Study Guide : Unit 3 Part 1 AP Chemistry

🧪 AP Chemistry – Unit 3: Intermolecular Forces and Properties

Subtopic 3.1 – Intermolecular Forces (IMFs)

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🧠 Concept Overview

Why do water and oil separate? Why does salt dissolve in water but not in gasoline? These behaviors all relate to intermolecular forces (IMFs)—the “stickiness” (intermolecular attraction) between molecules that affects boiling points, solubility, vapor pressure, and physical states.

Understanding IMFs helps explain:

• Why some substances are gases at room temperature while others are liquids or solids

• How molecular structure influences physical properties

• The behavior of mixtures and solutions

🎯 Learning Goals:

By the end of this, you should be able to:

• Identify LDF, dipole-dipole, and hydrogen bonding

• Compare strengths of these forces

• Explain how IMFs affect boiling/melting point, vapor pressure, and solubility

IMFs are not chemical bonds. They're attractive forces between molecules—weaker than covalent or ionic bonds, but still strong enough to determine key physical properties like melting point, boiling point, solubility, and vapor pressure.

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⚗️ Theory Breakdown

🧠 3 Types of IMFs (Weakest → Strongest)

1. London Dispersion Forces (LDFs)

• Present in all molecules, but the only IMF in nonpolar ones

• Caused by temporary fluctuations in electron distribution (temporary dipoles)

• Stronger in larger or more polarizable molecules (i.e., easier to distort their electron cloud)
  🧪 Examples: Noble gases (He, Ne), nonpolar molecules like O₂, CH₄

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2. Dipole-Dipole Interactions

• Occur in polar molecules

• Arise from permanent partial charges on molecules

• Stronger than LDFs, especially when molecular dipoles align well
  🧪 Examples: HCl, SO₂

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3. Hydrogen Bonding

• A stronger, special case of dipole-dipole interaction

• Occurs only when hydrogen is bonded to N, O, or F (highly electronegative atoms)

• Leads to unusually high boiling points and strong intermolecular attraction
  🧪 Examples: H₂O, NH₃, HF

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💡 Note: These are intermolecular forces (between molecules). Don’t confuse them with intramolecular forces like covalent or ionic bonds within molecules or compounds.

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🪄 Real-World Analogy

Think of molecules like partygoers:

• LDF: Everyone occasionally glances at each other—brief, weak attractions.

• Dipole-Dipole: People with strong opinions cluster together—permanent, directed attractions.

• Hydrogen Bonding: Close friends or couples stick together all night—stronger, more specific bonds.

These levels of “stickiness” affect whether people (molecules) can move freely (gas), flow together (liquid), or stay stuck in place (solid).

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📘 Worked Example

Question: Which molecule has the strongest intermolecular forces: CH₄, H₂O, or CO₂?

Step 1: Identify polarity and dominant IMF:

• CH₄: Nonpolar → only LDF

• CO₂: Linear and nonpolar overall → only LDF

• H₂O: Polar with hydrogen bonding (H–O)

✅ Answer: H₂O has the strongest IMFs due to hydrogen bonding.

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🚫 Common Misconceptions

• ❌ “All polar molecules have hydrogen bonding.”
    Only true if H is bonded directly to N, O, or F.

• ❌ “Ionic bonds are an IMF.”
    False — they're intramolecular (within compounds), not between molecules.

• ⚠️ “Larger molecules always have stronger forces.”
    Partially true for LDFs, but hydrogen bonding or polarity can outweigh size.
    For example, though CCl₄ is larger, H₂O has stronger IMFs because of hydrogen bonding

🧠 FLASHCARDS

• Q: What are intermolecular forces?
  A: Weak attractions between molecules (not bonds).

  Q: LDFs are present in...?
  A: All molecules (but dominant in nonpolar ones).

  Q: What causes LDFs?
  A: Temporary dipoles due to electron movement.

  Q: Which IMF is strongest?
  A: Hydrogen bonding.

  Q: What’s needed for H-bonding?
  A: H bonded to N, O, or F.

  Q: Dipole-dipole happens in...?
  A: Polar molecules.

  Q: Which has stronger IMFs: CH₄ or H₂O?
  A: H₂O (due to hydrogen bonding).

  Q: Does CO₂ have dipole-dipole forces?
  A: No, it’s nonpolar overall.

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❓ QUIZ

Multiple Choice (1 pt each)

• 1. Which substance exhibits hydrogen bonding?
     A. CH₄
     B. CO₂
     C. NH₃ ✅
     D. Cl₂

  2. What’s the dominant IMF in CH₄?
     A. Dipole-Dipole
     B. Ionic
     C. Hydrogen Bonding
     D. LDF ✅

  3. Which has the highest boiling point?
     A. Ne
     B. H₂
     C. H₂O ✅
     D. CO₂

  4. Which of the following has only LDFs?
     A. HCl
     B. SO₂
     C. CH₄ ✅
     D. HF

  5. True or False: LDFs are stronger in larger molecules.
     ✅ True

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Short Answer

• 1. Explain why water has a higher boiling point than CO₂.
     → H₂O has hydrogen bonding (stronger IMF) while CO₂ is nonpolar with only LDFs.

  2. What types of IMFs are present in NH₃?
     → LDF, dipole-dipole, and hydrogen bonding