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Bases

Bases. Who's on first?. Back to Our Color-Coded Scale. What about the right-hand side of this?. Some Questions. 1. If pH comes from [H + ], how do you have a pH in base, which doesn't have any H+? 2. What makes something a base in the first place?. Once Again, in Reverse.

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Bases

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  1. Bases Who's on first?

  2. Back to Our Color-Coded Scale What about the right-hand side of this?

  3. Some Questions 1. If pH comes from [H+], how do you have a pH in base, which doesn't have any H+? 2. What makes something a base in the first place?

  4. Once Again, in Reverse 2. What makes something a base in the first place? Arrhenius: makes OH- when put in water Bronsted: gets an H+ Two ways to make OH- in water:

  5. Once Again, in Reverse 2. What makes something a base in the first place? Arrhenius: makes OH- when put in water Bronsted: gets an H+ Two ways to make OH- in water: 1. Just put some hydroxide in! NaOH, or Ca(OH)2, or KOH... (note: these must be ionic, or it's not really OH-. CH3OH is not a base)

  6. Once Again, in Reverse 2. What makes something a base in the first place? Arrhenius: makes OH- when put in water Bronsted: gets an H+ Two ways to make OH- in water: 1. Just put some hydroxide in! NaOH, or Ca(OH)2, or KOH... (note: these must be ionic, or it's not really OH-. CH3OH is not a base) 2. Pull an H+ off water to turn it into OH-: • B- (aq) + H2O (l) → HB (aq) + OH- (aq)

  7. Strong vs Weak 2. Pull an H+ off water to turn it into OH-: B- (aq) + H2O (l) → HB (aq) + OH- (aq) Just like with acids, you can have strong and weak bases. Strong ones do this reaction 100% Common strong bases: OH- , O2-, H-, and NH2-

  8. Strong vs Weak 2. Pull an H+ off water to turn it into OH-: B- (aq) + H2O (l) → HB (aq) + OH- (aq) Just like with acids, you can have strong and weak bases. Strong ones do this reaction 100% Common strong bases: OH- , O2-, H-, and NH2- There's one tricky thing with oxide, though, if we write the equation out: O2- (aq) + H2O (l) → OH- (aq) + OH- (aq) You end up with two hydroxides. This will be relevant later.

  9. Back to Question 1 1. If pH comes from [H+], how do you have a pH in base, which doesn't have any H+? This goes back to the acids video: [H+]*[OH-] = 10-14 While both are 10-7 M for neutral water, this equation is always true. So if we add a bunch of base, the [OH-] will be higher, so [H+] wil be lower. Example: 0.1 M NaOH [OH-] = 0.1 M • [H+]*0.1 = 10-14 [H+] = 10-13 • pH = -log(10-13) = 13

  10. A “Shortcut” Method • Example: 0.1 M NaOH [OH-] = 0.1 M • [H+]*0.1 = 10-14 [H+] = 10-13 • pH = -log(10-13) = 13 If we define pOH to be like pH: pOH = -log[OH-] Then using some algebra, we get: pH + pOH = 14 So back to the same example: 0.1 M NaOH [OH-] = 0.1 M • pOH = -log(0.1) = 1 pH + 1 = 14 pH = 13

  11. Two places where you can run into trouble: • 1. Some hydroxide compounds have more than one hydroxide: • If I make 0.1 M Ca(OH)2, the [OH-] = 0.2 M • 2. Remember the thing about oxides? This is where it comes up” • If I make 0.1 M Na2O, the [OH-] = 0.2 M

  12. Summary • Bases make OH- in water, either by being OH-, or getting an H+ • Strong bases completely pull H+ off water • OH-, O2-, H-, and NH2- are the most common strong bases • pOH = -log[OH-] • Use pH + pOH = 14 or [H+]*[OH-] = 10-14 to get pH • BASE jumping is something else entirely.

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