Objectives: Understand how different definitions can explain the acidic / basic behavior of chemicals.
You’re probably already aware from what you’ve learned in other classes that acids and bases are all over the place. From the lemon you put in iced tea to the Drano put down the drain to keep your hair from clogging the drain, acids and bases are an important part of our lives. In this chapter, we’ll discuss what acids and bases are, as well as what they’re used for and how we can use them in chemistry.
What are acids and bases? Acids and bases are defined by several different models. Overall, solutions are said to be “acidic” if they have more acid molecules than base molecules, “basic” if they have more base molecules than acid, and “neutral” if they have equal quantities of both. However, unless we know what acids and bases are, these definitions are worthless. Let’s take a look:
Definitions: 1. Arrhenius Acids and Bases Acids: ionize to yield H+ in solution Arrhenius acids almost always start with the letter “H” in their formulas – this is the source of the H+ ion that comes off when you dissolve the compound. Common examples include the following: HNO3(l) H+(aq) + NO3(aq)-
Bases: ionize to yield OH- in solution Arrhenius bases always have “OH” in their formulas, indicating the presence of the hydroxide ion. Common examples include: NaOH(s) Na+1(aq) + OH-1(aq)
Definitions: 2. Bronsted-Lowry Acids and Bases Brønsted-Lowry acids are compounds that give H+ ions to other compounds. Brønsted-Lowry bases are the compounds that accept these H+ ions. Unlike with the Arrhenius definition of acids and bases, you can’t have a Brønsted-Lowry acid by itself – it needs a base to give its H+ ion to. As a result, we have come up with the idea of conjugate acid and base pairs.
Le Let’s take a look at the following acid-base process: HNO3 + H2O H3O+ + NO3-1 You can see that HNO3 has given a H+ ion to H2O. Thus, we can define HNO3 as the acid and H2O as the base. Likewise, if you look at the right side of the equation, H3O+ can give a H+ ion to NO3-1 – this allows us to define H3O+ as an acid and NO3-1 as a base. Looking a little deeper, you can see that HNO3 is an acid and that NO3-1, which is based on it, is a base. As a result, we say that “HNO3 is an acid, and NO3-1 is its conjugate base.” Likewise, we can say that “H2O is a base and H3O+1 is its conjugate acid.” Together, these are called “conjugate acid-base pairs.”
Definitions: Write equations to show Bronsted-Lowry hydrolysis reactions for each of the following species. CO32- (as a base) NH4+ (as an acid) HSO4- (as an acid OR a base) CO32- + H2O HCO3- + OH- NH4+ + H2O NH3 + H3O+ HSO4- + H2O H2SO4 + OH- HSO4- + H2O SO4-2 + H3O+
Definitions: Particles like HSO4-1 are called amphoteric. HSO4- (as an acid OR a base) What types of particles can be amphoteric??? HSO4- + H2O H2SO4 + OH- HSO4- + H2O SO4-2 + H3O+
Definitions: Particles like HSO4-1 are called amphoteric. What types of particles can be amphoteric??? 1. Water 2. Multi-hydrogen acids that have lost a hydrogen.
Definitions: Definitions: 1. Arrhenius Acids and Bases 2. Bronsted-Lowry Acids and Bases 3. Lewis Acids and Bases
Lewis acids and bases: Compounds that accept electron pairs are Lewis acids and compounds that donate electron pairs are Lewis bases. The reason this definition was invented is because the definition of acids and bases was formerly not very well served when speaking of a class of compounds called “amines” (of which ammonia is the one we’re currently most interested in). You see, when ammonia is dissolved in water, it behaves as a base despite the fact that it doesn’t have OH-1 ions to give away to anything.
This was explained by Gilbert Lewis in the following way: The electron pair on ammonia is donated to water and pulls of an H+ ion. Since ammonia donates an electron pair and water accepts it, ammonia is a Lewis base and water is a Lewis acid An important feature of Lewis acid-base theory: An Arrhenius or Brønsted-Lowry acid will always be a Lewis acid as well – all Lewis did was to increase the definition of “acid” and “base” to include amines while keeping the other acids and bases in the same categories.
Work on Bronsted-Lowry worksheet and finish for homework.