CHEM 18b Lecture Notes - Lecture 5: Analyte, Absorbance, Transmittance

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1 Apr 2017
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Lab 16:
Measurement of equilibrium constant
ICE tables
Beer’s law:
A = a b c
Transmittance and absorbance
Colorimeter and spectrometer
Transmittance, T = I/Io
Absorbance, A = -log T
A sample that absorbs one color transmits the other color across the color wheel
Equilibrium constant
Keq - very large number, Keq >> 1 Products favored
Keq - very small number, Keq << 1 Reactants favored
Displacement (exchange)
A + B → C + D
Keq < 1 reactants favored
Keq > 1 products favored
Combination (synthesis)
A + B → AB
Keq < 1 reactants favored
Keq > 1 not always products favored
Decomposition
AB → A + B
Keq < 1 not always reactants favored
Keq > 1 products favored
Le Chatelier’s Principle: When stress is put on a system, the equilibrium will shift to relieve the
stress
Lab 17:
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A sample that absorbs one color transmits the other color across the color wheel. Keq - very large number, keq >> 1. Keq - very small number, keq << 1. Le chatelier"s principle: when stress is put on a system, the equilibrium will shift to relieve the stress. Titrant: naoh, strong base, solution in the buret. The titration curve compares the ph to naoh concentration. At the equivalence point there is an equal number of moles of acid and base. Half neutralization point is where you are halfway through adding the moles needed to react with the acid. The endpoint of the reaction is where the indicator runs out so it is directly after the equivalence point. Indicator: phenolphthalein (hin), weaker acid than the analyte and undergoes a color change when deprotonated. Hin will only be deprotonated after all of the analyte is deprotonated! At half neutralization point mol of ha = mol naoh.

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