CHEMISTRY Matter and Change

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1 CHEMISTRY Matter and Change UNIT 18 Table Of Contents Section 18.1 Introduction to Acids and Bases Unit 18: Acids and Bases Section 18.2 Section 18.3 Section 18.4 Strengths of Acids and Bases Hydrogen Ions and ph Neutralization Objectives Identify the physical and chemical properties of acids and bases. Classify solutions as acidic, basic, or neutral. Compare the Arrhenius, Brønsted-Lowry, and Lewis models of acids and bases. Review Vocabulary Lewis structure: a model that uses electron-dot structures to show how electrons are arranged in molecules New Vocabulary acidic solution basic solution Arrhenius model Brønsted-Lowry model conjugate acid conjugate base conjugate acid-base pair amphoteric Lewis model Different models help describe the behavior of acids and bases. 1

2 Properties of Acids and Bases Acids taste sour. Bases taste bitter and feel slippery. Acids and bases are conductors of electricity. Acids and bases can be identified by their reactions with some metals and metal carbonates. Properties of Acids and Bases (cont.) Acids turn blue litmus red. Bases turn red litmus blue. Magnesium and zinc react with acids to produce hydrogen gas. Geologists identify limestone because it produces bubbles of carbon dioxide when exposed to hydrochloric acid. Properties of Acids and Bases (cont.) All water solutions contain hydrogen ions (H + ) and hydroxide ions (OH ). An acidic solution contains more hydrogen ions than hydroxide ions. A basic solution contains more hydroxide ions than hydrogen ions. Properties of Acids and Bases (cont.) The usual solvent for acids and bases is water water produces equal numbers of hydrogen and hydroxide ions in a process called selfionization. H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH (aq) The hydronium ion is H 3 O +. 2

3 The Arrhenius Model The Arrhenius model states that an acid is a substance that contains hydrogen and ionizes to produce hydrogen ions in aqueous solution, and a base is a substance that contains a hydroxide group and dissociates to produce a hydroxide ion in solution. The Arrhenius Model (cont.) Arrhenius acids and bases HCl ionizes to produce H + ions. HCl(g) H + (aq) + Cl (aq) NaOH dissociates to produce OH ions. NaOH(s) Na + (aq) + OH (aq) Some solutions produce hydroxide ions even though they do not contain a hydroxide group. The Brønsted-Lowry Model The Brønsted-Lowry Model of acids and bases states that an acid is a hydrogen ion donor, and a base is a hydrogen ion acceptor. The Brønsted-Lowry Model is a more inclusive model of acids and bases. The Brønsted-Lowry Model (cont.) A conjugate acid is the species produced when a base accepts a hydrogen ion. A conjugate base is the species produced when an acid donates a hydrogen ion. A conjugate acid-base pair consists of two substances related to each other by donating and accepting a single hydrogen ion. 3

4 The Brønsted-Lowry Model (cont.) Hydrogen fluoride a Brønsted-Lowry acid HF(aq) + H 2 O(l) H 3 O + (aq) + F (aq) HF = acid, H 2 O = base, H 3 O + = conjugate acid, F = conjugate base The Brønsted-Lowry Model (cont.) Ammonia Brønsted-Lowry base NH 3 (aq) + H 2 O(l) NH 4+ (aq) + OH (aq) NH 3 = base, H 2 O(l) = acid, NH 4+ = conjugate acid, OH = conjugate base Water and other substances that can act as acids or bases are called amphoteric. Monoprotic and Polyprotic Acids An acid that can donate only one hydrogen ion is a monoprotic acid Introduction to Acids and Bases Monoprotic and Polyprotic Acids Acids that can donate more than one hydrogen ion are polyprotic acids. Only ionizable hydrogen atoms can be donated. 4

5 The Lewis Model According to the Lewis model, a Lewis acid is an electron-pair acceptor and a Lewis base is an electron pair donor. The Lewis model includes all the substances classified as Brønsted-Lowry acids and bases and many more Section Check A Lewis acid is a(n). A. electron pair donor B. hydrogen ion donor C. electron pair acceptor D. substance that contains an hydroxide group 18.1 Section Check A conjugate acid is formed when: A. a base accepts a hydrogen ion B. an acid accepts a hydrogen ion C. an acid donates a hydrogen ion D. a base donates a hydrogen ion 5

6 Objectives Relate the strength of an acid or base to its degree of ionization. Compare the strength of a weak acid with the strength of its conjugate base. Explain the relationship between the strengths of acids and bases and the values of their ionization constants. Review Vocabulary electrolyte: an ionic compound whose aqueous solution conducts an electric current New Vocabulary strong acid weak acid strong base weak base acid ionization constant base ionization constant In solution, strong acids and bases ionize completely, but weak acids and bases ionize only partially. Strengths of Acids Acids that ionize completely are strong acids. Because they produce the maximum number of hydrogen ions, strong acids are good conductors of electricity. 6

7 Strengths of Acids (cont.) Acids that ionize only partially in dilute aqueous solutions are called weak acids. Strengths of Acids (cont.) With a strong acid, the conjugate base is a weak base. Equilibrium lies almost completely to the right in the equation because the conjugate base has a weaker attraction for the H + ion than does the base in the forward reaction. Strengths of Acids (cont.) In a weak acid, the ionization equilibrium lies to the far left in the ionization equation because the conjugate base has a greater attraction for H + ions than does the base in the forward reaction. Strengths of Acids (cont.) The equilibrium constant, K eq, provides a quantitative measure of the degree of ionization of an acid. The acid ionization constant is the value of the equilibrium constant expression for the ionization of a weak acid, K a. K a indicates whether products or reactants are favored at equilibrium. 7

8 Strengths of Acids (cont.) For weak acids, the products tend to be smaller compared to the un-ionized molecules (reactant). Weaker acids have a smaller K a. Strengths of Bases A base that dissociates completely into metal ions and hydroxide ions is known as a strong base. A weak base ionizes only partially in dilute aqueous solution. Strengths of Bases (cont.) The base ionization constant, K b, is the value of the equilibrium constant expression for the ionization of a base Section Check A solution with a small K b is a. A. weak acid B. weak base C. strong acid D. strong base 8

9 18.2 Section Check Where is the equilibrium point in the ionization equation for a strong acid? A. far right B. far left C. slightly right D. slightly left 18.3 Hydrogen Ions and ph Objectives Explain ph and poh. Relate ph and poh to the ion product constant for water. Calculate the ph and poh of aqueous solutions. Review Vocabulary Le Châtelier s principle: states that if a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress 18.3 Hydrogen Ions and ph ph and poh are logarithmic scales that express the concentrations of hydrogen ions and hydroxide ions in aqueous solutions. New Vocabulary ion product constant for water ph poh 18.3 Hydrogen Ions and ph Ion Product Constant for Water Pure water contains equal concentrations of H + and OH ions. The ion product constant of water, K w = [H + ][OH ]. The ion product constant for water is the value of the equilibrium constant expression for the selfionization of water. 9

10 18.3 Hydrogen Ions and ph Ion Product Constant for Water (cont.) With pure water at 298 K, both [H + ] and [OH ] are equal to M. K w at 298 K = K w and LeChâtelier s Principle proves [H + ] [OH ] must equal at 298 K, and as [H + ] goes up, [OH ] must go down Hydrogen Ions and ph ph and poh Concentrations of H + ions are often small numbers expressed in scientific notation. ph is the negative logarithm of the hydrogen ion concentration of a solution. ph = log [H + ] 18.3 Hydrogen Ions and ph ph and poh (cont.) poh of a solution is the negative logarithm of the hydroxide ion concentration. poh = log [OH ] The sum of ph and poh equals Hydrogen Ions and ph MEMORIZE THIS 10

11 18.3 Hydrogen Ions and ph MEMORIZE THIS 18.3 Hydrogen Ions and ph ph and poh (cont.) For all strong monoprotic acids, the concentration of the acid is the concentration of H + ions. For all strong bases, the concentration of the OH ions available is the concentration of the base. Weak acids and weak bases only partially ionize and K a and K b values must be used Hydrogen Ions and ph ph and poh (cont.) Litmus paper and a ph meter with electrodes can determine the ph of a solution Section Check In dilute aqueous solution, as [H + ] increases. A. ph decreases B. poh increases C. [OH ] decreases D. all of the above 11

12 18.3 Section Check What is the ph of a neutral solution such as pure water? A. 0 B. 7 C. 14 D Objectives Write chemical equations for neutralization reactions. Explain how neutralization reactions are used in acidbase titrations. Compare the properties of buffered and unbuffered solutions. Review Vocabulary stoichiometry: the study of quantitative relationships between the amounts of reactants used and products formed by a chemical reaction; is based on the law of conservation of mass New Vocabulary neutralization reaction salt titration titrant acid-base indicator end point salt hydrolysis buffer equivalence point buffer capacity In a neutralization reaction, an acid reacts with a base to produce a salt and water. Reactions Between Acids and Bases A neutralization reaction is a reaction in which an acid and a base in an aqueous solution react to produce a salt and water. A salt is an ionic compound made up of a cation from a base and an anion from an acid. Neutralization is a double-replacement reaction. 12

13 Reactions Between Acids and Bases (cont.) Reactions Between Acids and Bases (cont.) Titration is a method for determining the concentration of a solution by reacting a known volume of that solution with a solution of known concentration. Reactions Between Acids and Bases (cont.) In a titration procedure, a measured volume of an acid or base of unknown concentration is placed in a beaker, and initial ph recorded. A buret is filled with the titrating solution of known concentration, called a titrant. Reactions Between Acids and Bases (cont.) Measured volumes of the standard solution are added slowly and mixed into the solution in the beaker, and the ph is read and recorded after each addition. The process continues until the reaction reaches the equivalence point, which is the point at which moles of H + ion from the acid equals moles of OH ion from the base. An abrupt change in ph occurs at the equivalence point. 13

14 Reactions Between Acids and Bases (cont.) Chemical dyes whose color are affected by acidic and basic solutions are called acid-base indicators. Reactions Between Acids and Bases (cont.) An end point is the point at which an indicator used in a titration changes color. An indicator will change color at the equivalence point. Salt Hydrolysis In salt hydrolysis, the anions of the dissociated salt accept hydrogen ions from water or the cations of the dissociated salt donate hydrogen ions to water. 14

15 Salt Hydrolysis (cont.) Salts that produce basic solutions KF is the salt of a strong base (KOH) and a weak acid (HF). KF(s) K + (aq) + F (aq) Salt Hydrolysis (cont.) Salts that produce acidic solutions NH 4 Cl is the salt of a weak base (NH 3 ) and strong acid (HCl). When dissolved in water, the salt dissociates into ammonium ions and chloride ions. NH 4 Cl(s) NH 4+ (aq) + Cl (aq) Salt Hydrolysis (cont.) Salts that produce neutral solutions NaNO 3 is the salt of a strong acid (HNO 3 ) and a strong base (NaOH). Little or no salt hydrolysis occurs because neither Na + nor NO 3 react with water. Buffered Solutions The ph of blood must be kept in within a narrow range. Buffers are solutions that resist changes in ph when limited amounts of acid or base are added. A buffer is a solution made up of a weak species and its conjugate. 15

16 Buffered Solutions (cont.) Ions and molecules in a buffer solution resist changes in ph by reacting with any hydrogen ions or hydroxide ions added to the buffered solution. HF(aq) H + (aq) + F (aq) When acid is added, the equilibrium shifts to the left. Buffered Solutions (cont.) Additional H + ions react with F ions to form undissociated HF molecules but the ph changes little. The amount of acid or base that a buffer solution can absorb without a significant change in ph is called the buffer capacity. Buffered Solutions (cont.) A buffer is most effective when the concentrations of the conjugate acid-base pair are equal or nearly equal Section Check In a neutralization reaction, an acid and base react to form. A. salt and oxygen gas B. salt and ammonia C. salt and water D. precipitate and water 16

17 18.4 Section Check 18.1 Study Guide Introduction to Acids and Bases Solutions that resist changes in ph are called. A. titrants B. salts C. conjugate pairs D. buffers Key Concepts The concentrations of hydrogen ions and hydroxide ions determine whether an aqueous solution is acidic, basic, or neutral. An Arrhenius acid must contain an ionizable hydrogen atom. An Arrhennius base must contain an ionizable hydroxide group. A Brønsted-Lowry acid is a hydrogen ion donor. A Brønsted-Lowry base is a hydrogen ion acceptor. A Lewis acid accepts an electron pair. A Lewis base donates an electron pair Strengths of Acids and Bases 18.3 Hydrogen Ions and ph Study Guide Study Guide Key Concepts Strong acids and strong bases are completely ionized in a dilute aqueous solution. Weak acids and weak bases are partially ionized in a dilute aqueous solution. For weak acids and weak bases, the value of the acid or base ionization constant is a measure of the strength of the acid or base. Key Concepts The ion product constant for water, K w, equals the product of the H + ion concentration and the OH ion concentration. K w = [H + ][OH ] The ph of a solution is the negative log of the hydrogen ion concentration. The poh is the negative log of the hydroxide ion concentration. ph plus poh equals 14. ph = log [H + ] poh = log [OH ] ph + poh = A neutral solution has a ph of 7.0 and a poh of 7.0 because the concentrations of hydrogen ions and hydroxide ions are equal. 17

18 18.4 Neutralization CHAPTER 18 Acids and Bases Study Guide Chapter Assessment Key Concepts In a neutralization reaction, an acid and a base react to form a salt and water. The net ionic equation for the neutralization of a strong acid by a strong base is H + (aq) + OH (aq) H 2 O(l). Titration is the process in which an acid-base neutralization reaction is used to determine the concentration of a solution. Which type of acid accepts an electron pair? A. Arrhenius B. Brønsted-Lowry C. Lewis D. Dalton Buffered solutions contain mixtures of molecules and ions that resist changes in ph. CHAPTER 18 Acids and Bases CHAPTER 18 Acids and Bases Chapter Assessment Chapter Assessment What is the conjugate of a weak acid? A. strong acid B. strong base C. weak acid D. weak base In a solution with a ph of 4.0, which of the following is true? A. [H + ] > [OH ] B. [H + ] < [OH ] C. [H + ] = [OH ] D. none of the above 18

19 CHAPTER 18 Acids and Bases CHAPTER 18 Acids and Bases Chapter Assessment Chapter Assessment Salt hydrolysis is what type of reaction? A. single-replacement B. double replacement C. synthesis D. decomposition The strength of a weak acid is measured by. A. ion product constant B. base ionization constant C. poh D. acid ionization constant CHAPTER 18 Acids and Bases CHAPTER 18 Acids and Bases Chapter Assessment Chapter Assessment What is the poh of 0.250M HBr, a strong acid? A B C D Which of the following is a diprotic acid? A. CH 3 COOH B. HCl C. NH + 3 D. H 2 SO 4 19

20 CHAPTER 18 Acids and Bases CHAPTER 18 Acids and Bases Chapter Assessment Chapter Assessment What is the conjugate acid in the following equation? HCl(s) + H 2 O H 3 O + (aq) + Cl (aq) A. Cl B. H 3 O + C. H 2 O D. HCl A decrease in concentration of hydrogen ions when hydroxide ions are added to an aqueous solution is an example of. A. an Arrhenius acid B. a Brønsted-Lowry acid C. the equivalence point D. Le Châtelier s Principle CHAPTER 18 Chapter Assessment Acids and Bases Which of the following is a polar molecule? A. CH 4 B. PCl 5 C. H 2 O D. H 2 20