AP Chemistry.

Test Preparation AP Test Preparation
1

<li><p>Why do low molar mass substances that are non polar molecules H2, O2, N2, and CO2 have very low boiling points?</p></li>
<li><p>Rationalize why chalk (calcium carbonate) has a higher melting point than motor oil (large compounds made from carbon and hydrogen), which has a higher melting point than water, which engages in relatively strong hydrogen-bonding interactions.</p></li>
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<p>[I thought motor oil would have a higher melting and boiling point than chalk, because it is a network solid.]</p>

2

<p>1) I think lighter molar massed substances have lower boiling points because of some reason which I don’t remember.</p>

<p>2) I think chalk in an ionic bond while motor oil is a covalent bond. Ionic bonds are harder to break apart because they are stronger.</p>

<p>Take these answers with a grain of salt. I took AP Chemistry last year.</p>

3

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<li><p>Lighter the molecule, the more simple it is. A heavier molecule is usually more complex and they get entangled, causing a higher boiling point. Also, because they are nonpolar, they will have low melting points.</p></li>
<li><p>Chalk is ionic while hydrocarbons are covalent. Ionic bonds are stronger.</p></li>
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4

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<li> Molecules that are nonpolar only have London Dispersion forces between the molecules. In general, LDF’s are weak, so most nonpolar molecules have low BP, MP, dHvap, dHfus, etc. (And high vapor pressure) When comparing molecules with only LDF’s as interparticle forces, molecules that are more polarizable have stronger LDF’s. (Polarizability depends on the number of electrons and size of a molecule. Molar mass is a quick and dirty way to compare polarizability.) Molecules as small as those listed do <em>not</em> get entangled - its purely an electrostatic phenomenon.
2) Be careful. If you say ionic bonds are stronger than covalent bonds you will <em>not</em> earn points. Ionic bonds are <em>not</em> stronger than covalent bonds. In ionic substances, the interparticle forces are ionic bonds. In covalent compounds, the interparticle forces are the relatively weak intermolecular forces (London dispersion forces and sometimes dipole-dipole or hydrogen bonding.) When melting or boiling molecular compounds, covalent bonds are not broken.</li>
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