Ch.15 - Acid and Base EquilibriumWorksheetSee all chapters
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Ch.1 - Intro to General Chemistry
Ch.2 - Atoms & Elements
Ch.3 - Chemical Reactions
BONUS: Lab Techniques and Procedures
BONUS: Mathematical Operations and Functions
Ch.4 - Chemical Quantities & Aqueous Reactions
Ch.5 - Gases
Ch.6 - Thermochemistry
Ch.7 - Quantum Mechanics
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Ch.10 - Molecular Shapes & Valence Bond Theory
Ch.11 - Liquids, Solids & Intermolecular Forces
Ch.12 - Solutions
Ch.13 - Chemical Kinetics
Ch.14 - Chemical Equilibrium
Ch.15 - Acid and Base Equilibrium
Ch.16 - Aqueous Equilibrium
Ch. 17 - Chemical Thermodynamics
Ch.18 - Electrochemistry
Ch.19 - Nuclear Chemistry
Ch.20 - Organic Chemistry
Ch.22 - Chemistry of the Nonmetals
Ch.23 - Transition Metals and Coordination Compounds

When an acid and base undergo a neutralization reaction they form ions, which can create acidic, basic or neutral solutions. 

Acid – Base Neutralization

Cations (positive ions) can create either acidic or neutral solutions. 

Concept #1: Transition Metal Cations and Acidity.

Transcript

Hey guys, in this brand new video, we're going to take a look at salts. What we should realize here is whenever we have an acid or a base neutralizing one another, we’re going to usually form water as well as a second product. We call this second product a salt. Let's think of a simple basic type of acid-base reaction. Let's think of hydrochloric acid, a very common strong acid. It reacts with sodium hydroxide, NaOH. Remember, our products here are going to be, remember HCl is made up of H positive and Cl negative. NaOH is made up of Na positive and OH minus. Opposites attract. This positive and this negative are now attracted together. Whenever you have an H positive and OH minus combining, you form water. The Na positive and the Cl minus will be attracted to one another and they give us NaCl.
We're going to say NaCl is our salt. It's basically the ionic product that results from the neutralization between an acid and a base. We’re going to say that this salt here, which is made up of Na positive and Cl minus, this salt can make our solution acidic, basic or neutral, depending on the rules that we're going to see in a few seconds from these two ions. That's the whole purpose of this section. Remember, we're going to have to learn that salts are just ionic compounds. They're made up of a positive ion called a cation and a negative ion called an anion. Based on certain properties, these ions could be acidic, basic or neutral. Let's take a look at it.
We're going to say here, if we’re taking a look at our cations, we're going to break down cations into basically three categories. We have our transition metals, we have our main group metals, and then we have our positive amines. Remember, on your periodic table, the transition metals are the ones that are found within the pit. Remember, our main group metals are just group 1A, 2A, 3A and 4A metals. Then our positive amines. Remember, an amine is a compound that has nitrogen and hydrogen, or carbon, hydrogen and nitrogen. Let's look at with the first section. The rule says if your transition metal has a charge of plus 2 or higher, it's going to be acidic. We’re going to say if the charge is less than plus 2, then it’s going to be neutral. Let's think of an example here. Let's say we have zinc chloride and then we have silver bromide. Remember, zinc is always plus 2. When this thing breaks up into its ions, these are the ions we’re going to create. Zinc is a transition metal. It meets the requirement. It meets the minimum of a plus 2 charge. Because it meets the minimum of a plus 2 charge, it's going to be acidic. Silver bromide, when it breaks up, it’s going to break up into Ag+ plus Br-. Technically, if you remember your solubility rules, silver bromide is insoluble. It shouldn’t breakup, but we're just saying theoretically if it did break up.
Here, it's a transition metal. It doesn't meet the requirement. It has to be plus 2 or higher. This is only plus 1 so instead of being acidic, it would be neutral. For each of these rules that we’re going to go over, if they meet the requirements of them, they could be acidic or basic. If they don't meet the requirement, that means they’re automatically going to be neutral.

If the transition metal within an ionic compound has a charge of +2 or higher then it creates an acidic solution. If it’s less than +2 then it creates a neutral solution and is classified as a non-significant ion. 

Concept #2: Main Group Metal Cations and Acidity.

Transcript

If we go to the next one, we're going to say if your main group metal has a charge of plus 3 or higher, then it's going to be acidic. If it doesn't meet that minimum requirement of plus 3, it's going to be neutral. If we have aluminum acetate, remember we’re only focused on the metals for right now. We’ll learn about what do we do with the negative ions in a few minutes. So we have aluminum acetate here and then we have barium oxide. Remember, this will break up into Al three positive and we know it's three positive because aluminum is in group 3A. Don't worry about acetate for now. We'll talk about how we look at that in a few minutes. Aluminum is a main group metal. It meets the minimum requirement of plus 3, so it's going to be acidic. Barium oxide. Barium is in group 2A so it’s plus 2. Oxide is in group 6A, so it’s minus 2. This right here is a main group metal but it doesn't meet the minimum requirement of plus 3. Because of that, it'll be neutral. It won't be acidic.
Remember, transition metals, the ones in the pit have to be plus 2 or higher. But main group metals have to be plus 3 or higher. If they meet the requirements, then they’re acidic. If they don't meet those requirements, then they're going to be neutral.

If the main group metal within an ionic compound has a charge of +3 or higher then it creates an acidic solution. If it’s less than +3 then it creates a neutral solution and is classified as a non-significant ion. 

Concept #3: Positive Amines and Acidity.

Transcript

Now, positive amines are acidic. Let's think of some examples here. We could have NH4BR. We could have CH3NH3Cl. We could have C6H5NH3NO2, all different samples. What you should realize here is you just have to think of it like this. You can say that this first compound could be an amine. But what's messing it up? That bromine is messing it up. That one element there signifies that we have a salt. You should know to break it up into its ions. This breaks up into NH4 positive. This breaks up into BR negative. We have a positive amine. Therefore, that amine is automatically acidic. Remember, just tell yourself it could be an amine, it could fit either this description or this description but there's something that's interfering with it. What's that one thing? It's usually going to be at the end and it's going to be a negative anion. Just break it up into its two ions. You’ll have your positive amine, you’ll have your negative ion, your anion. Look at the positive amine. It’s automatically going to be acidic. This too, this could be an amine. But what's messing it up? The chlorine at the end. So it’s a salt, so break it up. We know it has to be a plus 1 because the halogens are in group 7A and they’re minus 1. Look, we just isolated another positive amine. Therefore, it's acidic just like this one is acidic.
Finally, the last one. We could say that this portion here could be our amine. But what's messing it up? NO2. Remember, NO2 is our nitrite ion. All those polyatomic ions you guys had to learn earlier on, you still have to remember them. Here, this is going to be C6H5NH3 plus 1. This is going to be NO2 minus 1. Again, let's not worry about the negative ions just yet. What's important for right now is that we have a positive amine isolated, so it's going to be acidic. Remember, if we have a transition metal, it needs to be plus 2 or higher to be acidic. If it's not, it's neutral.
For main group metals, metals in groups 1A, 2A, 3A and 4A. They have to be plus 3 or higher or they're going to be neutral. For the main group metals, if you really think about it, since the main group metals have to be plus 3 or higher, that automatically means that group 1A which are always plus one and group 2A, which are always plus two will always be neutral. So they're not going to meet the requirement.
If you see a salt question on your exam and it has a group 1A or 2A metal involved, automatically that metal is going to be neutral because it's not going to meet the minimum requirement. Group 3A is plus 3 so if you see aluminum or gallium or something, those will be acidic. It gets a little tricky in group 4A. If you have a periodic table near you, just take a look at it. In group 4A we have tin, Sn, and then we have lead. They’re in group 4A but remember, I said this several videos ago that these two even though they’re main group metals, they can act like transition metals. They can be either plus 2 or plus 4. Be careful. Which one are they giving you? If they’re giving you Sn plus 2 or Pb plus 2, they’re main group metals. They don't meet the minimum requirement of plus 3, so they would be neutral. But if you have Sn plus 4 or Pb plus 4, then they wouldn’t meet the minimum requirements and they would be acidic. That one is a little tricky, so remember that.

Amines are compounds with only nitrogen or hydrogen (i,e. NH3) or with carbon, nitrogen and hydrogen (i,e. CH3NH2). Positive amines within compounds create acidic solutions. 

Anions (negative ions) can create either basic or neutral solutions. 

Concept #4: Anions and Basicity. 

Transcript

Now finally, we've talked about the anions, well, double positive ions I mean. Now, we're going to talk about the anions and negative ions and this one's a little bit trickier and it requires you guys to remember the rules that you learned in terms of what's a weak or strong acid so the rules that I taught you guys are still in play. So, here we're going to say, if you have a negative ion then add an H+ to it, if you create an acid then your negative ion is basic. So, let's take a look and figure out a good example of this, let's say, we have BrO- and we have Br-, two negative ions. So, to both of them, we're going to add an H+. Now, remember, if you add an H+ to it you're going to create an acid, you just have to ask yourself what type of acid am I creating first, and remember, we said this earlier, there are two types of acid, there are binary acids, which have no oxygen and then there oxyacids or oxoacids, which do have oxygen, the first one HBRO has hydrogen, a nonmetal and oxygen it's an oxyacid all we have to do now is use the rules that we know, we take the number of oxygens, which is 1 and we subtract by the number of hydrogen's, which is also 1, you need a minimum of 2 oxygens left to be strong, here you have nothing left. So, we classify this oxyacid as a weak oxyacid and we just said, if you create a weak acid then your negative ion will be basic. So, since, we have a weak oxyacid here, this negative ion is basic. Now, let's look at the other one HBR, HBR has hydrogen connected to an electronegative element on Br and it has no oxygen so this is a binary acid and we said earlier that there are only 3 types of binary acids that are strong, the three strong binary acids are HCl, HBR, HI and look, we have HBR here. So, this would be a strong binary acid, and remember, we just said you have to create a weak acid so that your negative ion is basic, here we just created a strong acid, so that means that this negative on would actually be neutral because it fails to meet the requirement, basic ions are a little bit more difficult because we have to add an H+ to it and see what kind of acid we create because this all goes into the theory of, if you have a weak species then its conjugate will be stronger than it, it won't be strong at the end of the day but it'll be stronger than where it came from. So, because we start from a weak oxyacid here, that means that its conjugate base would be more basic and be a stronger base, that's why BrO- is basic, here we have a strong acid so that means that its conjugate base would be extremely weak because remember, whatever you are your opposites going to be very, very different from you. So, because we have a strong binary acid, strong acid in that you create a weak base, weak acid means you create a stronger base, the base will be stronger, it'll still be relatively weak compared to other strong bases but it will still be stronger than the acid it came from, okay? So, just keep applying this rule, focus more on the anions because it's a little bit more difficult like I said, you have to add an H+ and then follow the rules that we learned about when it comes to identifying acids and bases, as long as you can remember that you'll be able to answer these types of questions.

To determine if an anion is basic or neutral you must add an H+ to it. If adding an H+ creates a weak acid then your negative ion is basic, but if adding an H+ creates a strong acid then your negative ion is neutral. 

Example #1: Determine if the following compound will create an acidic, basic or neutral solution: NaOCl

Transcript

Hey guys, in this new video, we're going to put to practice some of the concepts we learned about salts when it comes to acid-base neutralization. Let's take a look at this first example. We’re going to say here: Determine if each of the following compounds will create an acidic, basic or neutral solution. We're going to have to break them up into their ions. We're going to take a look at the positive ion and see if it’s acidic or neutral. Then we’re going to take a look at the negative ion and see if it’s basic or neutral. For the first one, we're going to say it's NaOCL. Remember, this is easy to break up. The metal always breaks off by itself. We know that Na is plus 1 because it's in group 1A. Then our negative ion would be OCl-. Remember, if we take a look at the positive ion, Na is a main group metal. Main group metals have to be plus 3 or higher to be acidic. Because this is only plus 1, it fails to meet the requirement, so it's neutral.
Now let's take a look at the negative ion. For this negative ion, we're going to have to add an H plus to it. We get HOCl. Every time you add an H to it, you're going to create an acid. This one has an oxygen in it, so we just created an oxyacid. Do the math. If you do one oxygen minus one hydrogen, you have nothing left. This would be a weak oxyacid. Remember, what does the rule state? It’s if you create a weak acid, then your negative ion will be basic. Since we just created a weak oxyacid, that means that this negative ion is basic.
We have a neutral ion. We have a basic ion. Neutral means we can ignore it. That means it doesn't affect the solution at all. We're going to look at the basic ion. The basic ion will make our solution basic. It's as simple as that. If you have a neutral and a basic, it gives you basic. If you have a neutral and acidic, it gives you acidic. If you have neutral and neutral, you're going to be neutral overall. Then in some rare cases, we can have acidic and basic. We’ll take a look on how exactly do we approach that later on down this page.

Example #2: Determine if the following compound will create an acidic, basic or neutral solution: PbCl4

Transcript

So for the next one, we have PbCl4. We have to break this up into its ions. Just realize here, you see this little 4? That came from the lead. It’s lead four positive plus chloride ion, which is Cl-. All you’re going to say now is this is a main group metal because it’s in group 4A and it's plus four. Main group metals have to be plus 3 or higher to be acidic. This one is definitely within the requirement so it's going to be acidic. This Cl- add in H+ to it, you create an acid. Since it doesn't have any oxygens, it's a binary acid. Remember, HCl is one of the three strong binary acids. We're going to say here if you create a strong acid, then the negative ion has to be neutral because it goes against the rule that we talked about. So this here would be neutral, so we can ignore it. We said earlier if you have an acidic and a neutral, you’re acidic overall.
This is the approach you have to take with each of these. I want you to take a look at practice 1. For this one, you need to do the same thing. You need to break those two compounds into their ionic forms then use the rules that we've gone over. Once you do that, you can figure out if they're acidic, basic or neutral. Good luck on this first one, guys!

Practice: Determine if each of the following compounds will create an acidic, basic or neutral solution.

a) LiC2H3O2   b) C6H5NH3Br

Practice: Determine if each of the following compounds will create an acidic, basic or neutral solution.

a) Co(HSO4)2           b) Sr(HSO3)2

Practice: Determine if each of the following compounds will create an acidic, basic or neutral solution.

Example #3: Determine whether each compound will become more soluble in an acidic solution.

a) NaBr                                                             b) LiCl                                                               c) KIO

Example #4: Determine the pH of a 0.50 M NH4Cl solution. The Kb of NH3 is 1.75 x 10-5

Practice: Determine the pH of a 0.55 M NaCN solution. The Ka of hydrocyanic acid, HCN, is 4.9 x 10-10.