π Hydrogen Bonds working (12 MCQs)
π From Campbell Biology β’ 2. The Chemistry of Life β’ 12 questions available
What is Hydrogen Bonds working?
Definition:
Hydrogen bonds are weak to moderate electrostatic interactions between a hydrogen atom covalently bonded to an electronegative atom (like oxygen or nitrogen) and another electronegative atom, and they are crucial for many biological structures and functions, including the double helix of DNA, protein folding, and enzyme-substrate interactions, and they are stronger than van der Waals forces but weaker than covalent bonds, with bond energies typically between 1 and 40 kJ/mol.
Working:
Hydrogen bonds work through the partial positive charge on the hydrogen atom (due to the polar bond) and the partial negative charge on the electronegative atom (e.g., oxygen in water), leading to an electrostatic attraction; they are directional and depend on the distance and angle, with the energy described by ; they are essential for the properties of water (cohesion, surface tension), and in biomolecules, they provide specificity and stability, as in the base pairing (A-T and G-C) in DNA, and they are reversible, allowing dynamic interactions.
Example:
A simple example is water, where hydrogen bonds between molecules give water its high boiling point and surface tension; another example is the hydrogen bonds between complementary bases in DNA (A=T and Gβ‘C), which hold the two strands together, allowing replication and transcription, illustrating the critical role of hydrogen bonds in biology.
Reason:
Hydrogen bonds are fundamental to life, as they define the structure of water, proteins, and nucleic acids, and understanding them is essential for biochemistry, structural biology, and drug design, as they are involved in virtually all biological interactions.
π All Hydrogen Bonds working MCQs
Q1. A student compares two substances with similar molecular masses. Substance X has many OβH groups and boils at a much higher temperature than substance Y. Which explanation best accounts for this observation?
π Explanation: OβH groups can participate in hydrogen bonding when hydrogen is attached to an electronegative atom. Extensive intermolecular hydrogen bonding requires additional energy to separate molecules, increasing boiling temperature.
Q2. Two water molecules approach each other. In one orientation, the hydrogen attached to oxygen of molecule A points toward the oxygen of molecule B. What most accurately describes the interaction?
π Explanation: The oxygenβhydrogen bond is polar because oxygen attracts electrons more strongly. This gives hydrogen a partial positive charge and oxygen a partial negative charge, allowing neighboring molecules to attract through hydrogen bonding.
Q3. A protein region contains several polar groups positioned so that hydrogen bonds can form between nearby parts of the chain. If these interactions are disrupted without breaking covalent bonds, what is the most likely immediate consequence?
π Explanation: Hydrogen bonds help stabilize particular three-dimensional arrangements of biological molecules. Disrupting them can alter folding or local structure even though the covalent bonds connecting the atoms remain intact.
Q4. A researcher replaces several OβH groups in a molecule with CβH groups while keeping molecular size approximately constant. The molecule becomes less able to interact strongly with water. Which reasoning best explains the change?
π Explanation: OβH groups create strongly polarized bonds because oxygen is highly electronegative, allowing hydrogen bonding with water. CβH bonds are much less polarized and therefore generally contribute less effectively to hydrogen-bond interactions.
Q5. A student claims: βHydrogen bonds are simply weak covalent bonds because hydrogen is involved in both.β Which correction is most scientifically appropriate?
π Explanation: A hydrogen bond is primarily an intermolecular or intramolecular electrostatic attraction involving polarized groups, whereas a covalent bond involves shared electrons within a chemical structure. Confusing these interactions leads to incorrect structural predictions.
Q6. A scientist measures the boiling temperatures of four liquids and obtains the following simplified data: Liquid A, 40Β°C; Liquid B, 78Β°C; Liquid C, 100Β°C; Liquid D, 20Β°C. Liquid C contains molecules capable of extensive hydrogen bonding, while the others have comparable molecular sizes but much weaker intermolecular attractions. Which conclusion is best supported?
π Explanation: The data show that a substance capable of extensive hydrogen bonding can require substantially more energy to separate its molecules. Therefore, hydrogen bonding can raise boiling temperature when molecular sizes are otherwise comparable.
Q7. A graph shows that the boiling temperatures of four similarly sized molecular substances increase from A to B to C, while the number of available hydrogen-bonding sites increases in the same order. Which interpretation is most reasonable?
π Explanation: When molecular sizes are comparable, increasing opportunities for hydrogen bonding can strengthen the overall intermolecular attraction. More energy may then be needed to separate molecules, producing a higher boiling temperature.
Q8. A polar molecule contains one oxygen atom and two hydrogen atoms. A student argues that because the molecule has covalent OβH bonds, it cannot form hydrogen bonds with neighboring molecules. What is the flaw?
π Explanation: The presence of covalent OβH bonds actually creates the polarization needed for hydrogen bonding. The hydrogen becomes partially positive, allowing attraction to an electronegative atom carrying a partial negative charge.
Q9. A wet biological surface is treated with a chemical that replaces several polar groups with nonpolar groups. After treatment, water spreads less effectively across the surface. Which chain of reasoning best explains the observation?
π Explanation: Water interacts favorably with surfaces containing appropriately polarized groups because hydrogen bonding and related electrostatic interactions can occur. Replacing those groups with nonpolar groups reduces such interactions and can decrease wetting.
Q10. Two molecules have similar sizes. Molecule P can form four hydrogen bonds with neighboring molecules, while molecule Q can form only one. Assuming their other major intermolecular forces are comparable, which prediction is most reasonable?
π Explanation: Multiple hydrogen-bonding interactions can produce a substantial cumulative attraction among molecules. If molecular size and other important factors are comparable, the molecule with more effective hydrogen-bonding opportunities may require more energy to separate.
Q11. A student says, βIf one hydrogen bond is broken, the molecule itself must decompose.β Which scenario most directly disproves this reasoning?
π Explanation: Evaporation involves overcoming intermolecular attractions so molecules can move farther apart. The OβH covalent bonds within individual water molecules remain intact, demonstrating that hydrogen-bond disruption does not necessarily decompose molecules.
Q12. A researcher compares two compounds and finds that compound M forms hydrogen bonds with water, while compound N has a similar size but lacks suitable hydrogen-bonding groups. Which prediction is most defensible for their behavior in water?
π Explanation: Water is highly polar and can form hydrogen bonds with suitable polar functional groups. A similarly sized compound possessing such groups can therefore interact more favorably with water than a compound lacking appropriate hydrogen-bonding sites.