Boiling Point and Melting Point Determination

Molina, John Kenneth Q.

Ruiz, Christian

INTRODUCTION

MELTING POINT Temperature at which liquid and solid phases coexist in

equilibrium

BOILING POINT Temperature at which the vapor pressure of a liquid

equals the atmospheric pressure

Strong intermolecular forces of attraction results in high melting and boiling points.

INTRODUCTION

FACTORS AFFECTING MELTING POINT AND BOILING POINT

Molecular weight and size Branching Polarity Molecular Symmetry Intramolecular H-bonding

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INTRODUCTION

WHY DETERMINE MELTING POINT AND BOILING POINT?

Identification/characterization of a compound

Indication of the purity of a substance

METHODOLOGY A. DETERMINATION OF MELTING POINT

B. DETERMINATION OF BOILING POINT

A. open end of capillary tube

thermometer

cooking oil

heat in hot plate/stove

B.

open end of capillary tube

thermometer

beaker with cooking oilheat in hot plate/stove

RESULTS AND DISCUSSION

SAMPLEMELTING POINT RANGE (oC)

EXPERIMENTAL THEORETICAL

Benzoic Acid

112-115 121-122

Benzoic Acid-Urea

Mixture

90-100

RESULTS AND DISCUSSION

Sample Boiling Pt Range (oC)

Hexane 75.5 – 82

Distillate 1 (simple) 90 – 97

Distillate 2 (fractional) 68 - 86

RESULTS AND DISCUSSION

MELTING POINT Temperature at which a solid is converted into a liquid Occurs as molecules acquire enough energy, when

enough heat is supplied, to overcome the intermolecular forces of attraction that binds the molecules together

Influenced by the type packing of the compound Packing is a property that determines how well the

individual molecules in a solid fit together in a crystal lattice.

The tighter the fit, the more energy is required to break the lattice and melt the compound.

RESULTS AND DISCUSSION

MELTING POINT RANGE Span of temperature where the lower value indicates the

point where the solid first begin to liquefy and the higher value is the point where the entire solid completely melted into a liquid

Pure samples shows a 1-2oC in their melting point range. If the melting point range of a substance is 5oC or higher

and the melting point starts at lower temperature than the theoretical temperature of the pure sample, then the sample is impure.

RESULTS AND DISCUSSION

Substances with impurities will melt at lower temperatures than pure substances

IMPURITIES GENERALLY CAUSES:

1. Melting range depression – the lower end of the range drops

2. Melting range broadening – the range increases

Impurities disrupts the crystalline lattice arrangement of the molecules resulting in randomly oriented and weaker bonds, thus, lower temperature is needed for the substance to melt.

RESULTS AND DISCUSSION

BOILING POINT The boiling point is described as the temperature at

which the vapor pressure equals atmospheric pressure For this to happen, the forces holding the molecules

together, such as the dipole-dipole interactions, van der Waals forces, and hydrogen bonding have to be overcome (by adding heat).

The raise in temperature is the result of its miscibility with water, and subsequent exothermic reaction. From this we see that it takes more energy/heat to reach the boiling point of ethanol, and so the temperature needed to boil it is also higher.

RESULTS AND DISCUSSION

Dipole-dipole interactions: dipoles, or net polarities of molecules, are attracted to each others’ opposite charges, aligning themselves accordingly

Van der Waals forces (induced-dipole-induced-dipole interactions): temporary dipoles are caused by the electron cloud’s shifting of densities and cause partial positive and negative poles

Hydrogen bonds: a special group of dipole-dipole interactions that involve hydrogen and an electronegative atom- mostly oxygen, nitrogen, and fluorine atoms

CONCLUSION AND RECOMMENDATIONS

Boiling point and melting point are essential physical properties of a compound that differentiates it to another compound and aids in determining whether the said compound is pure or impure.

It is highly recommended that students who would replicate this experiment to avoid errors that could cause inaccuracy in the experiment.

We therefore conclude that the boiling points and melting points of substances are intrinsic property and are thus not affected by the amount of the substance present

GUIDE QUESTIONS AND ANSWERS

1. Give an explanation of the observed melting points.

the difference in the melting point of the two samples (excluding the unknown) was due to the difference in the crystal lattice – since with the benzoic acid – urea, urea acted as the contaminant, disturbing the crystal lattice of benzoic acid and thus making it easier to break the bond within the molecule.

GUIDE QUESTIONS AND ANSWERS

2. Give an explanation of the observed boiling points of the two distillates.

The distillate from fractional distillation had more ethanol component compared to that of simple distillation, so the distillate behaved more like ethanol and exhibited a boiling point closer to that of the alcohol.

GUIDE QUESTIONS AND ANSWERS

3. What effect would poor circulation of the melting point bath liquid have on the observed melting point?

Poor circulation can cause uneven heating of a sample. This may lead to inaccurate results. Observed melting point may be unusually higher than the actual.

GUIDE QUESTIONS AND ANSWERS

4. What effect would the incomplete drying of a sample have on the melting point?

When crystals are isolated by filtration from a solvent, it is important to allow complete drying/evaporation of the solvent in order to get a good melting range. Residual solvent functions as a contaminant and will depress/broaden the melting range for a crystal

GUIDE QUESTIONS AND ANSWERS

5. Three test tubes, labeled A, B, and C, contain substances with approximately the same melting points. How could you prove the test tubes contain three different compounds?

Mixed Melting Points:If an unknown candidate C melts at a temperature close

to that of two potential candidates A and B, you can identify it by taking C+A mixed melting point, and C+B mixed melting point. If C is equal to either candidate, one of these mixed melting points will not be depressed. If the mixture with C+A is not depressed, C = A. if the mixture with C+B is not depressed, C = B. If both mixtures are depressed, then C ≠ A or B.

GUIDE QUESTIONS AND ANSWERS

6. Which would be expected to have a higher boiling point – t-butyl alcohol or n-butyl alcohol? Explain.

Some factors affecting boiling/melting point:Molecular weightIntermolecular forces of attractionPolarityBranching/structure of molecule

GUIDE QUESTIONS AND ANSWERS

6. Which would be expected to have a higher boiling point – t-butyl alcohol or n-butyl alcohol? Explain.

Some factors affecting boiling/melting point:Molecular weightIntermolecular forces of attractionPolarityBranching/structure of molecule

GUIDE QUESTIONS AND ANSWERS

7. Calculate the vapor pressure of a solution containing 30mol % hexane and 70mol % octane at 90°C assuming Raoult’s Law is obeyed. (Given: vapor pressure of the pure compounds at °C: hexane=1330 torr; octane=253 torr).

Raoult’s Law:

= + o oT hex hex oct octP P P

1330 )(0.3) (253 )(0.7) 576.1 = ( + TP torr torr torr

REFERENCES• Boiling Point and Distillation. (2011). Retrieved on May 9, 2011 from

https://eee.uci.edu/programs/hongchem/RDGbpdistill.pdf• Melting point. (2011). In Encyclopedia

Britannica. Retrieved May 9, 2011, from Encyclopedia Britannica Online: http://www.britannica.com/EBchecked/topic/374185/melting-point

• Raoult’s Law (2010). In Wikipedia. Retrieved

May 9, 2011 from http://en.wikipedia.org/wiki/Raoult%27s_law• Bruice, P. (2007). Organic chemistry. 5th Ed. New Jersey: Pearson

Prentice Hall.• Laboratory Manual in Organic Chemistry, Department of Physical

Science and Mathematics, University of the Philippines – Manila, 2010