Structure 2.2.5-10 & 2.4 Flashcards
What does bond polarity result from?
Difference in electronegativities of bonded atoms
What is electronegativity?
Measure of the atom’s relative ability to attract a shared pair of electrons towards itself in a covalent bond
When does a polar bond form?
Between two different non-metal atoms with different electronegativites
What will the effect of separation of charge be (in polar bonds)?
Making the more electronegative atom appear as if it had a negative pole and the less electronegative atom appear as if it had a positive pole
What makes the bond dipole greater?
Greater electronegativity difference
In an oxygen-hydrogen bond, what exists around the oxygen molecule?
A permanent negative dipole, due to greater electronegativity
In an oxygen-hydrogen bond, what exists around the hydrogen molecule?
A permanent positive dipole, due to less electronegativity
What does molecular polarity describe?
The distribution of electrons throughout the whole molecule
What does molecular polarity depend on?
Polar bonds within the molecule
Molecular geometry
What makes a covalent molecule non-polar?
No polar bonds
If polar bonds are arranged symmetrically and cancel each other out
What makes a covalent molecule polar?
If polar bonds are arranged asymmetrically, so don’t cancel each other out
What do carbon and silicon form?
Covalent network structures
What do covalent network structures comprise of?
Atoms held together by covalent bonds in a continuous 3D lattice
What is an allotrope?
Different structural forms of the same element in the same physical state
Why do allotropes have different physical and chemical properties?
Different bonding and structural patterns
Structure of a diamond
Each carbon atoms are covalently bonded to four other carbon atoms in a tetrahedral shape, with each bond being of equal strength, and each atom packed in the most efficient way. Each carbon atom forms only single bonds
How are carbon atoms packed in a diamond?
Most efficient way
What are the characteristics of diamond? Why?
Very hard, due to strong covalent bonds
High melting point, due to strong covalent bonds
Poor electrical conductor, as no free moving electrons
Very good thermal conductor
Structure of graphite
Each carbon atom forms 3 single covalent bonds with other carbon atom, forming covalently bonded layers of atoms.
The fourth electron is delocalised between layers, and can travel across them.
London dispersion forces between layers are weak
Properties of graphite? Why?
High melting point, as carbon atoms are covalently bonded.
Can conduct electricity due to free fourth electron
Soft and slippery, as forces between layers are weak
What type of covalent structure is diamond?
Covalent network lattice
What type of covalent structure is graphite?
Covalent layer lattice
What is the electron domain geometry / molecular shape of carbon atoms in graphite?
Trigonal planar
What is the electron domain geometry / molecular shape of carbon atoms in diamond?
Tetrahedral
How do London Dispersion Forces form between layers of graphite?
As electrons move, instantaneous dipoles form. This induces a dipole in the above layer, creating LDF
How do delocalised electrons move in graphite?
Can travel across layers
Example of a fullerene of carbon
C60 buckminsterfullerene
Structure of buckministerfullerene
60 carbon atoms arranged into a sphere consisting of 6 and 5 membered rings. Due to incomplete bonding in each carbon, the unpaired electron is delocalised
Properties of C60 buckminsterfullerene
Low electrical conductivity as delocalised electron cannot flow easily between molecules
What is the structure of graphene?
Single layer of graphite.
Can be rolled into a nanotube
Properties of graphene (according to preliminary results)
Stronger than steel/diamond
Better electrical conductivity than copper
better thermal conductor than diamond
Structure of silicon
(Same as that of diamond)
Each silicon atom is covalently bonded to four other silicon atoms in a tetrahedral arrangement
How can silicon conduct electricity?
At high temperatures, some electrons from covalent bonds are freed, leaving behind a positive hole, which another electron can fill. This process results in the carrying of charge
Structure of silicon dioxide
Similar structure to diamond and silicon, based on tetrahedral network arrangement. Each silicon atom is bridged to 4 other silicon atoms via an oxygen bridge. All bonds are covalent, with no free electrons
Does silicon dioxide conduct electricity?
No, there are no free-moving electrons
What is the other form that silicon dioxide can exist?
Fused quartz, which is formed when molten silicon dioxide is rapidly cooled to form a glass (non-crystalline solid)
Appearance of diamond
Transparent, lustruous, crystal
Physical and chemical properties of diamond
Hardest known natural substance
Brittle
HIgh MP
Uses for diamond
Jewelry
Grinding and cutting glass
Appearance of graphite
Grey
Crystalline
Solid
Physical and chemical properties of graphite
Soft
Slippery
Brittle
Very high melting point
Uses for graphite
Dry lubricant
Pencil lead
Electrodes in electrolysis
Appearance of graphene
Transparent
Physical and chemical properties of graphene
Thinnest
Strongest
Flexible
Very high melting point
Uses for graphene
Many applications such as nanotechnology
Electrical conductivity of C60 fullerene
Non-conductor
Thermal conductivity of C60 fullerene
Poor conductor
Appearance of C60 fullerene
black powder
Appearance of C60 fullerene
Light
Strong
Low MP
C60 fullerene uses
Lubricants
Nanotechnology
Electrical conductivity of silicon
Semi-conductor
Appearance of silicon
Lustruous
Crystal
PHysical and chemical properties of silicon
Strong
Brittle
High melting point
Uses for silicon
Electronic
Silicon dioxide appearance
Transparent to opaque
Crystals
Physical and chemical properties of silicon dioxide
Strong
brittle
high MP
Uses for silicon dioxide
Quartz
Glass
Sand
Relationship between bond enthalpies and atomic radii
Lower atomic radii = higher bond enthalpy
What is melting point a good reflection of?
Strength of the forces that bond molecules together in a solid
General rule for higher melting point meaning…
Stronger intermolecular forces
Four types of intermolecular forces that hold molecules together in the solid/liquid phase
London dispersion forces
Dipole-induced dipole
Dipole-dipole
Hydrogen bonding
What happens to intermolecular forces and covalent bonds when heat is applied?
Intermolecular forces are disrupted, therefore the substance melts/boils
Strong covalent bonds that hold atoms together remain intact
What explains why non-polar molecules liquefying if the temperature is low enough?
London Disperson Forces, meaning there is some form of attractive force between non-polar molecules
What are London Disperson Forces?
Short range, intermolecular forces that are thought to be electrostatic in nature.
They are created by the electron cloud having moments of being unevenly distributed; which generates a temporary, instantaneous dipole ; which induces a dipole in the neighbouring molecule. These temporary dipoles can attract each other but (because they are not permanent) the attraction is weak
What substances experience London Dispersion Forces?
All molecular
What forces influence the strength of the London Dispersion Forces?
Size of the molecules
Shape of the molecules
Why does the size of the molecules impact LDF?
The greater molecular size/mass means more electrons, meaning greater instantaneous dipole, meaning stronger LDFs
Why does shape of molecule impact LDF?
More compact means smaller surface area between molecules, meaning a weaker attraction
What is the relationship between molecular size and boiling point?
Larger molecular size (mass)
More electrons
Larger instantaneous dipole
Stronger LDFs
More energy required to overcome intermolecular forces
Higher boiling point
What are dipole-induced dipole forces?
These forces of atraction occur when a mixture contains polar and non-polar substances.
The presence of a permanent dipole in the polar molecule, such as water, induces a temporary dipole in a
neighbouring non-polar molecule, such as oxygen.
What are dipole-dipole forces?
These forces of attraction occur between polar
molecules such as hydrogen chloride where the negative
pole on one molecule is atracted to the positive pole on
another molecule.
What is the relationship between dipole and melting point, in terms of dipole-dipole forces?
Greater dipole
Stronger attraction between molecules
Stronger intermolecular force between molecules
Higher melting point
What is hydrogen bonding?
When nitrogen/oxygen/flourine (highly electronegative atoms) bond with a hydrogen, the largest possible dipole is created.
Each molecule has one pair of lone electrons (of a NOF) which is attracted to the positive dipole on the adjacent molecule
Ranking of intermolecular strength of comparable molecular mass
LDF < dipole-dipole < hydrogen bonding
What properties of water can hydrogen bonding explain?
Relatively high melting and boiling points
Expansion on freezing
Good solvent peroperties
What do the physical properties of covalent substance depend on?
Whether they are covalent lattice or covalent molecular substances
Covalent lattice substances only contain covalent bonds (which require a lot more energy to break) than the relatively weak intermolecular forces between molecules
Volatility
Measure of how readily a substance vapourises (liquid -> gas)
Electrical conductivity
Measure of a substance’s ability to conduct electricity
Solubility
Measure of a solute’s ability to dissolve in a solvent
What does vaporising a molecular substance require?
The intermolecular forces between molecules to be overcome.
What is the volatility of molecular forces?
Generally high, as intermolecular forces are relatively weak so not much energy is required
What do stronger intermolecular forces mean for volatility?
Stronger intermolecular forces
More energy required to overcome bonds
Higher boiling point
Reduced volatility
What happens when a substance dissolves (forces of attraction)?
Forces of attraction form between the solvent and the solute
When is a covalent molecular substance more likely to dissolve?
If the intermolecular forces between solvent and solute are stronger than between molecules of the solute
What are polar molecular substances more likely to dissolve in?
Polar solvents, as like dissolve like.
For example, the polar molecular substance can form hydrogen bonds with water molecules, meaning the intermolecular forces between the solute and the solvent are stronger than those between the molecules of the solute
What are non-polar substances likely to dissolve in?
Non-polar solvents
What happens when ethanol is mixed with water?
The polar -OH group on the ethanol molecule can form a hydrogen bond with water; thus, the ethanol dissolve in water, forming an ion-dipole bond
What happens when polar molecular substances mix with water (highly polar)?
Either form hydrogen bonds OR ionise in the aqeous solution
What does the bonding triangle represent?
Bonding as a continuum between the three bonding times, with each being present to different degrees
What does the x-axis of the bonding triangle represent?
Mean electronegativity of bonded elements
What does the y-axis of the bonding triangle represent?
Electronegativity difference of bonded elements
What is not considered in the bonding triangle?
Molecular formula or bond order
What is chromatography?
A technique used to separate the components of a mixture based on their relative attractions involving intermolecular forces to mobile and stationary phases
What is the stationary phase?
A solid, or a liquid coating a solid, onto which the componets of a sample adsorb
What is adsorption?
The attraction or sticking of one substance to the surface of another
What is the mobile phase?
Gas or liquid which carries the components of a mixture along the stationary phase
What does the continual process of chromatographic methods rely on?
Continual process of adsorption of components onto the stationary phase and then desorption and dissolving back into the mobile phase
What are the factors that affect movement along the chromatography?
Attraction between component and stationary phase
Solubility of the component in the mobile phase
Mass of the component
What does a greater attraction to the stationary phase rely on?
Slower movement along chromatography
What does a greater solubility in the mobile phase mean?
Faster the movement
What does a greater mass of the component mean for paper chromatography?
Greater mass, slower movement
What are the two types of chromatography?
Paper chromatography
Thin-Layer Chromatography (TLC)
What is the stationary phase of the paper chromatography?
Chromatography paper
Polar -OH groups in the cellulose in the paper can for hydrogen bonds with components with the mixture
What is the mobile phase of the paper chromatography?
(Depends on the mixture being separated)
Polarity is often an important factor
What is the stationary phase of Thin Layer Chromatrography?
Uniform layer of silica (silicon dioxide) or alumina (aluminium oxide) coated onto glass/plastic/metal
Polar -OH groups on the surface of the silica and alumina can form hydrogen bonds with components in the mixture
What is the mobile phase of thin layer chromatography?
(Depends on the mixture being separated)
Polarity is often an important factor
Technique of paper chromatography
- Origin line is drawn towards the bottom of the stationary phase (using pencil)
- A small and concentrated spot of a sample of mixture (two or more components) is made on the origin line.
- Stationary phase is placed in a vessel containing the slovent starting at a depth below the origin line
- The solvent sweeps the componets upwards
What is the retardation factor / retention factor (Rf)?
Distance travelled by solute from the origin
/
Distance travelled by solvent from the original
Why are reactions heated to a constant mass?
To ensure no oxygen is remaining in the sample that would impact the result of the actual mass of product
