8.1 Exercises 1. What is the definition for acids and bases in terms of proton transfer (Brønsted-Lowry)?

Transcription

1 Topic 8 Acids and Bases Answers 8.1 Exercises 1. What is the definition for acids and bases in terms of proton transfer (Brønsted-Lowry)? An acid is a proton donor. A base is a proton acceptor. The acid species donates its proton to the base when a base is present to accept the proton. 2. Define, with an example, a) a Lewis acid A Lewis acid is an electron pair acceptor e.g., BF 3. The central boron atom has space in its valence shell for two more electrons. b) A Lewis base A Lewis base is an electron pair donor e.g., N 3.It is the central nitrogen atom that has the lone electron pair available for forming a new bond. 3. Acid-base characteristics can be used to classify compounds. a) What are conjugate acids and bases and why are these species labelled this way? Acid-base conjugate pairs are the acids and bases that are connected by the loss or gain of a proton. When an acid loses a proton, the conjugate species is the resulting product minus the + e.g. Cl Cl. It is called the conjugate base because the product now has a lone electron pair with which it can accept a proton the definition of a base. When a base accepts a proton, the conjugate species is the resulting product that contains an extra proton e.g. N 3 N + 4. It is called the conjugate acid because the product now has an acidic proton that it can donate the definition of an acid. The acid and its conjugate base or the base and its conjugate acid are known as a conjugate pair. Conjugate acid-base pairs: are the compounds on either side of the reaction arrow that are connected by the loss and gain of a proton. Their reverse roles come about because both the forward and back reactions are occurring. The conjugate base results from an acid losing its proton: the conjugate acid results from a base gaining a proton. b) ow are conjugate acids/bases identified? Conjugate species are identified by determining which products are the result of losing or gaining a proton from the reactants. The conjugate base will be on the product side and will have the same formula as the reactant acid minus a proton (i.e. less one + ). The conjugate acid will also be on the product side and will have the same formula as the reactant base plus a proton (ie with one extra + ). 1

2 c) Pentanoic acid, C 5 10 O 2 (C 3 C 2 C 2 C 2 COO), has 10 hydrogens present in the molecule. i. Are all of these hydrogens able to form hydrogen ions (protons), so giving pentanoic acid its acid classification? No, nine of the ten protons (those directly bonded to carbons) will not be lost (donated to a base) and therefore do not add to the acidic character of pentanoic acid. These C bonds are not polar or weak enough to be broken easily in order to donate the proton. The hydrogen atoms attached to the carbon atoms are not acidic. ii. Identify which proton(s) will be lost and explain your decision. Only the proton bonded to the oxygen in the carboxylic acid functional group ( COO) will be donated. This is because it is the only proton involved in a highly polarised bond that is, therefore, relatively easy to break. 4. Ammonia and hydrogen chloride are polar, gaseous molecules. They react to form dense white fumes. a) Write a balanced equation for this reaction. Cl(g) + N 3 (g) Cl (s) + N 4 + (s) (the Cl and N 4 + form a solid salt, N 4 Cl) b) Explain how this can be defined as an acid-base reaction? This is defined as an acid-base reaction because the Cl molecule donates an + ion that the ammonia molecule accepts. c) Indicate the Brønsted-Lowry conjugate pairs in this reaction. On the reactant side Cl is the acid and it loses its proton, +, to become the conjugate base on the product side, Cl. N 3 is the base on the reactant side and it accepts a proton during the reaction to become the conjugate acid, N + 4 on the product side. d) Are these species also Lewis acids and bases? Explain. Yes these species are also Lewis acids and bases. The acidic proton that is lost from the Cl reactant accepts electron density from the ammonium and therefore acts as a Lewis acid. The nitrogen atom in the ammonia molecule donates an electron pair to form a new bond as it accepts the proton. Therefore, N 3 acts as a Lewis base. e) When ammonia and hydrogen chloride dissolve separately in water, they ionize. Write two balanced equations to show this and use them to explain what ionization means. Cl(g) + 2 O(l) Cl (aq) + 3 O + (ag) N 3 (g) + 2 O(l) N + 4 (aq) + O (aq) Ionization is the process in which atoms or molecules become charged by the loss or gain of electrons. With respect to acid-base chemistry, ionization is the conversion of the reactant molecules into ions when in solution. The transfer of electrons occurs when the acid or base reacts with water. 5. The formulas of a number of molecules and ions are given below. Define each as a Brønsted-Lowry and/or Lewis acid or base. (int: some will fit into more than one category). 2 a) SO 3 Brønsted-Lowry Base and Lewis Base b) Cl Brønsted-Lowry Acid and Lewis Acid c) 2 O Brønsted-Lowry Acid, Lewis Acid, Brønsted-Lowry Base and Lewis Base 2

3 d) N 3 Brønsted-Lowry Base and Lewis Base e) O Brønsted-Lowry Base and Lewis Base f) N 4 + Brønsted-Lowry Acid and Lewis Acid g) Zn 2+ Lewis Acid h) Cl Transition metal cations can expand their outer electron arrangement by accepting electron pairs from Lewis bases, e.g., Zn O [Zn( 2 O) 6 ] 2+ Ni N 3 [Ni(N 3 ) 6 ] 2+ Therefore transition metal cations can act as Lewis acids. Brønsted-Lowry Base and Lewis Base i) SO 3 Brønsted-Lowry Acid, Lewis Acid, Brønsted-Lowry Base and Lewis Base j) 3 O + Brønsted-Lowry Acid and Lewis Acid 6. There are several acid-base conjugate pairs in the previous list. Write the reactions showing the proton transfer in each and indicate which of the pair is the conjugate acid and which is its conjugate base. SO 2 3 SO Acid: SO 3 2 Conjugate base: SO 3 Cl Cl + + Acid: Cl Conjugate base: Cl 2 O O + + Acid: 2 O Conjugate base: O And also: 2 O O + Base: 2 O Conjugate acid: 3 O + N N 4 + Base: N 3 Conjugate acid: N 4 + 3

4 7. Proton donors a) If O can act as a proton donor, give the name and formula for its conjugate base. O O O 2 is the oxide ion. b) Why do solutions of acids have so many properties in common? All Brønsted-Lowry acids in solution produce protons (by definition). It is these + ions which cause the acid reactivity and therefore acids have many properties in common. 8. Which of the following reactions would be classified acid-base? Answer yes or no, give a reason for your choice; and for yes indicate the acid and base species. a) 2 + Cl 2 2Cl No, there is no proton transfer or electron pair transfer occurring for this reaction. This is, in fact, a redox reaction please see Topic 9 for more details. b) 3 O + + O 2 2 O Yes, there is a proton transfer from the hydronium ion, 3 O +, to the hydroxide ion, O, to form two water molecules. The hydronium ion is therefore the acid and the hydroxide ion is the base. c) O O 2O Yes, there is a proton transfer from the water molecule to the oxide ion causing the formation of two hydroxide ions. Water is the acid and the oxide ion is the base. 9. Use Lewis Dot Structures (electron diagrams) to show that the following reactions are Lewis acidbase in nature: a) BF 3 + F F F F B + F B F F F F b) + + O + O O 10. The compound aminoethane, C 2 5 N 2, is a weak base. Write an equation to show its ionization in water and comment on the extent of this ionization. C 2 5 N O C 2 5 N O As a base, the aminoethane accepts a proton from the water. Because it is a weak base, the extent of ionisation is not great. The strength of an acid or base refers to how readily it undergoes or causes ionization. Weak bases do not readily cause ionisation; they prefer to exist in the neutral form. In the example above, the aminoethane prefers to exist as C 2 5 N 2 rather than C 2 5 N 3 +. The concept of "strong and weak acids and bases" is covered more fully in section 8.3. Relating this to the previous Topic 7 Equilibrium, the equilibrium lies on the reactant side as the concentration of the reactants at equilibrium is much larger than the concentration of the products. What will the size of the equilibrium constant, K, be for the reaction of a weak acid or base with water? 4

5 11. Protons in an aqueous solution are hydrated and this can be written as + (aq) or 3 O + (aq). Show the Lewis Dot Structure of a water molecule accepting a proton and the structure of the resulting ion. What is the name of this ion? O + O The ion formed is the hydronium ion. 8.2 Exercises 1. Write balanced equations for examples of the following general reactions: a) acid + metal 2 SO 4 (ag) + Mg(s) MgSO 4 (ag) + 2 (g) b) acid + carbonate 2Cl(ag) + CaCO 3 (s) CaCl 2 (aq) + CO 2 (g) + 2 O(l) c) base + acid Cl(aq) + NaO(aq) NaCl(aq) + 2 O(l) d) acid + hydrogencarbonate 2 SO 4 (aq) + 2NaCO 3 (s) Na 2 SO 4 (aq) + 2CO 2 (g) O(l) e) alkali + acid KO(aq) + ClO 4 (aq) KClO 4 (aq) + 2 O(l) 2. What property do alkaline bases have that other bases do not? Alkaline bases dissolve in water to yield hydroxide ions. Bases that are not alkaline do not dissolve in water. 3. Complete and balance the following reactions: a) 2 SO 4 (aq) + KO(s) K 2 SO 4 (aq) + 2 O(l) b) 2Cl(aq) + Ca(s) CaCl 2 (aq) + 2 (g) c) C 3 COO(aq) + N 3 (aq) C 3 COO (aq) + N + 4 (aq) d) 2NO 3 (aq) + CaCO 3 (s) Ca(NO 3 ) 2 (aq) + CO 2 (g) + 2 O(l) e) 3 2 O(l) + 6Na(s) 3Na 2 O(aq) (g) 4. What colour will solutions of the following acids and bases be in the presence of the indicators given? Acid Base Acid Base Base Acid Yelow Blue Yelow Blue Blue Yelow Colourless Pink Colourless Pink Pink Colourless 5

6 5. When vinegar and baking soda are mixed the reaction produces very vigorous effervescence. What is the balanced equation for this procedure given that vinegar is ethanoic (acetic) acid (C 3 COO) and baking soda is sodium bicarbonate (NaCO 3 )? C 3 COO(aq) + NaCO 3 (s) NaC 3 COO(aq) + CO 2 (g) + 2 O(l) The source of the effervescence (bubbling) is the CO 2 (g) that is produced. It escapes as gas when vinegar and baking soda are combined. In baking, the cake or bread mix rises as the result of an acid hydrogencarbonate reaction. The recipe will include baking soda (NaCO 3 a hydrogencarbonate compound) and the addition of an acid, perhaps in the form of lemon juice or vinegar. The reaction that occurs is therefore that between an acid and hydrogencarbonate to produce CO 2 (g). It is the CO 2 (g) that gets trapped within the mixture and causes it to rise. 6. Complete and balance the following equations and note which group they belong to. a) 2 SO 4 (l) + Ca(O) 2 (s) CaSO 4 (aq) O(l) acid/base b) Cl(g) + 2 O(l) Cl (aq) + 3 O + (aq) acid/base or ionization c) CN(l) + NaO(s) NaCN(aq) + 2 O(l) acid/base d) 2ClO 3 (aq) + Ba(O) 2 (s) Ba(ClO 3 ) 2 (aq) O(l) acid/base e) 2 3 PO 4 (aq) + 3CaCO 3 (s) Ca 3 (PO 4 ) 2 + 3CO 2 (g) O(l) acid/carbonate f) 2NO 3 (aq) + CuO(s) Cu(NO) 3 (aq) + 2 O(l) acid/base 8.3 Exercises 1. Define the descriptions weak and strong for acids and bases. The terms weak and strong are used to indicate the reactivity of acids and bases. This is measured by their degree of ionization upon dissolution in water, i.e. where the ionization equilibrium lies. A strong acid or base has an equilibrium favouring the ionic product side. A weak acid or base has the equilibrium favouring the reactant side. 2. What is meant by a dilute solution of a strong acid? A dilute solution indicates that there are few moles of acid present for a given volume of solution. If the acid is strong it means that most of the acid molecules will ionize when they react with water; the ionization equilibrium lies on the product side. A dilute solution of a strong acid is, therefore, one in which there are few moles of acid present for a given volume of solution, but that the majority of these moles of acid have lost their protons. 3. Conjugate acid-base pairs. a) Explain why a strong acid has a weak conjugate base. If the acid is strong then it readily donates its proton and the ionization equilibrium favours the product side: A(aq) + 2 O(l) A (aq) + 3 O + (aq) acid base conjugate base conjugate acid Since this equilibrium lies on the product side, the back reaction (in which the conjugate base accepts a proton) is disfavoured. The conjugate base must, therefore, be a weak base because it does not readily accept a proton to return back to the original acid. 6

7 b) Explain why a weak acid has a strong conjugate base. If an acid is weak it means that it does not readily donate its proton and the ionisation equilibrium favours the reactant side. A(aq) + 2 O(l) A (aq) + 3 O + (aq) acid base conjugate base conjugate acid The back reaction is therefore favoured; the one in which the conjugate base accepts a proton to form the original reactant acid, A. Therefore, the conjugate base of a weak acid is strong. 4. Write balanced chemical equations for the following species with water and state on which side the equilibrium lies. a) Ethanoic (acetic) acid is a weak acid. C 3 COO(l) + 2 O(l) C 3 COO (aq) + 3 O + (aq) Equilibrium lies to the left, i.e. on the reactant side b) Ammonia is weakly alkaline. N 3 (l) + 2 O(l) O (aq) + N + 4 (aq) Equilibrium lies to the left, i.e. on the reactant side c) ydrochloric acid is a strong acid. Cl(g) + 2 O(l) Cl (aq) + 3 O + (aq) Equilibrium lies to the right, i.e. on the product side d) Perchloric acid is a strong acid. ClO 4 (l) + 2 O(l) ClO 4 (aq) + 3 O + (aq) Equilibrium lies to the right, i.e. on the product side e) The hydroxide ion is a strong base. 2 - O (aq) + 2 O(l) 2 2 O(l) Equilibrium lies to the right, i.e. on the product side 5. You have two equimolar acidic solutions, one of which is more strongly acidic than the other. What methods could you use to tell them apart? What method could you use to determine their exact ps values? The more acidic solution will have better electrical conductance as it ionises to a greater extent in solution. A p meter can then be used to determine the exact p. 6. Could you perform a titration on each of the above acid samples to distinguish between the two? (int: think about how the equilibrium will be affected as the donated protons react with the base from the burette.) No, a titration would not distinguish between the two solutions. By titrating the equimolar samples of both acid solutions, the end point will be the same. Though it is true that the stronger acid will produce more + ions in solution for a given number of moles of acid, the weaker acid will continue to convert acid moles to + as the base is added. The equilibrium of the general reaction below will be pushed to the right and more products will form until the end point is again reached at the same volume addition of base as for the strong acid. A(aq) + 2 O(l) A (aq) + 3 O + (aq) Take acetic acid for example: 7

8 C 3 COO C 3 COO (aq) + + (aq) The base added from the burette reacts with the free + in solution. Initially there is little free + because C 3 COO is a weak acid and does not readily dissociate. owever as the base reacts with the + the system adjusts to oppose the change and produces more + by shifting the equilibrium and favouring the forward reaction. In this way the solutions of the stronger and weaker acids will produce the same end point regardless of their strength, so a titration will not distinguish between them. 7. Ammonia solution can effectively and quickly neutralize a sulfuric acid spill but it is classified as a weak alkali. Explain why, with an equation. N 3 (l) + 2 O(l) O (aq) + N + 4 (aq) As N 3 is a weak alkali, the equilibrium lies to the left. N 3 is classified as a weak alkali as it weakly ionizes upon dissolution in water. owever, for the same reasons as in the previous question, it is still able to neutralize an acid spill. As the 2 SO 4 (aq) reacts with the product hydroxide ion, the above system adjusts to oppose the change by favouring the forward reaction, producing more products. Therefore with enough ammonia solution the entire sulphuric acid spill can be neutralized. 8. Pure ethanoic acid a) Pure ethanoic acid does not conduct electricity. Explain why it is a non-conductor. A pure ethanoic acid sample will contain no charged species. Firstly it is a weak acid and therefore does not readily dissociate in water, but significantly in this case it cannot donate a proton without a base to accept it. In a pure ethanoic acid sample (i.e. with no other species present, including water) there will be no base present and therefore no ionisation takes place. Electricity is the movement of charged particles and if there are none present the solution will not be able to conduct electricity. b) After water is added to the pure acetic acid making it an aqueous solution, it is found to be an electrical conductor. Explain. When water is added to the ethanoic acid the acid is able to dissociate into acetate and hydronium ions. Though it is a weak acid the extent of dissociation is not large, but it still produces the charged species required for electrical conductivity. c) Write an equation which will help explain why the aqueous solution now conducts. C 3 COO(l) + 2 O(l) C 3 COO (aq) + 3 O + (aq) d) The solution is tested and found to turn blue litmus red. Name the species responsible for this colour change. The hydronium ion is the source of the proton and therefore the cause of the change of the litmus from blue to red. 9. Acid rain is a form of atmospheric pollution. This pollution is produced when acidic oxides, e.g. SO 2 and NO 2 are formed in combustion reactions in engines and industrial processes and then dissolve in rainwater to produce acids. a) What acids are formed by the dissolution (i.e., the dissolving) of SO 2 and NO 2? Sulfuric, 2 SO 4, and nitric, NO 3, acids are eventually produced by the processes of dissolving these oxides in water. Please note: Initially sulfurous acid, 2 SO 3, and nitrous acid, NO 2, are formed, but these are oxidized by the oxygen in the air to 2 SO 4 and NO 3. 8

9 b) Do these oxides produce weak or strong acids? Explain your answer. Sulphuric acid ( 2 SO 4 ) and nitric acid (NO 3 ) are strong acids as they readily undergo dissociation in the presence of water. c) Is acid rain dilute or concentrated? Explain why. Acid rain is dilute because there are very few moles of the acidic species per volume of solution (the rainwater). d) In light of your answer to (c) explain why acid rain still causes considerable damage. Though the concentration of the acid in rain is dilute, this does not indicate that the solution is not reactive. When the acid rain comes into contact with material it can corrode (exposed metal or limestone surfaces for example) it will still react and cause damage. Sulfuric and nitric acid are strong acids that will readily react with metals and carbonates, despite the fact that they may be in a dilute solution. 10. Where would you place, in Table 1 on page 238, the following compounds? Give a reason. a) nitric acid, NO 3 Nitric acid is a strong acid and should therefore be placed towards the top of the left-hand column. It falls between sulfuric acid and the hydronium ion. b) carbonic acid, 2 CO 3 Carbonic acid is a relatively weak acid and should be placed in the middle of the left-hand column. It lies between the hydronium ion and ethanoic acid. c) the hydrogencarbonate ion, CO 3 This species can react as either a base and gain a proton to become 2 CO 3 or it can lose its remaining acidic proton to become CO 2 3. It could therefore be placed in either column. It is amphoteric, i.e. it can act as an acid or a base depending on what it is reacting with. 11. Acid-base reactions. a) Write and equation for the acid-base reaction between ammonium ions and sulfate ions. Why does the reaction favour the reactants? N + 4 (aq) + SO 2 4 N 3 (aq) + SO 4 According to Table 1, N is a weak acid and SO 4 is an even weaker base than SO 4. Because both of the reactant species are weak, they do not readily react and the equilibrium will lie on the reactant side. b) Which of the following solutions will have the larger concentration of ions: 1 M Cl(aq) or 1 M C3COO(aq)? Explain your choice. Cl is a much stronger acid than C 3 COO. It therefore readily ionizes and produces more ions in solution than C 3 COO. 12. You discover that the labels have been rubbed off of your flasks containing ethanoic acid, sulfuric acid, ammonia solution and water. They all look the same but will obviously produce different results in your experiments and may even be hazardous if used incorrectly. As a chemist you have access to indicators, a p meter and a conductivity kit. What tests will you perform and what do you expect the results to be? Assuming all are aqueous solutions: Water: the use of a p meter should produce a reading close to neutral, p = 7. Furthermore, the conductivity of pure (deionized) water should be zero. 9

10 Ethanoic Acid solution: Bromomethyl Blue indicator will yield a yellow solution though this will not distinguish the solution from sulfuric acid. owever the acetic acid solution will conduct electricity but not to the same extent as sulphuric acid because it is a weaker acid. Sulphuric Acid solution: Bromomethyl Blue indicator will yield a yellow solution and it will conduct electricity to a greater extent than the acetic acid solution. Ammonia solution: Bromomethyl Blue indicator will yield a blue solution. 8.4 Exercises 1. The expression of hydrogen-ion concentration, [ + ], is bulky. A more widely used system is the p scale. Add the missing words. The p of a solution is the negative logarithm of the proton concentration. In equation form: p = log 10 [ + (aq)] 2. In expressing p, decimal expressions (long hand) for [ + ] are changed to the scientific notation form, i.e. 10 -x. Complete the following: 1x10-6 1x10-3 1x10-4 1x10-1 1x a) Which is the most acidic solution? Solution 4 b) Which is the most alkaline? Solution 5 c) Of solutions 2 and 3, which is the more acidic and why? Solution 2 is more acidic as the concentration of protons is ten times greater and the p is therefore one unit lower. d) What is the hydrogen ion concentration change, [ + (aq)], on going from solutions 4 to 2? On going from solution 4 to solution 2 the hydrogen ion concentration is decreasing by a factor of 100. Solution 2 is 100 times less concentrated than solution 4. e) What is the hydrogen ion concentration change, [ + (aq)], on going from solutions 3 to 2? On going from solution 3 to 2 the hydrogen ion concentration is decreasing by a factor of ten. Solution 3 is ten times less concentrated than solution 2. f) Which of the solutions would turn universal indicator red? Solution 4 g) Which of the solutions would turn bromomethyl blue a blue colour? Solution 5 10

11 3. Complete the following: In a neutral solution the concentration of [ + ] = 1.0 x 10-7 mol dm -3 and the p = 7. In acid solutions the p is less than 7, and in alkaline solutions the p is greater than 7. The more acidic a solution is, the lower the p. 4. Below is the p scale for some laboratory solutions and for some common substances found in the home. a) What is the [ + ] (mol dm 3 ) in each of these? i) soft drinks: 1 x 10-3 mol dm -3 ii) black coffee: 1 x 10-5 mol dm -3 iii) pure water: 1 x 10-7 mol dm -3 iv) toothpaste: 1 x 10-3 mol dm -3 v) gastric juices: approximately 1 x mol dm -3 vi) laundry detergent: 1 x mol dm -3 b) Which is the most acidic? Gastric Juices c) Which is the most alkaline? Laundry detergent d) Why would we not want the p of things like toothpaste and coffee to stray too far from neutral? Anything we ingest or that comes into contact with us should not be corrosive. Keeping common items such as toothpaste and coffee close to neutral ensures they will not be harmful to us when we use or ingest them. 5. Which of the following is not an acidic solution? A [ + ] = 1 x 10 5 mol dm 3 B Turns universal indicator orange and phenolphthalein colourless. C ydrogen ion concentration = mol dm 3 D p = 6.99 Answer: C. This solution has an + concentration of 1 x 10-8 mol dm -3 which is a p = 8; a basic solution. The remainder have a p < 7 (acidic solutions). 6. Which of the following changes will not cause a solution of p 4 to become a solution of p 5? A Diluting the solution by a factor of 10. B Increasing the [ + ] by a factor of 10. C Decreasing the [ + ] by a factor of 10. D Diluting the solution by a factor of 100. Answer: C If the solution above is of a strong acid then the answer could also be A. Being fully ionised, the number of + (aq) will not change and diluting will change their concentration by the required factor of ten. owever, if the solution is of a weak acid then diluting may not produce the corresponding dilution factor to the + (aq) present 11

12 the deprotonation equilibrium must be taken into account. It is the [ + (aq)] that is the determinant of the p and therefore answer C is preferred as it is correct for solutions of both strong and weak acids. On mixing a number of indicators together the chemist has a means of comparing the [ + (aq)] of acids or the [O (aq)] of bases. These indicator mixtures are called universal indicators, and contain individual indicators which change colour over a wide range of acidity and alkalinity. In universal indicators a certain colour indicates a certain 3 O + concentration. These colours are related to the p scale. It should be noted then that combinations of indicators can be used to determine the actual p of solution rather than whether it is simply acidic, neutral or basic. Universal indicator differentiates between ps of 1, 2, 3, 4, 5 and 6 with different colours, whereas an individual indicator phenolphthalein for example would simply be one colour (colourless) over this entire p range. The colours seen in universal indicator and the corresponding p: 1, 2, 3 REDS 4, 5, 6 YELLOWS 7 GREEN 8, 9, 10 BLUES 11, 12, 13 PURPLES Universal indicators are supplied with the colour code which will indicate the p. It should be noted that universal indicator can only give an approximate value for the p of the solution. p meters must be used to obtain a more accurate determination. 12