MELTING POINT DETERMINATION

Physical vs. Chemical Properties Chemical Properties – describe the chemical interactions of a substance with other compounds and involves chemical change Physical Properties – can be measured and observed without changing the chemical composition of a substance (no bond breaking!)

Physical Properties A few examples used in laboratory: Melting Point Boiling Point Index of Refraction Density Specific rotation Solubility

Uses of Physical Properties Part of an identification procedure As a relative measure of purity

What is a melting point? Definition – the temperature at which the liquid and solid phases exist in equilibrium without a change in temperature. Heat is required for the phase change so heat can be added or removed without changing the temperature as long as matter is changing from solid or liquid or the reverse. A melting point is also a function of atmospheric pressure.

How is a melting point reported? A melting point is reported as the temperature range from the first appearance of liquid to the disappearance of all solid For pure compounds, the range should not exceed 1oC.

Effect of Impurities Mole fraction of B Vapor Pressure of Mixture = all liquid Liquid + solid A Melting begins here – the melt composition is that of the eutectic Liq + solid B all solid Mole fraction of B Vapor Pressure of Mixture = (Mole Fraction of A) * (VP of pure A) + (Mole Fraction of B) * (VP of pure B)

Eutectic Mixture = a mixture of substances with a unique composition which exhibits a sharp melting range as though it were a pure compound – a constant melting mixture. Therefore, a solid with a sharp melting point (narrow melting range) is not always one pure compound.

Impure compounds most often have both – a wide melting range and a lower melting point than the pure compound WHY?? Impurities decrease vapor pressure, which decreases melting point (and increases boiling point.) Impurities “get in the way” of the intermolecular (between molecule) bonds that would be holding the solid together. Therefore, impurities weaken the intermolecular bonds holding the solid together, and consequently, it takes less energy to pull the molecules apart (which means it will melt at a lower temperature).

Mixed Melting Point Technique (Part of an identification procedure ...) If you suspect you know the identity of a solid – (1) Thoroughly mix some of it with the compound you think it may be. Determine the melting point. If the two solids are the same - there will be no depression of melting point and no widening of the range.

Factors Affecting Observed Melting Point Quantity of sample – Use the smallest sample you can clearly observe. Particle size/Packing - Small particles of reasonably uniform size – you may need to grind the sample and pack it without air pockets Rate of heating –1oC per minute These can cause a lag time in heat transfer to the solid or throughout the body of the solid!

Factors Affecting Observed Melting Point (cont’d) Chemical Characteristics of Sample – Some compounds decompose before melting – color changes or gas evolution – and some sublime. Purity of Sample

Thermometer Calibration Submersion line Graph of observed melting points of known compounds comparing them to the literature values

Steps for Melting Point (Range) Determination: Perfect your technique. You know (Table of Physical Properties) where pure benzoic acid should melt. Carefully determine the melting point of your purified sample. (Remember you will need to use this information in both Expt I and Expt II). Record the “unknown” letter in your notebook and run a “fast” determination. Cool melting point apparatus 10-15oC below the “approximate” melting point of the compound & repeat. Report the melt as a range.

Completion … You will not actually identify an unknown by melting point. The goal is to get the best/most accurate melting point. How do you know you did?

Include in your PROCEDURE: Instructions for operating the Electrothermal 9100 MP apparatus (pdf file on Moodle) Write instructions for how to prepare & run your samples (see pg. 38 & handout) Figure 2.16 on pg. 39 in textbook (shows how to properly prepare a MP sample) The data table @ bottom of the handout should go in your DATA & OBSERVATIONS section. You will be doing the following… 1) Determining the MP of ONE standard (pure) compound. 2) Determining the MP of ONE unknown compound. 3) Determining the MP of your purified benzoic acid (from Exp. 1)

Compounds to include in Table of Chemical Properties: Benzoic acid ‒ Salicylic acid ‒ 4-Nitrotoluene Urea ‒ Benzophenone ‒ Biphenyl Acetanilide ‒ Trans-Cinnamic acid Naphthalene ‒ 3,5-Dinitrobenzoic acid You will be doing the following: 1) Determining the MP for ONE standard (pure) compound. 2) Determining the MP for ONE unknown compound. 3) Determining the MP for your purified benzoic acid (from Exp. 1) 4) Determining the MP for the impure benzoic acid (from Exp. 1)

Remember: Weigh purified benzoic acid from last week and record mass in NB! Finish Calculations and Discussion sections for Exp. 1. Complete NB for Exp. 2.