Question

In general, as the molar mass of Group VIA/16 hydrogen compounds increases, does each of the following increase or decrease? (a) melting point (b) boiling point (c) heat of fusion (d) heat of vaporization

Short answer

(a) Increase, (b) Increase, (c) Increase, (d) Increase.

Step-by-step solution

Understanding the Trend in Molar Mass

Group VIA elements form hydrogen compounds that increase in molar mass as you move down the group. This affects their physical properties, such as melting point, boiling point, heat of fusion, and heat of vaporization.

Analyzing the Melting Point

Melting points generally increase as you move down the group due to stronger Van der Waals forces in compounds with higher molar mass. These forces require more energy to overcome, raising the melting point.

Analyzing the Boiling Point

The boiling point usually increases with higher molar mass for similar reasons as melting points -- stronger Van der Waals interactions between larger molecules result in a need for more energy for phase transition from liquid to gas.

Exploring the Heat of Fusion

Heat of fusion increases with molar mass because it requires more energy to disrupt the more substantial intermolecular forces present in compounds with larger molar mass during the phase transition from solid to liquid.

Investigating the Heat of Vaporization

Heat of vaporization also increases as molar mass increases. Similar to the boiling point, more energy is required to overcome the stronger intermolecular interactions during vaporization of heavier molecules.

Key concepts

Molar Mass

Molar mass is an essential concept in understanding the behavior of molecules. It refers to the weight of one mole of a substance and is usually expressed in grams per mole (g/mol). The molar mass impacts various physical properties of substances, especially their melting and boiling points, heat of fusion, and heat of vaporization.

For compounds formed by Group VIA elements with hydrogen, we notice that as these compounds become heavier (due to an increase in molar mass), their physical characteristics change. Heavier molecules tend to bond more strongly with each other, thanks to more substantial intermolecular forces present. This directly influences several phase change properties. Understanding molar mass is crucial as it helps predict how substances will behave under different conditions.

Phase Change

Phase changes refer to transitions between different states of matter: solid, liquid, and gas. These changes involve energy transfer and are significantly influenced by the molar mass of the molecules involved.

• **Melting Point:** As seen with Group VIA elements bonded with hydrogen, the melting point generally increases as the molar mass goes up. Heavier molecules have stronger intermolecular forces, demanding more energy to transition from solid to liquid.
• **Boiling Point:** Similar to melting points, boiling points rise with increasing molar mass. This is due to stronger interactions needing extra energy to shift from liquid to gaseous state.
• **Heat of Fusion and Vaporization:** Both these properties—measurements of the energy required to change states—also tend to increase with molar mass for similar reasons. Larger molecules need more energy for these transitions because of stronger intermolecular attractions.

Acknowledging these relationships helps predict how substances of different molar masses behave energetically.

Group VIA Elements

Group VIA elements include oxygen, sulfur, selenium, tellurium, and polonium. These elements form compounds with hydrogen—known as hydracids or hydrides—that display specific patterns in physical properties.

• The trend within Group VIA elements is clear: as you move down the group, the compounds formed have greater molar masses.
• This molar increase is a crucial factor in determining physical behaviors, such as higher melting and boiling points.
• Moreover, the strength of intermolecular attractions like Van der Waals forces becomes more pronounced with larger molecules.

Understanding the group trends helps in predicting and rationalizing the changes observed in various physical properties, simply by knowing the position of the element within the periodic table.

Van der Waals Forces

Van der Waals forces are weak intermolecular forces that come into play in molecules having non-polar covalent bonds. Despite being weaker than ionic or covalent bonds, they have significant effects on the physical properties of compounds as the molecular size increases.

• These forces are responsible for holding molecules together in the liquid and solid phases.
• As the molar mass grows, these forces become stronger, contributing to higher melting and boiling points.
• In Group VIA hydrogen compounds, larger molecules experience heightened Van der Waals forces, resulting in more energy needed to change their states.

Understanding Van der Waals forces allows students to comprehend why larger molecules demonstrate increased energy requirements for phase transitions, connecting molecular mass to observable properties like phase changes.