of the hydrogens is replaced by CH3, but otherwise it's the same. As it turns out, it reacts the same way:
NH2CH3 + H2O <-> NH3CH3(+) + OH(-)
Which again produces hydroxide and makes for a basic solution.
I hope I've been clear! Good luck!
Pye
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