Problem 28 - Chemistry and Chemical Reactivity

Question

When water is placed in a buret it forms a concave meniscus at the surface. In contrast, mercury (in a manometer for example) forms a convex meniscus (Figure \(12.22) .\) Explain why this phenomenon occurs, and why the two liquids give different results. Predict the meniscus shape if the buret is filled with ethylene glycol \(\left(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\right).\)

Step-by-Step Solution

Answer

Ethylene glycol would form a concave meniscus due to hydrogen bonding and adhesion to glass.

Step 1: Understanding Properties of Liquids

Water has strong hydrogen bonds due to its polar nature. These bonds result in strong cohesion (interaction between molecules of the same substance) and strong adhesion (interaction between molecules and different material, like glass). Mercury, however, is a metal and does not exhibit hydrogen bonding. It has weak adhesion with glass but strong cohesion within itself, due to metallic bonding properties.

Step 2: Analyzing Meniscus Shape

A concave meniscus occurs when adhesive forces are stronger than cohesive forces. In water, the molecules adhere strongly to the glass, drawing it up the side and forming a concave shape. In contrast, a convex meniscus is formed when cohesive forces are stronger than adhesive forces. For mercury, metallic bonds hold the molecules closely together, preventing them from adhering to the glass, forming a dome-like, convex shape.

Step 3: Predicting Ethylene Glycol Meniscus Shape

Ethylene glycol is similar to water in that it can form hydrogen bonds, as it is also a polar molecule. Given its adhesive nature to glass and similar intermolecular hydrogen bonding, ethylene glycol would form a concave meniscus when placed in a glass buret. Its behavior is expected to be more like water than mercury.

Key Concepts

Cohesion and Adhesion

Cohesion and adhesion are essential concepts in understanding how liquids interact with surfaces. Cohesion refers to the attraction between molecules of the same substance. For instance, in water, molecules are attracted to each other due to hydrogen bonds, a type of cohesion.
Adhesion, on the other hand, describes the interaction between molecules of a liquid and a different material, such as glass. This interaction causes water to "stick" to surfaces that it contacts. When the adhesive forces (between water and glass) are stronger than the cohesive forces (between water molecules), the liquid tends to "climb" up the walls of its container, resulting in a concave meniscus.
In mercury, cohesion is dominant due to strong metallic bonding between mercury atoms. Adhesion to glass is much weaker, which causes mercury to form a convex meniscus, showing limited climbing on the container walls.

Hydrogen Bonding in Liquids

Hydrogen bonding is a fascinating mechanism that plays a prominent role in determining the properties of certain liquids. It occurs when a hydrogen atom, covalently bonded to a highly electronegative atom like oxygen, experiences an attraction to another electronegative atom in a neighboring molecule. This strong intermolecular force is pivotal in distinguishing the behavior of water and similar liquids.
Water is a polar molecule, meaning it has positively and negatively charged poles. The hydrogen bonds formed in water lead to significant cohesion among water molecules, enhancing its surface tension and leading to characteristics like the meniscus shape observed in its container.

• Strong hydrogen bonds create a pronounced concave meniscus in water.
• When compared to other non-polar substances like mercury, the absence of hydrogen bonding results in significantly different interactions with glass, notably weaker adhesion.

Ethylene glycol, another polar molecule similar to water, forms hydrogen bonds, predicting a concave meniscus when placed in a glass container.

Properties of Water and Mercury

Water and mercury display starkly contrasting behaviors due to their distinct properties. Water, known for its unique hydrogen bonding capabilities, spreads readily on surfaces, driven by its adhesive forces to materials like glass. This quality is accentuated by the polar nature of water molecules, fostering strong adhesion and the formation of a concave meniscus.
Mercury, by contrast, does not engage in hydrogen bonding. It is a metallic element in liquid form, possessing strong cohesive forces due to the metallic bonds within itself. These forces overpower any weak adhesion it might have with materials like glass, thus generating a convex meniscus.

• Water's adhesive properties lead to a wetter surface and a concave meniscus.
• Mercury's strong cohesion leads to minimal spread on surfaces and a convex meniscus.

Understanding these properties helps predict behaviors of other liquids, such as ethylene glycol, which, akin to water, has polar characteristics and shows similar properties when in contact with glass.