Chapters 9 & 19 Chemistry 1L Cypress Creek High School Unit 13: Acids & Bases Chapters 9 & 19 Chemistry 1L Cypress Creek High School

Properties of Acids Taste sour Feel watery Are electrolytes: conduct an electrical current by forming H+ ions in solution React with metals Corrosive pH < 7 2

Arrhenius’ Definition of Acids Arrhenius defined acids as anything that produces H+ ions in solution Monoprotic acids – have 1 ionizable hydrogen (HCl) Diprotic acids – have 2 ionizable hydrogens (H2SO4) Triprotic acids – you guessed it…they have 3 (H3PO4) Produces H+ ions in solution 3

Bronsted-Lowry Definition of Acids  According to Bronsted-Lowry an acid is a chemical that is able to lose or "donate" a hydrogen ion (aka proton because the H+ contains no electrons or neutrons) during a reaction. Acid loses a hydrogen ion

Naming an Acid hydro ______ic acid ate ions: _________ic acid Binary acids (H + nonmetal): hydro ______ic acid Ternary acids (H + polyatomic ion): ate ions: _________ic acid ite ions: _________ous acid Name the following acids: HBr H2SO3 H3PO4 Write the formulas for the following acids: Hydrosulfuric acid Nitrous acid Chromic acid 5

Properties of Bases Taste bitter Feel slippery Corrosive Are electrolytes: conduct an electrical current by forming OH- ions in solution pH > 7 6

Arrhenius’ Definition of Bases Arrhenius defined Bases as anything that produces OH- ions in solution Produces OH- ions in solution 7

Bronsted-Lowry Definition of Bases A chemical species that is able to gain or "accept" a hydrogen ion (aka proton because the H+ contains no electrons or neutrons) during a chemical reaction. Base gains a hydrogen ion

Naming a Base Formula is either: Metal + OH- or Ammonium + OH- Name + hydroxide Name the following bases: NaOH NH4OH NH3 Ca(OH)2 Write the formulas for the following bases: Magnesium hydroxide Aluminum hydroxide 9

Conjugates Conjugate acid is formed when a base accepts a proton (H+) Conjugate base is formed when a acid donates a proton (H+) 10 10

Conjugate Acid/Base Pairs Acids and Bases are always reactants Conjugate acids and conjugate bases are always products. Conjugate acid is the product that remains when a base has accepted a proton. Conjugate base is the product that remains when an acid has donated a proton. Ex NH3 + H2O → NH4+ + OH- (base) (acid) (c.acid) (c.base)

Conjugate Practice NH3(g) + H3O+(aq)  NH4+(aq) + H2O(l) CH3OH(l) + NH2-(aq)  CH3O-(aq) + NH3(g) OH-(aq) + H3O+(aq)  H2O(l) + H2O(l) NH2-(aq) + H2O(l)  NH3(g) + OH-(aq) Base Acid C.Acid C.Base Acid Base C.Base C.Acid Base Acid C.Acid C.Base Base Acid C.Acid C.Base

Amphoteric Some substances can act as an acid or a base and are called amphoteric. Ex. H2O can act as both an acid and a base NH3 + H2O → NH4+ + OH- (base) (acid) H+ was lost by water HCl + H2O → H3O+ + Cl- (acid) (base) H+ was gained by water 13

Indicators Indicators are organic substances that change colors in an acid or a base (sometimes paper, sometimes liquids) Acids turn… Litmus paper – red Phenolphthalein – clear Universal indicator – yellow/orange/red Bases turn… Litmus paper – blue Phenolphthalein – magenta (hot pink) Universal indicator – dark green/blue/purple 14

The pH Scale pH is a mathematical scale in which the concentration of hydrogen ions (H+) in a solution is expressed as a number from 0 to 14. The pH scale is a convenient way to describe the concentration of H+ ions in acidic solutions, as well as the hydroxide ions in basic solutions.

pH and pOH Scale [H+] and [OH-] are inverses: the exponents always add up to -14 Ex: If [H+] = 1.0x10-3 M, then [OH-] = 1.0x10-11 M

Household Items & the pH Scale

Mathematical Definition of pH and pOH The letter “p” in front of pH and pOH refers to the negative log. A logarithm (log) is the power to which a base, such as 10, must be raised to produce a given number. For example: the logarithm of 1000 to base: 10 is 3, because 3 is the power to which ten must be raised to produce 1000: 103 = 1000

pH and pOH The pH of a solution equals the negative logarithm of the hydrogen ion concentration. Chemists have also defined a pOH scale to express the basicity of a solution. In an acidic solution, [H+] > [OH-] In a basic solution, [H+] < [OH-] pH + pOH = 14 pH pOH Stands for “power of hydrogen” Stands for “power of hydroxide” Shows the [H+] in a solution Shows the [OH-] in a solution pH = -log[H+] pOH = -log[OH-]

pH and pOH Calculations Notice the relationship between concentration and the pH/pOH value: Toothpaste has a hydrogen ion concentration of 10–10M, so its pH is 10. pH = -log[10-10] = 10 Pure water, which is neutral, has a pH of 7. That means its hydrogen ion concentration is 10–7M. 7 = -log[H+] ; [H+] = 10-7

Relating pH, pOH, [H+], & [OH-] 4 2 1x10-6 1.26x10-4 9.64 13.7 1.82x10-4 10 1x10-4 1x10-10 12 1x10-12 1x10-2 6 8 1x10-8 10.1 3.9 7.9x10-11 4.36 4.37x10-5 2.29x10-10 0.3 2.00x10-14 5.01x10-1 3.74 10.26 5.50x10-11

Strength of Acids and Bases The terms weak and strong are used to compare the strengths of acids and bases The terms dilute and concentrated are used to compare the concentration of solutions They do not mean the same thing! The combination of strength and concentration determines the behavior of the acid or base.

Strength of Acids and Bases Strength is Determined by how much they ionize (dissociate) in water Strong – ionize 100% in water Weak – only partially ionize in water Complete dissociation - all HCl compounds have separated into H+ and Cl- ions Partial dissociation - some HNO2 compounds have separated into H+ and NO2- ions, while some remain together 23

Strength of Acids and Bases Strong Acids: Only HNO3, HI, HBr, HCl, H2SO4, HClO4 Remember: NO, I Brought Claude SOme ClOthes All other acids are weak, and remain in equilibrium Ex: HNO2 ↔ H+ + NO2- Strong Bases: Group I or II metals Ex: NaOH, Mg(OH)2…etc. All other bases are weak, and remain in equilibrium NH3 + H2O ↔ NH4+ + OH- 24

Conjugate Strength Strong Acid = weak conjugate base Strong Base = weak conjugate acid Weak Acid = strong conjugate base Weak Base = strong conjugate acid Example: HCl + NH3 → Cl- + NH4+ (strong acid) (weak base) (weak c. base) (strong c. acid) 25

Acid-Base Reactions The reaction of an acid and a base to produce a salt and water is called a neutralization reaction.

Neutralization Reactions Sodium hydroxide (base) and hydrochloric acid (acid) react to form sodium chloride (salt) and water.

Neutralization Practice Neutralizations are double displacement reactions! Practice predicting products (always a salt and water): Example #1 __Ca(OH)2 + __H3PO4 → ? __Ca(OH)2 + __H3PO4 → __Ca3(PO4)2 + __H2O 3Ca(OH)2 + 2H3PO4 → Ca3(PO4)2 + 6H2O Example #2 __Fe(OH)2 + __HBr → ? __Fe(OH)2 + __HBr → __FeBr2 + __H2O Fe(OH)2 + 2HBr → FeBr2 + 2H2O 28

Acid-Base Titrations Titration is used in determining the concentration of an unknown acid or base. An indicator is added to the standardized acid (acid of known concentration). The unknown base is added slowly with a burette until the solution is neutralized and reaches the equivalence point (where [H+] = [OH-]). The equivalence point is not necessarily where pH = 7 Adding one more drop of base changes the color of the solution to pink. This is called the endpoint.

Titration Calculations MaVa(H+)= MbVb(OH-) Note: pH at the equivalence point is not always 7. Strong Acid + Strong Base, pH=7 Strong Acid + Weak Base, pH<7 Weak Acid + Strong Base, pH>7 Example: It took 75mL of NaOH to neutralize 50mL of 2M HCl. What is the concentration of the NaOH? MaVa(#H+) = MbVb(#OH-) (2M)(50mL)(1) = (x)(75mL)(1) x = 1.33M NaOH

Titration Practice It took 20mL of Ca(OH)2 to neutralize 25mL of 0.05M HCl. What is the concentration of the base? MaVa(#H+) = MbVb(#OH-) (0.05M)(25mL)(1) = (x)(20mL)(2) x = 0.031 M