Physical properties (melting point and boiling point) trend prediction in AP chemistry

Physical properties trend prediction such as melting point and boiling point appears every year in AP chemistry exam. Physical properties are determined by the chemical bonding at atomic level. When you encounter such question, first thing you need to do is to figure out what kind of chemical bonding you are dealing with. The stronger the bond is, the higher the melting point or boiling point it will be.

There are four types of chemical bonding as follows:

1. Ionic bond. Ionic bond exists in compounds typically formed between a metal and a non metal, where electron transfer occurs. Normally, if the electronegativity difference between the two elements is bigger than 1.7, the bond is considered to be ionic. Ionic compounds contain lattice structures. The strength of ionic bond is determined by the static interaction between the cation and anion. The higher charge the cation and anion carry, and the smaller the distance between the cation and the anion, the stronger the bond becomes. For example, in NaF and MgO, the ionic radii of sodium and magnesium, fluoride and oxygen are comparable. But since magnesium and oxygen ions carry 2 positive and negative charges while sodium and fluoride have only   1 charge. As a result, MgO has stronger bonding and higher melting and boiling points. In LiF and NaCl, all the ions carry one charge, but since lithium ion and fluoride ion have smaller radii than sodium ion and chloride ion, LiF has stronger bond than NaCl. LiF is expected to have higher melting and boiling points than NaCl.

2. Covalent bonds. Covalent bond exists in compounds formed between nonmetals, where electron sharing occurs. If the electronegativity difference is between 0 and 0.4, the bond is considered non polar and with a electronegativity difference between 0.4 and 1.7, the bond is considered polar covalent. Apart from covalent network crystals, the smallest unit of covalently bonded compounds is molecule. Inside the molecule, covalent bonds exist among the atoms, but what bind the molecule together is intermolecular forces. Physical properties of the compounds are determined by the strength of their intermolecular forces. There are three types of intermolecular forces as follows:

• Hydrogen bonding. Hydrogen bonding is the strongest form of intermolecular force. It occurs where F or O or N atoms are covalently bonded to a H atom. Due to the high electronegativity of F, O, N, They tend to attract strongly the H atoms from their neighbouring molecules. For example, HF, water and ammonia have exceptionally higher boiling points. It is worthwhile to point out that hydrogen bonding exists when F, O or N are directly bonded to H. In molecules such as CH3F where F and H are all directly bonded to C and No bond is between H and F, there is no hydrogen bonding.
• Dipole dipole interactions. Dipole dipole interactions occur in polar covalent bonds. The dipoles formed due to the unbalanced partial charges in the molecule tends to align the molecules so that the positive end of one molecule is close to the negative end of another molecule. Dipole dipole interaction is the second strongest form of intermolecular force. Here one thing to keep in mind is that polar covalent compounds contains covalent bond, but compounds contains polar covalent bond are not necessarily polar covalent molecules. It depends on their molecular geometry. If the molecule contains polar covalent bonds but is symmetrical, all the bond polarities will cancel and the overall molecule is non polar.
• London dispersion force. London dispersion force is the weakest interaction between molecules. It occurs in all the covalent molecules. But due to its weak strength, if the molecule is polar, we do not need to consider London dispersion force, because the dipole dipole interactions are much stronger. In non polar molecules, there is only London dispersion forces. London dispersion forces are formed due to the instant induced dipoles in the molecules. More electrons in the molecule will increase the chance to induce instant dipoles, making the London dispersion force stronger. As a result, the higher the molecular weight, the stronger the London dispersion force, the higher the melting points and boiling points in non polar covalent compounds. For example in the halogen series, F2, Cl2, Br2 and I2, fluorine and chlorine exist as gas, bromine is a liquid and iodine is a solid at room temperature.

As mentioned earlier, some covalent compounds exist as network covalent crystals, the whole compound exist as a giant molecule. To melt or boil, instead of breaking intermolecular forces, covalent bonds has be be broken. Thus, this type of compounds have super high melting and boiling points. Typical compounds belonging to this class are diamond, silicon dioxide (SiO2, the major ingredient of sand), SiC etc.

3. Metallic bond. Metallic bond exist in metals in their elemental form, where all the metal ions take the lattice structure and the electrons move freely among the metal ions. The strength of the metallic bonds depends on the distance of the metal ions, the smaller the radium of the metal, the stronger the metallic bond. Foe example, going down from the top to the bottom of the alkali and alkali earth groups, the metallic bond becomes weaker.

Hope the stuff above helps you to predict physical properties trends in AP chemistry exams.

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