AP Chemistry Unit 2 Study Notes

AP Chemistry 2.2: Intramolecular force and potential energy

Relate bond formation, bond breaking, and potential energy.

Aligned to Compound Structure and Properties from the current College Board AP Chemistry course outline. Exam weighting for this unit: 7%-9% of the multiple-choice score range listed by College Board.

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Intramolecular Force and Potential Energy Overview 2.1 Intramolecular vs. Intermolecular 2.2 What Happens When Two Atoms Approach Each Other? 2.3 The Ideal Bond Distance 2.4 What If the Atoms Get Too Close? 2.5 Potential Energy Curve 2.6 Bond Energy 2.7 Bond Strength vs. Bond Length 2.8 Why? 2.9 Breaking Bonds Requires Energy 2.10 Forming Bonds Releases Energy Topic 2 Test Interpretation

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Intramolecular Force and Potential Energy Overview Open

  • This topic explains why bonds have certain lengths and strengths using potential energy.

2.1 Intramolecular vs. Intermolecular Open

  • These words are easily confused.

  • Intramolecular forces

    • exist inside a particle , holding atoms together.

    • Examples:

    • covalent bonds

    • ionic bonding

    • metallic bonding

  • Intermolecular forces

    • occur between separate molecules .

  • Intermolecular forces are mainly Unit 3.

  • For Unit 2, we focus mostly on intramolecular attractions .

2.2 What Happens When Two Atoms Approach Each Other? Open

  • Imagine two isolated atoms moving toward each other.

  • When they are very far apart:

    • interaction is weak.

  • As they get closer:

    • each nucleus begins attracting electrons on the other atom.

  • Potential energy decreases.

  • Lower potential energy means:

    • greater stability

2.3 The Ideal Bond Distance Open

  • Eventually, the atoms reach a distance where attraction and repulsion balance.

    • This is approximately the:

    • bond length

  • At that distance, the potential energy reaches a minimum .

    • This is the most stable distance between those bonded atoms.

2.4 What If the Atoms Get Too Close? Open

  • If the atoms are pushed closer than the equilibrium bond length:

    • nucleus-nucleus repulsion becomes very strong

    • electron-electron repulsion also increases

  • Potential energy rises quickly.

    • Therefore atoms do not collapse into one another.

2.5 Potential Energy Curve Open

  • A typical potential-energy graph has:

    • x-axis: internuclear distance

    • y-axis: potential energy

  • The curve often looks like a valley.

  • At far distance:

    • PE ≈ 0

  • As atoms approach:

  • PE decreases.

  • At the minimum:

    • equilibrium bond length

  • If atoms get even closer:

  • PE rises sharply.

2.6 Bond Energy Open

  • Bond energy describes the amount of energy required to break a bond.

  • On a potential-energy graph, it corresponds to the difference between:

    • the energy at the bottom of the potential well

    • and

    • the energy of separated atoms.

  • Deeper potential well:

    • stronger bond

  • More energy is required to separate the atoms.

2.7 Bond Strength vs. Bond Length Open

  • For bonds between similar atoms:

    • stronger bonds are generally shorter

    • and:

    • weaker bonds are generally longer

    • Example:

    • C—C

    • C=C

    • C≡C

  • Bond order increases:

    • 1 → 2 → 3

  • Bond length:

    • decreases

  • Bond strength:

    • increases

2.8 Why? Open

  • More shared electron density between the nuclei produces stronger attraction.

    • Thus:

  • C≡C holds the nuclei closer than C—C.

2.9 Breaking Bonds Requires Energy Open

  • Important:

  • Breaking a chemical bond requires energy.

  • Students sometimes think breaking bonds releases energy.

    • It does not.

  • You must supply energy to overcome the attractive forces holding atoms together.

2.10 Forming Bonds Releases Energy Open

  • When atoms form a stable bond:

    • potential energy decreases.

  • The lost potential energy is released to the surroundings.

    • So:

    • breaking bonds → absorbs energy

    • forming bonds → releases energy

    • This becomes extremely important in thermochemistry later.

Topic 2 Test Interpretation Open

  • On a potential-energy curve:

    • minimum x-value location → bond length

    • deeper minimum → stronger bond

    • atoms too close → high repulsive energy

    • atoms infinitely far apart → essentially no bond