Melting Point and Acid-Base Indicator

CHEM 14BL

acid-base indicator does not provide

pH readings

end point

an indicator changes color in an acid-base reaction

how to correctly choose an acid-base indicator for an acid-base reaction

the indicator should change color (the end point) very close to the equivalence point pH

equivalence point

moles of acidic protons = moles of hydrozide

at the end point (with correct indicator), we can assume

MaVa = MbVb

volumetric buret contains what in a titration?

titrant, base

equivalence point does not equal

end point

equivalence point

intermolecular forces

molecules of crystals are aligned in a conformation that are held together by these

to break molecule conformations

an input of energy is needed

melting point

one way to find the identity and purity of an experimental product

use of melting point in an experiment to find identity

melting point does not always confirm the identity of any given compound by itself (many organic compounds have similar melting points)

use of melting point in an experiment to find purity

“sharpness” of melting point range indicates purity of compounds, pure compounds melt at a narrow temperature range but an impure compound melts over a wide temperature range

melting point range for a pure substance

very sharp melting point, 2 degrees celsius or less in range

melting point range equation

Tf - Ti (Tf = temperature where melting completed, Ti = first sign of melting)

if something has dipole-dipole forces

then they also have london forces

effects of soluble impurities on experimental melting point

1. melting point depression (the lower end of the melting point range (Ti) occurs below the literature melting point value

2. melting point range increases - usually 2-4 degrees celsius for small amounts of impurity (more than 5 degrees celsius if highly impure)