Flashcards in Deck 7 Deck (25):
What are the three types of common intermolecular forces?
2) Permanent dipole-dipole forces
3) van der Waals'forces
Define intermolecular force
An intermolecular force is an attractive force between neighbouring molecules.
Define permanent dipole-dipole force
A permanent dipole-dipole force is a weak attractive force between permanent dipoles in neighbouring polar molecules.
Define van der Waals'forces
van der Waals'forces are attractive forces between induced dipoles in neighbouring molecules.
What are van der Waals'forces?
van der Waals'forces are weak intermolecular attractions between very small, temporary dipoles neighbouring molecules.
What causes van der Waals'foces?
1) Caused by the movement of electrons in the shells. This movement unbalances the distribution of charge within the electron shells.
2)At any moment, there will be an instantaneous dipole across the molecule, this induces a dipole in neighbouring molecules.
3) The small attracions between the small induced dipoles attract one another causing weak intermolecular forces.
Why do the van der Waals' forces increase with increasing numbers of electrons?
- The greater the number of electrons, the larger the induced diples.
- The greater the attractive forces between molecules
Define hydrogen bond.
A hydrogen bond is a strong dipole-dipole attraction between:
- An electron-deficient hydrogen atom on one molecule and a lone pair of electrons in a highly electronegative atom on a different molecule.
For H-bonding what sot of molecules should you be looking for?
1) Molecules with O-H, N-H bonds.
Common examples: H2O,NH3,CH3OH,C2H5OH
Why is ice less dense than water?
- Ice has an open lattice with hydrogen bonds holding the water molecules apart.
- When ice melts, the rigid hydrogen bonds collapse, allowing the H2O molecules to move closer together.
Why does water have relatively high melting and boiling points?
- The hydrogen bonds are extra forces, over and above van der Waals'forces.
- These extra forces have to be overcome in order to melt or boil H2O. This results in H2O having higher melting and boiling points than would be expected from just van der Waals'forces.
What other properties does water have?(Apart from ice being less dense and having high melting/boiling points)
The extra intermolecular bonding from hydrogen bonds also explains the relatively high surface tension and viscosity of water.
In DNA, hydrogen bonds responsible for what?
The hydrogen bonds are responsible for holding together the double helix structure.
Define metallic bonding
Metallic bonding is the electrostatic attraction between positive metal ions and delocalised electrons.
Define delocalised electrons
Delocalised electrons are shared between more than two atoms.
Define giant metallic lattice
A giant metallic lattice is a three-dimensional strucure of positive ions and delocalised electrons, bonded together by strong metallic bonds.
What are the electrons in metals like compared to electrons in covalently bonded structures?
- A covalent bond has a localised pair of electrons positioned between the two atoms bonded together.
- In metallic bonding, the electrons are delocalised and are shared by all the positive ions in the structure.
Why do metals have high melting and boiling points?
- The attraction between the positive ions and negative delocalised electrons is strong.
- High temperatures are needed to break the metallic bonds and dislodge the ions from their rigid positions within the lattice.
Why are metals good conductors of electricity?
- The delocalised electrons can move freely anywhere within the metallic lattice.
What does malleable mean?
Malleable means: can be hammered into shape. Many metals can be pressed into shapes or hammered out into thin sheets.
What does ductile mean?
Can be drawn out or stretched. Ductility permits metals to be drawn out into thin wires.
Why are metals ductile and malleable?
Because the delocalised electrons can move, the metallic structure has a degree of give, which allows atoms or layers to slide over each other.
What are alloys?
Alloys are mixtures of metals.
- In the structure, the positive ions of one element simply replace the ions of another element.
Why is an alloy not a compound?
In a compound,there is always the same proprtion of atoms of each element. But metals can mix together in different proportions.