The boiling points, surface tensions, and viscosities of water and several alcohols are listed in the following table:
b. How do you explain the fact that propanol and ethylene glycol have similar molecular weights (60 vs. 62 amu), yet the viscosity of ethylene glycol is more than 10 times larger than propanol?

Verified step by step guidance

Identify the molecular structures of propanol and ethylene glycol. Propanol is C3H7OH, and ethylene glycol is C2H4(OH)2.

Consider the functional groups present in each molecule. Propanol has one hydroxyl group (-OH), while ethylene glycol has two hydroxyl groups.

Understand that the presence of hydroxyl groups leads to hydrogen bonding. Ethylene glycol, with two -OH groups, can form more hydrogen bonds compared to propanol.

Recognize that increased hydrogen bonding in ethylene glycol results in stronger intermolecular forces, which contribute to higher viscosity.

Conclude that despite similar molecular weights, the ability of ethylene glycol to form more hydrogen bonds due to its two hydroxyl groups leads to its significantly higher viscosity compared to propanol.

Verified video answer for a similar problem:

This video solution was recommended by our tutors as helpful for the problem above.

Video duration:

59s

Was this helpful?

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Molecular Weight and Viscosity

Molecular weight is the mass of a molecule, typically measured in atomic mass units (amu). While similar molecular weights can suggest comparable sizes, viscosity is influenced by intermolecular forces and molecular structure. Ethylene glycol, with its two hydroxyl (-OH) groups, forms stronger hydrogen bonds than propanol, leading to higher resistance to flow and thus greater viscosity.

Intermolecular Forces

Intermolecular forces are the attractive forces between molecules that significantly affect physical properties like boiling point and viscosity. Ethylene glycol exhibits stronger hydrogen bonding due to its structure, which increases its viscosity compared to propanol. Understanding these forces helps explain why substances with similar molecular weights can have vastly different physical properties.

Hydrogen Bonding

Hydrogen bonding is a specific type of strong dipole-dipole attraction that occurs when hydrogen is bonded to highly electronegative atoms like oxygen. In ethylene glycol, the presence of two -OH groups allows for extensive hydrogen bonding, resulting in a much higher viscosity compared to propanol, which has only one -OH group. This difference in bonding capabilities is crucial for understanding the viscosity disparity.

Recommended video:

Guided course

01:22

Hydrogenation Reactions

Related Practice

Textbook Question

Based on their composition and structure, list CH₂Cl₂, CH₃CH₂CH₃, and CH₃CH₂OH in order of (a) increasing intermolecular forces (c) increasing surface tension

Textbook Question

Liquids can interact with flat surfaces just as they can with capillary tubes; the cohesive forces within the liquid can be stronger or weaker than the adhesive forces between liquid and surface:

(b) Which of these diagrams, i or ii, rep- resents what happens when water is on a nonpolar surface?

Textbook Question

Name the phase transition in each of the following situations and indicate whether it is exothermic or endothermic: (c) Rubbing alcohol in an open container slowly disappears.

Textbook Question

Name the phase transition in each of the following situations and indicate whether it is exothermic or endothermic: (d) Molten lava from a volcano turns into solid rock.