Question

which compound would you expect to have a greater viscosity at a given temperature, ethane (\\(\text{c}_2\text{h}_6\\)) or pentane (\\(\text{c}_5\text{h}_{12}\\)), and why?

ethane because it has a smaller molar mass and is shorter.
ethane because it has the stronger intermolecular forces.
pentane because it has a larger molar mass and is longer.
pentane because it has the weaker intermolecular forces.

Explanation:

Define viscosity and its relationship to intermolecular forces

Viscosity measures a fluid's resistance to flow. Stronger Intermolecular Forces hold molecules together more tightly, increasing viscosity.

Compare the structures of ethane and pentane

Ethane \(\text{C}_2\text{H}_6\) is a short two-carbon alkane. Pentane \(\text{C}_5\text{H}_{12}\) is a longer five-carbon alkane.

Analyze dispersion forces and molecular size

Both are nonpolar hydrocarbons experiencing London dispersion forces. Larger, longer molecules have larger electron clouds and greater surface area contact.

Determine the relative viscosity

Using the Intermolecular Forces knowledge point, pentane has stronger dispersion forces due to its larger molar mass and longer chain. This leads to higher viscosity.

Match with the correct option

Pentane has a greater viscosity because it has a larger molar mass and is longer.

Answer:

• (A) Ethane because it has a smaller molar mass and is shorter.
• (B) Ethane because it has the stronger intermolecular forces.
• (C) Pentane because it has a larger molar mass and is longer. (Correct answer)
• (D) Pentane because it has the weaker intermolecular forces.