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Lab activity:

Intermolecular forces

Presented by: Alejandro Zuluaga

04/03/2012

Intermolecular forces
Objective:Determine if a liquid is a polar or non polar substance

Theorical frame:

In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact throughdipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in electronegativity between atoms in a compound and the asymmetry of the compound's structure. For example, a molecule of water is polar because of the unequal sharing of its electrons between oxygen and hydrogen in which the former has larger electronegativity than the latter, resulting in a "bent"structure, whereas methane is considered nonpolar because the carbon shares the electrons with the hydrogen atoms almost uniformly. Polarity underlies a number of physical properties including surface tension, solubility, and melting- and boiling-points.

Electrons are not always shared equally between two bonding atoms; one atom might exert more of a force on the electron cloud than the other.This "pull" is termed electronegativity and measures the attraction for electrons a particular atom has. The unequal sharing of electrons within a bond leads to the formation of an electric dipole: a separation of positive and negative electric charge. Partial charges are denoted as δ+ (delta plus) and δ− (delta minus). These symbols were introduced by Christopher Ingold and his wife HildaUsherwood in 1926.

Atoms with high electronegativities — such as fluorine, oxygen, and nitrogen — exert a greater pull on electrons than atoms with lower electronegativities. In a bond, this can lead to unequal sharing of electrons between atoms, as electrons will be drawn closer to the atom with the higher electronegativity.

Bonds can fall between one of two extremes — being completely nonpolar orcompletely polar. A completely nonpolar bond occurs when the electronegativities are identical and therefore possess a difference of zero. A completely polar bond is more correctly termed ionic bonding and occurs when the difference between electronegativities is large enough that one atom takes an electron from the other. The terms "polar" and "nonpolar" bonds usually refer to covalent bonds. Todetermine the polarity of a covalent bond using numerical means, the difference between the electronegativity of the atoms is taken. If the result is between 423 and 432 then, generally, the bond is polar covalent.

Example: The hydrogen fluoride, HF, molecule is polar by virtue of polar covalent bonds — in the covalent bond electrons are displaced towards the more electronegative fluorineatom.

Non-polar molecules:
A molecule may be nonpolar either because there is (almost) no polarity in the bonds (when there is an equal sharing of electrons between two different atoms) or because of the symmetrical arrangement of polar bonds.
Examples of household nonpolar compounds include fats, oil, and petrol/gasoline. Therefore (per the "oil and water" rule of thumb), most nonpolar moleculesare water-insoluble (hydrophobic) at room temperature. However, many nonpolar organic solvents, such as turpentine, are able to dissolve polar substances. When comparing a polar and nonpolar molecule with similar molar masses, the polar molecule in general has a higher boiling point, because of the dipole–dipole interaction between their molecules. The most common form of such an interaction isthe hydrogen bond, which is also known as the H-bond.
For example: - In the methane molecule (CH4) the four C–H bonds are arranged tetrahedrally around the carbon atom. Each bond has polarity (though not very strong). However, the bonds are arranged symmetrically so there is no overall dipole in the molecule.
* The oxygen molecule (O2) does not have polarity in the covalent bond because of...