1.5 solid structures Flashcards
what is a crystal structure?
a description of how ions, molecules or atoms are distributed in a solid or crystalline structure
(e.g giant ionic is a type of crystal structure)
what are the 4 main types of crystal structure you need to know?
- giant ionic
- giant covalent (/macromolecular)
- simple covalent (/molecular)
- metallic
what are the two giant ionic compounds you need to know?
- sodium chloride
- caesium chloride
what is the crystal coordination number of sodium chloride? why?
6.6
cause each sodium ion is bonded to 6 chloride ions and each chloride ion is bonded to 6 sodium ions
in a sketch of sodium chloride, which ion is drawn bigger? (chloride ion or sodium ion?)
chloride ion is drawn bigger than the sodium ion
what is the crystal coordination number of caesium chloride? why?
8.8
cause each caesium ion is bonded to 8 chloride ions and each chloride ion is bonded to 8 caesium ions
in a sketch of caesium chloride, which ion is drawn bigger? (chloride ion or caesium ion?)
chloride ion is drawn bigger than caesium ion
why is the crystal coordination number for CsCl larger than for NaCl?
Cs+ is larger than sodium so can create more bonds
what are the physical properties of giant ionic structures?
- high melting points
- generally very soluble
- generally brittle
- only conduct electricity when molten or dissolved
why do giant ionic structures have high melting points?
due to strong electrostatic forces of attraction between oppositely charged ions
(also there are lots of these bonds due to each bind being part of a giant lattice structure)
why are giant ionic structures generally very soluble?
as water is a polar molecule and each ion forms bonds with surrounding water molecules
why are giant ionic structures generally brittle substances?
they will smash when a force is applied to them as they have static bonds between ions
why do giant ionic structures only conduct electricity when in a molten or a dissolved state?
as ions are onlu free to move when in these states and hence carry a charge
what are the 2 giant covalent/macromolecular structures you need to know
- diamond
- graphite
what does diamond consist of? (bond? atom?)
covalently bonded carbon atoms
how many strong covalent bonds does each carbon make? in what arrangement?
each carbon makes 4 strong covalent bonds to a carbon atom in a tetrahedral arrangement
why is diamond described as a ‘hard’ structure?
cause it is arranged in a tetrahedral arrangement
does diamond conduct electricity? why?
no - bc all electrons are used up in bonding
why does diamond have an extremely high melting point?
bc lots of energy is required to break all of the strong C-C bonds within the giant covalent structure
is diamond soluble or insoluble? why?
insoluble
due to the absence of any polarity within the structure
what does graphite consist of? (bond? atom?)
covalently bonded carbon atoms
how many strong covalent bonds does each carbon make in graphite? in what arrangement?
each carbon makes 3 strong covalent bonds to a carbon atom in a trigonal planar arrangement
does graphite conduct electricity? why?
yes - because each carbon only makes 3 strong covalent bonds so there is one free moving electron which is delocalised per carbon atom
does graphite have a high melting point? why?
yes - bc lots of energy is required to break all of the strong C-C bonds within the layers
why is graphite considered a ‘soft’ material?
weak Van der Waals forces exist between layers - these are easily broken
is graphite soluble or insoluble? why?
insoluble
due to the absence of any polarity within the structure
what are the 2 simple covalent/molecular structures you need to know?
- iodine
- ice
in what state does iodine exist at at room temperature?
as a solid
what is the structure of iodine?
a crystal of iodine would have alternating layers of iodine molecules
what forces are found between molecules in iodine?
weak Van der Waals forces
why is iodine described as ‘volatile’?
the weak Van der Waal’s forces between molecules are easily broken and it means iodine molecules can exist in the gas phase with little heating
(if heated, goes from solid straight to gas)
what does volatile mean?
it can easily go into a gas state
- the bond between gases is weak
does iodine conduct electricity? why?
no bc it doesn’t have delocalised electrons in any physical state
why are simple covalent molecules ofyen described as ‘weak, crumbly or powdery’ substances?
due to the weak forces between I2 molecules
(VdW)
does iodine have a high or low melting and boiling point?
low
what is the shape of how water molecules are held in place in ice?
water molecules are held tetrahedrally in place
how many hydrogen bonds does each water molecule in ice form?
each water molecules in ice forms 2 hydrogen bonds between its lone pairs on the oxygen and 2 hydrogen bonds which involve its hydrogen atome
why is ice less dense than water?
the molecules are held further apart than in liquid water
ice has ‘gaps’ (filled with air/ trapped air)
what are the physical properties of simple covalent/molecular structures?
- generally low melting and boiling points
- can be soluble, depending on the molecule
- often described as ‘weak, crumbly or powdery’ substances
- generally do not conduct electricity
why do simple covalent structures generally have low melting and boiling points?
due to weaker IMF forces between molecules
what type of simple covalent molecules are soluble?
if a molecule can form hydrogen bonds or has a permanent dipole, then it is likely to be soluble
non-polar molecules are insoluble
why do simple covalent structures generally not conduct electricity?
due to the absence of any free moving electrons or ions
what is a metallic bond?
the attraction between the negative electrons in the delocalised electron sea and their attraction to the positive metal ions in the lattice
do metallic structures conduct electricity?
yes - due to free moving electrons which can carry charge
do metallic structures have high or low melting points? why?
they have high melting point
due to the strong electrostatic forces of attraction between the electrons and the ions in the lattice
how are metallic structures ductile and malleable?
as the metal ion layers are able to alide over each other without the structure snapping off due to the electron sea which acts like a ‘glue’ holding the structure together
why do group 2 metals have a higher melting point?
- the group 2 metal ions gave a higher charge density (2+)
- this results in a stronger attraction between the delocialised electrons and the positive metal ion
- this results in a higher melting point as more energy is requird to break the metallic bond
what is said about a charge of 2+ compared to a charge of +?
2+ has a higher charge density
why are group 2 metals better electrical conductors than group 1 metals?
as they donate 2 valence electrons per ion to the electron sea
- therefore there are more electrons in the electron sea, resultung in more electrons being ablr to carry a charge
why are ionic solids made up of oppositely charged ions packed around each other? (lattice)
as this maximises electrostatic attraction brtween the oppositely charged ions and minimises repulsion between ions with the same charge, ensuring maximum bond energy
what are the properties of ionic compounds determined by?
the strength of the electrostatic forces between the ions
what does the melting and boiling points of metals depend on?
the number of delocalised electrons per atom
generally the more delocaised electrons per atom, the higher the melting and boiling points
describe how the structures of sodium chloride and caesium chloride are similar
they are both cubic
describe how the structures of sodium chloride and caesium chloride are different?
- caesium ions are bigger than sodium ions
- co-ordination number 6.6 for sodium chloride and 8.8 for caesium chloride
name the force or bond that is being overcome when the solid changes into a gas : iodine
Van der Waals
name the force or bond that is being overcome when the solid changes into a gas : diamond
covalent bond
which out of caesium chloride and sodium chloride has the higher melting point?
sodium chloride has a higher melting point than caesium chloride
which out of iodine and ice has the higher melting point?
iodine has a higher melting point than ice
why does ice and water have different densities at 0.C?
- in ice hydrogen bonds hold the moleculee together in an open (tetrahedral) structure
- the molecules are further apart in ice than they are in water (so ice is less dense than water)
on formjn ice at 0.C, 50cm^3 of water expands to occupy 54.5cm^3. calculate the density of ice at 0.C?
volume is 50cm^3 so mass is 50g
density = 50/54.5 = 0.917 g cm^-1
explain why:
aluminium has a higher melting temperature than sodium.
you should refer to the nature of the bonding [3]
- bonding is metallic
- this is attraction between the sea/delocalised electrons and the positive ions
- Al3+ has more electrons in the sea than Na+ / Al3+ has a higher charge density than Na+
- more energy is needed to overcome forces in Al
