Atkins/Jones - 12.01-12.07 Aqueous Equilibrium
About this set
Created by:
RheaWong on February 22, 2012
Subjects:
Description:
12.1 Buffer Action
12.2 Designing a Buffer
12.3 Buffer Capacity
12.4 Strong Acid-Strong Base
12.5 Strong Acid-Weak Base and Weak Acid-Strong Base Titrations
12.6 Acid-Base Indicators
12.7 Stoichiometry of Polyprotic Acid Titrations
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32 terms
Terms | Definitions |
|---|---|
 Qualitative Analysis | The identification of substances and ions present in a aqueous solution sample. |
| "Mixed" Solutions | Solutions that contain a weak acid or a weak base and one of its salts. A means of stabilizing the pH of aqueous solutions such as blood plasma, seawater, etc. (i.e. buffers). |
| Buffer | A solution in which the pH resists changes when small amounts of strong acids or bases are added. |
| 12.1 Buffer Action Summary | A buffer is a mixture of a weak conjugate acid-base pair that stabilizes the pH of a solution by providing both a source and a sink for protons. |
| Acid Buffer | A buffer made of a weak acid and its salt, pH>7. |
| Basic Buffer | A buffer made of a weak base and its salt, pH<7. |
| Calculating the pH of a buffer solution (acid example) | Identify the weak acid and its conjugate base. Write the proton transfer equilibrium between them and rearrange the expression for Ka to give [H₃O⁺]. Calculate the pH. (Use tabled values and approximate conjugate acid/base concentrations by their initial values). |
| Calculating the pH change of a buffered solution (acid example) | Calculate the new concentration of acid (initial concentration minus how much reacts [based on rxn stoichiometry]). Do the same for the conjugate base. Calculate the pH from such. |
| Buffer composition | Buffers are often made with equal amounts of conjugate acids/bases. This means that, since the acids/bases are so weak, and dissociate so little, in calculating Ka, the concentrations can be set equal to their initial concentrations so that they cancel out. tl;dr when [HA]ini=[A⁻]ini, pH=pKa |
| Henderson-Hasselbalch Equation | pH = pKa + log [base]ini/[acid]ini |
| 12.2 Designing a Buffer Summary | The pH of a buffer solution is close to the pKa of the weak acid component when the acid and base have similar concentrations. |
| Buffer Capacity | The maximum amount of acid or base that can be added before the buffer loses its ability to resist large changes in pH. |
| Buffer capacity relies on... | A more concentrated buffer is more resistant to pH changes. The relative concentrations of weak acid/base also matter; when the weak base is at least 10% of the weak acid, the buffer better resists the addition of acid, and when the weak acid is at least 10% of the weak base, the buffer better resists the addition of base. |
| Buffer effectiveness | When the acid is 10× as abundant as the base ([acid] = 10[base], the pH is given by pH = pKa + log [base]/10[base] = pKa + log 1/10 = pKa - 1 When the base is 10× more abundant than the acid, then pH = pKa + log 10[acid]/[acid] = pKa + 1 Buffers act most effectively when the pH is within a range of ±1 unit of pKa. |
| 12.3 Buffer Capacity Summary | The capacity of a buffer is determined by its concentration and pH. A more concentrated buffer can react with more added acid or base than can a less concentrated one. A buffer solution is generall most effective when the pH is in the range pKa ± 1. |
| Analyte | The unknown sample in a titration. |
| Titrant | The solution of known concentration in a titration. |
| Stoichiometric p\Point | When the amount of OH⁻ or H₃O⁺ added as titrant is equal to the amount of H₃O⁺ or OH⁻ initially present in the analyte. |
| pH Curve | A plot of the pH of the analyte solution against the volume of titrant added during a titration. |
| Physiological Buffers (Blood) | Biological systems are highly reliant on pH buffers, ex. in the blood. Blood uses HCO₃⁻/H₂CO₃ in a ration of ~20:1, with most of the carbonic acid in the form of CO₂. |
| Alkalosis/Acidosis | Alkalosis: when the pH of blood rises above normal levels. Acidosis: when the pH of blood falls below normal levels. Reliant on the ratio of HCO₃⁻ to H₂CO₃ present. |
| How blood pH is maintained | H₂CO₃: exhalation as CO₂ HCO₃⁻: excretion in urine |
| 12.4 Strong Acid-Strong Base Summary | in the titration of a strong acid with a strong base or vice versa, the pH changes slowly initially, changes rapidly through pH=7 at the stoichiometric point, and then changes slowly again. |
| At the halfway point of an acid-base titration, since pH = 7... | [HA] = [A⁻] and thus pH = pKa |
| 12.5 Strong Acid-Weak Base and Weak Acid-Strong Base Titrations Summary | Halfway to the stoichiometric point, the pH is equal to the pKa of the acid. The pH is greater than 7 at the stoichiometric point of the titration of a weak acid and strong base. The pH is less than 7 at the stoichiometric point of the titration of a weak base and strong acid. |
| pH Meter | A device that uses a special electrode to measure H₃O⁺ concentration. |
| Acid-Base Indicator | A water-soluble organic dye with color that depends to pH. |
| How indicators work | An indicator changes color with pH because it is a weak acid that has one color in its acidic form and another color in its conjugate base form. Because it is a weak acid, it takes place in a proton transfer equilibrium and has its own Ka/pKa. |
| End Point of an Indicator | When the concentrations of its acid and base forms are equal, i.e. [acid]=[base]; the color change occurs when pH=pKin (in=indicator). |
| 12.6 Acid-Base Indicators Summary | Acid-base indicators are weak acids that change color close to pH = pKin; an indicator should be chosen so that its end point is close to the stoichiometric point of the titration. |
| Polyprotic Acids | Acids with more than one donatable H⁺. |
| 12.7 Stoichiometry of Polyprotic Acid Titrations Summary | The titration of a polyprotic acid has a stoichiometric point corresponding to the removal of each acidic hydrogen atom. The pH of a solution of a polyprotic acid undergoing a titration is estimated by considering the primary species in solution and the proton transfer equilibrium that determines the pH. |
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