More Flashcards
The structure Ana density of ice
In water in its liquid state, the hydrogen bonds break and reform easily as the molecules are moving about. When water freezes, the water molecules are no longer free to move about and the hydrogen bonds hold the molecules in fixed positions. The resulting three-dimensional structure, In order to fit into this structure, the molecules are slightly less closely packed than in liquid water. This means that ice is less dense than water and forms on top of ponds rather than at the bottom.
Election pair repulsion theory
each pair of electrons around an atom will repel all other electron pairs
• the pairs of electrons will therefore take up positions as far apart as possible to minimise repulsion.
This is called the electron pair repulsion theory.
Two, three, four, five, six pairs of electrons name plus bond angle
2: linear ,180*
3:trigonal planar ,120*
4:tetrahedral,109.5*
5:trigonal bipyramid ,90* and 120*
6:octahedral,90*
How many degrees of angle should deduct for a lone pair
About 2 *for every lone pair
Why in octahedral shapes do the 2 lore pairs end up at opposite poles of the cell
As lone pairs repel the most, they adopt a position furthest apart. This leaves a flat square-shaped ion described as square planar. The lone pairs are above and below the plane,
For shapes of Moeller hat do you do for ions
Add an electron for however many negative charge it is and device any electrons for positife charge
Energy changes on heating
Heating a solid
When you first heat a solid and supply energy to the particles, it makes them vibrate more about a fixed position. This slightly increases the average distance between the particles and so the solid expands.
Turning a solid to a liquid - also known as fusion
In order to turn a solid - into a liquid- you have to supply more energy. This energy is needed to weaken the forces that act between the particles, holding them together in the solid state. The energy needed is called the latent heat of melting, or more correctly the enthalpy change of melting. The temp doesn’t change in this process as heat energy is provided into breaking bonds
Enthalpy
Enthalpy is the heat energy change measured under constant pressure whilst temperature depends on the average kinetic energy of the partides and is therefore related to their speed
Heating a liquid
When you heat a liquid, you supply energy to the particles which makes them move more quickly - they have more kinetic energy. On average, the particles move a little further apart so liquids also expand on heating.
Liquid to gas - vapourisection
In order to tumn a liquid into a gas, you need to supply enough energy to break all the intermolecular forces between the particles. A gas consists of particles that are far apart and moving independently.
The energy needed is called the latent heat of vaporisation or more correctly the enthalpy change of vaporisation. As with melting. there is no temperature change during the process of boiling.
Heating a gas
As you heat a gas, the particles gain kinetic energy and move faster. They get much further apart and so gases expand a great deal on heating.
Crystal
Crystals are solids. The particles have a regular arrangement and are held together by forces of attraction. These could be any bond and force The strength of the forces of attraction between the particles in the crystal allects the physical properties of the crystals.
Ionic crystals
lonic compounds have strong electrostatic attractions between oppositely charged ions. Sodium chloride, NaCl, is a typical ionic crystal,. tonic compounds have high melting points.
This is a result of the strong electrostatic altractions which extend throughout the structure. These require a lot of energy to break in order for the ions to move apart from each other.
Metallic crystals
Metals exist as a lattice of positive ions embedded in a delocalised sea of electrons,. Again the attraction of positive to negative extends throughout the crystal. The high melting temperature is a result of these strong metallic bonds.
Molecular crystals
Molecular crystals consist of molecules held in a regular array by intermolecular forces. Covalent bonds within the molecules hold the atoms together but they do not act berween the molecules. Intermolecular forces are much weaker than covalent, ionic or metallic bonds, so molecular crystals have low melting temperatures and low enthalpies of melting.
Macromolecular crystals
Covalent compounds are not always made up of small molecules. In some substances the covalent bonds extend throughout the compound and have the typical property of a giant structure held together with strong bonds - a high melting temperature. including diamond and graphite.
Diamond and graphite
Diamond and graphite are both made of the element carbon only.
They are called polymorphs or allotropes of carbon. They are very different materials because their atoms are differently bonded and arranged. They are examples of macromolecular structures.
Structure and properties of diamond
The atoms form a giant three-dimensional lattice of strong covalent bonds, which is why diamond has the following properties:
• very hard material (one of the hardest known) |
• very high melting temperature, over 3700 K
• does not conduct electricity because there are no free charged particles to carry charge.
Has a tetrahedron shape
Graphite
Forms a trigonal planar shape with a delocalised electron that carries charge There is no covalent bonding berween the layers of carbon atoms.
They are held together by the much weaker van der waal forces Graphite is a soft material.
• It has a very high melting temperature and in fact it breaks down before it melts. This is because of the strong network of covalent bonds, which make it a giant structure.
• It conducts electricity along the planes of the hexagons.
