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Chemistry: Acid-Base Reactions

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About this Lesson

  • Type: Video Tutorial
  • Length: 14:23
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 153 MB
  • Posted: 07/14/2009

This lesson is part of the following series:

Chemistry: Full Course (303 lessons, $198.00)
Chemistry: Reactions in Aqueous Solutions (10 lessons, $14.85)
Chem: Solutions: Precipitation, Acid-Base, & Redox (3 lessons, $4.95)

This lesson was selected from a broader, comprehensive course, Chemistry, taught by Professor Harman, Professor Yee, and Professor Sammakia. This course and others are available from Thinkwell, Inc. The full course can be found at http://www.thinkwell.com/student/product/chemistry. The full course covers atoms, molecules and ions, stoichiometry, reactions in aqueous solutions, gases, thermochemistry, Modern Atomic Theory, electron configurations, periodicity, chemical bonding, molecular geometry, bonding theory, oxidation-reduction reactions, condensed phases, solution properties, kinetics, acids and bases, organic reactions, thermodynamics, nuclear chemistry, metals, nonmetals, biochemistry, organic chemistry, and more.

Dean Harman is a professor of chemistry at the University of Virginia, where he has been honored with several teaching awards. He heads Harman Research Group, which specializes in the novel organic transformations made possible by electron-rich metal centers such as Os(II), RE(I), AND W(0). He holds a Ph.D. from Stanford University.

Gordon Yee is an associate professor of chemistry at Virginia Tech in Blacksburg, VA. He received his Ph.D. from Stanford University and completed postdoctoral work at DuPont. A widely published author, Professor Yee studies molecule-based magnetism.

Tarek Sammakia is a Professor of Chemistry at the University of Colorado at Boulder where he teaches organic chemistry to undergraduate and graduate students. He received his Ph.D. from Yale University and carried out postdoctoral research at Harvard University. He has received several national awards for his work in synthetic and mechanistic organic chemistry.

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great series
11/07/2012
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These series have been extremely helpful to me since I am a veteran returning from Iraq to go back to school, having been out of school for a while I have forgotten most of this stuff. I wish the VA covered instructional materials like this, it's difficult to go back to school when you've forgotten most of your high school education.More schools should incorporate learning programs such as these into their curriculum.

Nopic_orng
great series
11/07/2012
~ nadine6

These series have been extremely helpful to me since I am a veteran returning from Iraq to go back to school, having been out of school for a while I have forgotten most of this stuff. I wish the VA covered instructional materials like this, it's difficult to go back to school when you've forgotten most of your high school education.More schools should incorporate learning programs such as these into their curriculum.

2845 - Acid-Base Reactions
What does the word "acid" mean to you? That may invoke images of some positively evil substance that can dissolve everything known to man. But in fact many acids, if you think about it, are very common things to you that are no more harmful than water. Vinegar you put on your salad. Vinegar is acetic acid. Citric acid in lemon juice, ascorbic acid, vitamin C, we're surrounded by acids varying in strength and their ability to do damage.
And when I talk about base, what does that invoke? Base is chalky, milky, something that neutralizes acid perhaps you've learned. What we want to do in this unit is define more precisely what exactly chemists mean when they talk about acids or bases.
Let's start out with acids. The Arrhenius definition of an acid is any substance that, when dissolved in water, increases the concentration of the H+ ion. Sometimes we just refer to this as a proton, or a hydronium ion, H+ ion. What kinds of substances are we talking about? Well certainly good candidates for this are things that have very polarized bonds between hydrogen and other elements. Now remember the Periodic Table again. Where we're going to find strong polarization in bonds is when we have hydrogen bound to a very electronegative element, such as chloride, for instance. Hydrochloric acid, this is also referred to as muriatic acid, the acid that people dissolve scum off of swimming pools with, so a very hardcore acid here, a very strong acid. We refer to this as an acid because, when it's dissolved in water, it generates the H+ ion. It dissociates into ions just like ionic materials dissociate into ions in aqueous solutions. But in particular, because we've increased the H+ concentration, we refer to hydrochloric acid as an acid, hydrobromic acid, the same story, again a very electronegative element. Nitric acid, another common acid, very strong, capable of dissolving all kinds of things, nitric acid contains an OH bond that's very strongly polarized. So once again, the bonding here, the electrons are very close toward oxygen. It's relatively easy to break that apart into two stable ions, the H+ ion and the nitrate ion.
Just as an aside, nitric acid very often you'll see written as HNO[3], and I've written it this way just simply to remind you that there is an OH bond that we're actually breaking in this process. Also, just as an aside to clear this up, when I write H+, remember that ions are very tightly solvated by water molecules. They're hydrated in solution. So remember on a molecular level, at least, this H+ is going to be surrounded by water molecules. Sometimes people will write it as H+, sometimes they'll write it as H[3]O+. All of this refers to the same type of species, just a solvated proton.
Not all acids, as we mentioned before, are strong acids. An example of a weak acid is acetic acid, or vinegar. This is the molecule acetic acid. And once again we see this OH bond, a very polarized bond in which the H+ can fairly readily come off and ionize as H+. So down here I've written a dissociation reaction. Acetic acid goes to the acetate ion and H+. What makes this different than HCl is when acetic acid is in water it primarily exists in the form of acetic acid. There's only a very small amount of H+ that's been formed by putting acetic acid in water. Regardless of how much acetic acid we put in. So the big difference is the degree to which it ionizes. Hydrochloric acid, or hydrobromic acid, nitric acid fully dissociates in water. Acetic acid only marginally dissociates in water. And so we make a distinction then between those former acids, which are strong, and acetic acid, or citric acid, or ascorbic acid, which are weak acids. They don't fully dissociate when we put them in water.
Now what about base? A base we'll define--again the Arrhenius definition of a base is any substance that we put in water that will raise the concentration of hydroxide ion. Of course obvious examples of this would be salts that contain hydroxide, calcium hydroxide, lithium hydroxide, magnesium hydroxide such as in Milk of Magnesia. All of these things dissolve to liberate hydroxide ions so would certainly be categorized as bases. Like with the acids, it's possible to have a base that does not fully dissociate into hydroxide ions. Certain salts will do this, metallic salts. Barium hydroxide would be an example that, for the most part, doesn't dissolve very much, but generates a small amount of hydroxide.
But other types of substances can generate hydroxide ion as well, for instance ammonia. Ammonia we recognize in the scent of urine, especially in babies. Ammonia has a very strong pungent odor. Also in rotting fish, amines have the same type of scent. Ammonia acts as a base by reacting with water to generate the ammonium ion and hydroxide. Again, notice the key here is that we generated the hydroxide ion, so ammonia would be classified therefore as a base. Like acetic acid, ammonia is not a strong base. It's a weak base, and this reaction only takes place only to a small degree. When you dissolve ammonia in water, the primary thing that you have is ammonia, but you generate a small amount of hydroxide. The concentration of hydroxide increases, therefore we consider ammonia again as a weak base.
Likewise there are going to be substances that will generate H+ that don't contain H+ itself. Just like ammonia didn't contain hydroxide, something like for instance carbon dioxide, this material will react in water. In fact, in soda water this reaction is happening. It will react in water to generate a small amount of H+. So we would consider carbon dioxide, in a sense, an acid as well, a weak acid. This has a special term. This is referred to as a Lewis acid. And Lewis acids we'll see are things that do not contain an H+ in and of themselves. They're not proton donors in other words. But nonetheless they fit the Arrhenius definition of an acid, meaning when you dissolve them in water, they increase the concentration of H+.
Chemists have many different ways of detecting whether something is an acid or a base. And one of them is to use something called an indicator. I have in a beaker here something called the universal indicator, which in fact is a mixture of several different types of molecules capable of changing color as we adjust the concentration of H+ and the concentration of hydroxide. So we're starting out right now with a purple solution. Let's make a mental note. That purple solution tells us that we have an excess of hydroxide ion. I'm now going to add a small amount of acid to this solution to do an example of what acids and bases do best. In fact this is why we spend so much time talking about them. They undergo a neutralization reaction. H+ will react with hydroxide to give us water. And so your early notions of acids and bases being complimentary to each other is true. An H+ and hydroxide will come together again to give the neutral molecule water.
So let's do that. I'm going to add a small amount of H+ to the solution, and watch what happens to our color. Notice now we've gone to green. And that indicates to us that we pretty much have a neutral solution. We now go on to yellow. And as I add a little bit more acid, eventually this will turn red. And while I'm thinking of it, let me ask one of my assistants to get me a straw. Again, make a mental note now that this red color is indicating an excess of H+ concentration. And notice one thing, that I can continue to do this reaction all day long. I can add now some more hydroxide. That will react with the H+. It will give us a neutralization. And if I keep adding hydroxide, that will take us back to purple. So we kind of missed the--there's the green color, and we'll add a little bit more in there, and that will just get us back to this purple color. And we can just go back again. I'll add some acid now. That will take us back to the red indicating an excess of H+, a little more, back to purple indicating an excess of hydroxide. Let's see, for the fun of it, if we can neutralize this, try to get back to the green color, if that's going to stay there.
The reason I wanted a straw is because one of the things I do when I breathe is I exhale carbon dioxide. And we said that carbon dioxide is capable of acting as an acid. Well let's see if that's really true. So we're back to an excess of hydroxide. Watch what happens now when I blow into this. So what you're seeing is the carbon dioxide, coming out of my breath, is reacting with the water to generate H+. Remember we saw earlier. And that H+ neutralized the hydroxide. We knew that happened because the color changed from purple through green, and then finally to yellow, indicating now that we have an excess of hydroxide. And of course we can get that back once again just by adding a little bit more base to take us back to the purple color.
Now carbon dioxide is only one example of many weak acids. And we have lots of other weak acids here, so let's take a moment just to explore a couple of other common things. Let's start out with lemon juice. Lemon juice contains citric acid. It also contains ascorbic acid, vitamin C. So what do you expect is going to happen then when we add a little bit of this lemon juice? Notice that we have gone to red. Remember red indicates a surplus of H+. So lemon juice is acidic. And we know that also by the sour taste that we feel. That's another indication, although you don't want to use that in the lab certainly. But things that taste sour to us tell us that it is, in fact, acidic material. We've talked about vinegar, so let's do a little bit with vinegar. Add a little bit of vinegar here. Once again, if I'm telling you the truth it should be an acid. There we see it. Again vinegar is something that gives us a sour taste. That just confirms for us that vinegar is an acid.
I'm going to have to go to a Styrofoam cup now. Let's look at a couple of bases that we're familiar with. In this case bleach, and Drano also could be in this category, something that cleans your drains is, in fact, not an acid at all. It's a very strong base just like bleach is. Again you'll notice that instant color change to a purple color. So the sodium hypochlorite in bleach is reacting with the water to give us hydroxide.
And finally, let's look at something that's all too important to all of us, especially as we get older, and that is excess stomach acid. You've certainly heard about this on television. So let's use an indicator once again. In fact I'll do it in a beaker here so you can see it better. So let's suppose that that's your stomach and you've had just a little bit too much fish and chips. So I'll put just a touch of vinegar in there. And now our indicator tells us that indeed we have excess stomach acid. Of course in the stomach we also have HCl to help break down food, so there's going to be plenty of acid there. Milk of Magnesia is just magnesium hydroxide. Hydroxide again, something that liberates hydroxide is going to be a base. Pour this in. Mix this up. Eventually, there's the color change. We go from acidic to basic. So the Milk of Magnesia has neutralized our excess stomach acid and we feel much better.
Reactions in Acqueous Solutions
Reactions Involving Solutions
Acid-Base Reactions Page [1 of 3]

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