Why are some molecules polar, and how do the forces between molecules set boiling points?
Polarity and intermolecular forces: determine molecular polarity from shape and bond polarity, and compare dispersion, dipole-dipole and hydrogen-bonding forces and their effect on properties.
A focused Virginia SOL Chemistry answer on polarity under CH.3: how bond polarity and molecular shape combine to make a molecule polar or nonpolar, the three intermolecular forces (dispersion, dipole-dipole, hydrogen bonding), and how they set boiling and melting points and solubility.
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What this topic is asking
The bonding standard CH.3 asks you to decide whether a molecule is polar or nonpolar from its bond polarity and shape, and to compare the three intermolecular forces and how they set physical properties. Polarity explains why some substances dissolve in water and others do not, and intermolecular forces explain boiling points, melting points and volatility.
Bond polarity versus molecular polarity
The deciding factor is shape (from VSEPR). In carbon dioxide, the two C-to-O dipoles point in exactly opposite directions in a linear molecule, so they cancel and the molecule is nonpolar. In water, the bent shape means the two O-to-H dipoles do not cancel, so water is polar. Symmetry cancels dipoles; asymmetry (often from lone pairs) leaves a net dipole.
The three intermolecular forces
Intermolecular forces act between separate molecules and are much weaker than the bonds inside a molecule. The SOL focuses on three:
- London dispersion forces are present in all molecules. They arise from momentary, random shifts in the electron cloud that create temporary dipoles. They are weak individually but grow with the number of electrons, so larger molecules have stronger dispersion forces. In nonpolar substances they are the only force.
- Dipole-dipole forces act between polar molecules, where the partial positive end of one molecule attracts the partial negative end of another. They are stronger than dispersion forces for molecules of similar size.
- Hydrogen bonding is an especially strong dipole-dipole attraction that occurs when hydrogen is bonded directly to a small, highly electronegative atom: nitrogen, oxygen or fluorine. It is the strongest of the three and explains water's unusual properties.
How the forces set properties
This is why water (hydrogen bonding) boils at while a similar-sized nonpolar molecule such as methane (dispersion only) boils far below room temperature. It also explains why oil (nonpolar) does not dissolve in water (polar).
Try this
Q1. Which intermolecular force is present in liquid nitrogen, ? [1 point]
- Cue. London dispersion forces only, because is nonpolar.
Q2. Will a nonpolar substance such as oil dissolve well in water? Explain. [1 point]
- Cue. No; "like dissolves like", and oil is nonpolar while water is polar, so they do not mix.
Exam-style practice questions
Practice questions written in the style of VDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SOL (multiple choice)1 marksWhich molecule is nonpolar even though it contains polar bonds? (A) (B) (C) (D) Show worked answer →
The answer is (C) .
Carbon dioxide has two polar C-to-O bonds, but the molecule is linear and symmetrical, so the two bond dipoles point in opposite directions and cancel, leaving the molecule nonpolar overall. Water (A) is bent and ammonia (B) is pyramidal, so their dipoles do not cancel; (D) has a single polar bond and is polar.
The trap is assuming polar bonds always make a polar molecule; if the shape is symmetrical, the dipoles can cancel.
SOL (tech-enhanced, ordering)2 marks(a) Identify the strongest type of intermolecular force present in liquid water. (b) Explain why water has an unusually high boiling point for such a small molecule.Show worked answer →
A 2-point item on intermolecular forces.
(a) Force (1 point): hydrogen bonding.
(b) Explanation (1 point): water molecules form strong hydrogen bonds (between the partially positive H of one molecule and the partially negative O of another). These take extra energy to overcome, so water boils at a much higher temperature than its small size alone would suggest.
Markers reward naming hydrogen bonding and linking the strong attraction to the high energy needed to separate the molecules. Hydrogen bonding occurs when H is bonded to N, O or F.
Related dot points
- Types of chemical bonds: explain ionic, covalent and metallic bonding in terms of valence electrons and electronegativity, and predict bond type from the periodic table.
A focused Virginia SOL Chemistry answer on bonding under CH.3: why atoms bond to reach a stable octet, how ionic, covalent and metallic bonds form, and how to predict the bond type from electronegativity difference and position on the periodic table.
- Lewis structures and molecular geometry: draw electron-dot (Lewis) structures for simple molecules and use VSEPR to predict molecular shapes.
A focused Virginia SOL Chemistry answer on structure under CH.3: drawing electron-dot (Lewis) structures for simple molecules, counting bonding and lone pairs, and using VSEPR to predict shapes such as linear, bent, trigonal planar and tetrahedral.
- States of matter and kinetic molecular theory: describe solids, liquids and gases in terms of particle arrangement and motion, and state the assumptions of kinetic molecular theory.
A focused Virginia SOL Chemistry answer on the states of matter under CH.4: how particles are arranged and move in solids, liquids and gases, the link between temperature and average kinetic energy, and the assumptions of kinetic molecular theory.
- Solutions, solubility and concentration: describe solutes, solvents and the dissolving process, the factors that affect rate of dissolving and solubility, and how to read a solubility curve.
A focused Virginia SOL Chemistry answer on solutions under CH.5: solute and solvent, the dissolving process and like dissolves like, the factors that change the rate of dissolving and solubility, saturated and unsaturated solutions, and reading a solubility curve.
- The periodic table and periodic trends: describe the organization of the periodic table and the trends in atomic radius, ionization energy, electronegativity and reactivity across periods and down groups.
A focused Virginia SOL Chemistry answer on the periodic table under CH.2: how it is organized into groups, periods, metals, nonmetals and metalloids, and the trends in atomic radius, ionization energy, electronegativity and reactivity and why each runs the way it does.
Sources & how we know this
- 2018 Science Standards of Learning - Chemistry — Virginia Department of Education (2018)
- Chemistry Curriculum Framework — Virginia Department of Education (2018)