Unit 4: Acids and Bases Topic A: Definitions of Acids and Bases and the Relative Strength of Acids and Bases

Transcription

1 1 Unit 4: Acids and Bases Topic A: Definitions of Acids and Bases and the Relative Strength of Acids and Bases In this topic we will examine: Various definitions of acids and bases BrønstedLowry definitions The meaning of ph (an introduction) Conjugate acidbase pairs Relative strength of BrønstedLowry acids and bases The levelling effect Hydrolysis of salts (part 1) A.1: Definitions of Acids and Bases Aqueous acids and bases may be defined in a number of ways. Some definitions are more inclusive than others. A.1.1: The Operational Definitions of Acids and Bases The operational definition of acids and bases defines these substances according to observed chemical and physical properties. According to this definition, acids: In contrast to this, bases: Taste sour. Example: citric acid which is in lemon juice Conduct electric currents. That is, they are good electrolytes. Cause certain dyes to change colour in a specific way. Example: they cause blue litmus to turn red. Produce H 2(g) when reacted with certain metals. Example: the reaction between aqueous HCl and solid Zn. React with bases in such a way that all the above properties are lost, except for the ability to conduct an electric current. Taste bitter. Feel slippery. Example: soap. Conduct electric currents. That is, they are good electrolytes. Cause certain dyes to change colour in a specific way. Example: they cause red litmus to turn blue. React with acids in such a way that all the above properties are lost, except for the ability to conduct an electric current. While these definitions give a useful method of identifying acids and bases, they do not provide us with an explanation of their behaviour.

2 2 A.1.2: The Arrhenius Definitions of Acids and Bases Svante Arrhenius defined acids and bases (and salts) on the molecular level. According to the Arrhenius definitions: Acids release H + ions in aqueous solutions. Bases release OH ions in aqueous solutions. An example of an Arrhenius acid is hydrochloric acid, HCl: HCl (aq) H + (aq) + Cl (aq) An example of an Arrhenius base is sodium hydroxide, NaOH: NaOH (aq) Na + (aq) + OH (aq) The Arrhenius definitions of acids and bases are able to explain why a reaction between them causes them to lose their physical properties (except for the ability to conduct an electric current). Consider the reaction between hydrochloric acid and sodium hydroxide: HCl (aq) + NaOH (aq) NaCl (aq) + H 2 O (l) The result of the reaction is an aqueous solution of sodium chloride. Because such a solution contains a dissolved ionic solid (or salt), it is able to conduct and electric current. In general, by the Arrhenius definitions: ACID + BASE SALT + WATER Because neither salts nor water fall under the Arrhenius definition of acids or bases, they are classified as neutral substances. As a result, the reaction between an acid and a base is often referred to as a neutralization reaction. Another example is the reaction between hydrofluoric acid and barium hydroxide: 2HF (aq) + Ba(OH) 2(aq) BaF 2(aq) + 2H 2 O (l) acid + base salt + water In very general terms, the Arrhenius definitions define any compound having a chemical formula starting with one or more hydrogens as an acid (e.g. HF, H 2 SO 4, etc.). Similarly, any compound having a chemical formula ending with one more hydroxides is a base (e.g. LiOH, Ba(OH) 2, etc.). The Arrhenius definitions also tell us that any ionic compound made up of ions other than H + or OH ions is neither an acid nor a base, but a salt. Eliminating the spectator ions in either of the neutralization reactions above gives the following net ionic equation: H + (aq) + OH (aq) H 2 O (l)

3 3 While Arrhenius definitions help to explain some of the chemical behaviour of acids and bases, it does not explain why solutions of a large number of compounds (such as NH 3 and NaF) are able to partially neutralize acids such as HCl. Similarly, the Arrhenius definitions are unable to explain why some compounds (such as NH 4 NO 3 ) are able to partially neutralize bases such as NaOH. Hebden Reference: Section IV.1 Practice Exercises: Complete the following exercises from the Hebden text: Pages 110 to 112 Exercises 1 to 3. A.1.2.1: Some Common Acids and Bases In Section IV.2, Hebden outlines some common acids and bases, their uses, and some alternate names for them. These include: sulphuric acid (battery acid) hydrochloric acid (muriatic acid) nitric acid acetic acid (ethanoic acid, vinegar) sodium hydroxide (caustic soda, lye) potassium hydroxide (caustic potash) aqueous ammonia (a common household cleaner) You will need to be familiar with the acids and bases listed above and some of their uses. In addition to those above, you also need to be familiar with: carbonic acid (carbonated water) citric acid (found in oranges, lemons, etc.) ascorbic acid (vitamin C) acetylsalicylic acid (A.S.A; found in pain relievers) Perhaps unknown to Hebden is that it is actually possible to obtain sulphuric acid in concentrations greater than 100%! SO 3(g) will dissolve in 98% H 2 SO 4 solutions. When added to water, the dissolved SO 3(g) combines with water: SO 3(g) + H 2 O (l) H 2 SO 4(aq) Concentrated H 2 SO 4 that has SO 3(g) dissolved into it is called oleum. If time permits, I will tell you why some frozen french fries are a good source of vitamin C.

4 4 In this section, Hebden discusses the fact that NaOH (and KOH) rapidly absorb water from the air. He uses the term deliquescent to describe such substances. A better term for this property of some substances is hygroscopic. You need to know this term. Notice that acetic acid (CH 3 COOH (aq) ) does not fit in with the Arrhenius definition of an acid; its chemical formula does not start with an H. While commonly considered to be an acid, acetic acid solutions are rather poor electrolytes. This too, goes against the definition of an acid as we have described thus far. On the other hand, aqueous ammonia (NH 3(aq) ) does not fit in with the Arrhenius definition of a base. Ammonia solutions may be quite basic, yet the chemical formula for ammonia does not end with an OH. As well, it is a poor electrolyte, meaning that the concentration of dissolved ions is rather low. In the next section, we will examine a different way of defining acids and bases. This method of defining acids and bases is much more inclusive and explains the acidproperties of substances such as acetic acid and the base properties of substances such as ammonia. Hebden Reference: Section IV.2 Practice Exercises: Complete the following exercises from the Hebden text: Page 114 Exercises 5 to 9. A.1.3: The BrønstedLowry Definitions of Acids and Bases Johannes Brønsted and Thomas Lowry simultaneously presented a more general method of defining acids and bases. A major flaw of Arrhenius definitions is that H + ions do not independently exist in aqueous solutions. Once released into a solution, they bond with other molecules that are present. Very often these are water molecules. For example, in a solution of HCl: HCl (aq) + H 2 O (l) H 3 O + (aq) + Cl (aq) (i) H 3 O + is called the HYDRONIUM ion, or a hydrated proton. Production of H 3 O + is the result of transferring a proton (H + ) from the HCl to H 2 O molecules. H + (aq) + H 2 O (l) H 3 O + (aq) In equation (i), HCl is said to be donating a proton to another chemical substance.

5 5 According to the BrønstedLowry Definition an acid is any substance that donates a proton in an aqueous solution. According to the BrønstedLowry Definition, a base is any substance that accepts a proton in aqueous solution. In equation (i), H 2 O is acting as a BrønstedLowry (BL) base. The BL definitions extend to a number of substances that the Arrhenius definitions do not define as either an acid or a base. For example, aqueous ammonia is a BL base because NH 3 accepts protons from water molecules: NH 3(aq) + H 2 O (l) NH 4 + (aq) + OH (aq) Notice that in equation (i), H 2 O acts as a BL base, while in equation (ii) above, it is acting as a (BL) acid. Substances that can act as either a BL acid or a BL base (depending on the conditions of the solution) are said to be AMPHIPROTIC. As we will see, many substances are amphiprotic. With the exception of H 2 O, all other amphiprotic substances that we will encounter are anions containing a hydrogen atom. For example, (ii) Some amphiprotic substances: HPO 4 2, HS, HCO 3, etc. To attract a positive proton, a negative charge is required. Water, being extremely polar, does not need a net negative electric charge. Of course, a substance cannot donate protons if it does not contain hydrogen. Comparing equations (i) and (ii), you may have noticed that different kinds of arrow were used. This is due to the fact that many aqueous solutions of acids and bases form equilibrium systems. Some do not. We will address this later. Looking at the reverse reaction in equation (ii), we see that in the reverse reaction a proton is being donated to the hydroxide ion by the ammonium ion, NH 4 + (aq) + OH (aq) NH 3aq) + H 2 O (l) acid + base base + acid In EVERY BrønstedLowry acidbase reaction, an acid and a base react to form another acid and another base. We can write two general equations for the reaction between a BL acid and a BL base. Either: or, ACID + BASE ACID + BASE ACID + BASE ACID + BASE

6 6 The type of arrow used depends on whether the reaction goes to completion or establishes a state of ionic equilibrium. This is determined by the nature of the acids and bases involved. We will study this indepth later. A.1.3.1: A Different Definition of Neutral Under the BL definitions of acids and bases, neutral is defined differently than under the Arrhenius definitions. Definition of Neutral: Any aqueous solution in which [H 3 O + ] = [OH ] is defined to be NEUTRAL. In pure water at any temperature, [H 3 O + ] = [OH ]. Thus, pure water is ALWAYS neutral. As we will see, the role temperature plays in determining the [H 3 O + ] and [OH ] in pure water is very important. With a different definition of neutral comes a new definition of acidic and basic. Definitions of Acidic and Basic: Any solution in which [H 3 O + ] > [OH ] is an ACIDIC solution. Any solution in which [H 3 O + ] < [OH ] is a BASIC solution. A.1.3.2: The ph Scale An Introduction Whether a solution is acidic, neutral, or basic (a.k.a. alkaline) is often indicated by a measure of its ph. For now, all that you need to know is that ph is a measure of the [H 3 O + ] in a solution. Its exact mathematical definition does not concern us just yet, but it will become a major part of this course in due time. Typically, measures of ph range from ph = 0 (very acidic) to ph = 14 (very basic). Basic solutions are sometimes said to be alkaline.

7 7 At 25 C, and ONLY at 25 C, a ph of 7.0 is considered neutral ([H 3 O + ] = [OH ]). For the purposes of this course, you may always assume a temperature of 25 C unless told otherwise. Hebden Reference: Sections IV.3 and IV.4. Practice Exercises: Complete the following exercises from the Hebden text: Pages 115 to 119 Exercises 10 to 14. Provincial Exam Examples: Definitions of Acids and Bases 1. Which of the following is a general characteristic of Arrhenius acids? A. They produce H + in solution. C. They accept an H + from water. B. They turn bromthymol blue to a blue colour. D. They react with H 3 O + ions to produce H Which of the following is generally true of acids, but not for bases? A. ph 7 C. release H + in solution B. conduct current when in solution D. cause indicators to change color. 3. Which of the following is a property of all acidic solutions at 25 C? A. They have a ph less than 7.0. B. They have a ph greater than 7.0. C. They cause phenolphthalein to turn pink. D. They release hydrogen when placed on copper metal. 4. A substance that produces hydroxide ions in solution is a definition of which of the following? A. an Arrhenius acid C. a BrønstedLowry acid B. an Arrhenius base D. a BrønstedLowry base. 5. Which of the following represents the complete neutralization of H 3 PO 4 by NaOH? A. H 3 PO 4 + NaOH NaH 2 PO 4 + H 2 O B. H 3 PO 4 + 3NaOH Na 3 PO 4 + 3H 2 O C. H 3 PO 4 + 2NaOH Na 2 HPO 4 + 2H 2 O D. H 3 PO 4 + NaOH NaH + HPO 4 + H 2 O

8 8 6. Which of the following represents the neutralization reaction between Ca(OH) 2(s) and HCl (aq)? A. H 2 O (l) H + (aq) + OH (aq) B. Ca 2+ (aq) + 2Cl (aq) CaCl 2(aq) C. Ca(OH) 2(s) + 2HCl (aq) CaCl 2(aq) + 2H 2 O (l) D. Ca 2+ (aq) + 2OH (aq) + 2H + (aq) + 2Cl (aq) CaCl 2(aq) + 2H 2 O (l) 7. Which of the following household products could have a ph = 12.0? A. soda pop C. lemon juice B. tap water D. oven cleaner 8. Identify the common acid in the stomach. A. nitric acid C. perchloric acid B. sulphuric acid D. hydrochloric acid 9. Which of the following is a typical ph value for dishwashing solutions? A. 2.0 C B. 4.0 D A BrønstedLowry acid is defined as a substance that A. releases H + (aq). C. accepts a proton. B. releases OH (aq). D. donates a proton. 11. What is a general characteristic of all BrønstedLowry bases? A. They all accept H +. B. They all accept OH. C. They will all turn litmus a pink color. D. They will all react with acids to produce H 2 gas. 12. Which of the following represents the reaction of H 2 PO 4 acting as an acid? A. H 2 PO 4 + H 2 O H 3 PO 4 + OH B. H 2 PO 4 + H 2 O H 3 O + + H 3 PO 4 C. H 2 PO 4 + H 2 O H 3 O + + HPO 4 2 D. H 2 PO 4 + H 2 O H 4 PO OH

9 9 13. Consider the following equilibrium: HS + H 3 BO 3 H 2 BO 3 + H 2 S The two species acting as BrønstedLowry bases in the equilibrium are A. HS and H 2 S C. H 3 BO 3 and H 2 S B. HS and H 2 BO 3 D. H 3 BO 3 and H 2 BO In which of the following is HSO 3 acting as a BrønstedLowry acid? A. HSO 3 + H 2 O H 2 SO 3 + OH B. NH 3 + HSO 3 NH SO 3 C. HSO HPO 4 H 2 SO 3 + PO 4 D. H 2 C 2 O 4 + HSO 3 HC 2 O 4 + H 2 SO Write a chemical reaction showing an amphiprotic anion reacting as a base in water. A.2: Conjugate AcidBase Pairs and Relative Acid Strength As mentioned previously, every BrønstedLowry acidbase reaction has an acid and a base on each side of the reaction equation. (iii) HNO 3(aq) + H 2 O (l) H 3 O + (aq) + NO 3 (aq) acid base acid base (iv) HF (aq) + H 2 O (l) H 3 O + (aq) + F (aq) acid base acid base (v) CO 3 2 (aq) + H 2 O (l) HCO 3 (aq) + OH (aq) base acid acid base Each combination of 1 acid and 1 base with chemical formulae that differ by one proton constitute a conjugate acidbase pair. The conjugate pairs in the above reactions are: (iii) and (iv) and (v) and

10 10 It is VERY IMPORTANT that you remember that a conjugate pair differs by one, and only one, proton. e.g., the conjugate base of H 2 CO 3 is HCO 3 and NOT. Amphiprotic substances are tricky as they all have BOTH a conjugate acid AND a conjugate base. e.g. H 2 O: conjugate acid: H 2 PO 4 : conjugate acid: conjugate base: conjugate base: Notice that in equation (iii) a oneway arrow ( ) was used, while in (iv) and (v), a reversible arrow ( ) was used. Use of the two different arrows indicates the degree to which each substance ionizes in an aqueous solution. HNO 3 completely ionizes in an aqueous solution. Therefore, a solution of HNO 3 contains only H 3 O +, NO 3, and H 2 O molecules (i.e., no HNO 3 molecules exist in their original form in solution). Acids which completely ionize in aqueous solutions are called STRONG ACIDS. There are 6 strong acids that you MUST know: Perchloric acid HClO 4 Hydroiodic acid HI Hydrobromic acid HBr Hydrochloric acid HCl Nitric acid HNO 3 Sulphuric acid H 2 SO 4 Strong acids will react to completion with any BrønstedLowry base present in solution with it. e.g., HCl (aq) + F (aq) HF (aq) + Cl (aq) acid base acid base Acids that do not completely ionize in aqueous solutions are called WEAK ACIDS. In aqueous solutions, weak acids form an equilibrium system. e.g., HCN (aq) + H 2 O (l) H 3 O + (aq) + CN (aq) acid base acid base Equilibrium systems such as the one above exhibit the same characteristics as all other equilibrium systems we have encountered.

11 11 It is important that one does not associate acid strength with concentration. They are distinctly different things. We can have: concentrated solutions of strong acids dilute solutions of strong acids concentrated solutions of weak acids dilute solutions of weak acids The Chemistry 12 Data Booklet contains a table of Relative Strengths of BrønstedLowry Acids and Bases on page 6. Notice that in each entry, the acid is shown to dissociate into a single H + ion and the conjugate base of the acid. e.g., HCOOH H + + COOH This shows that the acid can donate a proton to a BrønstedLowry base when in solution. If the only base present is H 2 O, then the reaction is: HCOOH (aq) + H 2 O (l) H 3 O + (aq) + COOH (aq) The seventh entry from the top is: This may also be written as: H 3 O + H + + H 2 O H 3 O + + H 2 O H 3 O + + H 2 O This tells us that H 3 O + (aq) and H + (aq) are equivalent. The twoway arrow above means that the reaction may be read left to right or right to left. The H 3 O + ions on the left side of the above equations represents the hydronium ions produced by the ionization of any one of the six strong acids., e.g., HNO 3(aq) + H 2 O (l) H 3 O + (aq) + NO 3 (aq). Because all six strong acids completely ionize to form solutions of H 3 O +, all six strong acids have equal strength when in an aqueous solution. The levelling effect is the name given to the effect water has on the strong acids which causes them to all have the same strength. As we will see, the levelling effect also applies to solutions of strong bases. Another result of the complete ionization of the strong acids is that H 3 O + is the strongest acid that can exist in an aqueous solution. As discussed at other times in this course, the ability of a solution to conduct an electric current depends upon the concentration of dissolved ions in the solution. As strong acids (and strong bases) are 100% ionized, they are good electrolytes. Solutions of weak acids (and weak bases) are only partially ionized. This makes them weak electrolytes. As we move down the left side of the table, the acids get weaker and weaker.

12 12 On the right side of the table are the conjugate bases of the acids, with the weakest of the bases on the top, the strongest on the bottom. The six bases at the top are the conjugate bases of the strong acids. These are so weak that they will not act as BrønstedLowry bases. The conjugate base of sulphuric acid (HSO 4 ) is, in fact, a rather strong weak acid (see the eleventh entry from the top). From the table we can conclude the following: The stronger an acid is, the weaker is its conjugate base. The three strongest bases listed are OH, O 2 and NH 2. Being anions, basic solutions containing these anions must also contain cations (Why?). As we saw in Unit 3, an ion must be introduced into a solution in the form of a soluble compound. The OH, O 2, and NH 2 ions are only considered as strong bases when combined with one of seven different metal ions: Li +, Na +, K +, Rb +, Cs +, Sr 2+ and Ba 2+. The 21 Strong Bases LiOH Li 2 O LiNH 2 NaOH Na 2 O NaNH 2 KOH K 2 O KNH 2 RbOH Rb 2 O RbNH 2 CsOH Cs 2 O CsNH 2 Sr(OH) 2 SrO Sr(NH 2 ) 2 Ba(OH) 2 BaO Ba(NH 2 ) 2 You need to know these!! On page 122, Hebden states that Mg(OH) 2, Ca(OH) 2, Fe(OH) 3 and Zn(OH) 2 are strong bases. He is absolutely incorrect. In an aqueous solution, each of the amides and oxides listed above will first dissociate and then the anion released will react with water. For example, (i) NaNH 2(s) Na + (aq) + NH 2 (aq) then NH 2 (aq) + H 2 O (l) NH 3(aq) + OH (aq) (ii) Na 2 O (s) 2Na + (aq) + O 2 (aq) then O 2 (aq) + H 2 O (l) 2OH (aq) The hydroxides dissociate to release OH, NaOH (s) Na + (aq) + OH (aq) Because the reactions of the NH 2 and O 2 ions with water are not reversible, the strongest base that exists in water is OH. The reaction of the hydroxide ions from any strong base will accept a proton

13 13 from any available acid. This reaction is given in the third entry from the bottom of the table when read left to right: H 2 O H + + OH If the only acid available is water, the reaction is: OH + H 2 O H 2 O + OH The twoway arrow in the above equation means that the reaction may be read in either direction. In the presence of any acid, the OH from a strong base will react to completion: NaOH (aq) + HF (aq) H 2 O (l) + NaF (aq) NOTE: It is common for students to think that NH 3 is a strong base because of its location on the very bottom of the left side of the table. This is absolutely incorrect. The presence of NH 3 at this location shows that it is the conjugate acid of NH 2. Mind you, it is such a weak acid that it will not act as an acid at all, in the same way that the conjugate bases of the strong acids are too weak to ever act as bases. The ranking of the bases on the table lead to the following conclusion: The stronger a base is, the weaker is its conjugate acid. In a solution of a weak BrønstedLowry base, the base only partially reacts with water to form OH ions. For example: CN (aq) + H 2 O (l) HCN (aq) + OH (aq) The above equation represents an equilibrium system that behaves just as all other equilibrium systems we have examined. We can write four general equations to represent the reactions between BrønstedLowry acids and bases with water. Strong Acid: Weak Acid: Strong Base: Weak Base: HA (aq) + H 2 O (l) H 3 O + (aq) + A (aq) HA (aq) + H 2 O (l) H 3 O + (aq) + A (aq) B (aq) + H 2 O (l) HB (aq) + OH (aq) B (aq) + H 2 O (l) HB (aq) + OH (aq)

14 14 We have already discussed the fact that the strength of a BrønstedLowry acid is a measure of the degree to which it will ionize in an aqueous solution. Since the result of the ionization of an acid is the formation of H 3 O +, we can conclude that: For any given concentration, the weaker an acid is, the lower the [H 3 O + ]. Similarly: For any given concentration, the weaker a base is, the lower the [OH ]. A.3: The Equilibrium Position of BrønstedLowry Acid/Base Reactions We have discussed the idea that the products of every BrønstedLowry acidbase reaction are also a BrønstedLowry acid and base. During reaction of a strong acid and any base (weak or strong), the reaction will always go to completion. HA (aq) + B (aq) HB (aq) + A (aq) strong base acid base acid Similarly, the reaction of a strong base with any acid (weak or strong), the reaction will go to completion. HA (aq) + B (aq) HB (aq) + A (aq) acid strong acid base base In either case, the reaction reaches a final position of static equilibrium with only products present. In the reaction between a weak acid and a weak base, the reaction will reach a state of dynamic equilibrium, with reactants and products present in appreciable concentrations. Whether the products are present in the greater concentration or the reactants are present in the greater concentration at equilibrium depends on the relative strength of the acids and bases present in the equilibrium system. Consider the general reaction between a weak acid and a weak base: HA (aq) + B (aq) HB (aq) + A (aq) acid base acid base Both of the acids and both of the bases in the above equation are weak substances.

15 15 If HA is the stronger of the two acids, then will be the weaker base. If HB is the stronger of the two acids, then will be the weaker base. The result of every weak acid/weak base reaction is that the stronger acid and stronger base always appear on the same side of the reaction equation, and the weaker acid and weaker base always appear on the same side of the equation. In every weak acid/weak base reaction, the stronger of the weak substances will react to a greater extent than will the weaker of the weak substances. We know this because the stronger a substance is, the more closely it will react to completion. In the equation above, if HA is the stronger of the two acids, B will be the stronger of the two bases. Since they react to a greater degree than do HB and A, HB and A will be present in greater concentrations than HA and B at equilibrium. That is, the products will be favoured at equilibrium. Conversely, if the stronger of the two weak acids and the stronger of the two weak bases are on the product side of the reaction equation, they will react to the greater degree and the reactants will be present in greater concentrations at equilibrium (i.e., the reactants will be favoured ). Determining which of the acids is stronger in the equilibrium resulting from a weak acid/weak base reaction is simply a matter of locating each on the table. Usually there is no need to locate the weak bases, as we know that the stronger of the weak acids has the weaker conjugate base. Example: Consider the following equilibrium: HNO 2(aq) + CN (aq) HCN (aq) + NO 2 (aq) To determine whether the products or reactants are favoured at equilibrium, we simply need to locate HNO 2 and HCN on the table of relative acid strengths. We find the following: HNO 2(aq) + CN (aq) HCN (aq) + NO 2 (aq) stronger weaker This result also tells us that CN is a stronger base than NO 2. Since the reactants are the stronger of the substances, the products are favoured at equilibrium. In every acidbase reaction, the side of the reaction with the weaker acid and weaker base is favoured at equilibrium.

16 16 Practice Examples: Complete the following acidbase equilibrium reaction equations and state whether the reactants or products are favoured at equilibrium. (a) HIO 3(aq) + SO 3 2 (aq) (b) HCN (aq) + H 2 BO 3 (aq) (c) HF (aq) + HS (aq) Hebden Reference: Sections IV.5 and IV.6. Practice Exercises: Complete the following exercises from the Hebden text: Pages 119 to 125 Exercises 15 to 27. Page 133 Exercises 38, 39, 43, and 44. Provincial Exam Examples: AcidBases Pairs and Relative Acid Strength 1. Identify a conjugate pair from the following equilibrium: PO HCO 3 HPO CO 3 2 A. CO and PO 4 B. PO 3 4 and HCO 3 C. PO and HPO 4 D. HCO 2 3 and HPO 4 2. What is the conjugate base of H 2 PO 4? A. OH 2 C. HPO 4 3 B. PO 4 D. H 3 PO 4 3. The conjugate base of HBO 3 2 is 2 A. BO 3 3 B. BO 3 C. HBO 3 D. H 2 BO 3 more on next page

17 17 4. Consider the following reaction: HCN + CH 3 NH 2 CN + CH 3 NH 3 + Which of the following describes a conjugate acidbase pair in the equilibrium above? Acid Base A. CN HCN B. + CH 3 NH 3 CN C. HCN + CH 3 NH 3 D. + CH 3 NH 3 CH 3 NH 2 5. a) Write the formula equation to represent the complete neutralization reaction between household vinegar (acetic acid) and drain cleaner (sodium hydroxide). b) Write the formula for the conjugate base of the reactant acid. 6. An acidbase reaction occurs between HSO 3 and IO 3. a) Write the equation for the equilibrium that results. b) Identify one conjugate acidbase pair in the reaction. c) State whether reactants or products are favoured, and explain how you arrived at your answer. 7. The two reactants in an acidbase reaction are HNO 2(aq) and HCO 3 (aq). a) Write the equation for the above reaction.

18 18 b) Define the term conjugate acidbase pair. 8. Which of the following is the weakest base? A. F C. CN B. HS D. IO 3 9. List the bases C 2 O 4 2, NH 3, and PO 4 3 in order from strongest to weakest. A. PO 4 3 NH 3 C 2 O 4 2 B. C 2 O 4 2 NH 3 PO 4 3 C. NH 3 PO 4 3 C 2 O 4 2 D. PO 4 3 C 2 O 4 2 NH Water has the greatest tendency to act as an acid with which of the following? A. Cl C. H 2 PO 4 B. NO 2 D. CH 3 COO 11. When comparing equal volumes of 0.10 M HNO 3 with 0.10 M HNO 2, what would be observed? A. The ph values would be the same. B. The electrical conductivities would be different. C. The effects on blue litmus paper would be different. D. The volumes of 0.10 M NaOH needed for neutralization would be different. 12. Which of the following 1.0 M solutions would have the highest electrical conductivity? A. HI C. HCN B. HF D. HNO Which of the following solutions will have the lowest electrical conductivity? A. 1.0 M HI C. 1.0 M NaOH B. 1.0 M H 2 S D. 1.0 M NaNO 3

19 Consider the following equilibrium: HF (aq) + HPO 4 2 (aq) F (aq) + H 2 PO 4 (aq) For the above equilibrium, identify the weaker acid and determine whether reactants or products are favoured. Weaker Acid Side Favoured A. HF Products B. HF Reactants C. H 2 PO 4 Products D. H 2 PO 4 Reactants 15. Which of the following is the strongest acid that can exist in an aqueous solution? A. O 2 C. H 3 O + B. NH 2 D. HClO Consider the following equilibria: I. CH 3 COOH + OCN HOCN + CH 3 COO II. CH 3 COOH + ClO HClO + CH 3 COO a) In equation I above, the reactants are favoured. Identify the stronger acid. b) In equation II above, the products are favoured. Identify the stronger acid. c) Consider the following reaction: HOCN + ClO OCN + HClO Does this reaction favour the reactants or products? Explain.

20 20 A.4: Hydrolysis of Salts (Part 1) According to Arrhenius: Acid + Base Salt + Water This is called a neutralization reaction. However, in many cases where we react an acid and base in equivalent amounts, the resulting salt solution may be slightly acidic (ph 7.0 at 25 C) or slightly basic (ph 7.0 at 25 C). For example, the neutralization reaction between nitric acid and ammonium hydroxide is: HNO 3(aq) + NH 4 OH (aq) NH 4 NO 3(aq) + H 2 O (l) The resulting solution of NH 4 NO 3 is slightly acidic. The neutralization of acetic acid by sodium hydroxide is: CH 3 COOH (aq) + NaOH (aq) NaCH 3 COO (aq) + H 2 O (l) The resulting NaCH 3 COO solution is slightly basic. The above results can be very easily explained if one examines the table of relative acid strengths. In the case of the NH 4 NO 3 solution, we can see that NH 4 + is a weak acid: NH 4 + (aq) + H 2 O (l) H 3 O + (aq) + NH 3(aq) In the case of the NaCH 3 COO solution, CH 3 COO is a weak base: CH 3 COO (aq) + H 2 O (l) CH 3 COOH (aq) + OH (aq) When an anion or cation reacts with water as an acid or a base, it is said to hydrolyze. Similarly, if a salt contains an ion that reacts with water as an acid or a base, that salt is said to hydrolyze. The equation showing an ion acting as an acid or a base is called a HYDROLYSIS REACTION EQUATION. As we have seen, a large number of BrønstedLowry bases are anions. In addition, some anions and a few cations hydrolyze as BrønstedLowry acids. Any salt containing an ion that hydrolyzes will form a solution that may be acidic or basic. So which ions hydrolyze and which ones do not?

21 21 Cations That Hydrolyze Neither the Group I nor the Group II metal ions hydrolyze. In fact, for our purposes, we only need to concern ourselves with four cations that hydrolyze, all as acids. These are NH 4 +, Al 3+, Fe 3+ and Cr 3+. We have already examined the hydrolysis of NH 4 +. The other three cations hydrolyze in a manner similar to one another. Because of the large positive charge on these ions, water molecules (being very polar) are strongly attracted to them, forming a complex hydrated ion. Cr 3+ (aq) + 6H 2 O (l) Cr(H 2 O) 6 3+ (aq) Al 3+ (aq) + 6H 2 O (l) Al(H 2 O) 6 3+ (aq) Fe 3+ (aq) + 6H 2 O (l) Fe(H 2 O) 6 3+ (aq) Each of these complex ions will donate a proton to a water molecule and produce a H 3 O + ion. Cr(H 2 O) 6 3+ (aq) + H 2 O (l) H 3 O + (aq) + Cr(H 2 O) 5 (OH) 2+ (aq) Al(H 2 O) 6 3+ (aq) + H 2 O (l) H 3 O + (aq) + Al(H 2 O) 5 (OH) 2+ (aq) Anions That Hydrolyze Fe(H 2 O) 6 3+ (aq) + H 2 O (l) H 3 O + (aq) + Fe(H 2 O) 5 (OH) 2+ (aq) None of the anions that are conjugate bases of the six strong acids hydrolyze EXCEPT HSO 4. HSO 4 hydrolyzes as a weak acid in water as shown below: HSO 4 (aq) + H 2 O (l) H 3 O + (aq) + SO 4 2 (aq) The conjugate base of HSO 4 is SO 4 2. It will hydrolyze as a pathetically weak base (remember, the stronger an acid is, the weaker is its conjugate base): SO 4 2 (aq) + H 2 O (l) HSO 4 (aq) + OH (aq) As we know, the anions OH, NH 2 and O 2 are all strong bases in solution. ALL OTHER anions not containing a hydrogen atom will tend to hydrolyze as weak bases. For example: SO 3 2 (aq) + H 2 O (l) HSO 3 (aq) + OH (aq) A difficulty arises when dealing with anions containing hydrogen atom. As we saw earlier, such ions are amphiprotic, and could hydrolyze as and acid or as a base!

22 22 To determine whether an amphiprotic ion hydrolyzes primarily as an acid or as a base requires the use of mathematical methods that we will learn later in this course. Examples: Predict whether the following salts form solutions that are acidic, basic or neutral. Write the corresponding hydrolysis reaction equation(s) when applicable. (a) NaBr (b) KNO 2 (c) Fe(NO 3 ) 3 (d) NH 4 CN Provincial Exam Examples: The Hydrolysis of Salts 1. Consider the following reaction: NO 2 (aq) + H 2 O (l) HNO 2(aq) + OH (aq) This reaction represents which of the following? A. the titration of NO 2 C. the ionization of HNO 2 B. the hydrolysis of NaNO 2 D. the dissociation of NaNO 2 2. Which of the following describes the net ionic equation reaction for the hydrolysis of NH 4 Cl (s)? + A. NH 4 (aq) + Cl (aq) NH 4 Cl (s) + B. NH 4 Cl (s) NH 4 (aq) + Cl (aq) C. Cl (aq) + H 2 O (l) HCl (aq) + OH (aq) + D. NH 4 (aq) + H 2 O (l) H 3 O + (aq) + NH 3(aq)

23 23 3. Which of the following is the net ionic equation describing the hydrolysis of KCN? A. K + (aq) + H 2 O (l) KOH (aq) + H + (aq) B. KCN (aq) + H 2 O (l) K + (aq) + CN (aq) C. CN (aq) + H 2 O (l) HCN (aq) + OH (aq) D. CN (aq) + H 2 O (l) 2H + (aq) + CNO (aq) It is now time to complete and submit the Unit 4 Topic A Handin Assignment.