Criteria to determine which substance has the highest boiling point PDF

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Criteria to determine which substance has the highest boiling point (b.p) or melting point (m.p) • •

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Any network covalent solid (e.g SiC or SiO2) will have the highest b.p When comparing any ionic substance with a network solid, the ionic substance will have the second highest b/p. (e.g NaCl versus SiC). If a network covalent solid is not present then the ionic substance will have the highest b.p. Any metallic substance that is present with a network covalent solid and an ionic compound will have the third highest b.p. (e.g Na versus SiC or NaCl). If the metallic substance is present with only one of these it will have the second highest b.p. Any molecular substance that is not network covalent substances will have the lowest b.p. Of these, to determine the substance with the highest boiling point, identify the types and relative strengths of the intermolecular forces (IMF) present:

London Force • Present in all molecules • Count the number of electrons; if no other IMF are present the substance with the greatest number of electrons will have the highest b.p. • If substances have the same number of electrons (isoelectronic) then determine if other IMF forces are present Dipole-Dipole forces • Present in polar substances, in addition to London forces, thus this substance has the higher b.p. • If both substances are polar, then determine if Hydrogen bonds are present Hydrogen Bonds • Present if molecules contain a H-F, H-O or H-N bond • This substance will have the higher b.p., since it contains all three types of IMF i.e London Forces, Dipole-dipole and Hydrogen bonds Note for substances that are isoelectronic but do not contain hydrogen bonds, the substance with the higher b.p. will be the one that is most polar

Exercises 1. Write the type of bonding present in each of the substances below. List the substances in order of increasing b.p. Ag, PH3, LiCl, SiO2 2. The b.p. of C2H6 is -87 °C, while that of CH3F is -78 °C. Account for this difference. 3. C2H5F and C2H5OH are two molecular substances with different boiling points. Identify the types of IMF present, determine which substance has the higher boiling point and explain why.

Chemical Bonding Bonding between Atoms or Ions Covalent • Sharing of electrons between 2 non-metallic atoms • Occurs in molecular substances • The strongest type of bonding Ionic • Ions are formed when electrons are transferred from one atom to another, usually metal atoms lose electrons to form cations and non-metal atoms gain electrons to form anions • Ionic bonds are formed by the attraction of oppositely-charged ions to each other • The second strongest type of bond Metallic • Positive ions attract valence electrons which are free to move from one empty valence orbital to another • The third strongest type of bonding Bonding amongst Molecules London (Dispersion) Forces • Occurs among all molecular substances • The attraction of positive nuclei of one molecule to the electrons of another molecule (and vice versa) • The strength of these forces depends on the number of electrons that a substance contains, such that the greater the number of electrons the stronger the London forces among the molecules of that substance Dipole-Dipole Forces • Only occurs among polar molecules • The partial positive end of one polar molecule is attracted to the partial negative end of another molecule (and vice versa) Hydrogen Bonds • A special type of dipole-dipole force (about 10 times stronger) only occurs among molecules that contain a H atom which is directly bonded to a highly electronegative atom (F,O,N) i.e. that molecule at least one H-F, H-O, H-N bond...