Water, Hydrogen Bonding, and Noncovalent Interactions - Practice Flashcards

These flashcards cover water's structure and role in biology, hydrogen bonding, solubility, hydrophobic effects, noncovalent interactions, osmotic concepts, and basic acid–base chemistry from the notes.

What percentage of body weight is water typically present in humans?

About 55% to 80%, depending on metabolic status, age, and gender.

Where is water predominantly found in the body (main compartments)?

In the cytoplasm, interstitial spaces, and blood (i.e., inside cells, between cells/tissues, and in the circulatory system).

Name two physiological roles of water related to the brain and temperature regulation.

Water is needed for brain hormone and neurotransmitter synthesis and regulates body temperature via sweating and respiration.

Why is water considered an ideal biological solvent?

Water’s dipole moment and ability to form hydrogen bonds give it a high dielectric constant and enable dissolution of polar and charged substances.

Describe the molecular structure of water and why it is polar.

H2O: oxygen is more electronegative, giving partial negative charge on O and partial positive on H; bent geometry (~105°) with two lone pairs, leading to a dipole.

What is a hydrogen bond donor and what is a hydrogen bond acceptor? Give an example in water.

Donor: the atom (usually H) covalently bonded to an electronegative atom that donates the hydrogen; Acceptor: the electronegative atom with a lone pair that accepts the hydrogen. In water, the O–H bond can donate, and the lone pairs on O can accept.

What is the typical energy range for hydrogen bonds and how does it compare to covalent bonds?

Hydrogen bonds: about 4–6 kJ/mol (neutral) or 6–10 kJ/mol (when a charge is involved); covalent O–H bonds are about 420 kJ/mol, much stronger.

How many hydrogen bonds can a water molecule form in liquid water at any moment?

Approximately 3.4 on average; up to 4 possible.

How does a covalent O–H bond differ from a hydrogen bond?

Covalent O–H involves sharing electrons between atoms; hydrogen bond is a noncovalent electrostatic attraction between a hydrogen attached to an electronegative atom and another electronegative atom, with no electron sharing.

Why does water have a high boiling point and high surface tension?

Because of the extensive, directional hydrogen-bond network that requires substantial energy to break and that promotes cohesion and adhesion.

What is special about hexagonal ice compared to liquid water?

Hexagonal ice forms a rigid lattice with four hydrogen bonds per molecule, making it less dense than liquid water, so ice floats; low entropy in the solid state.

What kind of substances dissolve well in water and why?

Charged and polar substances dissolve well because they can form hydrogen bonds or ion–dipole interactions with water (e.g., salts, sugars, amino acids); nonpolar lipids do not dissolve well.

What is a solvation (hydration) shell?

The shell of water molecules surrounding a dissolved ion or molecule, stabilizing and solvating it.

Define the hydrophobic effect and its role in micelle formation.

Nonpolar regions are expelled from water to minimize disruption of the water hydrogen-bond network, driving aggregation; amphipathic molecules form micelles with hydrophobic tails inward and polar heads outward.

What is an amphipathic molecule?

A molecule that has both a polar (hydrophilic) region and a nonpolar (hydrophobic) region.

What is Van der Waals interaction and its role in biology?

Weak, noncovalent forces arising from proximity and induced dipoles; they contribute to steric complementarity and stabilize structures like DNA base stacking and protein packing.

What are ionic interactions?

Attractions between permanently charged species and between ions and permanent dipoles (e.g., between ammonium and carboxylate groups).

What holds together DNA base pairs, and how many hydrogen bonds link A–T and G–C?

Hydrogen bonds stabilize base pairing; A–T pairs are held by 2 hydrogen bonds, G–C pairs by 3 hydrogen bonds.

What is the ionic product of water (Kw) at 25°C?

Kw = [H+][OH−] = 1.0 × 10^−14.

Define pH and explain its relation to pOH and Kw.

pH = −log10[H+]; pOH = −log10[OH−]; at 25°C, pH + pOH = 14 and Kw = 10^−14.

If [H+] = 1.0 × 10^−5 M, what is [OH−] and the pH?

[OH−] = Kw/[H+] = 1.0 × 10^−14 / 1.0 × 10^−5 = 1.0 × 10^−9 M; pH = 5.

What is proton hopping and why is it important in water?

Protons move rapidly between water molecules via the hydrogen-bond network (Grotthuss mechanism), giving water high proton conductivity.

What is osmosis and distinguish endosmosis from exosmosis?

Osmosis is the movement of water across a semipermeable membrane to equalize solute concentrations; endosmosis is water entering a cell (hypotonic environment), exosmosis is water leaving (hypertonic environment).