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Chapter 18

# CHEM 1C Chapter Notes - Chapter 18: Ionic Compound, Chemical Equation, Lewis Acids And Bases

Department
Chemistry
Course Code
CHEM 1C
Professor
Eric Potma
Chapter
18

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Chapter 18: Aqueous Ionic Equilibrium
18.1 The Danger of Antifreeze
18.2 Buffers: Solutions That Resist pH Change
- Buffer = resists pH change by neutralizing added acid or added base and contains either:
1. significant amounts of both a weak acid and its conjugate base or
2. significant amounts of both a weak base and its conjugate acid
- Buffer can maintain nearly constant pH
- Weak acid neutralizes added base. Weak base neutralizes added acid
Calculating the pH of a Buffer Solution
oCommon ion effect = solution contains two substances that share a common ion
The Henderson-Hasselbalch Equation
oMake the same x is small approximation.
oConsider the equilibrium concentrations of HA and A to be essentially identical to the initial
concentrations of HA and A.
o
oHenderson-Hasselbalch Equation – only if x is small enough
Calculating pH Changes in a Buffer Solution
oAdding a small amount of strong acid to a buffer converts a stoichiometric amount of the base
to the conjugate acid and decreases the pH of the buffer
oAdding a small amount of strong base to a buffer converts a stoichiometric amount of the acid
to the conjugate base and increases the pH of the buffer
1. The stoichiometry calculation: calculate how the addition changes the relative amounts of acid and
conjugate base.
2. The equilibrium calculation: calculate pH based on the new amounts of acid and conjugate base
oThe Stoichiometry Calculation
Table like ICE table to keep track. Consequence of 1:1 stoichiometry of neutralization
Use the stoichiometry of the neutralization equation to calculate the changes in the
amounts (in moles) of the buffer components upon addition of the acid or base
oThe Equilibrium Calculation
ICE table and x is small approximation
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Use the new amounts of buffer components to work an equilibrium problem to find pH
Buffers Containing a Base and Its Conjugate Acid
oCalculate pH of solution same as before but must find pKa of conjugate acid
18.3 Buffer Effectiveness: Buffer Range and Buffer Capacity
Relative Amounts of Acid and Base
oBuffer most effective when concentrations of acid and conjugate base are equal
oBuffer becomes less effective as the difference in the relative amounts of acid and conjugate
base increase
oIn order for a buffer to be reasonably effective, the relative concentration of acid and conjugate
base should not differ by more than a factor of 10
Absolute Concentrations of the Acid and Conjugate Base
oA buffer is most effective when the concentrations of acid and conjugate base are high
oMore dilute the buffer components, the less effective the buffer
Buffer Range
oLowest pH for effective buffer occurs when the base is one-tenth as concentrated as the acid
oHighest pH for effective buffer occurs when the base is ten times as concentrated as the acid
oThe effective range for a buffering system is one pH unit on either side of pKa
Buffer Capacity
oBuffer capacity = the amount of acid or base that we can add to a buffer without causing a
large change in pH
oBuffer capacity increases with increasing absolute concentrations of the buffer components
oOverall buffer capacity increases as the relative concentrations of the buffer components
become more similar to each other
18.4 Titrations and pH Curves
- acid-base titration = a basic (or acidic) solution of unknown concentration reacts with an acidic (or basic)
solution of known concentration.
- indicator = a substance whose color depends on the pH
- equivalence point = the point in the titration when the number of moles of base is stoichiometrically equal
to the number of moles of acid
- point of inflection in pH curve is where the equivalence point is
- exact shape of pH curve depends on strength of acid or base being titrated
The Titration of a Strong Acid with a Strong Base
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