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Unit 7: Acids and Bases

Unit 7: Acids and Bases. Acids and Bases: The Basics. Acid comes from the Latin word, acidus, which means “sour.” Ascorbic acid: C 6 H 8 O 8 – Citrus fruits Acetic acid: CH 3 COOH – Vinegar Hydrochloric acid: HCl – Toilet bowl cleaners Carbonic acid: H 2 CO 3 - Soda

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Unit 7: Acids and Bases

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  1. Unit 7: Acids and Bases

  2. Acids and Bases: The Basics • Acid comes from the Latin word, acidus, which means “sour.” • Ascorbic acid: C6H8O8 – Citrus fruits • Acetic acid: CH3COOH – Vinegar • Hydrochloric acid: HCl – Toilet bowl cleaners • Carbonic acid: H2CO3 - Soda • Sulfuric Acid: H2SO4 – Fertilizers

  3. Strength of Acids • Strong acid: an acid that dissociates completely in aqueous solution. They are considered strong electrolytes. • Examples: HCl, HNO3, H2SO4, HBr • Acids tend to produce H3O+ (Hydronium ion = Hydrogen ion) • H3O+ is the same thing as H+

  4. Strength of Acids • Weak acid: an acid that does not dissociate completely in aqueous solution. • Examples: H3PO4, CH3COOH, H2CO3

  5. Strength of Acids

  6. Strength of Bases • Bases have a bitter taste and a slippery feel. • Also known as “alkaline” • Sodium Hydroxide (NaOH) – drain cleaners • Sodium bicarbonate (NaHCO3) – baking soda • Potassium carbonate (K2CO3) – ashes

  7. Strength of Bases • Strong base: A base that completed dissociates in water and yields aqueous OH- ions. • Weak bases: A base that does not produce a large number of hydroxide ions (does not dissociate completely).

  8. Acids and Bases • Strong acids and strong bases are considered strong electrolytes, because a large concentration of ions are produced.

  9. Bronsted-Lowry: Acid and Bases • In the Bronsted-Lowry definition, an acid is any chemical that donates a hydrogen ion, H+ (H3O+), and a base is any chemical that accepts a hydrogen ion. B.A.A.D - “Bases Accept, Acids Donate”

  10. Consider what happens when hydrochloric acid is mixed with water: …resulting in a 3rd hydrogen bonded to oxygen HCl donates a H+… Hydrogen ion H3O+ HCl behaves as an acid (proton donor) and water behaves as a base (proton acceptor). Acids, when dissolved in water, release hydrogen ions.

  11. Ammonia behaves as a base by accepting a H+ from water, which, in this case, behaves as an acid. Hydroxide ion Bases tend to increase the concentration of hydroxide ions.

  12. Hydroxide ion

  13. Amphoteric • Amphoteric: A substance that is capable of acting as either an acid or a base • It acts as a base when combined with something more strongly acidic than itself. • It acts as an acid when combined with something more strongly basic than itself. • Water has the ability to react with itself.

  14. Keep in mind… • Acid-base interactions are almost seen as a behavior. For instance, water can behave as a base and as an acid. • An ammonium ion may donate a H+ back to OH- to reform ammonia and water.

  15. Forward and reverse acid-base reactions proceed simultaneously and can therefore be represented by the double arrow.

  16. Identify the acid or base behavior for each participant in the reaction: H2PO4 + H3O+↔ H3PO4 + H2O Forward:  • H2PO4-accepts a H+ to become H3PO4 H2PO4-behaves as a base • H3O+donates a H+ to become H2O H3O+ behaves as an acid

  17. Identify the acid or base behavior for each participant in the reaction: H2PO4- + H3O+↔ H3PO4 + H2O Backwards:  • H3PO4donates a H+ to become H2PO4- H3PO4behaves as a acid • H2Oaccepts a H+ to become H3O+ H2O behaves as a base

  18. Conjugate Acid-Base Pairs • In any acid-base equilibrium (↔), it involves the transfer of H+. • An acid and a base that only differs in the presence or absence of a H+ are called a conjugate acid-base pair. • Every acid has a conjugate base • For example: H2O (acids donate) can become OH- • Every base has a conjugate acid • For example, H2O (bases accept) can become H3O+

  19. Conjugate Acid-Base Pairs donates H+ HNO2(aq)+ H2O (l)↔ NO2-(aq)+ H3O+(aq) Acid Base Conjugate Base Conjugate Acid Accepts H+ HNO2donates an H+ and becomes its conjugate base, NO2- H2O accepts an H+ and becomes its conjugate acid, H3O+

  20. Conjugate Acid-Base Pairs Accepts H+ Conjugateacid base HCl + H2O H3O+ + Cl- Conjugate base acid Donates H+

  21. + 1- Acid Dissociation H Cl HCl Conjugate base Acid Conjugate pair Kelter, Carr, Scott, Chemistry A World of Choices 1999, page 280

  22. Conjugate Acid-Base Pairs Donates H+ acid Conjugate base NH3 + H2O NH4+ + OH- Conjugate acid base Accepts H+

  23. What is the conjugate base of each of the following acids: • Step 1: Remember that acids DONATES an H+ • Step 2: Its conjugate base always has an extra negative charge • HClO4 • ClO4- • H2S • HS- • HCO3- • CO32-

  24. What is the conjugate acid of the following bases? • Step 1: Remember that all bases ACCEPT an H+ • Step 2: Its conjugate acid always has one LESS negative charge • CN- • HCN • H2O • H3O+ • HCO3- • H2CO3

  25. Neutralization Reactions • Neutralization reactions occur between an acid and a base. • These reactions often produce a salt, created from the positive ion of the base and the negative ion from the acid. ACID BASE SALT HCN + NaOH  NaCN + H2O HNO3+ KOH  KNO3 + H2O

  26. Neutralization Reactions De-ice roads • Predict the salt that is produced in the following: 2 HCl + Ca(OH)2 ____________ + 2 H2O HF + NaOH  ____________ + H2O CaCl2 NaF Prevents tooth decay

  27. What is pH?

  28. What is pH? • In this reaction, a water molecule gains a H+ and the second water molecule must lose a H+. • In pure water, the number of H+ = the number of OH- • The concentration of H+ and OH- each is extremely low – about 1 x 10-7 M

  29. What is pH? • [H3O+][OH-] = Kw • [1.0 x 10-7][1.0 x 10-7] = Kw • 1.0 x 10-14 = Kw • The dissociation constant of water, Kw, means that no matter WHAT is dissolved in water, the product of H+ and OH- always equals 1.0 x 10-14

  30. What is pH? If a small amount of HCl is added to water, it dissociates and increases the H+ from 1.0 x 10-7 to 1.0 x 10-5. Therefore, the OH- concentration decreases so that the product of H+ and OH- is still equal to Kw.

  31. Sample Problem • What is the concentration of H+ ions if the concentration of OH- ion is 1.0 x 10-3 M? [H3O+][OH-] = Kw [H3O+][1.0 x 10-3 M] = 1.0 x 10-14 M [H3O+] = [H3O+] = 1.0 x 10-11 M 1.0 x 10-14 M 1.0 x 10-3 M

  32. What is pH? • In an acidic solution, [H3O+] > [OH-] • In a basic solution, [H3O+] < [OH-] • In a neutral solution, [H3O+] = [OH-]

  33. Sample Problem • How does adding ammonia, NH3, to water make a basic solution when there are no hydroxide ions in the formula for ammonia? NH3 + H2O  NH4+ + OH- Ammonia increases the OH- concentration, thereby lowering the H+ concentration. Because [H+] < [OH-], the solution is basic.

  34. What is pH? • The pH scale is a numeric scale used to describe acidity. pH = -(log[H3O+]) Consider a neutral solution, [H+] = 1.0 x 10-7 M pH = -(log [1.0 x 10-7]) pH = -(-7) pH = 7

  35. What is pH? • Acidic solutions have greater H+ concentrations, which lowers its pH. • Acidic solutions: pH < 7 Consider [H+] = 1.0 x 10-4 M pH = -(log [H+]) pH = -(log [1.0 x 10-4 M]) pH = -(-4) pH = 4

  36. What is pH? • Basic solutions have pH values > 7, because its H+ concentrations are less. Consider [H+] = 1.0 x 10-8 M pH = -(log [H+]) pH = -(log [1.0 x 10-8 M]) pH = -(-8) pH = 8

  37. What is pH?

  38. Rainwater is Acid & Ocean Water is Basic

  39. Rainwater is Acidic: pH 5-6 • The source of this acidity is carbon dioxide, the same gas that gives fizz to soda pop. • There are 760 billion tons of CO2 in the atmosphere that undergo this reaction: CO2 (g) + H2O (l)  H2CO3 (aq) Carbonic acid • Carbonic acid lowers the pH, accelerating the erosion of land and historical artifacts.

  40. What is classified as ACID RAIN? • Acid rain: rain that has a pH < 5. • Source: Airborne pollutants that are absorbed by atmospheric moisture, most commonly – sulfur dioxide. • Sulfur dioxide is readily convert to sulfur trioxide… 2 SO2 (g) + O2 (g)  2 SO3 (g) • …which reacts with water to form sulfuric acid. SO3 (g) + H2O (l)  H2SO4 (aq) • Acid rain affects vegetation and ecosystems.

  41. ACID RAIN CYCLE

  42. Impact of Acid Rain: Midwest • Midwest: The ground contains calcium carbonate (basic), which often neutralizes the acid rain before much damage is done.

  43. Liming • In order to rains the pH of acidified lakes and rivers by adding calcium carbonate – a process called liming. • Long-term solution: Prevent the sulfur dioxide and other pollutants from entering the atmosphere in the first place. • Shift fossil fuels to nuclear and solar energy

  44. Impact of Acid Rain: Northeast • Northeast: The ground contains LITTLE CaCO3. The effect of acid rain on lakes and rivers accumulate.

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