In this unit we will look at how the concept equilibrium is applied to acids and bases. Our objective is to look at two definitions of acids and bases. The Bronsted-Lowry definition and the Arrhenius definition and look at specific examples for each of them. Here we see examples acid found in foods. We have citric acid found in fruit. Lipoic acid found in Broccoli and Lactic acid found in yogurt. These are just a few of the examples where acid is present and food. We also see bases in many substances, as many cleaners contain a base. We also see bases such as sodium bicarbonate or calcium carbonate president tums, Rolaids or other antacid tablets. Acids and bases have additional properties beyond those that define them as an acid or base according to either definition. Acids tend to have a sour taste such as that seen in lemons or vinegar. They can dissolve many medals and they neutralize bases. A base tend to be bitter, slippery, and has the ability to neutralize acid. In fact the reaction between an acid-base is called a neutralization reaction because it produces a salt and water. Now the salt may have acidic or basic properties which we will explore and the next uni. But the acidity and basicity of that resulting salt is much less then that of the original acid or base solutions. Here we have four acids pictured. We see we have two binary acids meaning they contain two elements HF in HBr and we have to oxyacids because they are formed from the polyatomic ions of nitrite and nitrate. However, what we see in common in all four off of these acids is that they all have a hydrogen present that can be removed and it is this dissociation or ionization of this hydrogen ion that lends to the acidity at this molecule. Arrhenius acid is a substance that produces H+ in aqueous solution. Here we show HBr and we show that it completely dissociate or ionizes into the H+ and Br- ions. We represent this in the chemical reaction by showing a one direction arrow. This indicates that the reaction goes completely from reactants to products. We show that we have an H+ in solution but remember we're in aqueous solution and we're not going to see free H+ ions floating around. What we will see are H_3O+ ions, which means exactly the same as H+. So this the H_3O+ is what's truly present the solution. The H+ is just an abbreviation, but they mean exactly the same thing. Remembered that we also know that hydrogen has one proton and one electron. When it loses one electron to form H+, all that remains is a single proton; and so we also refer to this H+ as a proton. Here we have an Arrhenius Base that shows KOH dissociating into K+ and OH- ions. This also is a strong base and this is indicated by the presences of the one direction arrow rather than an equilibrium arrow. Now, we can look at the Bronsted-Lowry definitions of acids and bases. We have conjugate acid/base pairs that differ by a single H+. If I look at NH_3, which is acting as a base because it's acting as a proton acceptor I look at it partner the conjugate acid at that base and I see it is NH_4+ because it has accepted that proton. Now NH_4+ can behave as an acid because they can behave as a proton donor. Together the NH_3 and the NH_4+ behave as a conjugate acid base pair. When I look at the next of water, in this case, water is acting as an acid. It acting as a proton donor because it giving up a proton and becomes OH-. Water and OH- are now a conjugate acid-base pair. Water is also amphoteric and behaves as a base in some cases. It depends on who is partnered with because when we look at defining what an acid and what is its conjugate base and vice versa we're looking at it in the context of a chemical reaction. If we look at a reaction we can go through and define everything is either an acid or base. The first thing I want to do is look at my HF and see what substance is most similar to that on the product side. I see I go from HF to F-. That's going to be one to conjugate acid/base pair. Also I see that have H_2O going to H_3O+, and that will be my other conjugate acid/base pair. And so what I want to do is look at HF and F- and see how it's changing. As I that HF is donating a proton to something else, I'm not worried about where the proton is going, I just know that HF is donating a proton. We form F-, so HF is acting as an acid. F- is now willing to accept a proton. So it's acting as a base because they differ from one another by a single proton, a single H+ ion, they make a conjugate acid-base pair. I can look at the same thing when I look at water. Here water is accepting a proton going for H_2O to H_3O+ therefore water is acting as a base. When I look at H_3O+ plus, it's now willing to donate a proton. So it's behaving as an acid. And together H_2O and H_3O+ make a conjugate acid-base pair because they differ by a single proton. We can look at another reaction such as NH_3, ammonia, plus water in equilibrium with ammonium ion plus hydroxide ion. We can do the same thing that we did on the previous example. We want to identify the pair's, so I have NH_3 and NH_4+ and I have H_2O and OH-. When I look at water in this example I see that water is donating a proton and becoming OH-, therefore H_2O is acting as an acid in this case. OH- is willing to accept a proton. So it acting as a base and together H_2O and OH- make a conjugate acid/base pair. I can also look at NH_3 and NH_4+ plus. NH_3 is willing to accept a proton which makes it a base. NH_4+ is willing to donate a proton which makes it an acid and because NH_3 and NH_4+ differ by a single proton they make a conjugate acid/base pair. Now let's look at an example. What is the conjugate acid at HSO_4-. The answer is H_2SO_4. Because it asking for the conjugate acid of HSO_4- it implies that HSO_4 is acting as a base. If it's acting as a base then it is going to be a proton acceptor. If HSO_4- accepts a proton then it become H2SO_4. Now, we could also look at example were HSO_4- is acting as an asset. And when it acts as an asset it's going to act as a proton donor. When it donates a proton it will go to SO_4^2-. So we still have a conjugate acid/base pair HSO_4- and H2SO_4. As well as another conjugate acid/base pair from HSO_4 to SO_4^2-. The differences is, are we treating HSO_4- as a base or are we treating HSO_4- as an acid. Next we will look at pH and K_w. This K value is the same that we look at in the previous unit.
