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ACIDS AND BASES

ACIDS AND BASES. WHAT IS AN ACID?. An acidic solution contains more hydrogen ions (H + ) than hydroxide (OH - ) ions They must be in aqueous solutions Example: H 2 O  H + + OH -. More of these. WHAT ARE THE PROPERTIES OF AN ACID?. Acidic solutions taste sour

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ACIDS AND BASES

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  1. ACIDS AND BASES

  2. WHAT IS AN ACID? • An acidic solution contains more hydrogen ions (H+) than hydroxide (OH-) ions • They must be in aqueous solutions • Example: H2O  H+ + OH- More of these

  3. WHAT ARE THE PROPERTIES OF AN ACID? • Acidic solutions taste sour • Citric acid in lemons and limes, acetic acid in vinegar • Able to conduct electricity • That’s why there is battery acid • Cause litmus paper to turn red • Able to neutralize bases

  4. WHAT IS A BASE? • A basic solution contains more hydroxide ions (OH-) than hydrogen (H+) ions • They must be in aqueous solutions • Example: H2O  H+ + OH- More of these

  5. WHAT ARE THE PROPERTIES OF A BASE? • Basic solutions taste bitter • Basic solutions feel slippery • Bases are one of the ingredients of soaps and detergents • Bases also conduct electricity • Turns litmus paper blue • Able to neutralize acids

  6. INDICATORS • Scientists have a number of ways to measure if something is acidic or basic • These are called indicators

  7. ARRHENIUS ACID • The first person to determine how water became acidic or basic • Svante Arrhenius • Arrhenius acid: a compound that donates a hydrogen ion (H+) • Arrhenius base: a compound that donates a hydroxide ion (OH-)

  8. EXAMLES • Acids: • HCl, HF, HI, H2SO4, HNO3 • Bases • NaOH, KOH, Ca(OH)2,

  9. HOW IT WORKS • The actual molecule that makes aqueous solutions acidic is called the hydronium ion (H3O+) • H2O + HCl H3O+ + Cl- (acid solution) • Abbreviated: HCl  H+ + Cl- • NaOH Na+ + OH- (basic solution) • NOTE: Acids and bases only donate 1 H+ or 1 OH- at a time

  10. TRY THESE • For each of the following, draw the reaction for the acid or base. Indicate whether it is an acid or base: • HCl ? • KOH  ? • H2SO4  ? • Mg(OH)2  ?

  11. ANSWERS • HCl  H+ + Cl- • KOH  K+ + OH- • H2SO4  H+ + HSO4- • Mg(OH)2  MgOH+ + OH-

  12. WATER: THE SPECIAL CASE • Most compounds either donate a hydrogen ion (H+) or a hydroxide ion (OH-). • Water is an example of a compound that can do both • H2O + H2O  H3O+ + OH- • Since water can act as an acid or a base it is called: amphoteric

  13. HOW DO WE MEASURE ACID? • We have a scale that measures how acidic or basic a substance is: • pH scale • Scale goes from 0-14 • The smaller the number the more acidic • The bigger the number the more basic • pH of 7 is neutral • Each pH number is 10x stronger than the previous one • pH 4 is 10X stronger than pH 5

  14. pH Scale

  15. HOW DO WE FIGURE OUT THE CONCENTRATION OF AN ACID? • So far we have a scale that measures how acidic or basic: pH scale • Yet how does this relate to our previous measurement: the mole • Molarity: the number of moles of solute dissolved per liter of solution, also known as molar concentration • Has the unit of molarity (M)

  16. FURTHER DEFINITION • Solution: a uniform mixture of solids, liquids or gases; also called homogenous • Solute: one or more substances dissolved in a solution • The stuff you put into the mixture • Solvent: the substance that dissolves a solute to form a solution • The liquid that you add the stuff to

  17. EXAMPLE • Molarity of an acid is always based on the concentration of the H+ ion • Also written as [H+] • REMINDER: Acids and bases always donate 1 H+ or 1 OH- at a time • If you have 2 moles of HCl and you add it to 1L of water, what is the molarity of H+ in the solution?

  18. EXAMPLE • HCl  H+ + Cl- • 2 moles of HCl = 2 moles of H+ • 2moles = 2M solution 1 Liter • NOTE: To get molarity(M), you must have the solution in moles and Liters (L) • Molarity will also be a conversion (moles/L)

  19. LET’S TRY SOME • If you have 25.0g of sodium hydroxide in 250mL of water, what is the molarity(M) of the hydroxide ion? • If you add 155 g of hydrochloric acid in 500mL of water, what is the molarity(M) of the hydronium ion? • You add 330g of chromic acid to 100mL of water. What is the molarity(M) of the hydronium ion?

  20. ANSWERS • 2.5M • 8.5M • 28M

  21. HOW DO WE DO DILUTIONS? • A lot of times, when working with acids, you want to reduce the concentrations so they are not so strong. • HOW DO WE DO THIS?

  22. EXAMPLE • You have 500mL of a 6M solution of HCl. How do you dilute this solution to a 0.5M solution? • Here’s the formula • (V1)(M1) = (V2)(M2)

  23. ANSWER • (500mL)(6M) = (V2)(0.5M) • 3000mL•M = V2(0.5M) 0.5M 0.5M • 6000mL = V2 • Therefore, you have to add enough water to bring the volume to 6000mL

  24. TRY THESE • If you have 100mL of a 12M solution of NaOH. What is the final volume needed to make a 0.25M solution? • You start with 50mL of a 3.5M solution of HF. What is the final volume needed to make a 1.0M solution? • You put 22.8g of HCl in 250mL of water. How much water do you need to make it a 1.0M solution?

  25. ANSWERS • 4800mL • 175mL • 625mL

  26. HOW DO WE CONVERT BETWEEN pH AND MOLARITY • We have two scales to measure the amount of H+ in a solution • pH • Molarity (M) • There has to be a way to convert between pH and molarity

  27. pH FORMULA • The formula to convert between pH and concentration is: pH = -log[H+] • Let’s examine what these mean

  28. EXAMINING THE FORMULA • pH: the scale of how acidic or basic • log: is a math conversion using the base power of 10 (we will examine this on your calculator) • [H+]: the molar concentration of hydronium ions in solution

  29. LET’S TRY AN EXAMPLE • If you have a solution that has a 1.0x10-3 M of H+, what is the pH? • Step 1: Type 1.0x10-3 into your calculator • Step 2: Press the log button • Step 3: Take the negative of the answer • You should get pH = 3

  30. LET’S GO THE OTHER WAY • If you have a solution that has a pH of 3.5, what is the concentration of H+? • Step 1: Take the inverse log of the negative of the number • TI-30X: Type 2nd LOG and then enter the number • Casio: Type in the number and type shift log • Step 2: Make sure you enter the negative of the pH • [H+] = 3.16x10-4 M

  31. TRY THESE • A solution has a pH of 8.25. What is the molar concentration of H+? • A solution has a pH of 1.44. What is the molar concentration of H+? • If you have [H+] = 2.45x10-9 M, what is the pH? • If you have [H+] = 1.5x10-2 M, what is the pH?

  32. ANSWERS • 5.62X10-9 M • 3.63x10-2 M • 8.61 • 1.04

  33. USING pH IN A DILUTION • You have a 3250mL of a solution with a pH of 3.55. What final volume do you need to make the concentration of the solution 2.5x10-5 M?

  34. ANSWER • V1= 3250mL • M1= pH 3.55 = 2.82 x 10-4 M • V2= ? • M2= 2.5x10-5 M • V1M1 = V2M2 • (3250mL)(2.82 x10-4 M) = V2 (2.5x10-5 M) • V2 = 3.7x104 mL or 37L

  35. USING pH IN A DILUTION • You have 2.75L of a sample with a pH of 5.83. What final volume do you need to create a solution with a neutral pH?

  36. ANSWER • V1= 2.75L • M1= pH 5.83 = 1.48 x 10-6 M • V2= ? • M2= pH 7.00 = 1.00x10-7 M • V1M1 = V2M2 • (2.75L)(1.48 x10-6 M) = V2 (1.00x10-7 M) • V2 = 40.7L

  37. NEUTRALIZATION REACTIONS • So far . . . • An acid produces H+ • A base produces OH- • What do you get when you combine an acid with a base?

  38. NEUTRALIZATION REACTIONS • Answer: You get water and a salt. • Example: HCl + NaOH H2O + NaCl • When you mix an acid and a base, this is called a neutralization reaction. • Reminder: Pure water is pH 7

  39. YOU DON’T ALWAYS FULLY NEUTRALIZE • When you mix an acid and a base, you don’t produce all water and a salt. • For example, if you mix 250mL of 1.5M HCl with 250mL of 1.0M NaOH, will this reaction be fully neutralized? What is the new concentration?

  40. ANSWER: NO • Since the acid is more concentrated than the base, only part of the acid is neutralized. • How do we find out the new concentration of the solution after we mix the acid and base?

  41. NEUTRALIZATION • Step 1. Find out the moles of HCl • 250mL of 1.5M HCl • 0.250L | 1.5moles = 0.375 moles HCl 1L • Step 2. Find out the moles of NaOH • 250mL of 1.0M NaOH • 0.250L | 1.0moles = 0.250moles NaOH 1L

  42. NEUTRALIZATION • Step 3. Figure how much acid or base you have left by subtracting • 0.375moles H+ – 0.250 moles OH- • 0.125 moles of HCl were not neutralized • Step 4. Figure out the new molarity • 0.125 moles is now in 500mL (250mL acid + 250mL of base) • 0.125moles/0.500L = 0.25M HCl

  43. TRY THIS ONE • You mix 300mL of 2.5M HCl with 150mL of 0.75M KOH. What is the acid concentration of the solution?

  44. ANSWER • Total moles of HCl: • 0.300L| 2.5 moles = 0.75moles H+ 1 L • 0.150L| 0.75 moles = 0.1125moles OH- 1 L • 0.75moles – 0.1125 moles = 0.6375 moles • 0.6375 moles = 1.42M 0.450L

  45. TRY THIS ONE • You have 320mL of pH 4.5 solution. If you add 220mL of a 0.0032M solution of sodium hydroxide. What is the new concentration?

  46. ANSWER • 1.29x10-3 M

  47. FULL NEUTRALIZATION • Not only can you discover the new concentration, you can also figure out how much of an acid or a base you need to fully neutralize a sample. • Remember to fully neutralize a sample, you need to make it pH 7. • This means all the H+ reacts with all of the OH-

  48. EXAMPLE • You have 25.00mL of 3.2M HCl. How much 1.0M NaOH do you have to add to make the solution fully neutral? • Step 1. Find out how much H+ you have • 0.025L | 3.2moles = 0.080moles H+ 1L • Therefore you need 0.080 moles OH- to fully neutralize

  49. ANSWER • Step 2. How much NaOH do you need to add? • 0.080 moles | 1L = .080L 1 mole • Final answer: You need to add 80mL of 1.0M NaOH to fully neutralize 25.00mL of 3.2M HCl.

  50. SHORTCUT • Since you are doing a full neutralization, you can also use the dilution formula: V1M1 = V2M2 • Try out the previous problem to see if you get the same answer. • NOTE: This only works on FULL neutralization

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