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Acids and Bases

Acids and Bases. Chapter 16. Pre-Chapter Questions. What is meant by the term acid? Name two products you think are acidic. What is meant by the term base? What do you know about the pH scale?. Properties of acids and bases. Acids. Bases. Bitter pH >7 Conduct electricity

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Acids and Bases

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  1. Acids and Bases Chapter 16

  2. Pre-Chapter Questions • What is meant by the term acid? Name two products you think are acidic. • What is meant by the term base? • What do you know about the pH scale?

  3. Properties of acids and bases Acids Bases Bitter pH >7 Conduct electricity Dissolved in water Used in cleaners Soaps / slippery Magenta/pink in phenolphthalein • Sour • pH <7 • Conduct electricity • Dissolved in water • Clear in phenolphthalein • Corrode metals

  4. What are acids and bases? • Simplest view… Arrhenius: • An acid produces hydrogen ions (H+) in an aqueous solution • A base produces hydroxide ions (OH-) in an aqueous solution • more General Bronsted or Thomas Lowry, they would say that: • An acid is a proton (H+) donor • A base is a proton acceptor

  5. Bronsted-Lowry Model • HA (aq) + H2O (l)  H3O+ (aq) + A- (aq) • Acid base conjugate conjugate acid base • A conjugate acid-base pair is a pair of substances related to each other by the donating and accepting of a single proton. • Oh, by the way… • H3O+ is called the hydronium ion

  6. Strength of acids • A strong acid is an acid that completely ionizes (dissociates) when dissolved • A weak acid most of the acid stays in tact

  7. STRONG ACIDS • HCl • HBr • HI • H2SO4 • HNO3 • HClO4 • In general 2 more O than H in parent

  8. strong acid / weak conj base The conjugate base of a strong acid doesn’t compete much with H2O for the proton - water is a stronger base

  9. Water just can’t decide… • Water can be both an acid and a base • Amphoteric substance • Water can even react with itself! • In fact, [H+] and the [OH-] are the same • [H+] = [OH-] = 1.0 x 10-7 • FYI… [ ] means molarity, or concentration

  10. H2O(l) + H2O(l) <--> H3O+(aq) + OH-(aq) • What is the equilibrium constant for this reaction? • K = [H+][OH-] = 1 x10-14 • This equilibrium constant is given a special name and notation. • Kw the ion-product constant for water. • This is an important number. It means that in any aqueous solution at 25˚C, no matter what the solution contains, the product of [H+] and [OH-] must equal 1 x 10-14.

  11. THEREFORE… • If [H+] = [OH-], the solution is neutral. • If [H+] > [OH-], the solution is acidic • If [H+] < [OH-], the solution is basic. • In any case, the product of [H+] and [OH-] will be 1 x 10-14.

  12. pH Scale • pH is based on a logarithm, base 10 • pH = -log [H+] • Remember: [ ] means concentration

  13. Determining Acid Strength • When dealing with acids and bases, we’re dealing with really small numbers for concentrations. • Need some other way to describe acid strength. • pH Scale!

  14. A word on sig figs • The rules for sig figs are a bit different for pH, since it’s a log function.

  15. The pH scale

  16. What about bases? • Just do the same thing, only with OH-. • pOH = -log [OH-] • pH + pOH = 14.00

  17. Undoing the pH

  18. Salt Hydrolysis • hydrolysis occurs when certain salts dissolve in water to form solutions that have acidic or basic properties. Positive ionnegative ionresulting solution strong base strong acid strong base weak acid weak base strong acid weak base weak acid

  19. Salt hydrolysis examples • KCl • NaHCO3 • Fe(NO3)3 • Bi(C2H3O2)3 • CaHSO4

  20. Hydrolysis • How does it work? • The conjugate base of a weak acid is a stronger base than water... so most of the protons end up as undissociated acid. • NaNO2 - the Na+1 from a strong base, so Na+1 doesn’t interact with water, NO2-1 is from a weak acid, so a somewhat strong base (stronger than H2O) so what happens? • NO2-1 + H2O <--> HNO2 + OH-1 (basic)

  21. Weak acid equilibrium • Weak acids dissolved in water are in equilibrium with their conjugate base • Ka is the equilibrium constant for weak acids • The Ka expression is similar in form to Keq for gases, products over reactants to the power of coefficients.

  22. Weak acid examples • See worksheet... • Do worksheet... also calculate the pH of each solution

  23. Measuring pH • Indicators • substances that exhibit different colors in acidic and basic solutions • litmus Paper • pH Meters • Cabbage juice

  24. Some Chemical indicators

  25. Titrations and Buffers • A few definitions… • Titration – delivering a measured volume of a solution of known concentration into the solution being analyzed • Titrant – a standard solution • Buret – device used for accurate measurement of the delivery of a liquid • Stoichiometric point (equivalence point) – when just enough titrant has been added to react with all of the solution being analyzed

  26. Consider the following… • How many moles of H+ can be donated by the following acids: • HCl • H2CO3 • H3PO4 • So, if I have 1 mol of each acid, I have a different amount of H+ • Need to consider how many H’s or OH’s are in the compounds when reacting them together.

  27. Titration Calculations • A titration is a neutralization reaction. • A base reacting with an acid producing water and a salt. • FOR ANY NEUTRALIZATION REACTION • moles of H+ = moles of OH- • Determine the volume of 0.100 M NaOH needed to titrate 50.0 ml of HNO3.

  28. All about buffers • Buffered solution – resists a change in its pH when either and acid or a base has been added • Presence of a weak acid and its conjugate base buffers the solution • Blood

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