Flashcards in Intermolecular Forces Deck (21)
What are intermolecular forces?
Weak interactions between dipole of different molecules.
Permanent dipole dipole interactions
What properties do intermolecular forces control?
What properties do the covalent bonds control?
Identity and chemical reactions of molecules
What is the bond enthalpy of London forces?
What is the bond enthalpy for permanent dipole dipole interactions?
What is the bond enthalpy for hydrogen bonds?
What are London forces?
Weak intermolecular forces that exist between all molecules. Act between induced dipoles
How do London forces work?
Movement of electrons produces a changing dipole in a molecule.
At any point, an instantaneous dipole will exist, but the position constantly changes.
The instantaneous dipole induces a dipole on the neighbouring molecule
The induced dipole induces further dipoles on the neighbouring molecules, which then attract one another.
Induced dipoles are only temporary
How strong are London forces?
The weakest forces.
The more electrons in each molecule, the larger the instantaneous and induced dipoles, the greater the induced dipole dipole interaction, the stronger the attractive forces between the molecules.
What is the relationship between relative strength of London forces and the boiling point?
The stronger the London forces, the higher the boiling point
What are permanent dipole dipole interactions?
Intermolecular forces that act between the permanent dipoles in different polar molecules.
What is the relationship between the boiling point and permanent dipole dipole forces?
If there are permanent dipole dipole interactions between the molecules, more energy to required to break the permanent dipole dipole interactions between the molecules.
What is a simple molecular substance?
Made up of small units containing a definite number of atoms with a definite molecular formula
What is a simple molecular lattice?
A regular structure for simple molecules, where the molecules are held in place by weak intermolecular forces and the atoms within the molecules are strongly covalently bonded
Properties of simple molecular substances
Low melting and boiling point
Non polar solutes are soluble in non polar solvent
Polar solutes are soluble in polar solvents
Non polar solutes are insoluble in polar solvents.
Insulators of electricity.
Why do simple molecular substances have low melting points
Weak intermolecular forces can be broken by a small amount of energy
Covalent bonds do not break.
Why are non polar substances soluble in non polar solvents?
Intermolecular forces form between molecules and solvent
Interactions weaken intermolecular forces in simple molecular lattice which break and compound dissolves
Why are non polar substances insoluble in polar solvents?
There is little interaction between lattice molecules and solvent molecules
Polar solvent intermolecular bonding is too strong to be broken
Why are polar simple molecular substances dissolve in polar solvents?
Polar compounds attract and bond with polar water molecules.
Polar solvent surrounds simple polar molecular lattice and the attractive forces between solvent and solute are stronger than forces between molecular lattice and between solvent
When is it hard to predict the solubility of polar molecular substances?
Depends on dipole strength
Some molecules have hydrophobic and hydrophilic parts, which are polar and non polar