11.2
Intermolecular vs Intramolecular Forces
Intermolecular forces (IMF) are electrostatic attractions arising from charge-charge interactions between molecules. The strength of the intermolecula…
Chemical substances form when atoms or ions interact electrostatically.
For example, one oxygen atom and two hydrogen atoms covalently bond to form a molecule of water. Such bonding forces that hold the atoms together within a molecule are called intramolecular forces.
Intramolecular forces dictate chemical properties like stability and types of chemical bonds. The three basic types are ionic, covalent, and metallic bonds.
An ionic bond is formed by the transfer of valence electrons from a metal to a nonmetal atom, which results in an electrostatic attraction between the oppositely charged ions.
A covalent bond is formed when nonmetal atoms share their valence electrons.
Lastly, metallic bonding results from the interaction between the array of positive metal ions and a shared pool of delocalized valence electrons.
However, electrostatic interactions do not only exist within a molecule but also between molecules.
For example, in water—whether solid, liquid, or gaseous—the molecules interact via electrostatic, nonbonding interactions dictating the state of matter. These interactions are called intermolecular forces and influence various physical properties, such as melting and boiling points.
Intermolecular forces can be categorized into several types. Strong ion–dipole forces occur between ions and polar molecules; dipole–dipole forces exist between polar molecules, with hydrogen bonding being a special type of dipole–dipole force; and finally, the weakest of all—dispersion forces—exist in all molecules, polar and nonpolar, and are a result of temporary dipoles.
Intermolecular forces are weak because small or partial charges are interacting over large distances, as compared to intramolecular forces, which are strong owing to large electrostatic interactions over short distances.
For example, in liquid water, the molecules are separated by an average distance of about 300 picometers, characteristic of the comparatively weaker intermolecular forces.
Consequently, it takes heating water to only 100 °C to overcome these intermolecular forces and transition liquid-phase water molecules into the vapor phase.
In contrast to this, the length of the O–H bond in water is 96 picometers, characteristic of the stronger intramolecular bonds. It takes heating water to around 1000 °C, much more than its boiling point, to break this intramolecular bond.
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Q1: What is the difference between intramolecular and intermolecular forces?
Intramolecular forces hold atoms together within a molecule, such as ionic, covalent, and metallic bonds. Intermolecular forces are electrostatic attractions between molecules. Intramolecular forces are much stronger because large charges interact over short distances, while intermolecular forces are weaker due to small or partial charges interacting over larger distances.
Q2: Why does water boil at a lower temperature than the energy needed to break its chemical bonds?
Water boils at 100°C because this temperature provides enough thermal energy to overcome intermolecular forces between water molecules, which are separated by about 300 picometers. Breaking the O–H covalent bonds within water molecules requires much more energy and temperatures around 1000°C, demonstrating that intermolecular forces are significantly weaker than intramolecular bonds.
Q3: What are the main types of intermolecular forces?
Intermolecular forces include ion–dipole forces between ions and polar molecules, dipole–dipole forces between polar molecules (with hydrogen bonding as a special type), and dispersion forces in all molecules. Dispersion forces are the weakest type, arising from temporary dipoles in both polar and nonpolar molecules.
Q4: How do intermolecular forces affect the physical properties of substances?
Intermolecular forces dictate physical properties including melting point, boiling point, and density. Substances with stronger intermolecular forces have higher boiling and melting points. For example, water with strong intermolecular forces boils at 100°C, while hexane with weaker forces boils at 68.73°C, illustrating how intermolecular forces melting point and boiling point are directly related.
Q5: What are the three basic types of intramolecular bonds?
The three basic intramolecular bonds are ionic bonds, formed by electron transfer between metals and nonmetals; covalent bonds, formed by electron sharing between nonmetal atoms; and metallic bonds, resulting from interactions between positive metal ions and a shared pool of delocalized valence electrons.
Q6: Why are intermolecular forces weaker than intramolecular forces?
Intermolecular forces are weaker because they involve small or partial charges interacting over relatively large distances between molecules. In contrast, intramolecular forces involve large electrostatic interactions over very short distances within molecules. For example, breaking one mole of HCl covalent bonds requires 430 kilojoules, while overcoming intermolecular forces in liquid HCl requires only 17 kilojoules.
Q7: How do state changes relate to intermolecular forces?
When a liquid is heated, thermal energy overcomes intermolecular forces holding molecules in place, causing the liquid to boil and convert to gas. Melting ice disrupts intermolecular forces between solid water molecules, rearranging them into liquid form, but does not break intramolecular bonds within individual H2O molecules, demonstrating that state changes affect only intermolecular interactions.