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Experiment No. 3
Transcript of Experiment No. 3
Melting Point Determination
1. To determine the melting point of citric acid, urea, benzoic acid, oxalic acid and salicylic acid.
2. To compare the effects of structures and functional groups on the melting points of organic compounds.
When heat is added to a solid, the particles in the Rigid Structure begin to move faster. At a temperature called melting point, the particles in the solid gain sufficient energy to overcome the attractive forces that hold them together. The particles in the solid separate and move about in random patterns. The substance is melting, changing from a solid to a liquid. (Timberlake, p-235)
Results and Observation - Benzoic acid
Capillary with sample is fixed to the thermometer with a small slice of rubber tubing, and dipped in the oil bath and heated.
110°C - 122°C
121°C - 123°C
Slowly melted at the first 5 minutes and 7 seconds and totally melted at 5 mins. and 12 secs. During the melting, a scent of fried french fries can be strongly smelled.
Results and Observation - Oxalic acid
79°C - 84°C
102°C - 105°C
1st: starts to shrink/melt ( 3 minutes and 40 seconds)
2nd: melted (3 minutes and 48 seconds)
III. Questions for Analysis and Interpretation
1. Which of the samples has the highest melting point? How would you account for this? (discuss on the basis of Van der Waals interaction and Hydrogen Bonding)
III. Questions for Analysis and Interpretation
2. Discuss the results obtained by comparing the structures, functional groups, the melting point range and the intermolecular forces of the different samples.
2. State the different factors affecting melting points.
1. Give a brief statement about the specificity of the melting points of pure substances.
The melting point of pure substances depends at precisely defined temperature, and depends on pressure too, usually specified at standard pressure.
The different factors that affects melting points are the following:
- are forces of attraction or repulsion which act between neighboring particles (atoms, molecules or ions).
- are substances inside a confined amount of liquid, gas, or solid, which differ from the chemical composition of the material or compound. Impurities are either naturally occurring or added during synthesis of a chemical or commercial product.
Van der Waals forces
- the sum of the attractive or repulsive forces between molecules (or between parts of the same molecule) other than those due to covalent bonds, or the electrostatic interaction of ions with one another, with neutral molecules, or with charged molecules.
Molecular structure (Molecular geometry)
- the three-dimensional arrangement of the atoms that constitute a molecule. It determines several properties of a substance including its reactivity, polarity, phase of matter, color, magnetism, and biological activity.
BMLS - 1A
Almost all of the sample have carboxylic acid functional groups except urea and most exhibit hydrogen bonding. The samples have very high melting point varying from 100°C upwards mainly because most of them have high polarities and participates in hydrogen bonding.
Benzoic acid has the higher melting point because carboxylic acid is a highly polar organic functional group which is a combination of carbonyl ( C=O ) and hydroxyl ( O-H ). A dipole allows carboxylic acid participates in hydrogen bonding which is stronger than a van der Waals interaction.
A melting point can be used to identify a substance and to get an indication of its purity. The melting point of a solid is the temperature at which the solid exists in equilibrium with its liquid state under an external pressure of one atmosphere. Both the melting point range (the interval between the beginning of liquefaction and complete liquefaction) and the temperature of complete liquefaction are valuable indicators of the purity of the solid compound. A pure crystalline organic compound usually possesses a sharp melting point and it melts completely over a narrow temperature range.
II. Drawing Set-up
Hydrogen bonds are a special type of dipole-dipole attraction. The bonds between the hydrogen atom and nitrogen, oxygen or fluorine are exceptionally short and polar. Dipole-dipole attractions between these specific bonds are particularly strong.
To determine if a molecule can hydrogen bond, draw its dot structure and look for H-F, H-O, and H-N covalent bonds. These specific bonds are so polar that they form particularly strong dipole-dipole forces.The hydrogen bond is stronger than a van der Waals interaction, but weaker than covalent or ionic bonds. With an increasing number of H-bonds there will be an increasing melting and boiling point, because more amount of energy is required to disorganize these ordered states in which molecules are arranged due to their intermolecular forces.
Van der Waals forces, relatively weak electric forces that attract neutral molecules to one another in almost all organic liquids and solids. Solids that are held together by van der Waals forces characteristically have lower melting points and are softer than those held together by the stronger ionic, covalent, and metallic bonds.
May 2, 2015 Saturday