S2.4 - from models to materials Flashcards
What is chemical bonding?
Chemical bonding is best described as a continuum between the ionic, covalent and metallic models, not a discrete model.
- Shown by the bonding triangle.
What does position of triangle depend on?
Electronegativity values
What is the bonding triangle?
Used to illustrate the types of chemical bonding as a continuum.
What goes on the x-axis of the bonding triangle?
Average electronegativity
What goes on the y-axis in the bonding triangle?
Difference in electronegativity
Where do elements fit in the bonding triangle?
Along the x-axis
What are the properties and bonding of Na, Mg and Al2O3?
Properties:
- High melting and boiling points
- Electrical conductors when molten
Bonding:
- Ionic metallic oxides
What are the properties and bonding of SiO2?
Properties:
- High melting and boiling points
- Electrical conductors
Bonding:
- Giant covalent oxides of the metalloid silicon
What are the properties and bonding of P, SO2?
Properties:
- Low melting and boiling points
- Non-conductors of electricity
Bonding:
- Covalent non-metallic oxides
What does the composition of a compound determined by?
Position of a compound in the bonding triangle is determined by the relative contributions of the three bonding types to the overall bond.
What is AlCl3 classed as?
It should come out as more covalent, suggesting a lower melting point.
What is a composite material?
Heterogeneous mixture made up of at least 2 materials, which are present at separate phases.
What happens to the properties of substances in a composite material?
Each element keeps its own properties, leading to a final product with enhanced properties designed for a particular purpose.
What are examples of composite materials?
Reinforcement bar - concrete and steel bars
Adobe bricks - mud and hay
What is an alloy?
Homogeneous mixtures containing at least 1 metal held together by metallic bonding.
What are some examples of alloys?
Steel - iron + carbon
Bronze - copper + tin
Brass - copper + zinc
Why are alloys hard?
Alloys are hard as the layers are different sizes and distorted, meaning that they do not slide or change shape when a force or pressure is applied.
How can factors affect the strength of a metallic bond?
Number of delocalised electrons - more electrons means a stronger electrostatic attraction, which requires more energy to overcome.
Charge of cations - higher charge means more delocalised electrons moving through the lattice with smaller radii as ENC increases, causing a stronger attraction.
Radius of cation - smaller radius means a stronger attraction between the electron and nucleus.
What are trends of metals down the group and across the period?
Down the group - larger radii means weaker electrostatic attraction, so a lower melting point as less energy is needed to overcome this.
Across the period - larger ENC and smaller ionic radius means larger charge density and stronger attraction, so a higher melting points as more energy is needed to overcome this.
What is a polymer?
Large molecules (macro-molecule) made up of repeating sub-units (monomers).
What are properties of polymers?
- Covalent bonding
- Unique ability to form stable bonds with itself, H and other elements.
How do the side chains affect the polymer?
Different side chains change the type of IMF formed, which affects the polymer made and the properties it has.
What is a repeating unit?
A ‘snapshot’ at a part of chain which has the part that repeats.
What are the two types of polymers and what are some examples?
Natural polymers - protein, starch and DNA
Synthetic polymers - plastic
What are the properties of plastic?
- Light-weight
- Low reactivity
- Water-resistant
- Strong
- Non-biodegradable
- Made out of cellulose, starch and lignin
What does compostable mean?
Can be broken down along with food and plant matter in specific conditions.
What does biodegradable mean?
Can be broken down by microorganisms
What does non-biodegradable mean?
Cannot be broken down by natural processes.
What does reusable mean?
Can be used in the same format over and over again.
What does recyclable mean?
Can be melted down and changed into something else.
What is an addition polymer?
Requires a double bond to be broken to make a new covalent bond with neighboring molecules to form a chain.
How does polymerization link to atom economy?
Has 100% atom economy - no waste products
What are the disadvantages of addition polymers?
- Requires huge amounts of energy which is expensive and bad for the environment.
How do condensation polymers form?
Condensation polymers form from the reaction between functional groups in each monomer.
- A small molecules, usually water or ammonia is released.
What can undergo condensation reactions?
- The monomer can be 2 different monomers, or the same monomer with 2 different functional groups in the structure.
What happens when a carboxylic acid reacts with an alcohol?
It forms polyester.
- It has dipole-dipole bonding between the polymer chains as they are polar molecules.
- Stronger than polyethene which has LDF’s.
What is the repeating unti?
Part of the molecule that can be seen repeating along the length of a polymer chain.
- Produced after the removal of the small molecule (water or ammonia).
What happens when a carboxylic acid reacts with an amine?
It reacts to form polyamides (peptide bonds on biology forming polypeptides from protein).
- It has hydrogen bonding, dipole-dipole bonding and LDF’s meaning it is very strong.
- It causes an amide link.
What is hydrolysis?
When enzymes break down macro molecules using a chemical reaction.
- Each bond formed in the condensation reaction takes a molecule of water to break it.
- In a lab setting, an acid is used to speed this up.
- In a biological setting, an enzyme is used to speed this up.
