- Created by: Kyra Mullarney
- Created on: 28-05-19 11:05
Induced dipole-dipole interactions (London forces)
- London forces are weak intermolecular forces that exist between all molecules, whether polar or non-polar.
- Movement of electrons produces a changing dipole in a molecule.
- At any instant, an instantaneous dipole will exist, but its position is constantly shifting.
- The instantaneous dipole induces a dipole on a neighbouring molecule.
- the induced dipole induces further dipoles on neighbouring molecules, which then attract one another.
- Induced dipoles are only temporary.
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Strength of induced dipole-dipole interactions
The more electrons in each molecule:
- the larger the instantaneous and induced dipoles.
- the greater the induced dipole-dipole interactions
- the stronger the attractive forces between molecules
- Larger numbers of electrons means larger induced dipoles so more energy is needed to overcome the intermolecular forces so the boiling point increases.
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Permanent dipole-dipole interactions
- Act between the permanent dipoles in different polar molecules.
- Extra energy is needed to break the additional permanent dipole-dipole interactions.
- This means the boiling point of molecules with permanent dipole-dipole inetarctions is higher.
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Simple molecular substances
- A simple molecular substance is made up of simple molecules - small units containing a definite number of atoms with a definite molecular structure such as neon, hydrogen, water, and carbon dioxide.
- In the soild state, simple molecules form a regular structure called a simple molecular lattice.
- The molecules are held in place by weak intermolecular forces.
- The atoms within each molecule are strongly bonded together by covalent bonds.
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Boiling and melting points
- In simple molecular lattice, the weak intermolecular forces can be broken even by the energy present at low temperatures.
- Simple molecular substances have low melting and boiling points.
- When a simple molecular lattice is broken apart during melting only the weak intermolecular forces break. The covalent bonds are strong and do not break.
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Solubility of non-polar simple molecular substance
- When a simple molecular compound is added to a non-polar solvent, such as hexane, intermolecular forces form between the molecules and the solvent.
- The interactions weaken the intermolecular forces in the simple molecular lattice. The intermolecular forces break and the compound dissolves.
- Non-polar simple molecular substances tend to be soluble in non-polar substances.
- When a simple molecular substance is added to a polar solvent there is little interaction between the molecules in the lattice and the solvent molecules.
- The intermolecular bonding within the polar solvent is too strong to be broken.
- Simple molecular substances tend to be insouble in polar solvents.
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Solubility of polar simple molecular substances
- Polar covalent substances may dissolve in polar solvents as the polar solute molecules and the polar solvent molecules can attract each other.
- Solubility depends upon the strength of the dipole and can be hard to predict.
- Some compounds, such as ethanol, contain both polar and non-polar parts in their structure and can dissolve in both polar and non-polar solvents.
- Some biological molecules have hydrophobic and hydrophilic parts. The hydrophilic part will be polar and contain elecytronegative atoms that can interact with water. The hydrophobic part will be non-polar and comprised of a carbon chain.
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- There are no mobile charged particles in simple molecular structures.
- With no charged particles that can move, there is nothing to complete an electrical circuit.
- Simple molecular structures are non-conductors of electricity.
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