CHEMISTRY NOTES: Structures, Shapes, Polarity and IMF s

Transcription

1 CHEMISTRY NOTES: Structures, Shapes, Polarity and IMF s DRAWING LEWIS STRUCTURES: RULES 1) Draw the skeleton structure for the molecule. The central atom will generally be the least electronegative element in the compound often it will be the first element given in the formula. 2) Determine the total number of valence electrons in the molecule. If the molecules is an ion: add e s if negative and subtract e s if positive. 3) Since each dash represents a bond with 2 e s, subtract 2 from the total electrons for every bond in the molecule. 4) Distribute the e s so that each outside atom (except H, which only gets 2 e s) gets 8 e s. The outside atoms will always follow the octet rule, except of course H. EXAMPLES NH3 BrCl4 1 SO2 GaI3 5+3(1) = 8 7+4(7)+1 = (6) = (7) = = = = = 18 5) Any remaining e s go on the central atom. no e s left 6) Look at the central atom. If it has: - 8 or more e s you are done - less than 8 e s: - if the central atom is C, N, O, S or P bonded to C, N, O, S or P, add another bond for each pair of e s short of 8 e s on the central atom remove a pair of e s from the outside atom you draw the extra bond to. - if the central atom is any other element you are done. central atom (N) has 8 e s this one is done central atom (Br) has 12 e s; this one would be done except it is an ion, so proceed to next step central atom (S) only has 6 e s; needs another bond central atom (Ga) only has 6 e s; Ga and I cannot have multiple bonds. this one is done 7) If the molecule is an ion, put [ ] around the drawing with the charge outside the brackets in the upper right. this one is done 8) If you added another bond you may have resonance. If there is another possible atom in which the extra bond could be drawn to you have resonance and must draw all the possibilities and put a between all the drawings. this one is done

2 DETERMINING SHAPES OF MOLECULES: To determine the shape of the molecule you need to look at the central atom in the Lewis structure and determine two things: o coordination number the number of atoms bonded to the central atom o lone pairs of electrons the pairs of unbonded electrons on the central atom Once you determined the coordination number and lone pairs, you can determine the electronic shape (electron pair geometry) and the of the molecule. o electronic shape need the sum of the coordination number and the lone pairs o use the coordination number and the lone pairs separately It should be noted that the extra bonds in the multiple bonds do NOT count in determining the shapes. Refer to the following table to determine shapes. total e pairs coordination electronic shape on central atom number lone pairs 2 linear 2 0 linear 3 trigonal planar 3 0 trigonal planar 2 1 bent 4 0 tetrahedral 4 tetrahedral 3 1 trigonal pyramidal 2 2 bent 5 0 trigonal bipyramidal 5 trigonal bipyramidal 4 1 teeter totter 3 2 T-shape 2 3 linear 6 0 octahedral 6 octahedral 5 1 square pyramidal 4 2 square planar DRAWING MOLECULAR SHAPES Linear Trigonal Planar Tetrahedral Trigonal Bipyramidal Octahedral To draw other shapes in the same family, you just need to remove one of the lines except for in the trigonal bipyramidal family. In the trigonal bipyramidal family, you need to remove lines off the 3 equator lines since lone pairs require more room than bonded pairs. It is also acceptable to rotate the drawings to fit what you are doing. There are a few shapes that are best drawn as follows: bent trigonal pyramidal T-Shape square pyramidal square planar formula NH3 BrCl4 1 SO2 GaI3 SeF4 Lewis Structure coordination number lone pairs total e pairs electronic shape tetrahedral octahedral trigonal planar trigonal planar trigonal bipyramidal trigonal pyramidal square planar bent trigonal planar teeter totter

3 DETERMINING POLARITY OF MOLECULES: Let s look at a water molecule. According to the Lewis structure for H2O, the resulting is bent. Lewis structure molecular shape The electronegativity of O is 3.5 and H is 2.1, resulting in a polar covalent bond. If we draw in the dipole moment for this bond and show the partial charges, we get: If we apply this to the molecule in its we get: If you were to draw a line through the middle of the molecule, you can see there is a separation of charges there is a negatively charged side and a positively charged side: Therefore, H2O is what we call a polar molecule. Now, let s look at CCl4. According to the Lewis structure, CCl4 is a tetrahedral molecule. Lewis structure molecular shape The electronegativity for C is 2.5 and Cl is 3.0, resulting in a polar covalent bond. Drawing in the dipole moment for this bond and showing the partial charges we get: If we apply this to the molecule in its we get for the dipole moments: and this for the partial charges: With this molecule there is no way to draw a line to separate the charges. In fact, since the molecule is symmetrical, all the dipole moments will cancel each other out. CCl4 is an example of a nonpolar molecule.

4 DETERMINING POLARITY OF MOLECULE: The polarity of a molecule ultimately is determined by the shape of the molecule and whether or not it is symmetrical. Start by determining if the bonds in the molecule are polar or nonpolar covalent bonds. Check the electronegativity of the elements and if the difference between the two elements is: difference bond type nonpolar covalent polar covalent 2.0 and up ionic If the bond type is in the ionic range, treat the bond as if it is polar when determining the polarity of the molecule. NOTE: If ALL the bonds are nonpolar covalent, the molecule will be nonpolar regardless of the shape. Next, check the shape of the molecule you need to determine if the shape is symmetrical or not. The following shapes are symmetrical if ALL the outside atoms are the same: linear trigonal planar tetrahedral trigonal bipyramidal linear (in the trigonal planar family) octahedral square planar If the shape is symmetrical, the molecule will be nonpolar. Otherwise, the molecule is polar. The one exception to this is for molecules of just 2 atoms when this occurs, the polarity of the molecule will be the same as the polarity of the bond. SUMMARY: If the bonds are: nonpolar covalent molecule is nonpolar polar covalent and the shape is: o symmetrical molecule is nonpolar o not symmetrical molecule is polar formula NCl3 BrCl4 1 SO2 H2CO SeF4 SF6 Lewis Structure bond type nonpolar covalent trigonal pyramidal polar covalent polar covalent polar covalent polar covalent polar covalent square planar bent trigonal planar teeter totter octahedral symmetrical? no yes no no no yes polarity of molecule nonpolar nonpolar polar polar polar nonpolar

5 DETERMINING INTERMOLECULAR FORCES (IMF) BETWEEN MOLECULES: There are basically four different types of IMFs between particles of matter: o London Forces found between every particle o Dipole-Dipole found between polar molecules o Hydrogen Bonding found between polar molecules which have H bonded to a N, O or F o Ionic Bonds found between ions, not found between molecules Ionic bonds by far is the strongest of the four listed. The other three are quite weak in comparison. However, the order of the other three in decreasing order of strength is hydrogen bonding, dipole-dipole and then London Forces. The strength of London Forces will increase as molecules get heavier since there are more electrons to generate the forces. Dipole-dipole increases with larger differences between electronegativities of the elements. To determine the type of forces (London Forces, dipole-dipole and hydrogen bonding) between molecules you need to check the polarity of the molecule. If the molecule is: o nonpolar London Forces only o polar with H bonded to N, O or F hydrogen bonding o polar without H bonded to N, O or F dipole-dipole Like mentioned before, everything has London Forces, including molecules with dipole-dipole and hydrogen bonding. However, dipole-dipole and hydrogen bonding are more dominant and therefore we generally don t list London Forces with molecules having these forces. formula NCl3 BrCl4 1 SO2 H2CO SeF4 SF6 polarity of molecule nonpolar nonpolar polar polar polar nonpolar DOMINANT IMF London Forces London Forces dipole-dipole dipole-dipole dipole-dipole London Forces formula NH3 GaI3 CH3OH C2H6 polarity of molecule polar nonpolar polar nonpolar hydrogen hydrogen DOMINANT IMF London Forces London Forces bonding bonding PROPERTIES BASED ON IMF s: There are several important properties molecules have that are dependent on the type of forces between the particles. There are 4 we will be concerned with: solubility, vapor pressure, melting (freezing) point and boiling point. SOLUBILITY: In order for a solution to form the substances must be able to interact with each other. To do so, they must have similar IMF s. GENERAL RULE: Like dissolves like. o Polar substances will dissolve polar substances. o Nonpolar substances will dissolve nonpolar substances. o They do not dissolve each other.

6 To determine if a substance will dissolve in another substance, you must check the polarity of the substances. If they are the same, dissolving occurs. Determine whether the following molecules will dissolve in water or CCl4. Remember from a previous page in these notes, H2O is polar and CCl4 is nonpolar. formula NH3 GaI3 SO2 H2CO SeF4 SF6 polarity of molecule polar nonpolar polar polar polar nonpolar dissolve in H2O CCl4 H2O H2O H2O CCl4 VAPOR PRESSURE: The vapor pressure of a liquid is defined as the pressure generated by the vapor above a liquid at equilibrium. The amount of vapor that escapes from a liquid depends on the strength of the IMF s between the particles of the liquid. o The stronger the IMF, the less vapor that will escape low vapor pressure. o The weaker the IMF, the more vapor that will escape high vapor pressure. formula H2O CCl4 C2H6 SeF4 IF3 polarity of molecule polar nonpolar nonpolar polar polar DOMINANT IMF hydrogen bonds London Forces London Forces dipole-dipole dipole-dipole vapor pressure low high high intermediate intermediate Which one of each of the following pairs has the higher vapor pressure? Both are polar, but H2O has hydrogen bonding, H2S has dipole-dipole. H2S has higher vapor pressure. CCl4 is nonpolar with London Forces. NH3 is polar with hydrogen bonds. CCl4 has higher vapor pressure. Both are nonpolar with London Forces. Propane is larger and therefore has stronger London Forces. Ethane has higher vapor pressure. ethane propane

7 BOILING POINT: The boiling point of a liquid is defined as the temperature at which the liquid s vapor pressure is equal to atmospheric pressure. Therefore, it is very much dependent on the vapor pressure of the liquid. The vapor pressure of a liquid will increase as temperature increases. The higher the vapor pressure a liquid has the less heat it needs for its vapor pressure to become equal to atmospheric pressure. Therefore, the higher the vapor pressure, the lower the boiling point. Summary: The stronger the IMF, the lower the vapor pressure, the higher the boiling point. The weaker the IMF, the higher the vapor pressure, the lower the boiling point. EXAMPLE: Which of the following pairs of substances has the higher boiling point? Both are polar, but H2O has hydrogen bonding, H2S has dipole-dipole. H2O has the lower vapor pressure and, therefore, the higher boiling point. CCl4 is nonpolar with London Forces. NH3 is polar with hydrogen bonds. NH3 has the lower vapor pressure and, therefore, the higher boiling point. Both are nonpolar with London Forces. Propane is larger and therefore has stronger London Forces. Propane has the lower vapor pressure and, therefore, the higher boiling point. ethane propane MELTING (FREEZING) POINT: In order to melt a substance the forces between the particles of the substance must be broken. The stronger the forces the more heat is needed to do so, and, therefore the higher the melting point. Summary The stronger the IMF, the higher the melting point. The weaker the IMF, the lower the melting point. NaCl H2O has hydrogen bonds between molecules. NaCl has ionic bonds between the ions. The ionic bonds are stronger and therefore NaCl has the higher melting point. H2CO is polar with dipole-dipole while GaI3 is nonpolar with London Forces. H2CO has the stronger forces between particles and therefore the higher melting point. O=C=O SO2 is polar with dipole-dipole. CO2 is nonpolar with London Forces. SO2 has the stronger forces and therefore the higher melting point. O=C=O Both are nonpolar with London Forces between molecules. SF6 being larger will have stronger forces and therefore have a higher melting point.

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