2. Motor oils are comprised of hydrocarbons along with other carbon-based molecu

Question

2. Motor oils are comprised of hydrocarbons along with other carbon-based molecules. They are lubricants and must have a relatively high viscosity, but not so high that they cannot flow. Motor oils are thus rated based on their viscosity. The viscosities for a series of hydrocarbons are shown at right. Substance Formula Viscosity (kgm-s CH,CH CH CH CH CH, CH CH,CH CH. CH CH CH CH CH CH CH CH CH CH CH CH,CH CH CH CH,CH,CHCH CH CIH CH,CH,CH CH CH CIL CHCHCH, 142 x10 Hexane Heptane 326 × 10-4 4,09 x 10-4 5.42 × 10.4 Octane Nonane Decane a. Explain the observed trend in viscosity using intermolecular forces. Be very specific- What IMFs are present and how do they determine viscosity? of veggie oil versus biodiesel.Which is more viscous? Veggie oil or biodiesel? Why? NaOH or KOH vegetable ol gycerol

Explanation / Answer

2. For the given hydrocarbons, the viscosity increases as the number of carbon atoms increases in the hydrocarbon chain. This happens because of intermolecular forces such as dipole-dipole bonding, hydrogen bonding and dispersion forces.

For long hydrocarbon chains (low mobility), the molecules are able to pack with each other and stay close by, thus increasing the magnitude of the intermolecular forces, resulting in higher viscosity. However, for small chain hydrocarbons (high mobility), the molecules fins it difficult to align with each other and thus, the packing is weak. Hence, intermolecular forces are weak and viscosity is less.

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