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Acids and Bases Acids and Bases Acid and Base Strengths Lewis Acids and Bases Page [1 of 2] … to find a Brønsted-Lowry acid as something that donate a proton and a Brønsted-Lowry base as something that can accept a proton. Now let’s look at the most modern definition of acid and bases. It’s very general, but you’re going to see that there are analogies, and, one thing to remember is that all Brønsted-Lowry acids and bases also happen to fit into the new definition. And the new definition is due to G. N. Lewis, the same Lewis that gave us Lewis dot structures, and we call this most modern definition of acids and bases Lewis acids and bases. So let’s start by talking about a Brønsted-Lowry base, like ammonia. What can it do? Well, it can accept a proton, here’s the proton, to form ammonia. We can now look at the analogous reaction, where the ammonia reaction with something like . What’s ? Well, it’s trigonal planar boron trifluoride, and the boron, you’ll recall has only 6 electrons in its valent shell. And so, when we have a reaction with ammonia and , and you’ll just have to take my word for it that these react with each other, they’d form a species that looks like this. And we call the bond between nitrogen and boron a coordinate covalent bond, because the electrons come from just one of the partners. So, in contrast to, say, difluorine, where each atom brings and electron to form the bond, here all the electrons are coming from nitrogen. Now, this is mostly an historical idea. The coordinate covalent bond is an accounting method. And this bond is mostly like very other bond. In other words, it is the sharing of electrons between two atoms to form what we call a sigma bond. So let’s get to the definition. The definition of a Lewis acid something that accepts a pair of electrons. And so, here is accepting a pair of electrons. It’s getting this electron pair, or getting to share this electron pair on the ammonia, and a Lewis base is something that donates a pair of electrons. So the ammonia is donating its pair of electrons to the boron to share them to form this coordinate covalent bond. Now, it’s clear that ammonia and proton are also Lewis bases and Lewis acids, respectively. So ammonia here donates the lone pair to the proton, so the proton is accepting that lone pair, and so the proton is an acid in the Lewis sense, as well, and the ammonia is the Lewis base. Let’s look at some other examples of Lewis acid-base reactions. When we study equilibria, one of the equilibria that we look at is silver chloride, a sparingly soluble salt, dissolving to form silver cations and chloride anions in solution. And we can increase the solubility of silver chloride by adding ammonia to the water. What’s going on there? Well, when we add aqueous ammonia, what we’re adding is , and the lone pair on nitrogen can stick to the silver in this sort of form, donate a pair of electrons the silver. The silver, being a cation, is somewhat electron deficient, certainly compared to the neutral atom, so it’s looking for electron density. So think about it – silver wants electrons, it accepts an electron pair, so silver is acting as the Lewis acid. And then ammonia is still the Lewis base, and so we can think of these sorts of reactions as Lewis acid-base reactions. They’re clearly not Brønsted-Lowry acid-base reactions, but they are, in the most general sense, Lewis acid-base reactions, or acid-base reactions in the Lewis sense, to give these sort of complex ions. And we can still call the bond between nitrogen and silver a coordinate covalent bond. Well, what that has to do with making the solubility of silver chloride greater is that Le Chatelier’s Principle says if we decrease the concentration of silver plus, we’re going to shift the equilibrium to the right. And we affect that shift by adding ammonia, which complexes the silver cation that’s in solution to form this silver ammonia complex, and so decrease the concentration of free silver cations in aqueous solution. And so that draws more of the silver chloride into solution. So that’s another example. As a tribute to Professor Harmon, we’ll look at this example. This is osmium pentamine aquo, and here the osmium 2 plus – so even more charge on the central ion. And it actually coordinates 5 ammonias and 1 water. And still we have coordinate covalent bond, where the osmium is acting as the Lewis acid. It’s accepting electrons from the ammonia, which is the Lewis base. Now, I’ll pose a question. What would happen if we had an osmium 3 plus in the middle, instead of an osmium 2 plus? Would we expect that osmium 3 plus should be a stronger Lewis acid or a weaker Lewis acid? And if we think about Lewis acidity as being associated with wanting electrons, we’re saying that something that accepts a pair of electrons makes it a Lewis acid, then the more charge on the central ion should make this a stronger Lewis acid. In other words, it wants electron density more. And that’s exactly what happens. So osmium 3 plus is a stronger Lewis acid. It holds these ammonias more tightly than does the osmium 2 plus. Acid anhydrides of the main group, things like carbon dioxide, are also Lewis acids, and the carbon here is perfectly happy. It’s got its octet and it has 0 formal charge, but it still reacts with water to form this sort of species, where carbon is acting as the Lewis acid. The oxygen is acting as the Lewis base, so the oxygen in water is acting as the Lewis base, donating a pair of electrons to form this intermediate, which then rearranges to – what’s this? This is carbonic acid. So the reaction of carbon dioxide and water to form carbonic acid is another example of a Lewis acid- base reaction. Now, you’ll notice that carbonic acid is now a Brønsted-Lowry acid. This thing can give up a proton. Carbonic acid is the acid in soda, and this thing can donate a proton to water. So this is a Brønsted-Lowry acid, so Acids and Bases Acid and Base Strengths Lewis Acids and Bases Page [2 of 2] what we have here is a Lewis acid-base reaction, giving rise to the formation of a Brønsted-Lowry acid. And that idea is exactly what goes on in this product, baking powder. Now, you might have thought – you were in the kitchen one day and you didn’t have any baking powder, and you did have baking soda, and you thought, “Well, baking powder, baking soda, they both have ‘baking’ in the name. I’ll just substitute.” And there are some substitutions you can make, for instance, you can substitute honey for sugar. You can’t do it tablespoon for tablespoon, but there are conversions that allow you to substitute honey for sugar. Let me tell you, do not, without making some modifications, substitute baking soda for baking powder. And the reason is – what’s the role of baking soda when you’re baking? And the answer is it reacts with an acid in the solution, or in the mixture somewhere, to form carbon dioxide. And carbon dioxide is what inflates your bread or cake. So, in the case of bread that has yeast in it, the yeast eats the sugar and forms carbon dioxide, and what inflates your cake. But here we have a chemical way to make carbon dioxide to inflate you cake or bread. So, if you have buttermilk, for instance. Buttermilk has lactic acid in it. It reacts with the baking soda to make carbon dioxide, and that’s what inflates your buttermilk biscuits. So what’s in baking powder? Baking powder has two components in it: it’s baking soda plus something else, and that something else is some acid. Now, it’s not acetic acid, like vinegar, but it’s a solid acid, because baking powder is a powdery solid. There are no liquids in here. What do you need to do? Well, if you do want to make the substitution, what you have to do is, if you want to put baking soda in for baking powder, you have to make sure you add some acid, as well. You could add some lemon juice, or some vinegar or something like that, or buttermilk. But, in baking powder, the acid is a Lewis acid, and that Lewis acid is aluminum sulfate, or sodium aluminum sulfate. I’m going to do it for aluminum sulfate. The point is, it’s a source of aluminum 3 plus. So aluminum 3 plus is liberated when we take aluminum sulfate, which is ()3, and dissolve it in water to make aluminum 3 plus and sulfate anions. And aluminum 3 plus is a Lewis acid. Why is it a Lewis acid? It’s highly charged. And remember, things that are highly charged want more electron density. So they’re going to react with whatever Lewis bases are in the pot, and the Lewis base that’s in the pot is a bunch of water. Water has lone pairs.
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