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Chemical Bonding - Intermolecular vs Intramolecular Forces

Grade 9IB

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Intramolecular forces are the strong forces of attraction that hold atoms together within a molecule. The main types are covalent, ionic, and metallic bonds. For example, the bond in Cl−Cl\text{Cl}-\text{Cl} is intramolecular.

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Intermolecular forces (IMF) are the relatively weak attractive forces between neighboring molecules. These forces determine physical properties such as boiling point, melting point, and solubility.

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The three primary types of intermolecular forces include London Dispersion Forces (temporary dipoles), Dipole-Dipole interactions (permanent dipoles), and Hydrogen Bonding (a strong dipole-dipole interaction involving H\text{H} bonded to N\text{N}, O\text{O}, or F\text{F}).

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Strength Comparison: Intramolecular bonds are significantly stronger than intermolecular forces. Breaking an intramolecular bond (like the covalent bond in H2O\text{H}_2\text{O}) requires much more energy (≈460 kJ/mol\approx 460 \text{ kJ/mol}) than overcoming intermolecular forces (≈20−40 kJ/mol\approx 20-40 \text{ kJ/mol}).

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Phase Changes: When a substance changes state from liquid to gas, only the intermolecular forces are overcome. The intramolecular bonds remain intact. For instance, boiling water produces H2O\text{H}_2\text{O} vapor, not hydrogen and oxygen atoms.

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Electronegativity and Polarity: The polarity of a molecule, which dictates the type of IMF, is determined by the electronegativity difference Δχ\Delta \chi between atoms and the molecular geometry.

📐Formulae

Δχ=∣χA−χB∣\Delta \chi = |\chi_A - \chi_B|

F=kq1q2r2F = k \frac{q_1 q_2}{r^2}

μ=q×d\mu = q \times d

δ+⋯δ−\delta^+ \cdots \delta^-

💡Examples

Problem 1:

Explain why the boiling point of Water (H2OH_2O) is 100∘C100^\circ \text{C}, while the boiling point of Methane (CH4CH_4) is −161.5∘C-161.5^\circ \text{C}, despite having similar molecular masses.

Solution:

H2OH_2O exhibits strong Hydrogen Bonding, whereas CH4CH_4 is non-polar and only exhibits weak London Dispersion Forces.

Explanation:

Because Hydrogen bonds are much stronger than London Dispersion forces, significantly more thermal energy is required to overcome the intermolecular attractions in water to transition it from liquid to gas. This results in a much higher boiling point for H2OH_2O.

Problem 2:

When Hydrogen Chloride (HClHCl) dissolves in water, it dissociates into ions. Identify the force that holds the HH and ClCl atoms together in a single HClHCl molecule.

Solution:

Polar Covalent Bond (Intramolecular Force).

Explanation:

The atoms within a single HClHCl molecule are held together by the sharing of electrons. Since Chlorine is more electronegative (χCl≈3.0\chi_{\text{Cl}} \approx 3.0) than Hydrogen (χH≈2.1\chi_{\text{H}} \approx 2.1), the bond is specifically a polar covalent intramolecular force.

Problem 3:

Which requires more energy: boiling 1 mole1 \text{ mole} of liquid water or decomposing 1 mole1 \text{ mole} of water into Hydrogen and Oxygen gas?

Solution:

Decomposing water into Hydrogen and Oxygen gas.

Explanation:

Boiling involves overcoming intermolecular Hydrogen bonds (≈41 kJ/mol\approx 41 \text{ kJ/mol}), while decomposition involves breaking intramolecular covalent O−HO-H bonds (≈463 kJ/mol\approx 463 \text{ kJ/mol}). Intramolecular forces are much stronger than intermolecular forces.