Chapter 19: Acids and Bases

Transcription

1 Chapter 19: Acids and Bases 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: 1) Arrhenius definition of acids and bases: Acids are compounds that give off H + ions (also called hydronium ions, H 3 O + ions, or simply protons ) when you dissolve them in water. Bases are compounds that give off OH - (hydroxide) ions when you dissolve them in water. Examples of Arrhenius acids and bases: 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: HNO 3(l) H + (aq) + NO 3(aq) - 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)

2 Properties of acids and bases: It s fairly easy to tell whether something is an acid or a base based on observable properties. Here are just a few properties put into a simple table that you can easily remember. Please, however, always remember that these properties are guidelines and aren t always going to be true for all acids and bases! Property Acid Base Neutral taste smell reactivity sour (lemon or vinegar) may burn nose with metals to form H 2 bitter (baking soda, Alka Seltzer) usually none (except NH 3 ) oils and organic compounds sweet, salty, bitter, no flavor none, or a chemical smell varies by compound conductivity in water in water some do texture of solution sticky slippery/soapy oily, sticky, watery, slippery

3 Indicators: As you might imagine, it s not a great idea to go around touching, smelling, or eating suspected acids and bases in the lab. As a result, we need a better way to determine whether something is an acid or a base. Indicators are chemical compounds that change color when the acidity of the solution changes. Historically, indicators have been naturally occurring pigments occurring in vegetables and flowers. More recently, indicators are manmade. These include: o Litmus: It turns red in acid and blue in base. o Phenolphthalein: Clear in acid and pink in base.

4 ph: Up until now, we ve talked about ways of telling whether a compound is acidic or basic. However, we haven t yet discussed how we can determine how acidic or basic it is. Because strongly acidic solutions have different properties than weakly acidic solutions, this is an important thing to be able to figure out. To figure out how acidic a solution is, we use the ph scale: Using the ph scale: Acids fall between ph = 0 and ph = 7. Bases fall between ph = 7 and ph =14 Neutral solutions have a ph of exactly (though people sometimes erroneously refer to solutions as neutral if they re between 6 and 8). more next page

5 Calculating ph: You ve probably learned at some point that ph is determined by the following equation: ph = -log[h + ] where [H + ] is the molarity of the H + ion in the solution. For strong acids, [H + ] is the same as the molarity of the acid solution. Sample: What s the ph of a M HCl solution? Answer: ph = -log[h + ]. Since HCl is an acid, we just use the molarity of HCl in this equation. Since log[0.0045] = 2.35, that s the ph of this solution! Though this makes sense for acids, you may also recall (from a few minutes ago) that bases (which have lots of OH - ions) also have ph values. Why is this? As it turns out, water behaves as both an acid and a base. We can see this from the following equilibrium: H 2 O H + + OH - K w = 1.00 x 10-7 As we know from our studies of equilibria, this means that whenever water is present, there will be some H + and some OH - ions present. That means that even if there is a very high concentration of OH - ions, there will still be a very tiny amount of H + ions present that we can use to find the ph.

6 In the real world, of course, this is simpler. Due to how the math works out (and it does work out, just trust me on this one), we can express the basicity of a solution using the equation: poh = - log[oh - ] which is the identical equation for finding ph, except that it s for a base. As a result, if we have a basic solution we don t need to find the concentration of the H + that s present instead we can just figure out the concentration of OH - ions by looking at how much base is present. Once we do this, we can use the following simple expression to figure out the ph of a basic solution: ph + poh = 14 Sample: What s the ph of a 3.41 x 10-4 M NaOH solution? Answer: Since NaOH is a base, we will have to first find the poh and then find ph from the poh. poh = -log[oh - ] = -log[3.41 x 10-4 M] = Since ph + poh = 14, we can find that: ph = 14 ph = 10.53

7 Titrations: How do we find the concentration of H + or OH - ions in the laboratory? We do this by doing a neutralization process called a titration. Here s the basic idea: Let s say that we have an acidic solution but don t know how much acid is dissolved in it. If we add a base to this solution, the acid will be slowly neutralized. At the point where the ph is equal to , the amount of base that we added will be equal to the amount of acid that was in the solution to start with. The equation that we use to describe titrations: M 1 V 1 = M 2 V 2, where M 1 is the molarity of the acid, V 1 is the volume of the acid, M 2 is the molarity of the base, and V 2 is the volume of the base. Example: If it takes 50 ml of M NaOH to neutralize 30 ml of an acid with unknown concentration, what was the concentration of this acid? o Using M 1 V 1 = M 2 V 2, we find that (x)(30 ml) = ( M)(50 ml); solving this, we find that x = M, which is the original concentration of our acid. When we do a titration, how can we tell if the solution is neutral? Indicators are chemical compounds that turn one color in an acid and turn a different color in base. When you do a titration, you look for the endpoint of the titration: This is when the color of the indicator changes and tells you that you re done. Common indicators you should know for the SOL: o Litmus: Red in acid, blue in base. o Phenolphthalein: Colorless in acid, pink in base.