Question

Consider the following enthalpy changes: $$\begin{aligned} \mathrm{F}^{-}+\mathrm{HF} \longrightarrow \mathrm{FHF}^{-} & \Delta H=-155 \mathrm{kJ} / \mathrm{mol} \\ \left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{O}+\mathrm{HF} \longrightarrow\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{O}--\mathrm{HF} & \Delta H=-46 \mathrm{kJ} / \mathrm{mol} \\\ \mathrm{H}_{2} \mathrm{O}(g)+\mathrm{HOH}(g) \longrightarrow \mathrm{H}_{2} \mathrm{O}--\mathrm{HOH}(\text { in ice }) & \Delta H=-21 \mathrm{kJ} / \mathrm{mol} \end{aligned}$$ How do the strengths of hydrogen bonds vary with the electronegativity of the element to which hydrogen is bonded? Where in the preceding series would you expect hydrogen bonds of the following type to fall?

Short answer

In conclusion, the strength of hydrogen bonds varies with the electronegativity of the element to which hydrogen is bonded. From the given enthalpy changes, we can rank the hydrogen bond strengths as follows: 1. FHF- (∆H = -155 kJ/mol) - Strongest hydrogen bond, since it involves the highly electronegative fluorine atom. 2. CH3C=OHF (∆H = -46 kJ/mol) - Weaker hydrogen bond, since it involves an oxygen atom. 3. H2O--HOH (∆H = -21 kJ/mol) - Weakest hydrogen bond, the interaction between two water molecules where there may be some shielding of the charges going on.

Step-by-step solution

Understanding hydrogen bonds

A hydrogen bond is a type of electrostatic attraction that occurs between a hydrogen atom, which is covalently bonded to a highly electronegative atom (such as fluorine, oxygen, or nitrogen), and another electronegative atom. The strength of hydrogen bonds depends on the electronegativity of the atoms involved and the distance between them. Step 2: Electronegativity trends in the periodic table

Electronegativity trends

Electronegativity is the ability of an atom to attract electrons towards itself. The general trend in the periodic table is that electronegativity increases up and to the right (towards fluorine). Therefore, Fluorine (F) is the most electronegative element, followed by Oxygen (O) and Nitrogen (N). Step 3: Comparing hydrogen bond strengths

Comparing hydrogen bond strengths

Recall that the strength of hydrogen bonds depends on the electronegativity of the atoms involved and the distance between them. 1. HF: The hydrogen bond in this molecule is between a fluorine atom and a hydrogen atom. Since fluorine is the most electronegative element, this hydrogen bond is expected to be the strongest. 2. H2O: The hydrogen bond in this molecule is between an oxygen atom and a hydrogen atom. Oxygen is less electronegative than fluorine, so the hydrogen bond in this case will be weaker than the one in HF. 3. (CH3)2CO: The hydrogen bond in this molecule is between an oxygen atom and a hydrogen atom. Since the oxygen atom is less electronegative than fluorine, the hydrogen bond will be weaker than that in HF but similar to the one in H2O. As the electronegativity of the element to which hydrogen is bonded increases, the strength of the hydrogen bond increases. From the given enthalpy changes in these reactions, we have: HF: ∆H = -155 kJ/mol H2O: ∆H = -21 kJ/mol (CH3)2CO: ∆H = -46 kJ/mol Step 4: Ranking the hydrogen bond strengths

Ranking hydrogen bond strengths

Based on the enthalpy values and electronegativity trends, we can rank the hydrogen bond strengths as follows: 1. FHF- (∆H = -155 kJ/mol) - Strongest hydrogen bond, since it involves the highly electronegative fluorine atom. 2. CH3C=OHF (∆H = -46 kJ/mol) - Weaker hydrogen bond, since it involves an oxygen atom. 3. H2O--HOH (∆H = -21 kJ/mol) - Weakest hydrogen bond, the interaction between two water molecules where there may be some shielding of the charges going on. In conclusion, the strength of hydrogen bonds varies with the electronegativity of the element to which hydrogen is bonded. In the given series, hydrogen bonds between the highly electronegative fluorine atom will be the strongest, followed by those between oxygen, and the weakest will be between the less electronegative oxygen in the H2O-HOH interaction.

Key concepts

Enthalpy Changes and Hydrogen Bonding

Enthalpy change, denoted as \( \Delta H \), is a measure of heat absorbed or released during a chemical reaction. When discussing hydrogen bonds, it's crucial to connect the concept of enthalpy changes to the strength of these bonds.

Hydrogen bonding occurs when a hydrogen atom covalently attached to a highly electronegative atom, like fluorine, oxygen or nitrogen, has an electrostatic attraction to another electronegative atom. Stronger hydrogen bonds usually result in larger (more negative) enthalpy changes, reflecting greater energy released when the bond forms. Therefore, analyzing \( \Delta H \) values provides insights into bond strength. For instance, in the provided exercise, a \( \Delta H \) of -155 kJ/mol for the fluoride ion and hydrogen fluoride reaction indicates a much stronger bond compared to -21 kJ/mol for water molecules forming ice.

It's imperative to remember, heat transfer and bond strengths go hand in hand; stronger bonds release more heat when formed, illustrating why enthalpy changes are a key factor in analyzing hydrogen bond strengths.

Electronegativity Trends in Chemical Bonding

Electronegativity is pivotal when exploring chemical bonding, especially hydrogen bonding. It describes an atom's tendency to attract and hold onto electrons. Within the periodic table, a trend emerges where electronegativity increases as one moves up a group and across a period towards fluorine.

The strength of a hydrogen bond is significantly influenced by the electronegativity of the elements involved; the more electronegative the atom bonded to hydrogen, the stronger the bond. This is due to the greater ability of such atoms to polarize the bond, creating a stronger attraction to the hydrogen's positive charge.

For example, the bond between hydrogen and fluorine is stronger than between hydrogen and oxygen, as reflected by their respective enthalpy changes. Simplified, electronegativity trends help anticipate the relative strength of hydrogen bonds, highlighting the fundamental role of electronegativity in understanding chemical bonding.

Navigating the Periodic Table

The periodic table is not just a collection of element symbols; it's a map of chemical properties and trends, crucial for understanding concepts like electronegativity and bond strength. Each column (group) and row (period) signifies elements with shared characteristics, anchoring the predictive power of chemistry.

Hydrogen's position in the periodic table, alongside the electronegative elements like fluorine and oxygen it bonds with, is fundamental to our understanding of bond strength variation. By examining the periodic trends, students can infer the relative strength of hydrogen bonds based on the position of the elements involved — knowledge that is as practical as it is theoretical.

Ultimately, grasping the layout and underlying principles of the periodic table is beneficial for predicting how elements will interact, especially when it comes to forming hydrogen bonds and other types of chemical bonding.

Chemical Bonding and Hydrogen Bonds

Chemical bonding is at the heart of chemistry, defining how atoms connect to form molecules. One specific type, the hydrogen bond, is a dipole-dipole interaction that is stronger than most. Yet, it remains weaker than ionic or covalent bonds.

In simplicity, a hydrogen bond between two water molecules might be pictured as a 'handshake' whereas a bond involving a highly electronegative fluorine atom resembles a 'tight hug' — a metaphor that conveys the variability in bond strength. It's the disparity in electronegativity and partial charges that enforces or weakens this 'handshake'.

Understanding the molecular geometry and electron distribution within molecules is essential for grasping hydrogen bonding. The small size of the hydrogen atom allows it to come into close proximity with electronegative elements making the bond stronger. Educating students about these intricate details ensures a comprehensive understanding of how molecules interact through hydrogen bonds and other chemical bonds.