C2 - Elements, Compunds And Mixtures Flashcards
Definition of ‘pure’ in everyday life
The word ‘pure’ is often used to mean clean or natural
Definition of ‘pure’ in chemistry
In chemistry, a substance is pure if it’s completely made up of a single element or compound
How to distinguish between pure and impure substances:
Every pure substance has a specific melting point and boiling point. You can test the purity of a sample by comparing the actual melting point or boiling point to the expected value. If a substance is impure, the melting point will be too low and the boiling point will be too high
Definition of relative atomic mass
The relative atomic mass of an element is the average mass of one atom of the element compared to 1/12 of the mass of one atom of Carbon-12
Definition of relative formula mass
The relative formula mass of a compost is all the relative atomic masses in its formula added together
Definition of an empirical formula
An empirical formula of a compound tells you the smallest whole number ratio of atoms in the compound E.g. C2H6 —> CH3
Describe a ‘formulation’
Formulations are useful mixtures with a precise purpose that are made by following a ‘formula’. Each component is presented in a measured quantity, and contributes to the formulation’s properties. E.g. alloy size
Describe the process of filtration
Filtration is used to separate an insoluble solid from a liquid. You put filter paper into a funnel and pour your mixture into it. The liquid part of the mixture runs through, leaving behind a solid residue
Describe the process of crystallisation
Crystallisation separates a soluble solid from a solution Method: 1. Pour the solution into an evaporating dish and gently heat the solution. Some of the solvent will evaporate and the solution will get more concentrated 2. Once some of the solution has evaporated, or when you see crystals form, remove the dish from the heat and leave to cool 3. The salt should start to form crystals as it becomes insoluble in the cold, highly concentrated solution 4. Filter the crystals out of the solution, and leave them in a warm place to dry.
Describe the process of simple distillation
Simple distillation is used for separating out a liquid from a solution:
- Pour your sample of seawater into the distillation flask.
- Set up the apparatus as shown in the picture. Connect the bottom end of the condenser to a cold tap using rubber tubing. Run cold water through the condenser to keep it cool.
- Gradually heat the distillation flask. The part of the solution that has the lowest boiling point will evaporate - in this case, the water
- The water vapour passes into the condenser where it cools and condenses. It then flows into the beaker where it is collected
- Eventually you will end up with just salt in the flask.
Describe the process of fractional distillation
Fractional distillation is used to separate a mixture of liquids
Method for fractional distillation of crude oil:
- Put your mixture into the flask. Attach a fractionating column and condenser above the flask as shown in the picture
- Gradually heat the flask. The different liquids will all have different boiling points - so they will evaporate at different temperatures
- The liquid with the lowest boiling point evaporates first. When the temperature on the thermometer matches the boiling point of this liquid, it will reach the top of the column
- Liquids with higher boiling points might also start to evaporate. But the column is cooler towards the top, so they will only get part of the way before condensing and running back down towards the flask.
- When the first liquid has been collected, raise the temperature until the next one reaches the top.
Describe thin-layer chromatography
In TLC, the stationary phase is a thin laye of a solid (e.g. silica gel) on a glass or plastic plate. The mobile phase is a solvent (e.g. ethanol).
Method:
- Draw a line near the bottom of the plate using a pencil as pencil marks are insoluble. Put a spot of the mixture to be separated on the line.
- Put some of the solvent into a beaker. Dip the bottom of the plate (not the spot) into the solvent
- Put a watch glass over the speaker to stop any solvent from evaporating away
- The solvent will start to move up the plate. When the chemicals in the mixture dissolve in the solvent, they will move up the plate too.
- You will see the different chemicals in the sample separate out, forming spots at different places on the plate
- Remove the plate from the beaker before the solvent reaches the top. Mark the distance the solvent has moved (the solvent front) in pencil.
Describe paper chromatography
Paper chromatography is very similar to TLC, but the stationary phase is a sheet of chromatography paper
The mobile phase is a solvent such as ethanol (just like in TLC)
What is the mobile phase?
The mobile phase is where the molecules can move. This is always a liquid or a gas
What is the stationary phase?
Where molecules cannot move. This can be a solid or a really thick liquid.
Formula for Rf value
Rf = Distance travelled by solute/Distance travelled by solvent
How to interpret a chromatogram?
The chromatogram from gas chromotography is a graph, each peak on the graph represents a different chemical:
The distance along the x axis is the retention time - this can be looked up to find out what the chemical is
The relative areas under the peaks show you the relative amounts of each chemical in the sample
Theres one peak for each chemical, which means a sample of pure substance will produce a single peak
Describe the properties of metals
High melting points and boiling points
High density
Strong
Malleable
Good conductors of heat and electricity
Describe the properties of non-metals
Low melting and boiling points
Weak
Brittle
Lower densities than metals
Weak conductors of heat and electricity
Why are metals on the left hand side of the periodic table?
Metals were usually on the left hand side because of the bonding in solid metals - elements on the left of the table normally get a full outer shell by losing electrons so metal atoms find it easy to become positive ions
What does the group number correspond to?
Number of electrons in the outer shell
What does the period number correspond to?
Number of electron shells in the atom
Describe the structure of ionic compounds
Ionic compounds always have a giant ionic lattice structure. The ions form a closely packed regular lattice. There are very strong electrostatic forces of attraction between oppositely charged ions, in all directions
How do simple molecules join together?
Through covalent bonds
Describe the arrangement of chemical bonds in giant covalent structures?
Giant covalent structures contain many covalent bonds.
They are very similar to giant ionic lattices except there are no charged ions
The atoms are bonded to each other by strong covalent bonds
This means they have very high melting and boiling points
Describe the structure of polymers
Polymers are formed when lots of small molecules called monomers join together.
They’re a type of covalent molecule - but they behave differently to simple covalent substances because of the long, thin shapes of their molecules
Strong covalent bonds hold the atoms together in polymer chains
Describe the structure in metals
All metals have the same basic properties, due to the special type of bonding that exists in metals/
In metals, the outer electrons of each atom can move freely. The atoms become positive ions in a ‘sea’ of delocalised electrons
Metallic bonding is the electrostatic attraction between these ions and electrons. The ions are surrounded by the electrons, so the attraction acts in all directions.
Describe covalent bonding
When non-metals combine together, they form covalent bonds by sharing pairs of electrons.
This way, both atoms feel that they have a full outer shell.
Covalent bonds are strong because there’s a strong electrostatic attraction between the positive nuclei of the atoms and the negative electrons in each shared pair
Describe ionic bonding
Ions are charged particles - they can be single atoms or groups of atoms.
When atoms lose or gain electrons to form ions, their main purpose is trying to achieve a full outer shell (stable electronic structure)
When metals form ions, they lose electrons to form positive ions
When non-metals form ions, they gain electrons to form negative ions
When a metal and non-metal react together, the metal can lose electrons to form a positively charged ion and the non-metal can gain electrons to from a negatively charged ion.
These oppositely charged ions are then strongly attracted to one another by electrostatic forces and form an ionic bond.
Explain chemical bonding?
A chemical bond is a lasting attraction between atoms, ions or molecules that enables the formation of chemical compounds.
The bond may result from the electrostatic force of attraction between oppositely charged ions as in ionic bonds or through the sharing of electrons as in covalent bonds.
What are the limitations of a dot and cross diagram?
Fails to illustrate the 3D arrangements of the atoms and electron shells
Doesn’t indicate the relative sizes of the atoms
What are the limitations of the ball and stick model?
Fails at indicating the movement of electrons
The atoms are placed far apart from each other, which in reality is not the case as the gaps between atoms are much smaller
What are the limitations of using a two-dimensional representation of a particle?
Fail to illustrate the relative sizes of the atoms and bonds
Cannot give you an idea of the shape of a molecule and what it looks like in 3D space
What are the limitations of using a three-dimensional representation of a particle?
Only illustrate the outermost layer of the compound
Are difficult and time-consuming to draw
How many covalent bonds can carbon form?
Four
Why is there a vast array of natural and synthetic organic compounds in chemistry?
Due to the ability of carbon to form families of similar compounds, chains and rings.
Properties of diamond:
Diamond has a giant covalent structure in which:
Each carbon atom is joined to four other carbon atoms by covalent bonds
The carbon atoms have a regular lattice arrangement
There are no free electrons thus cannot conduct electricity
The rigid structure makes diamond very hard
Properties of graphite:
Graphite has a giant covalent structure in which:
Each carbon atom is joined to three other carbon atoms by covalent bonds
The carbon atoms form layers with a hexagonal arrangement of atoms
The layers in graphite can slide over each other because the forces between them are weak. This makes graphite slippery, so it is useful as a lubricant.
Each carbon atom has one non-bonded outer electron, which becomes delocalised, thus allows graphite to conduct electricity
Properties of graphene:
Its structure resembles a single layer of graphite.
Graphene has a very high melting point and is very strong because of its large regular arrangement of carbon atoms joined by covalent bonds.
Like graphite, graphene conducts electricity well because it has delocalised electrons that are free to move through its structure.
Properties of fullerenes:
Fullerenes are forms of carbon, and include nanotubes and buckyballs:
Nanotubes:
A nanotube resembles a layer of graphene, rolled into a tube shape.
Nanotubes have high tensile strength, so they are strong in tension and resist being stretched.
Like graphene, nanotubes are strong, and they conduct electricity because they have delocalised electrons.
Buckyballs:
Buckyballs are spheres or squashed spheres of carbon atoms.
They are made up of large molecules but do not have a giant covalent structure.
Weak intermolecular forces exist between individual buckyballs.
Little energy is needed to overcome these forces, so substances consisting of buckyballs are slippery and have lower melting points than graphite or diamond.
What are the typical dimensions of a nanoparticle?
1-100 nanometres
1nm = 0.000000001m
What is the relationship between the surface area to volume ratio and the properties the material has?
As particles decrease in size, the size of their surface area increases in relation to their volume - thus the surface area to volume ratio increases.
As this increases, properties such as melting point and boiling point are decreased as more of their atoms are available to interact with substances they come into contact with.
Describe how the properties of nanoparticles relate to their uses.
They have a large surface area to volume ratio - thus can make good catalysts.
They don’t leave marks on skin - thus are used in cosmetic products such as sun creams and deodorants
Smaller fullerenes are absorbed more easily by the body than most particles - this suggests that they could deliver drugs right into the cells
Nanotubes can conduct electricity - they can be used in tiny electric circuits for computer chips
What are the risks associated with nano particles?
The way they affect the body isn’t fully understood yet however we have learnt that prolonged exposure can lead to lung inflammation and heart problems.
