IMF

weakest force. Dispersion forces happen between nonpolar molecules. It comes from the random movement of electrons around the atoms. Occasionally the electrons are closer to 1 atom than the other and this creates a charge on the molecule for a split second. These forces do not last and so are very weak.

medium strength force between molecules. Dipole-dipole forces happen between polar molecules. The slight positive and slight negative charges on the polar molecules happen because the atoms do not share electrons equally. These slight positive charge on one molecule attracts the slight negative charge on another molecule. These are permanent forces.

the strongest of the IMF's. These are very much like dipole-dipole forces as they are between 2 polar molecules. The difference is that in hydrogen bonding, there must be a hydrogen bonded to nitrogen, oxygen or fluorine. The bond between hydrogen and nitrogen, oxygen and fluorine is so polar the slight charges on the molecule are larger than with normal polar molecules. This makes the slight charges stronger so the positive end of one molecule can attract the negative end of the other molecule with more force.

The bigger the molecules the stronger the IMF's. With more electrons (larger atom) the electrons can create a larger partial charge and therefore stronger attractions between the molecules.

To melt or boil a substance, molecules need to be pulled apart. The stronger the IMF the more heat is necessary to pull the molecules apart. Molecules with stronger IMF's need more energy (heat) to pull the molecules apart so they tend to be solid at room temperature while substances with weaker IMF's need less energy (heat) to pull the molecules apart which makes them liquids or gases at room temp.

Ionic substances have full positive and negative charges in a molecule (cation and anion). When two ionic molecules come together the cation of one molecule is attracted to the anion of the other molecule. Because these are full charges, the force holding the molecules together is very strong. Covalent molecules have no charges or partial charges. This means the forces holding the molecules together are weaker. In nonpolar molecules there are dispersion forces holding the molecules together which are usually the weakest type of force and in polar molecules there are either hydrogen bonding or dipole-dipole forces.

CCl4 would have the largest dispersion force and therefore boil at a higher temperature. (It is a dispersion force because of the tetrahedral shape and the surrounding atoms are the same.) Because Cl is a larger atom than F, the dispersion forces would be greater in CCl4. Larger molecules tend to have larger IMF's due to having more electrons. More electrons can lead to more imbalance of charge and stronger attractive forces.

HCl is covalent and has hydrogen bonds where as RbCl is ionic. Ionic molecules have stronger IMF's because there are full positive and negative charges which make the molecules very difficult to separate.

CHBr3 would have the higher melting point. Both molecules are covalent and have dipole-dipole IMF's but CHBr3 is the larger of the two molecules. Larger molecules have stronger IMF's holding them together.

KF should have the higher melting point. It is a binary ionic compound. These have the highest melting points. K(NO3) is a polyatomic ionic compound. These tend to have lower melting points. According to Google, KF melts at 1576℉ where as K(NO3) melts at 633℉.

Although NH3 is a smaller molecule than PH3, NH3 has hydrogen bonding between the molecules. PH3 only had dipole-dipole IMF's. Hydrogen bonds are stronger than dipole-dipole so NH3 molecules are harder to pull apart and therefore have a higher boiling point.

Argon is a larger atom than neon. Larger molecules have stronger IMF's, are harder to pull apart and so have higher melting and boiling points.

CF4 is nonpolar. It has a tetrahedral shape and the atoms around the central atom are the same making it nonpolar. This means that CF4 will have dispersion forces holding the molecules together. CH2F2 is polar meaning it will have dipole-dipole forces holding the molecules together. Both molecules are about the same size but dipole-dipole IMF's are stronger than dispersion IMF's so CH2F2 will have a higher boiling point. According to Google, CF4 has a boiling point of -198℉ and CH2F2 has a boiling point of -62℉.

C (largest molecule)

A (largest molecule. Note NH3 would not be hydrogen bonding not dispersion)

B (largest molecule)

A (NH3 has hydrogen bonding. These are the most polar)

C (NH3 has hydrogen bonding. These are the most polar)

B (least symmetrical and C-F bond is very polar. All others are nonpolar.)

E (only molecule with hydrogen bonded to O, N or F)

A (only molecule with hydrogen bonded to O, N or F)

B (only molecule with hydrogen bonded to O, N or F)

B (Intermolecular forces are between molecules and that is what is broken when melting or boiling)

C (substances mix with each other only if their polarity is the same. Nonpolar mixes with nonpolar and ionic/polar mix with ionic/polar.)

always polar... either binary or polyatomic, does dissolve, high melting point because all the ions need to pull away from each other

positive

does not dissolve, low melting point because there aren't any bonds being broken just separated