Acids, Bases and Salts. Chapters 19

Transcription

1 Acids, Bases and Salts Chapters 19

2 Acid - Base Theories Section 19.1

3 What are common examples of acids and bases?

4 What properties do you know about acids and bases?

5 Arrhenius acids In 1887 A swedish Chemist, Svante Arrhenius defined acids and bases in a new way Acids are hydrogen containing compounds that separate to yield hydrogen ions (H+) in aqueous solution Not all compounds that contain hydrogen are acids, and not all hydrogen in acids has to form hydrogen ions The hydrogen atom that can form a hydrogen ion is described as ionizable This means it must be bonded to a very electronegative element

6 What happens to the hydrogen ion? The hydrogen does not simply float around in water - instead it joins with a water molecule to form a hydronium ion A hydronium ion (H3O+) is the ion that forms when a water molecule gains a hydrogen ion

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8 Arrhenius acids and weak polar bonds In some acids, they contain hydrogen atoms that do not ionize To recognise which hydrogen is ionizable you need to look at the structural formula, and establish which hydrogen is bonded to an electronegative element

9 Arrhenius Bases In an Arrhenius base, compounds ionize to yield hydroxide ions in aqueous solutions Sodium hydroxide is a common example of a base

10 NaOH can be dangerous! It is easy to make a concentrated solution, because it is easy to dissolve in water This solution is extremely caustic Sodium hydroxide is made from reacting sodium with water

11 What do you use this for? Milk of magnesia is made of magnesium hydroxide This is not very soluble in water, and their solutions are always very dilute Although most bases are generally caustic, the low solubility make this base safe to consume

12 Bronsted-Lowry acids and bases The Arrhenius definition of acids does not include some substances that have acidic or basic properties Sodium carbonate (Na2CO3) and Ammonia (NH3) act as bases in aqueous solutions, but do not contain OH- ions. In 1923 a new definition of bases was proposed by Johannes Bronsted and Thomas Lowry. They proposed that an acid is a hydrogen ion donor and a base is a hydrogen ion acceptor

13 Example

14 Conjugate acids and bases This reaction is reversible

15 Conjugate acid and base pairs A conjugate acid is the ion or molecule formed when a base gains a hydrogen ion It is an acid because it can donate the hydrogen ion in the reverse reaction A conjugate base is the ion or molecule that remains after an acid loses a hydrogen ion It is a base because it can accept a hydrogen ion in the reverse reaction A conjugate acid is always paired with a base, and a conjugate base is always paired with an acid

16 Amphoteric substances Some substances can behave as both acids and bases Water can BOTH donate hydrogen ion and accept hydorgen ions An amphoteric substance is something that can act as either an acid or a base How water behaves for example, depends on the other reactant

17 Lewis Acids and Bases Gilbert lewis proposed a third definition for acids and bases An acid accepts a pair of electrons during a reaction, a base donates a pair of electrons A Lewis acid is a substance that can accept a pair of electrons to form a covalent bond A lewis base is a substance that can donate a pair of electrons to form a covalent bond This is a broader definition than the other two definitions, and includes all of the acids and bases covered by the other definitions

18 Example

19 Example

20 Overview

21 Key questions How did Arrhenius define an acid and a base? How do acids and bases differ in the Bronsted -Lowry theory? How did lewis define an acid and a base?

22 Hydrogen Ions and Acidity Section 19.2

23 Hydrogen ions from water? Two water molecules can react to produce hydrogen ions via a process called self - ionization This occurs when the water molecules have enough energy for a reaction to occur

24 Self ionization water This is a reversible reaction, but the reverse reaction (formation of H2O is favored) Self ionization is therefore not very common The concentration of H+ and OH- ions in pure water at 25 C is 1 x Very small concentrations Any aqueous solution with equal concentrations of hydrogen and hydroxide solutions are a neutral solution

25 Ion product constant for water The Kw value is the product of the concentration of H+ ions and OH- ions - the ion product constant In an acidic solution, the conc. of H+ ions is great than that of OH-, meaning greater than 1 x 10-7 In a basic solution the conc. of H+ ions is less than that of OH- ions, meaning less than 1 x 10-7.

26 ph We use ph as a simple way of expressing the strength of an acid; it is much simpler than discussing hydrogen ion molarity Proposed by Soren Sorensen - ranges from 0 to 14 ph = -log[h+] In pure water [H+] = 1 x 10-7, and ph is 7 (neutral) ph < 7 = acidic, [H+] > 1 x 10-7 ph > 7 - basic [H+] < 1 x 10-7

27 Relationship between ph, [H+], and [OH-]

28 Why use scientific notation? It makes everything easier! When writing a concentration with a coefficient of 1.0, the ph is simply the exponent If the ph is not a whole number, it is not this simple, and you need to use the antilog function of your calculator to get an accurate hydrogen ion concentration Example - milk of magnesia has ph 10.5 = 3.2 x You can also use the knowledge that [H+] x [OH-] = 1 x to calculate [H+] and ph

29 How do you measure ph? Acid- Base indicators An indicator is an acid or a base that dissociates in a certain ph range They work because they form different colors in solution This change occurs in a narrow ph range HIn: The acidic form of the indicator - found at low phs In-: The basic form of the indicator - found at high phs

30 Indicators do not give precise measurements instead you can make a rough estimate This chart shows the position at which the color change occurs for indicators At lower phs you would see the acid color, whereas at higher phs you would see the basic color

31 Universal indicator solution

32 Limitations of an indicators usefulness They only work at a specific temperature - 25 C At any other temperatures the ph of a color change can vary Colored solution affect the usefulness of an indicator Dissolved salts can also sometimes react with the indicator These issues can be overcome with indicator strips These are pieces of paper that have been soaked in indicator solution and then dried It is often important to test the ph of soil prior to planting - why?

33 Why are these plants different?

34 ph Meters ph meters are probes that can make fast, continuous measurements of ph These are more accurate that measurements taken with an indicator solution They can be accurate to within 0.01 ph of the true ph value They are unaffected by color, or cloudiness The only issues can occur with calibrating

35 Strengths of Acids and Bases Section 19.3

36 Why is not dangerous to drink lemonade, even though it contains citric acid? Acids can be classed as strong or weak depending upon how much they ionize in water A strong acid ionizes completely in aqueous solution HCl is a strong acid A weak acid only ionizes a little bit in water. With ethanoic acid, less than 1% of ethanoic acid ionizes

37 Strengths of common acids and bases

38 Dissociation of a strong acid

39 Dissociation of a weak acid

40 The acid dissociation constant Ka: The ratio of the concentration of the dissociated forms of an acid to the concentration of the undissociated form This shows the fraction of the acid that ionizes Weak acids have small Ka values Strong acids have large Ka values This is like an equilibrium constant, but water is removed in dilute aqueous solutions

41 The base dissociation constant Just as there are strong and weak acids, there are strong and weak bases A Strong Base dissociates fully to form metal ions and hydroxide ions in aqueous solution Some, such as MgOH2 do not dissolve well, but those that do fully dissociate A Weak Base reacts with water to form the conjugate acid of the base and hydroxide ions Ammonia is a common example of a weak base NH3(aq) + H2O(l) Ammonia Water NH4+(aq) + OH (aq) Ammonium ion Hydroxide ion

42 Base dissociation constant The concentration of water is constant in dilute solutions, so [H2O] can be removed when calculating the base dissociation constant (Kb) Kb is the ratio of the conjugate acid times by the hydroxide, divided by concentration of base [NH4+] [OH ] Keq = [NH ] [H O] 3 2 Keq [H2O] = Kb = [NH4+] [OH ] [NH3]

43 Concentration vs Strength (they are different!!) Acids and Bases are strong or weak based on the degree to which they ionize in water Comparing Concentration and Strength of Acids Acidic solution Hydrochloric acid Concentration Quantitative (or Molar) Relative 12M HCl Concentrated Strength Strong Gastric juice 0.8M HCl Dilute Strong Ethanoic acid 17M CH3COOH Concentrated Weak Vinegar 0.2M CH3COOH Dilute Weak

44 Neutralization reactions Section 19.4

45 What happens when you mix an acid and a base together? A neutralization reaction takes place During a neutralization reaction, a salt and water are always produced Remember - a salt is any ionic compound made of an anion and cation

46 Mole ratios and acid-base reactions Neutralization is often used to describe the point at which a reaction has gone to completion At completion, the acid and base solutions must have equal numbers of hydrogen and hydroxide ions The mole ratio can be used to determine how much acid/base is required for neutralization to occur

47 How can you work out the concentration of an unknown acid or base? Titration A measured volume of a solution of known concentration is added to a known volume of a solution of unknown concentration If you want to find the concentration of an acid, you titrate into it a base of known concentration If you want to find the concentration of a base, you titrate into it an acid of known concentration You want to add just enough acid/base to observe a color change - to the nearest drop!

48 Titration curves

49 Interpreting titration curves Standard solution - the solution of known concentration in the biuret - can be either an acid or a base Equivalence point - the point in titration when neutralization occurs End point - the point where the color change occurs These are not necessarily the same thing! The end point is the visible change in indicator color that occurs near the equivalence point Neutralization occurs when the number of moles of hydrogen ions equals the number of ions of hydroxide ions

50 Salts in Solution Section 19.5

51 Are all salt solutions neutral?

52 Not all titration curves are the same

53 Why is there a difference when you titrate a strong acid or a weak acid with a strong base? The equivalence point is at a higher ph with a weak acid because of salt hydrolysis Salt hydrolysis is when the cations or anions of a dissociated salt remove hydrogen ions from water or donate hydrogen ions to water The result is that a solution becomes either acidic or basic

54 Hydrolysis of the salt of a weak acid and strong base Sodium ethanoate is the salt produced from the combination of the weak acid ethanoic acid and the strong base sodium hydroxide Sodium ethanoate ionizes completely The ethanoate ion is a Bronsted-lowry base, so it can accept a hydrogen ion This ion reacts with water to form an ethanoic acid and hydroxide ions Because the concentration of OH- increases, the solution becomes basic CH3COONa(aq) CH3COO (aq) + Na+(aq) Sodium ethanoate CH3COO (aq) + H+ acceptor Brønsted-Lowry base H2O(l) H+ donor Brønsted-Lowry acid Ethanoate ion Sodium ion CH3COOH(aq) + OH (aq) (makes the solution basic)

55 Hydrolysis of the salt of a strong acid and weak base Ammonium chloride is the salt of a strong acid (HCl) and the weak base Ammonia Ammonium chloride will ionize completely NH4Cl(aq) NH4+(aq) + Cl (aq) The ammonium ion is a Bronsted - lowry acid In solution it donates a hydrogen ion to a water molecule The product is ammonia molecules and hydronium ions Due to the increase in H3O+ ions, the solution becomes acidic NH4+(aq) H+ donor Brønsted-Lowry acid + H2O(l) H+ acceptor Brønsted-Lowry base NH3(aq) + H3O+(aq) (makes the solution acidic)

56 General rules for hydrolysis Strong acid + Strong base Neutral solution Strong acid + Weak base Acidic solution Weak acid + Strong base Basic solution

57 How can you control the ph of a solution? You can use a buffer solution A buffer is a solution in which the ph remains fairly constant when small amount of acid or base are added They are made of a solution of a weak base and one of its salt One component of the buffer acts as a hydrogen ion acceptor, whereas the second component can react with the hydroxide ions Both components can work to mop up any hydrogen and hydroxide ions that are added to the solution

58 What does all this mean? Ethanoic acid and its salt sodium ethanoate works as a buffer solution When you add H+ ions to this solution, the ethanoate ion from the salt sodium ethanoate react with the H+. Ethanoic acid is formed, but this is a weak acid, only ionizing slightly in water, causing only a small change in ph Eventually all the H+ ion acceptors of H+ ion donors become used up, and the buffer can no longer control the ph of the solution The Buffer capacity is the amount of acid or base that can be added to a buffer before a large change in ph occurs. CH CH33COO COO (aq) (aq) ++ H H++(aq) (aq) Ethanoate Ethanoateion ion Hydrogen Hydrogen ion ion CH CH33COOH(aq) COOH(aq) Ethanoic Ethanoicacid acid