Properties of Aqueous Solutions of Acids and Bases. CHAPTER 10 Acids, Bases and Salts. Properties of Aqueous Solutions of Acids and Bases

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1 CAPTER Acids, Bases and Salts Properties of Aqueous Solutions of Acids and Bases Strong and Weak Acids Acids are substances that generate in aqueous solutions. Strong acids ionize 0% in water. That is, the strong acids completely ionized (or almost completely ionize) to produce when in dilute aqueous solution. The ions produced in aqueous solutions produce acidic solutions. Acidic solutions have a p lower than Properties of Aqueous Solutions of Acids and Bases There are only seven strong acids. 1. Cl hydrochloric acid 2. Br hydrobromic acid. I hydroiodic acid 4. N nitric acid 5. Cl chloric acid 6. Cl 4 perchloric acid 7. 2 S 4 sulfuric acid Learn these strong acids! Properties of Aqueous Solutions of Acids and Bases An acid cannot be an acid unless it is in solution. It must be able to produce and that can happen only in solution. ften, and correctly so, acids are named with the symbol (aq) because acids are more commonly dissolved in water. 4 Properties of Aqueous Solutions of Acids and Bases Weak acids are soluble in water but ionize significantly less than 0% in water. Typically ionize % or less! All weak inorganic acids ionize reversibly or in equilibrium reactions. This is why they ionize less than 0%. Properties of Aqueous Solutions of Acids and Bases Aqueous acidic solutions have the following properties: 1. They have a sour taste. 2. They change the colors of many indicators. Acids turn blue litmus to red. Acids turn bromothymol blue from blue to yellow.. They react with metals to generate hydrogen, 2(g). 5 6

2 Properties of Aqueous Solutions of Acids and Bases 4. They react with metal oxides and hydroxides to form salts and water. 5. They react with salts of weaker acids to form the weaker acid and the salt of the stronger acid. 6. Acidic aqueous solutions conduct electricity. Properties of Aqueous Solutions of Acids and Bases Strong Bases, Insoluble Bases, and Weak Bases Characteristic of common inorganic bases is that they produce ions in solution. Strong bases ionize 0% in water. 7 8 Properties of Aqueous Solutions of Acids and Bases The Strong Soluble Bases Strong soluble bases are ionic metal hydroxides. Strong soluble bases dissolve in water to produce hydroxide ions ( ). The hydroxide ions produced in water creates basic solutions. The p of basic solutions are greater that 7. Properties of Aqueous Solutions of Acids and Bases There are only eight strong soluble bases. 1. Li lithium hydroxide 2. Na sodium hydroxide. K potassium hydroxide 4. Rb rubidium hydroxide 5. Cs cesium hydroxide 6. Ca() 2 calcium hydroxide 7. Sr() 2 strontium hydroxide 8. Ba() 2 barium hydroxide Learn these strong bases! 9 Properties of Aqueous Solutions of Acids and Bases Aqueous basic solutions have the following properties: 1. They have a bitter taste. 2. They have a slippery feeling.. They change the colors of many indicators Bases turn red litmus to blue. Bases turn bromothymol blue from yellow to blue. 4. They react with acids to form salts and water. 5. Aqueous basic solutions conduct electricity. The ydronium Ion (ydrated ydrogen Ion) The protons that are generated in acidbase reactions are not present in solution by themselves. Protons are surrounded by several water molecules. ow many varies from solution to solution. (aq) is really ( 2 ) n Where n is a small integer

3 The ydronium Ion (ydrated ydrogen Ion) Chemists normally write the hydrated hydrogen ion as and call it the hydronium ion. = The BrØnstedLowry Theory J.N. BrØnsted and T.M. Lowry developed this more general acidbase theory in 192. An acid is a proton donor ( ). A base is a proton acceptor. Two examples to illustrate this concept. Br 2 Br acid base N 2 N 4 base acid 1 14 The BrØnstedLowry Theory Acidbase reactions are the transfer of a proton from an acid to a base. Cl N N acid base 4 Cl The BrØnstedLowry Theory An important part of BrØnstedLowry acidbase theory is the idea of conjugate acidbase pairs. Two species that differ by a proton are called acidbase conjugate pairs. Note that coordinate covalent bonds are often made in these acidbase reactions The BrØnstedLowry Theory The BrØnstedLowry Theory For example we can use this reaction: N 2 N 1. Identify the reactant acid and base. 2. Find the species that differs from the acid by a proton, that is the conjugate base.. Find the species that differs from the base by a proton, that is the conjugate acid. N is the acid, conjugate base is N 2 is the base, conjugate acid is N 2 N 17 18

4 The BrØnstedLowry Theory The BrØnstedLowry Theory Find the species that differs from the base by a proton, that is the conjugate acid. N 2 N Reactant acid Reactant base Conjugate acid Conjugate base Acidbase reactions are the transfer of a proton from an acid to a base. Br 2 Br acid base 0 % Br 2 Br acid base base acid The BrØnstedLowry Theory The BrØnstedLowry Theory An important concept in BrØnstedLowry theory involves the relative strengths of acidbase pairs. Weak acids have strong conjugate bases. Weak bases have strong conjugate acids. The weaker the acid or base, the stronger the conjugate partner. The reason why a weak acid is weak is because the conjugate base is so strong it reforms the original acid. Similarly for weak bases The BrØnstedLowry Theory Weak acid and water:.... F : F weak weak strong strong acid base conjugate acid conjugate base (the donor) (the acceptor) The weak acid, F, produces a strong conjugate base a very good proton acceptor, F. The base, 2, produces a strong conjugate acid a very good proton donor,. The BrØnstedLowry Theory Weak base and water: N N ( aq) 2 ( l) 4 ( aq) ( aq) N 2 Small amount N 4 Large amount base acid acid base 2 24

5 The BrØnstedLowry Theory N 2 N4 Since N is a weak base, N 4 must be a strong acid. N 4 gives up to reform N. Compare that to Na Na (aq) (aq) Na must be a weak acid or it would recombine to form Na Remember Na ionizes 0%. Na is a strong base. The BrØnstedLowry Theory N : : N 4.. weak base weak acid conjugate base conjugate acid (the acceptor) (the donor) The weak base, N, produces a strong conjugate acid. The acid, 2, produces a strong conjugate base The BrØnstedLowry Theory Amines are weak bases that behave similarly to ammonia. The functional group for amines is an N 2 group attached to other organic groups. The BrØnstedLowry Theory Conjugate acidbase pairs are species that differ by a proton. Some examples: N 2 N4 C N2 2 CN The Autoionization of Water Water does not do this extensively. [ ] = [ ] 1.0 x 7 M Autoionization is the basis of the p scale. The Autoionization of Water Water can be either an acid or base in BronstedLowry theory. Consequently, water can react with itself. This reaction is called autoionization. ne water molecule acts as a base and the other as an acid. 2 base 1 2 acid 2 acid 1 base

6 Amphoterism ther species can behave as both acids and bases. Species that can behave as an acid or base are called amphoteric. Proton transfer reactions in which a species behaves as either an acid or base is called amphiprotic. For binary acids, acid strength increases with decreasing X bond strength. For example, the hydrohalic binary acids Bond strength has this periodic trend. F >> Cl > Br > I Acid strength has the reverse trend. F << Cl < Br < I 1 2 The same trend applies to the VIA hydrides. Their bond strength has this trend. 2 >> 2 S > 2 Se > 2 Te The acid strength is the reverse trend. 2 << 2 S < 2 Se < 2 Te The acid leveling effect masks the differences in acid strength of the hydrohalic acids. The strongest acid that can exist in water is. Acids that are stronger than merely react with water to produce. Consequently all strong soluble acids have the same strength in water. I 2 I essentially 0% 4 Br, which should be a weaker acid, has the same strength in water as I. Br 2 Br essentially 0% Acid strength differences for strong acids can only be distinguished in nonaqueous solutions like acetic acid, acetone, etc. The strongest acid that can exist in water is. Cl 2 Cl Cl is strong enough that it forces water to accept. 5 6

7 The strongest base that can exist in water is. N 2 2 N N 2 is strong enough to remove from water. Using our knowledge of BrØnstedLowry theory, it is possible to construct a relative ranking of acid and base strengths (and their conjugate partners.) The reason that stronger acids and bases cannot exist in water is that water is amphiprotic. 7 8 Ternary acids are hydroxides of nonmetals that produce in water. Consist of,, and a nonmetal. Cl 4 P 4 Cl P 9 40 Cl 4 P 4 Acidic protons Cl P Cl P Bonds which must break for these compounds to be acidic 41 42

8 It is a very common mistake for students to not realize that the s are attached to atoms in ternary acids. Just because chemists write them as Cl 4. Cl P Remember that for binary acids, acid strength increased with decreasing X bond strength. Ternary acids have the same periodic trend. Strong ternary acids have weaker bonds than weak ternary acids. For example, compare acid strengths: N 2 < N 2 S < 2 S 4 This implies that the bond strength is: N 2 > N 2 S > 2 S Ternary acid strength usually increases with: 1. an increasing number of atoms on the central atom and 2. an increasing oxidation state of central atom. Effectively, these are the same phenomenon. Every additional atom increases the oxidation state of the central atom by 2. For ternary acids having the same central atom: the highest oxidation state of the central atom is usually strongest acid. For example, look at the strength of the Cl ternary acids. Cl< Cl 2 < Cl < Cl 4 weakest strongest Cl oxidation states AcidBase Reactions in Aqueous Solutions There are four acidbase reaction combinations that are possible: 1. Strong acids strong bases 2. Weak acids strong bases. Strong acids weak bases 4. Weak acids weak bases Let us look at one example of each acidbase reaction. AcidBase Reactions in Aqueous Solutions 1. Strong acids strong bases forming soluble salts This is one example of several possibilities hydrobromic acid calcium hydroxide The molecular equation is: 2 Br (aq) Ca() 2(aq) CaBr 2(aq) 2 2 (l) 47 48

9 AcidBase Reactions in Aqueous Solutions The total ionic equation is: 2 (aq) 2Br (aq) Ca2 (aq) 2 (aq) Ca2 (aq) 2Br (aq) 2 2 (l) The net ionic equation is: 2 (aq) 2 (aq) 2 2 (l) or (aq) (aq) 2 (l) AcidBase Reactions in Aqueous Solutions 1. Strong acidsstrong bases forming insoluble salts There is only one reaction of this type: sulfuric acid barium hydroxide The molecular equation is: 2 S 4(aq) Ba() 2(aq) BaS 4(s) 2 2 (l) This net ionic equation is the same for all strong acid strong base reactions that form soluble salts AcidBase Reactions in Aqueous Solutions The total ionic equation is 2 (aq) S 4 2 (aq) Ba2 (aq) 2 (aq) BaS 4(s) 2 2 (l) The net ionic equation is: AcidBase Reactions in Aqueous Solutions 2. Weak acids strong bases forming soluble salts This is one example of many possibilities: nitrous acid sodium hydroxide The molecular equation is: N 2(aq) Na (aq) NaN 2(aq) 2 (l) 2 (aq) S 4 2 (aq) Ba2 (aq) 2 (aq) BaS 4(s) 2 2 (l) AcidBase Reactions in Aqueous Solutions The total ionic equation is: Reminder there are types of substances that are written as ionized in total and net ionic equations. 1. Strong acids 2. Strong bases. Strongly water soluble salts N 2(aq) Na (aq) (aq) Na (aq) N 2 (aq) 2 (l) The net ionic equation is: AcidBase Reactions in Aqueous Solutions. Strong acids weak bases forming soluble salts This is one example of many. nitric acid ammonia The molecular equation is: N (aq) N (aq) N 4 N (aq) N 2(aq) (aq) N 2 (aq) 2 (l) 5 54

10 AcidBase Reactions in Aqueous Solutions The total ionic equation is: (aq) N (aq) N (aq) N 4 (aq) N (aq) The net equation is: AcidBase Reactions in Aqueous Solutions 4. Weak acids weak bases forming soluble salts This is one example of many possibilities. acetic acid ammonia The molecular equation is: (aq) N (aq) N 4 (aq) C C (aq) N (aq) N 4 C C (aq) AcidBase Reactions in Aqueous Solutions The total ionic equation is: C C (aq) N (aq) N 4 (aq) C C (aq) The net ionic equation is: Acidic Salts and Basic Salts Acidic salts are formed by the reaction of polyprotic acids with less than the stoichiometric amount of base. For example, if sulfuric acid and sodium hydroxide are reacted in a 1:1 ratio. 2 S 4(aq) Na (aq) NaS 4(aq) 2 (l) The acidic salt sodium hydrogen sulfate is formed. C C (aq) N (aq) N 4 (aq) C C (aq) Acidic Salts and Basic Salts Acidic Salts and Basic Salts If sulfuric acid and sodium hydroxide are reacted in a 1:2 ratio. 2 S 4(aq) 2Na (aq) Na 2 S 4(aq) 2 2 (l) The normal salt sodium sulfate is formed. Similarly, basic salts are formed by the reaction of polyhydroxy bases with less than the stoichiometric amount of acid. If barium hydroxide and hydrochloric acid are reacted in a 1:1 ratio. Ba() 2(aq) Cl (aq) Ba()Cl (aq) 2 (l) The basic salt is formed

11 Acidic Salts and Basic Salts If the reaction is in a 1:2 ratio. Ba() 2(aq) 2Cl (aq) BaCl 2(aq) 2 2 (l) The normal salt is formed. Acidic Salts and Basic Salts Both acidic and basic salts can neutralize acids and bases. owever the resulting solutions are either acidic or basic because they form conjugate acids or bases. Another example of BrØnstedLowry theory. This is an important concept in understanding buffers Acidic Salts and Basic Salts An acidic salt neutralization example is: NaS 4(aq) Na (aq) Na 2 S 4 (aq) 2 (l) A basic salt neutralization example is: Ba()Cl (aq) Cl (aq) BaCl 2(aq) 2 (l) A Review of Strong Electrolytes This chapter details the equilibria of weak acids and bases. We must distinguish weak acids and bases from strong electrolytes. Weak acids and bases ionize or dissociate partially, much less than 0%. In this chapter we will see that it is often less than %! Strong electrolytes ionize or dissociate completely. Strong electrolytes approach 0% dissociation in aqueous solutions A Review of Strong Electrolytes Strong Water Soluble Acids (the seven strong acids) A Review of Strong Electrolytes Strong Water Soluble Bases (eight strong bases) N ( l) N 2 0% ( l) or 0% ( l) (aq) (aq) N N (aq) (aq) K Sr() 2 0% (s) K 2 0% 2(s) (aq) Sr 2 (aq) (aq) 2 (aq) 65 66

12 A Review of Strong Electrolytes The calculation of ion concentrations in solutions of strong electrolytes is easy. Example 1: Calculate the concentrations of ions in M nitric acid, N. N ( l) M 2( l ) 0% (aq) M N (aq) M A Review of Strong Electrolytes Example 2: Calculate the concentrations of ions in M strontium hydroxide, Sr() 2, solution. 2 2 Sr() 2(s) Sr(aq) 2 (aq) M M M 0.040M ( ) The Autoionization of Water Pure water ionizes very slightly. The concentration of the ionized water is less than onemillionth molar at room temperature. The Autoionization of Water We can write the autoionization of water as a dissociation reaction similar to those previously done in this chapter. 2( l) 2( l) (aq) (aq) Because the activity of pure water is 1, the equilibrium constant for this reaction is: [ ][ ] K = c The Autoionization of Water Experimental measurements have determined that the concentration of each ion is 1.0 x 7 M at 25 o C. Note that this is at 25 o C, not every temperature! We can determine the value of K c from this information. K = [ ][ ] 7 7 ( 1.0 x )( 1.0 x ) c = = 1.0 x 14 The Autoionization of Water This particular equilibrium constant is called the ionproduct for water and given the symbol K w. K w is one of the recurring expressions for the remainder of this chapter and future equilibrium chapters. [ ][ ] Kw = 14 = 1.0 x 71 72

13 The Autoionization of Water Example : Calculate the concentrations of and in M Cl. The Autoionization of Water Use the [ ] and K w to determine the [ ]. Cl 2 Cl M M 0.050M = Thus the M. The and K w will allow us to calculate [ ]. = = = M = 7 74 The Autoionization of Water The Autoionization of Water The increase in [ ] from Cl shifts the equilibrium and decreases the [ ]. Remember from Chapter 17, increasing the product concentration, in this case it is [ ], causes the equilibrium to shift to the reactant side. This will decrease the [ ] because it is a product! The [ ] from Cl is 0.050M The [ ] from K w is 2.0 M. 1 The overall[ ] = M M The Autoionization of Water Now that we know the [ ] we can calculate the [ ]. 14 K = [ ][] = 1 w Since[ ] M. [ ] 14 1 [ ] = [0.050 M ] [ ] 14 1 = [ ] = M The p and p scales A convenient way to express the acidity and basicity of a solution is the p and p scales. The p of an aqueous solution is defined as: p = log 77 78

14 The p and p scales In general, a lower case p before a symbol is read as the negative logarithm of the symbol. Thus we can write the following notations. p = log pag = log Ag and so forth for other quantities. The p and p scales If either the [ ] or [ ] is known, the p and p can be calculated. Example 4: Calculate the p of a solution in which the [ ] =0.00 M. p = log p = log p = 1.52 [ ] 2 (.0 ) The p and p scales Example 5: The p of a solution is What is the concentration of? = log[ ] log[ ] = [ ] = p = log[ ] [ ] = M The p and p scales A convenient relationship between p and p may be derived for all dilute aqueous solutions at 25 0 C. [ ][ ] = Taking the logarithm of both sides of this equation gives: log log = The p and p scales Multiplying both sides of this equation by 1 gives: [ ] ( log [ ]) log = Which can be rearranged to this form: p p = The p and p scales Remember these two expressions!! = p p =

15 The p and p scales The usual range for the p scale is 0 to 14. [ ] = 1.0 M to [ ] p = 0 14 = 1.0 M p = And for p the scale is also 0 to 14 but inverted from p. p = 0 has a p = 14 p = 14 has a p = 0 to 14 [ ] = 1.0 M up to [ ] p = = 1.0M p = 0 The p and p scales Example 6: Calculate the [ ], p, [ ], and p for a M N solution. Is N a weak or strong acid? What is the [ ]? 0% N 2 N M M 0.020M 2 = 2.0 M 2 p = log ( 2.0 M ) p = The p and p scales Example 6: Calculate the [ ], p, [ ], and p for a M N solution. N (aq) (aq) N (aq) K = = w = = = 5.0 p = log 5.0 = 12.0 p = = 1.7 [ ] = M 1 M The p and p scales To help develop familiarity with the p and p scale we can look at a series of solutions in which [ ] varies between 1.0 M and 1.0 x 14 M. [ ] [ ] p p 1.0 M 1.0 x 14 M x M 1.0 x 11 M x 7 M 1.0 x 7 M x 12 M 5.0 x M x 14 M 1.0 M The p and p scales Example 7: Calculate the number of and ions in one liter of pure water at 25 0 C mol? ions = 1.00L L 16? ions = ions There are an equivalent number of 2 ions ions 1mol An Introduction to s are used to quantitate solutions. It is a procedure that will allow us to go into the laboratory and collect the data that is necessary to perform calculations

16 In order to perform a acidbase neutralization titration, we need the following: An accurate method of measuring the amount of acid or base that is used during the neutralization process. A known volume of an acid or base that will be used to neutralize an unknown concentration of acid or base. A method of determining the exact amount (moles) of unknown acid or base that was neutralized during the procedure. The first criterion for titration is provided by using an instrument known as a buret. This graduated glass tube is very accurate and is usually marked in graduations of 0. ml. (There are variations on the value of increments of volume on the burets.) The second criterion for titration is provided by using a standardized acid or base to neutralize the unknown concentration of acid or base. Standardized means that we have used a very accurate method of determining the molarity of the acid or base that we will use to neutralize the unknown concentration of acid or base. Finally, an indicator will be added to our standard acid or base that will turn colors when it has neutralized a stoichiometric amount of the unknown concentration of acid or base. That is, when the exact number of ions or ions of the unknown acid or base have reacted with our standard, the color of the reaction solution will change Such a procedure could be acid/base titrations: Example: We want to determine the concentration of a sulfuric acid solution using a standardized sodium hydroxide solution. The reaction stoichiometry is: 2 S 4 (aq) 2 Na (aq) 2 2 (l) Na 2 S 4 (aq) We have: 1. A buret that holds a volume of 50.0 ml and is graduated in 0.01 ml increments. 2. An indicator that is colorless in acidic solutions but is pink in basic solutions.. A standardized solution of 1. M sodium hydroxide solution

17 The procedure: We fill the graduated cylinder to the 50.0 ml mark. Some burets are marked 0.0 ml to 50.0 ml from the top to the bottom of the cylinder, and others are marked 50.0 ml to 0.0 ml from the top to the bottom of the cylinder. We are using a cylinder which has 50.0 ml marked at the top of the cylinder and 0.0 ml at the bottom of the cylinder. We are using 1. M sodium hydroxide as our standard solution (the known concentration) We will use a few drops of phenolphthalein as our indicator. Phenolphthalein is colorless in acidic solutions but will turn very light pink when the acid has been neutralized We will take a 50.0 ml sample of our unknown concentration sulfuric acid and add (dropwise) the basic standard until our acid sample solution turns very light pink. Theoretically, the sample volume of the sulfuric acid will still be colorless at exactly to point in which the number of moles have been completely neutralized by the ions of our standard. When the exact number of ions in the acid have been neutralized by the of the standard base, we have reached the equivalence point. The equivalence point = (all) If we add more ions than are needed to exactly neutralize the ions in the sample acid, then the solution becomes basic and the color changes to pink. We want to add the sodium hydroxide until the solution is very, very slightly pink (the end point) and hopefully, the end point will equal the equivalence point End point = equivalence point when you have used only the numbers of ions necessary to neutralize the exact number of ions in the acid. If you titrate until the solution is very noticeably pink, you have a basic solution and have overtitrated the acid. This result will give you an incorrect concentration of your unknown acid. The concentration value of the unknown will be greater than true value of its concentration think about it. Results of titration: Data collected. After a few minutes of adding the sodium hydroxide, the 50.0 ml sample of sulfuric acid solution turns very slightly pink. The value on the buret is 2.8 ml. 1 2

18 Determination of concentration of sulfuric acid. ow many ml of sodium hydroxide was used? started: 50.0 ml 1. M Na solution finished: 2.8 ml 1. M Na solution used: 17.2 ml 1. M Na What is the concentration of the sulfuric acid? 2 S 4(aq) 2 Na (aq) 2 2 (l) Na 2 S 4(aq) 1. mol Na 1.00mol 2S LNa 1.00 L sol ' n 2.00 mol Na = mol of S neutralized Class Questions: We know that the numbers of moles of 2 S 4, determined from the titration, were in a 50.0 ml sample: 1. An acidic solution has a p = 1.2. What are the [ ], [ ], and the p of the solution? mol 2S4 M = LS soln ' mol S LS soln ' 2 4 = = M S soln ' Class Questions: 2. A barium hydroxide solution has a p = 1.0. What are the [ ], [ ], [Ba 2 ], and the p of the solution? Class Questions:. Calculate the [ ], p, [ ], and p for a M N solution. Is N a weak or strong acid? What is the [ ]? 7 8

19 Class Questions: 4. What is the p of a solution that has g of Na added to 500. ml of 2? Class Questions: 5. What is the p, the [ ], [ ], and [S 4 2 ] of a 0.00 sulfuric acid solution? 9 1 Class Questions: 6. A small pond has a p of 5.9. owever, the p of the pond must be at least 6.50 in order to stock the pond with fish. If the pond has 6.00 million liters of water, what mass of CaC must be added to change the p of the pond to 6.50? The molar mass of CaC is 0.1 g/mol. The neutralization reaction is CaC Ca 2 C Class Questions: 7. What volume of M Ba() 2 is required to neutralize ml of Cl with a p = 2.1? Class Questions: Class Questions: 8. It took 2.5 ml of Na with a p of 1.2 to completely neutralize a 0.0 ml sample of an unknown Cl solution. What is the molarity of the Cl solution? 9. A titration of a 50.0 ml sample of M Cl solution was preformed by using 6.0 ml of M Na. What is the p of the reaction solution?

20 End of Chapter Many medicines are deliberately made as conjugate acids or bases so that they become active ingredients after passage through the stomach. 115