lec 3 - BL/CH 401 Lecture#3 Acids Bases and Buffers 1...

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BL/CH 401 Lecture #3 - Acids, Bases, and Buffers 1. Principle of Ionization of Weak Acids: The Fundamental Concept of Buffers is: A Buffer Resists Change pH buffers resist change in pH when either acid (H+) or base (OH-) is added to it. Chemicals which are pH buffers are weak acids or bases Acids = Proton (H+) donors Bases = Proton Acceptors {*Figure 1*} Acids and their conjugate bases are in equilibrium. Since equilibria are related to the properties of the reactants and products, so for weak acids, the tendency to give up its proton determines its buffering property. This tendency to ionize can be put in terms of an equation for the equilibrium: {*Figure 2*} Where [ ] = Molar concentration; K = Ionization constant Simplest example is water (H 2 O): {*Figure 3*} But since [H 2 O] (water concentration) = Constant (55.5 M), Kw = [H+][OH-] = 10 -14 M In pure water, [H+] = [OH-] = 10 -7 M
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To make this easier to use, the pH scale was invented. pH = -log [H+]; thus when [H+] = 10 -7 M, pH = 7 This is called Neutral pH because it is in the middle of the pH scale. At pH greater than neutral, the solution is alkaline; while at pH less than neutral, the solution is acid. {*Figure 4*} 2. Titration of a Weak Acid illustrating its Ionization and Buffering Property: {*Figure 5*} All weak acids have titration curves like this one. Bases (like ammonium, NH4+) are also weak acids and have similar titration curves. The position where the Buffering zone is on the pH scale is related to the chemical nature of the weak acid : Acetic acid ionizes in the Acidic portion of the pH scale {*Figure 6*}
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