17 - Additional Aqueous Equilibria

17 - Additional Aqueous Equilibria - Chapter 17: Additional...

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Chapter 17: Additional Aqueous Equilibria 17.1Buffer Solutions buffer is a chemical system that resists change in pH; buffer solutions Buffer Action o buffer must contain a weak acid that can react with added base and a weak base that can react with added acid and that the weak acid and base do not react with each other o buffers usually consists of approximately equal quantities of a weak acid and its conjugate base or a weak base and its conjugate acid The pH of Buffer Solutions o 1) Using [H 3 O + ] in K a expression if the K a and concentrations of conjugate acid and base are known o 2) Using Henderson-Hasselbalch Equation good for calculating pH of buffer containing known concentrations of conjugate base and acid can be used to determine ratio of conjugate base to conjugate acid concentrations needed to achieve a buffer of given pH to derive Henderson-Hasselbalch Equation 1) write acid ionization constant expression for weak acid, HA, and solve for [H 3 O + ]; A - = conjugate base of HA 2) convert [H 3 O + ] to pH and take logarithm of each side 3) multiply both sides by equation by 1 and use –log(x)=log(1/x) 4) -logK a =pK a 5) pH = pK a + log ([conjugate base]/[conjugate acid] rules for using equation 1) value of [conjugate base]/[conjugate acid] ratio must be between 0.1 and 10 2) [conjugate acid] and [conjugate base] must each exceed the K a factor of 100 or more a buffer’s pH = pK a of weak acid when concentrations of acid and conjugate base are equal a buffer for maintaining desired pH can be chose by looking at pK a values to have comparable quantities of both acid and conjugate base, the ratio of conjugate base to conjugate acid cannot be much smaller than 1:10 or bigger than 10:1 the pH range is limited to about one pH unit above or below the pK a of conjugate acid the weaker the acid, the larger its pK a when acid (H 3 O + ) is added to a buffer, the acid reacts with the conjugate base of buffer to form its conjugate acid: conjugate base + H 3 O + added conjugate acid of buffer base + water when base (OH - ) is added to a buffer, the base reacts with conjugate acid of buffer, which is converted into its conjugate base: conjugate acid in buffer + OH - added conjugate base of the buffer acid + water The pH Change on Addition of an Acid or a Base to a Buffer o pH changes when acid or base is added because of shifts in concentration of conjugate acid or conjugate base in buffer o extent of pH change depends on amount of acid or base added and on amounts of conjugate acid or conjugate base remaining in buffer to offset additional amounts of acid or base to be added o change in pH of a buffer when acid or base is added to it can be calculated: 1) by using the K a expression with K a value 2) by using the Henderson-Hasselbalch equation Buffer Capacity
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This note was uploaded on 09/28/2009 for the course CHEM 1A taught by Professor Nitsche during the Fall '08 term at University of California, Berkeley.

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17 - Additional Aqueous Equilibria - Chapter 17: Additional...

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