Intermolecular Forces Flashcards
Define intermolecular forces
Intermolecular forces are the forces that act between molecules. They are weaker than the bonds inside molecules (like covalent or ionic bonds), but they still play an important role in how substances behave.
Define intramolecular forces
**Intramolecular forces are the forces that hold atoms together within a molecule. These forces are much stronger than intermolecular forces, and they are responsible for the chemical bonds that form between atoms. **There are three main types of intramolecular forces:
Covalent Bonds: These occur when two atoms share electrons. For example, in a water molecule (H₂O), the oxygen atom shares electrons with two hydrogen atoms to form covalent bonds.
Ionic Bonds: These happen when one atom gives away electrons to another atom, creating oppositely charged ions that are attracted to each other. For example, in sodium chloride (NaCl), sodium gives up an electron to chlorine, and the oppositely charged ions stick together.
Metallic Bonds: These occur between metal atoms, where the electrons are shared freely among all the atoms in a “sea” of electrons. This explains why metals are good conductors of electricity and heat.
Define temporary dipole
Temporary Dipole: A temporary dipole occurs when the electrons in a molecule are unevenly distributed for a brief moment, creating a small, temporary negative charge on one side of the molecule and a positive charge on the other side. This happens because electrons are always moving around, and sometimes they can gather more on one side, creating an uneven charge distribution.
Define induced dipole
Induced Dipole: An induced dipole is a temporary dipole created in a molecule when it is close to another molecule with a permanent or temporary dipole. The electric field from the dipole can cause the electrons in the nearby molecule to shift, creating a new temporary dipole in that molecule, even if it wasn’t originally polarized.
Define London dispersion forces
London dispersion forces are the weakest type of intermolecular force. They occur due to temporary changes in the distribution of electrons around molecules.
How do London dispersion forces work?
Temporary Dipoles: At any moment, the electrons in a molecule can be unevenly distributed, creating a temporary negative charge on one side of the molecule and a temporary positive charge on the other. This is called a temporary dipole.
Induced Dipoles: When one molecule with a temporary dipole comes near another molecule, the fluctuating charge can cause the second molecule’s electrons to shift, creating an induced dipole in that molecule.
Attraction: The positive side of one dipole is attracted to the negative side of another, creating a weak force that holds the molecules together.
Where do London dispersion forces exist?
London dispersion forces exist in all molecules, even in nonpolar molecules (which don’t have permanent dipoles), but they are most important in nonpolar molecules, where they are the only type of intermolecular force.
Justify if London dispersion forces can have a big impact on the physical properties of substances.
Even though they are weak, these forces can still have a big impact on the physical properties of substances, like boiling and melting points. For example, larger molecules with more electrons have stronger London dispersion forces.
What determines the strength of Van der Waals forces?
- The Mr of the molecule and its shape.
- The larger the Mr of the molecule, the stronger the intermolecular forces.
- Straight chain molecules experience stronger van der waals forces than branched-chain molecules as they can pack much closer together. This reduces the distance over which the force acts, making the intermolecular force stronger.
Where do Van der waals forces act?
They act between organic alkane chains and are affected by the chain length and any branching. As the chain length of the alkane increase, so does the Mr of the molecule. This results in stronger intermolecular forces between the chains, and so the compound has a higher boiling point as a result.
What can weaken the Van der Waal forces?
Branching of alkane chains weakens van der waals forces between the chains as they are less able to pack tightly together. This means the distance over which the intermolecular forces act is increase, weakening the attractive forces. Therefore, branched-chain alkanes have lower boiling points than straight-chain alkanes.
Define the term permanet dipole
A permanent dipole occurs in a molecule when there is a consistent, uneven distribution of electrons between two atoms.
Why is there a consistent, uneven distribution of electrons between two atoms in permanent dipoles?
This happens because one atom is more electronegative (stronger at attracting electrons) than the other, causing the electrons to spend more time closer to the more electronegative atom.
As a result:
The more electronegative atom becomes slightly negative (δ−), and the less electronegative atom becomes slightly positive (δ+).
This creates a permanent separation of charge across the molecule, with one end being positive and the other being negative.
For example, in a water molecule (H₂O), oxygen is more electronegative than hydrogen, so it pulls the shared electrons closer to itself. This gives oxygen a partial negative charge (δ−) and hydrogen a partial positive charge (δ+), creating a permanent dipole in the molecule.
Define the term hydrogen bonding
Hydrogen bonding is the strongest type of intermolecular force that occurs when a hydrogen atom is bonded to a very electronegative atom (like oxygen, nitrogen, or fluorine), and the hydrogen atom is attracted to a lone pair of electrons on another electronegative atom nearby.
How does hydrogen bonding work?
Here’s how it works:
Hydrogen Bond Donor: The hydrogen atom, which is bonded to an electronegative atom (like oxygen or nitrogen), becomes partially positive (δ+). This is because the electronegative atom pulls the shared electrons away from the hydrogen.
Hydrogen Bond Acceptor: The partially positive hydrogen is attracted to the lone pair of electrons on a nearby electronegative atom (like oxygen, nitrogen, or fluorine) in another molecule. This creates the hydrogen bond.
For example, in a water molecule (H₂O), the hydrogen atoms are bonded to oxygen, which is highly electronegative. The hydrogen atoms have a partial positive charge, and they form hydrogen bonds with the lone pairs of electrons on the oxygen atoms of other water molecules. This strong attraction between the hydrogen and oxygen atoms in different molecules is called hydrogen bonding.
Hydrogen bonds are stronger than other types of dipole-dipole interactions but weaker than covalent or ionic bonds. They play a significant role in the properties of water, like its high boiling point and surface tension.
What happens to the boiling and melting points of molecules?
Have much higher melting and boiling points compared to similar-sized molecules without hydrogen bonding, This shows how the type of intermolecular force heavily influences the physical properties of a substance.
Explain why water, which has a simple molecular structure has an unsually higher boiling point for the size of the molecule.
Due to the presence of hydrogen bonds that require a lot of energy to be overcome. Hydrogen bonds also result in ice haveing a much lower density than liquid water because they hold the molecules in a rigid structure with lots of air gaps.
Why is hydrogen bonding responsible for alcohols having a much higher boiling points than alkanes with a similar Mr value?
This is because the lone electron pair on the oxygen atom is able to form hydrogen bonds with a hydrogen on another alcohol molecule.
Why is water a popular choice of solvent?
Its hydrogen bonding capabilities alow it to dissolve some ionic compounds by solvating the individual ions, and to dissolve some alchohols by forming hydrogen bonds with their hydroxyl group.
Fill in the blanks
Water and alcohols are ___ solvents for the dissolving of some ___ ___ such as halogenoalkanes that cannot form hydrogen bonds.
Water and alcohols are poor solvents for the dissolving of somepolar molecules such as halogenoalkanes that cannot form hydrogen bonds.
When are non-aqueous solvents used?
Used for compounds which have the same tpye of intermolecular force.
If hydrogen fluoride is the only hydrogen halide that forms hydrogen bonds between molecules. What does this mean?
This gives it the highest boiling point because hydrogen bonds are much stronger than van der waals and permanent dipole forces.
What happens to the halides of the hydrogen halides as you move down group 7? What does this mean??
The boiling point increase because the halide increases ins size, their number of electrons also increases. This means more van der waals forces form between molecules, so more energy is required to separate them.
