S2: Models of bonding and structure Flashcards
What is a polymer?
Macromolecule made up of repeating units called monomers linked with covalent bonds.
Three main structural types of polymers
Linear, Branched, Cross-linked
Explain why plastics are electrical insulators
Do not possess mobile charged particles, so they do not conduct electricity
Explain why plastics are unreactive and resistant to chemical attack
Made of long chains with strong covalent bonds which are very stable. They require lots of energy to overcome, so are inert.
Explain why many polymers melt over a range of temperatures
Chain length of polymers vary, so longer chains will melt at higher temps because they require lots of energy to overcome many bonds and vice versa
Describe how the strength of London forces changes as the carbon chain length increases
London forces become stronger in longer chains, because they have a larger number of electrons and a larger electron cloud
Explain why plastics are lightweight
Polymers are loosely packed and therefore have a low density
Explain why plastics do not dissolve in water
They are non-polar molecules
What is addition polymerisation?
The process of joining two monomers (generally the same) with covalent bonds
Describe how propene forms polypropene in terms of bonds broken and formed
The double bond between carbons in propene are broken and they form a single bond instead, then each C atom forms a covalent bond with another monomer at each end.
What is condensation polymerisation?
Bonding of two or more monomers (usually different) with reactive functional groups to make a polymer and a small molecule as a biproduct (e.g. H2O or HCl)
What is a functional group?
Groups found in organic compounds which give it chemical and physical properties
Organic vs Inorganic compounds
Organic compounds contain C atoms and almost always C-H bonds, Inorganic do not contain C atoms
Atom economy
Measure of amount of reacting materials that are incorporated in useful products
What is a polyamide?
Condensation polymer formed from a dicarboxylic acid and a diamide
What are 2 examples of polyamides?
Nylon, proteins
Which type of link forms in a polyamide?
Amide link
What is a polyester?
Condensation polymer formed between dicarboxylic acid and diol (2 -OH groups)
What are 2 examples of a polyester?
Terylene, PET
Which type of link froms in a polyester?
Ester link
How can biological condensation polymers be broken down?
Hydrolysis
Environmental impact of addition vs condensation polymers
Addition are often non-biodegradable, Condensation are sometimes biodegradable (because they can be hydrolised)
Instead of discreet types of bonding, how is bonding described in the bonding triangle?
A continuum between ionic, covalent and metallic models.
Where are the most metallic elements found + what is the most metallic?
At the bottom left of the periodic table, Cs
Where are the most covalent elements found + what is most covalent compound?
At the top right of periodic table, F2
What is the most ionic compound and why?
CsF - has the largest difference in metallic and covalent
What are the limitations of the bonding triangle?
Only applicable to binary compounds and elements
Does not take into account allotropes or structural isomers
Properties of covalent compounds
- Lower melting and boiling points.
- Often insoluble in polar solvents.
- Typically non-conductive as there are no free-moving ions.
Properties of polar covalent substances
- Soluble in polar solvents like water
- Higher fixed points than non-polar compounds
- They conduct electricity in the solution state
What are alloys?
A mixture of a metal with one or more other metal/non metal
Properties of alloys
Strong, corrosion resistant, electricaly conductive, lower melting point than pure metals
Why are alloys harder than pure metals?
The different sized ions create an irregular structure, which makes it more difficult for ions to slide past each other.
What is bronze made of?
Copper and Tin
What makes alloys corrosion resistant
Some elements can be added which form an oxide layer and prevent rusting e.g. Chromium in stainless steel
The irregular structure also makes it more difficult for corrosive agents to penetrate.
Why do pure metals have higher melting points than alloys?
All cations are the same size in pure metals, so a regular structure forms and ions pack together closely, forming stronger bonds between cations and delocalised electrons
Ionic bond
Strong electrostatic force of attraction between a cation and anion
Properties of ionic compounds + explanations
High melting and boiling points - because of strong electrostatic forces of attraction
Soluble in polar solvents - because charged ions are attracted to polar molcecules
Conduct electricity when molten or in solution - because ions are free to move
Lattice enthalpy
Enthalpy change when one mole of a solid ionic compound is broken down into its gaseous constituent ions
What 2 factors affect the lattice enthalpy / bond strength of an ionic compound?
Charge of ions - ions with a larger charge will form stronger attractions
Ionic radius - smaller ionic radius means the ions are closer together and form stronger attractions
Is lattice enthalpy endo or exothermic?
Endothermic - because bonds are broken
Explain why ionic compounds are hard and brittle
Strong electrostatic attractions mean that lots of force is required to slide layers over each other, and when they do, same-charge ions are adjacent which causes crystal to shatter.
Metallic bond
Electrostatic forces of attraction between lattice of cations and sea of delocalised electrons
Properties of metals + explanation
Conduct electricity - delocalised electrons are mobile and can carry charge
Conduct heat - cations vibrate and transfer heat energy, and delocalised electrons help with heat transfer
Malleable and ductile - layers can slide over each other because of non-directional bonding without disrupting bonding
Relatively high fixed points because of strong forces of attraction
2 factors affecting strength of a metalic bond
Cation charge - higher charge means attractions are stronger
Ionic radius - smaller radius means positive nuclei and e- are closer so attractions are stronger
Covalent bonds
Strong electrostatic forces of attraction between two atom’s nuclei and a shared pair of electrons
What is VSEPR?
Valence shell electron pair repulsion - states that there is greater repulsion when a lone pair is present
State the differences in bond length and strength between single, double and triple bonds
Single bonds are the longest and weakest, triple are the shortest and strongest.
Properties of simple molecular substances + explanations
Low melting and boiling points - molecules held together by weak intermolecular forces
Bad conductors of electricity - no mobile charged particles present
Sometimes solube - polar substances are polar in polar solvents and vice versa for non-polar
Why does NH3 have higher melting point than CH4?
NH3 forms hydrogen bonds between molecules, whereas CH4 only forms london forces
Examples of Macromolecular substances
SiO2, allotropes of Carbon
What are the allotropes of carbon?
Diamond, Fullerene C60, Graphite, Graphene
Properties of macromolecular substances + explanations
Bad electrical conductors (except graphite and graphene) - because no mobile charged particles are present
High melting and boiling points - because of many strong covalent bonds
Insoluble - because of stong covalent bonds
Coordinate covalent bonds
Where one atom donates both bonding electrons to form a covalent bond
How do sigma bonds form?
Direct overlap of two atomic orbitals (can be either s or p)
How do pi bonds form?
Sideways overlap of two p orbitals
How does electron density differ betwen sigma and pi bonds?
In sigma bonds, electron density is concentrated between the 2 nuclei.
In pi bonds, electron density is concentrated above and below the plane of the 2 nuclei.
What type of bonds does a single covalent bond consist of?
1 sigma
What type of bonds does a double covalent bond consist of?
1 sigma, 1 pi
What type of bonds does a triple covalent bond consist of?
1 sigma, 2 pi
What is a resonance structure?
When there is more than one position for a multiple bond in a molecule
In a resonance hybrid structure, describe the bond strength and length of resonance bonds
Bond strengths and lengths are the same
What is the reason for resonance?
Instead of electrons being fixed to one atom, regions of delocalised electrons form above and below the plane of the molecule where delocalised pi electrons are found
Formula for formal charge
no. of valence electrons - no. of non-bonding electrons - 1/2 no. of bonding electrons
What is a covalent network solid / giant covalent solid
Regular, usually 3D, arrangement of atoms covalently bonded which extends through a substance
Describe the hybridisation of C atom in methane
1) one electron in 2s sub level is excited to the 2p sublevel
2) one 2s and 2p orbitals hybridise to form 4 sp3 hybrid orbitals which all have the same energy levels (which are between that of 2p and 2s)
What type of hybridised orbitals does Ethene (C2H4) form?
3 sp2 orbitals (one p orbital is left unhybridised)
If a carbon atoms forms a triple bond and only has 2 electron domains, what type of hybridisation would occur?
2 sp orbitals would form (two p orbitals left unhybridised)
How many hybridised orbitals do sp3, sp2 and sp have?
sp3 - four
sp2 - three
sp - two
What molecular shapes do sp3, sp2 and sp hybrids have?
sp3 - tetrahedral
sp2 - trigonal planar
sp - linear
Describe the bonding between the first two carbon atoms in but-1-ene in terms of orbitals on these atoms.
Three sp2 orbitals and one p orbital left unhybridised for each carbon.
- sp2 orbitals have direct overlap to form a sigma bond
- unhybridised p orbitals have sideways overlap to form a pi bond
Describe the bonding between the two carbon atoms in Ethyne (C2H2) in terms of
orbitals on these atoms.
Two sp orbitals and two unhybridised p orbitals for each carbon atom.
- sp orbitals have direct overlap to form one sigma bond
- unhybridised p orbitals have sideways overlap to form 2 pi bonds
