#AP Chemistry: Intermolecular Forces - Your Ultimate Guide 🚀

Hey AP Chem student! Let's make sure you're feeling super confident about intermolecular forces (IMFs). This guide is designed to be your go-to resource, especially the night before the exam. We'll break down everything you need to know, make it easy to remember, and give you some killer strategies for test day. Let's get started!

#What Are Intermolecular Forces?

• Intermolecular Forces: Forces between molecules. Think interstate highways – they connect different states.
  • Types: London dispersion forces, dipole-dipole forces, hydrogen bonds, and ion-dipole forces.
• Intramolecular Forces: Forces within a molecule. Think intramural sports – they're within one school.
  • Types: covalent bonds, metallic bonds, ionic bonds.

🤔 Memory Aid: "Inter" means between, while "intra" means within. Easy peasy!

#Why are IMFs weaker than Intramolecular forces?

IMFs are weaker because they act over larger distances. Remember Coulomb's Law? Closer charges = stronger attraction. Intramolecular forces involve the simultaneous attraction of electrons between two nuclei within a molecule, making them much stronger.

Image Courtesy of Khan Academy

#London Dispersion Forces (LDFs)

London Dispersion Forces (LDFs) are the weakest type of IMF, but they're also everywhere! They exist between ALL molecules, including nonpolar molecules and noble gases.

• Key Point: If you're asked about IMFs, always include LDFs in your answer. They're easy to forget!

#How do LDFs Work?

At any given moment, electrons can shift in a molecule, creating a temporary dipole. This temporary dipole induces a dipole in neighboring molecules, leading to a weak attraction.

In other words, LDFs are a result of Coulombic interactions between temporary and fluctuating dipoles.

#LDF Strength

The strength of LDFs increases with the size of the molecule. More electrons = more polarizable electron cloud = stronger temporary dipole.

#Dipole-Dipole Interactions

Dipole-dipole attractions occur between polar molecules with permanent dipoles. These forces are stronger than LDFs.

The dipoles in HCl lead to a positive side and a negative side that are attracted to each other. Image Courtesy of EMedicalPrep

#Key Points

• Polarity Matters: The more polar the molecule, the stronger the dipole-dipole attraction.
• Coulomb's Law: Decreasing the distance between dipoles increases the interaction strength.
• Boiling/Melting Points: Stronger dipole-dipole attractions lead to higher boiling and melting points.

#AP Question Example

Here's a real AP question that combines LDFs and dipole-dipole forces:

The table above gives the molecular structures and boiling points for the compounds CS2 and COS. In terms of the types and relative strengths of all intermolecular forces in each compound, explain why the boiling point of CS2 (l) is higher than that of COS (l).

Sample Response: CS2 and COS both have London Dispersion Forces, but since COS is a polar molecule, it also exhibits dipole-dipole forces. However, the London Dispersion Forces in CS2 are so strong that they overpower the strength of both the LDFs and the dipole-dipole forces in COS. Therefore, CS2 has a higher boiling point.

#Hydrogen Bonding

Hydrogen bonding is a super strong type of dipole-dipole attraction. It only occurs when hydrogen is bonded to F, O, or N (Fluorine, Oxygen, or Nitrogen).

#Why is it so strong?

The high electronegativity difference and small size of F, O, and N lead to very strong attractions.

• The hydrogen (δ+) is attracted to the lone pairs on the F, O, or N (δ-).
• Molecules with hydrogen bonds have very high boiling points (e.g., water).

#Ion-Dipole Forces

Ion-dipole attractions occur in mixtures of ionic compounds and polar molecules. They're stronger than dipole-dipole and hydrogen bonding.

Image Courtesy of ck12.org

#How do they work?

• Ions (cations or anions) are attracted to the oppositely charged end of a polar molecule's dipole.
• Example: NaCl dissolving in water. The positive Na+ is attracted to the negative oxygen of water, and the negative Cl- is attracted to the positive hydrogens of water.

#Ion-Ion Attractions

Ion-ion attractions are the strongest type of attraction. They occur in ionic compounds, where full charges are present (not partial charges).

• These attractions hold ions together in a solid crystal lattice.
• They are responsible for the high melting and boiling points of ionic compounds.
• Example: The attraction between Na+ and Cl- in solid NaCl.

#Summary of IMFs

Here's a handy diagram to help you determine the dominant IMF:

Image Courtesy of Pearson Education

#Final Exam Focus 🎯

• High-Priority Topics:
  • Identifying types of IMFs in different substances.
  • Relating IMF strength to boiling points, melting points, and other physical properties.
  • Understanding the role of polarity and polarizability.
  • Applying Coulomb's Law to IMFs.
• Common Question Types:
  • Multiple-choice questions asking you to identify the strongest IMF in a given substance.
  • Free-response questions asking you to explain differences in boiling points based on IMFs.
  • Questions that combine IMFs with other concepts like thermodynamics or solutions.
• Last-Minute Tips:
  • Time Management: Don't spend too long on any one question. If you're stuck, move on and come back later.
  • Common Pitfalls: Watch out for confusing inter and intramolecular forces. Make sure to include LDFs in all your answers. Don't confuse hydrogen bonds with covalent bonds.
  • Strategies: Read the question carefully and identify key words. Draw diagrams to help you visualize the IMFs. Always explain your reasoning clearly and concisely.

#Practice Questions

Practice Question

#Multiple Choice Questions

1. Which of the following substances is expected to have the highest boiling point? (A) CH4 (B) H2O (C) NaCl (D) CO2

2. Which type of intermolecular force is primarily responsible for the relatively high boiling point of water? (A) London dispersion forces (B) Dipole-dipole forces (C) Hydrogen bonding (D) Ion-dipole forces

3. Which of the following molecules does NOT exhibit hydrogen bonding? (A) NH3 (B) HF (C) CH3OH (D) CH3Cl

#Free Response Question

The following data is given for two substances:

Substance	Molar Mass (g/mol)	Boiling Point (°C)
Butane (C4H10)	58	-0.5
Ethanol (C2H5OH)	46	78.3

(a) Draw the Lewis structures for both butane and ethanol. (b) Identify all the intermolecular forces present in a sample of each substance. (c) Explain why the boiling point of ethanol is significantly higher than that of butane, even though butane has a larger molar mass.

#Scoring Breakdown

(a) Lewis Structures (2 points)

• 1 point for correct Lewis structure of butane
• 1 point for correct Lewis structure of ethanol

(b) Intermolecular Forces (2 points)

• 1 point for identifying LDFs in both butane and ethanol
• 1 point for identifying hydrogen bonding in ethanol

(c) Explanation of Boiling Points (3 points)

• 1 point for stating that both have LDFs
• 1 point for stating that ethanol has hydrogen bonding, which is stronger than LDFs
• 1 point for relating the strength of IMFs to the higher boiling point of ethanol