Look at what gets broken
U4.1 classified changes by whether a new substance forms. AP topic 4.4 goes one level deeper and asks which interactions are broken and formed:
- Chemical change: chemical bonds化学键 (covalent, ionic, or metallic) are broken or formed, and atoms rearrange.
- Physical change: only the intermolecular forces分子间作用力 between particles change (dispersion forces, dipole–dipole forces, hydrogen bonds); each molecule stays intact.
So when water boils, the hydrogen bonds between water molecules are broken, while the O–H bonds inside every H₂O molecule are untouched; only when water decomposes into H₂ and O₂ are O–H covalent bonds broken.
How much energy
Chemical bonds are generally much stronger than intermolecular forces. Compare the energies per mole:
| Process | Interactions broken | Energy (kJ/mol) |
|---|---|---|
| Ice melting | Some hydrogen bonds | 6.01 |
| Water boiling | Hydrogen bonds | 40.7 |
| H₂O decomposing into H₂ and ½O₂ | 2 O–H covalent bonds | about 926 |
| H₂ + Cl₂ → 2HCl | The H–H and Cl–Cl bonds | about 678 |
The enthalpies of fusion and vaporization come from the CRC Handbook; the bond energies are average bond enthalpies (Brown et al., Chemistry: The Central Science). Physical changes typically involve a few to a few tens of kJ/mol, while the bonds broken in a chemical reaction typically take hundreds of kJ/mol.
A gray area: dissolving an ionic compound
When NaCl dissolves in water, the ionic bonds between Na⁺ and Cl⁻ are broken (the lattice energy is about 786 kJ/mol), while ion–dipole interactions form between the ions and water molecules, releasing almost as much energy; the net enthalpy of solution is only +3.88 kJ/mol.
- By whether a new substance forms: none does, and evaporating the water gives NaCl back, so it looks physical.
- By whether chemical bonds are broken: ionic bonds are, so it looks chemical.
AP accepts that such processes can be classified either way. Rather than asking only which kind of change it is, the exam expects you to describe the interactions broken and formed.
Some salts react further after they dissolve. When sodium acetate dissolves, the CH₃COO⁻ ions take H⁺ from some water molecules, acting as a Brønsted–Lowry base (U4.8) to form a little CH₃COOH and OH⁻, so the solution is basic:
That step breaks O–H bonds and forms new ones, so it is a chemical change.
Compare six processes, the interactions broken in each, and the energy needed:
Physical change
- Broken: some of the hydrogen bonds between water molecules (needs about 6.01 kJ/mol)
The water molecules themselves (their O–H covalent bonds) are unchanged; only their arrangement changes: a physical change.
Physical change
- Broken: the hydrogen bonds between water molecules (needs about 40.7 kJ/mol)
The water molecules separate, but no O–H bond inside any molecule breaks: a physical change.
Classified either way
- Broken: the ionic bonds between Na⁺ and Cl⁻ (needs about 786 kJ/mol)
- Formed: ion–dipole interactions between the ions and water molecules (releases about 782.1 kJ/mol)
The ionic bonds broken are strong, but the ion–dipole interactions formed are almost as strong, so the enthalpy of solution is only +3.88 kJ/mol. No new substance forms, yet ionic bonds are broken, so some books call it physical and others chemical. On the AP exam, say exactly which interactions are broken and formed.
Chemical change
- Broken: 2 O–H covalent bonds (needs about 926 kJ/mol)
- Formed: an H–H bond and half an O=O bond (releases about 683.5 kJ/mol)
The atoms rearrange into new substances, H₂ and O₂: a chemical change.
Chemical change
- Broken: the H–H and Cl–Cl bonds (needs about 678 kJ/mol)
- Formed: 2 H–Cl bonds (releases about 862 kJ/mol)
Old bonds break and new ones form, producing a new substance, HCl: a chemical change.
Chemical change
- Broken: 4 C–H bonds and 2 O=O bonds (needs about 2642 kJ/mol)
- Formed: 2 C=O bonds and 4 O–H bonds (releases about 3450 kJ/mol)
Combustion breaks and forms covalent bonds: a chemical change.
Energy needed to break these interactions (kJ/mol, log scale)
Example 1: Boiling versus electrolysis
Boiling water and electrolyzing water both produce bubbles. Compare the two processes in terms of the interactions broken and the energy involved.
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Identify: what boiling breaks. In boiling the H₂O molecules are unchanged, and the bubbles are water vapor; what’s overcome is the hydrogen bonding and other intermolecular forces between molecules. The energy needed is the enthalpy of vaporization, about 40.7 kJ/mol.
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Identify: what electrolysis breaks. In electrolysis the O–H covalent bonds inside the H₂O molecules break and the atoms recombine into H₂ and O₂, so the bubbles are new substances. Each H₂O molecule has 2 O–H bonds (bond energy about 463 kJ/mol), so per mole of H₂O (in kJ):
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Compare the energies.
Breaking the O–H bonds takes over 20 times the enthalpy of vaporization.
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Connect to the type of change. Boiling overcomes only intermolecular forces: a physical change. Electrolysis breaks covalent bonds and forms new substances: a chemical change. Covalent bonds are much stronger than intermolecular forces, which is why the energies differ so much.
Example 2: Two changes of iodine
When iodine is heated, the violet-black solid turns into a violet vapor; at very high temperatures, I₂ molecules in the vapor break apart into iodine atoms. Explain how the interactions broken differ in the two processes.
- Identify: what sublimation breaks. I₂(s) → I₂(g): there are I₂ molecules before and after; what’s overcome is the dispersion forces between I₂ molecules, and the I–I covalent bond stays intact: a physical change.
- Identify: what dissociation breaks. I₂(g) → 2I(g): the I–I covalent bond (151 kJ/mol) breaks, forming new particles, iodine atoms: a chemical change.
- Compare the strengths. The I–I covalent bond is much stronger than the dispersion forces between I₂ molecules, so breaking it takes more energy.
- Connect to temperature. More energy means a higher temperature: iodine sublimes with gentle heating, but I₂ molecules break apart noticeably only at very high temperatures.
- The method. For any change of one substance, ask: “Is what’s broken the attraction between molecules, or a bond inside them?” The first is a physical change; the second is a chemical change.
- Ethanol boiling
The hydrogen bonds between ethanol molecules are overcome; the molecules are unchanged.
- Ethanol burning
C–H, C–C, O–H, and O=O covalent bonds break, forming CO₂ and H₂O.
- Candle wax melting
Only a change of state.
- Dry ice subliming
The dispersion forces between CO₂ molecules are overcome.
- Electrolyzing molten NaCl into Na and Cl₂
Ionic bonds are broken and new substances form.
- Sugar dissolving in water
The attractions between sucrose molecules are broken; the molecules are unchanged.
- Hydrogen peroxide decomposing
O–O and O–H bonds break, forming H₂O and O₂.
- Steam condensing into water
Hydrogen bonds form between molecules; the molecules are unchanged.
- I₂ molecules splitting into iodine atoms
The I–I covalent bond breaks.
- Iodine subliming
The dispersion forces between I₂ molecules are overcome.
- Compressing water vapor at constant temperature until droplets form
Condensing forms hydrogen bonds between molecules; the water molecules are unchanged.
Common mistakes
Mistake 1: “When water boils, its molecules break into hydrogen and oxygen, so the bubbles are H₂ and O₂.” Boiling overcomes only the hydrogen bonds between water molecules; the bubbles are water vapor. Breaking O–H bonds takes far more energy.
Mistake 2: “Physical changes involve no energy change.” Melting, boiling, and dissolving all absorb or release energy, usually less than chemical reactions do. An energy change alone can’t tell the two kinds of change apart.
Mistake 3: “Intermolecular forces are just weak chemical bonds.” Chemical bonds are inside molecules, holding atoms together; intermolecular forces act between molecules. Breaking chemical bonds gives new particles; overcoming intermolecular forces leaves the original molecules.
Quick check
Which process requires overcoming only intermolecular forces?
Bromine evaporating takes Br₂ molecules from liquid to gas, overcoming only dispersion forces. The other three break chemical bonds and form new substances.
The enthalpy of vaporization of water is 40.7 kJ/mol, and the average O–H bond enthalpy is 463 kJ/mol. Which statement is best?
Both values are in kJ/mol, so they can be compared. 40.7 is far less than 463, showing that intermolecular forces are much weaker than covalent bonds; boiling breaks no covalent bonds.
Some books call dissolving NaCl in water a physical change and others a chemical change. Which explanation is best?
Both views have a basis: ionic bonds are broken (like a chemical change), but no new substance forms (like a physical change). A is false; C leaves out the ionic bonds.
What is overcome when iodine sublimes?
After sublimation the iodine is still I₂ molecules, so the I–I bond is intact. I₂ is nonpolar, so only dispersion forces act between its molecules.
Which pair of energies is most likely to be for a physical change and a chemical change, in that order?
Physical changes (overcoming intermolecular forces) typically take a few to a few tens of kJ/mol; chemical changes (breaking bonds) typically take hundreds.
A student says, “Ice absorbs heat when it melts, so melting is a chemical change.” Which rebuttal is best?
Decide by whether intermolecular forces or chemical bonds are broken. A is factually wrong; B is Mistake 1 in another form; C gives an irrelevant reason.
Hydrogen peroxide decomposing into water and oxygen is a chemical change. Which reason is best?
A chemical change is identified by bonds breaking and forming to make new substances. A gas appearing, more disorder, and heat release can all happen in physical changes too (boiling, for example), so they aren't valid reasons.
A vapor is compressed at constant temperature. At first the pressure rises as the volume decreases; once droplets appear, the volume keeps decreasing but the pressure stays constant. What is happening during the constant-pressure stage?
During this stage gas molecules keep condensing into liquid, so there are fewer gas molecules in the container, and the pressure stays at the vapor pressure for that temperature. Condensing only forms intermolecular forces: a physical change.
AP-style free response
The table gives the energies of some interactions:
| Interaction | Energy (kJ/mol) |
|---|---|
| Enthalpy of vaporization of H₂O | 40.7 |
| Average O–H bond enthalpy | 463 |
| Lattice energy of NaCl | 786 |
| Enthalpy of solution of NaCl | +3.88 |
(a) Which kind of interaction is overcome when water boils? Use the data to show that water molecules don’t decompose when water boils.
(b) Calculate the energy needed to break all the O–H bonds in 1 mol of gaseous water molecules, and identify what kind of change this is.
(c) Breaking the ionic bonds when NaCl dissolves takes about 786 kJ/mol, yet the enthalpy of solution is only +3.88 kJ/mol. Explain why the two values differ so much.
(d) Use your answer to (c) to explain why “Is dissolving NaCl in water a physical or a chemical change?” has no single answer.
Model answer and scoring
(a) The hydrogen bonds between water molecules (intermolecular forces) are overcome. The enthalpy of vaporization, 40.7 kJ/mol, is far less than the O–H bond enthalpy of 463 kJ/mol, so boiling doesn’t supply enough energy to break covalent bonds, and the water molecules stay intact.
(b) Each H₂O molecule has 2 O–H bonds: 2 × 463 = 926 kJ. Breaking covalent bonds lets the atoms recombine (for example into H₂ and O₂), a chemical change.
(c) Breaking the ionic bonds takes about 786 kJ/mol, but as the ions enter the water, ion–dipole interactions form and release almost as much energy (about 782 kJ/mol). The two largely cancel, so the net enthalpy of solution is small.
(d) Dissolving breaks ionic bonds, which makes it look chemical; but no new substance forms, and evaporating the solution gives NaCl back, which makes it look physical. So the process should be described by the interactions broken and formed rather than given a single label.
Scoring: (a) hydrogen bonds, comparing 40.7 with 463; (b) 926 kJ, a chemical change; (c) ion–dipole interactions form, releasing energy that nearly cancels the lattice energy; (d) both points of view explained.
Summary
- Chemical changes break or form chemical bonds; physical changes only change intermolecular forces, and the molecules stay intact.
- Chemical bonds are generally much stronger than intermolecular forces: changes of state take a few to a few tens of kJ/mol, breaking bonds hundreds.
- Dissolving an ionic compound breaks ionic bonds but forms no new substance, so it can be classified either way; describe the interactions broken and formed.
- An energy change on its own doesn’t distinguish physical from chemical changes.
AP tip: To explain why boiling doesn’t decompose water molecules, state both that intermolecular forces are overcome (name which kind) and that they are much weaker than covalent bonds, ideally with numbers.