C2 - elements, compounds and mixtures Flashcards
1
Q
who made the first proper periodic table?
A
in 1869, Dmitri Mendeleev took the 50 or so elements known at the time and arranged them into his Table of Elements (with various gaps)
2
Q
how did Mendeleev order the elements?
A
in order of atomic mass
3
Q
why did Mendeleev leave gaps?
A
to keep elements with similar properties in the same vertical groups he had to swap one or two elements round and leave a few gaps
4
Q
why were the gaps Mendeleev left clever?
A
they predicted the properties of so far undiscovered elements; when they were found and fitted the pattern, it helped confirm Mendeleev’s ideas
5
Q
how does the modern periodic table show elements?
A
in order of ascending atomic number
6
Q
what does an element’s group number correspond to?
A
the number of electrons that an element has on its outer shell
7
Q
what does an element’s period number correspond to?
A
the number of shells of electrons an element has
8
Q
what are the electron shell rules?
A
- electrons occupy shells (sometimes called energy levels)
- the lowest energy levels are always filled first
- only a certain number of electrons are allowed in each shell (1st: 2, 2nd: 8, 3rd: 8 etc)
9
Q
which elements form ions most easily?
A
those in groups 1,2,6 and 7
10
Q
what do group 1 and 2 elements do?
A
they are metals, they lose electrons to form positive ions (cations)
11
Q
what do group 6 and 7 elements do?
A
they are non-metals, they gain electrons to form negative ions (anions)
12
Q
what do ions with opposite charges do?
A
form ionic bonds
13
Q
what happens when a metal and a non-metal react together?
A
the metal can lose electrons to form a positively charged ion and the non-metal can gain electrons to form a negatively charged ion; these oppositely charged ions are then strongly attracted to one another by electrostatic forces and form an ionic bond
14
Q
how can you show ionic bonding?
A
using dot and cross diagrams
15
Q
what structure do ionic compounds have?
A
ionic compounds always have a giant ionic lattice structure, the ions form a closely packed regular lattice and there are very strong electrostatic forces of attraction between oppositely charged ions, in all directions
16
Q
what are some properties of ionic compounds?
A
- high melting and boiling points
- solid ionic compounds don’t conduct electricity, but when an ionic compound melts, the ions are free to move and will carry an electric charge
- many dissolve easily in water, this causes the ions to separate and are all free to move in the solution so they’ll carry an electric charge
17
Q
how do covalent bonds join atoms?
A
sharing electrons
18
Q
what happens when non-metal atoms combine together?
A
they form covalent bonds by sharing pairs of electrons
19
Q
what does each covalent bond provide?
A
one extra shared electron for each atom
20
Q
why are covalent bonds strong?
A
there’s a strong electrostatic attraction between the positive nuclei of the atoms and the negative electrons in each shared pair
21
Q
how can you show covalent bonding?
A
using dot and cross diagrams
22
Q
what structures do substances formed with covalent bonds usually have?
A
simple molecular structures
23
Q
what are some properties of simple molecular structures?
A
- low melting and boiling points
- most are gases or liquids at room temperature
- they don’t conduct electricity
24
Q
why do simple molecular structures have low melting and boiling points?
A
the atoms within the molecules are held together by very strong covalent bonds; however, the forces of attraction between these molecules are very weak, and its these weak intermolecular forces which need to be overcome in order to melt or boil a simple covalent compound
25
what do ball and stick models show?
how the atoms in covalent molecules are connected
26
what do giant covalent structures contain?
many covalent bonds
27
what are giant covalent structures?
they are similar to giant ionic lattices except that there are no charged ions, the atoms are bonded to each other by strong covalent bonds
28
what are some properties of giant covalent structures?
- very high melting and boiling points
- they don't conduct electricity (even when molten- except for graphite, graphene and fullerenes)
29
why can carbon form lots of different types of molecule?
because carbon atoms can form up to four covalent bonds and bond easily to other carbon atoms to make chains and rings
30
what are some properties of diamond?
- pure diamonds are lustrous and colourless
- each carbon atom forms four covalent bonds in a very rigid giant covalent structure, which makes diamond very hard (ideal as cutting tools)
- very high melting point
- doesn't conduct electricity
31
what are some properties of graphite?
- graphite is black and opaque, but still kind of shiny
- each carbon atom only forms three covalent bonds, creating sheets of carbon atoms which are free to slide over each other
- the layers are held together weakly so can be rubbed off onto paper to leave a black mark (which is how a pencil works)
- graphite is ideal as a lubricating material
- high melting point
- conducts electricity (has delocalised electrons)
32
what is a single sheet of graphite called?
graphene
33
what are some properties of graphene?
- extremely strong
- so thin that its transparent
- very light
- better than graphite at conducting electricity
34
what are fullerenes?
large carbon molecules
35
what do different fullerenes contain?
different numbers of carbon atoms
36
how are carbon atoms in fullerenes arranged?
in rings (similar to those in graphite)
37
what are some properties of fullerenes?
- shaped like hollow balls or tubes
- conduct electricity (delocalised electrons)
- high melting and boiling points but not as high as those of diamond and graphite
38
what are nanoparticles?
really tiny particles (1-100nm across)
39
what is an example of a nanoparticle?
fullerenes, which include nanotubes (tiny hollow carbon tubes)
40
what are some features of nanoparticles?
- high SA:V
- nanoparticles can modify the properties of materials
- good catalysts
- nanotubes conduct electricity
41
what are some examples of nanoparticles modifying the properties of materials?
- some sun creams and deodorants are using nanoparticles as they do the job and don't leave white marks on the skin
- nanotubes can be used in tiny electric circuits for computer chips
- when added to plastic in sports equipment, they can make it much stronger and more durable, without adding much mass
- silver nanoparticles are added to the polymer fibres used to make surgical masks and wound dressings as it gives the fibres antibacterial properties
42
why is it important that any new nanoparticles are tested thoroughly?
the way they affect the body isn't fully understood, so thorough testing can help to minimise risk
43
what are plastics?
long-chain molecules called polymers
44
how are polymers formed?
when lots of small molecules called monomers join together, this reaction is called polymerisation and usually needs high pressure and a catalyst
45
what are plastics usually made from?
they're usually carbon based and their monomers are often alkenes
46
what determines the properties of polymers?
forces between molecules
47
what holds the atoms together in polymer chains?
strong covalent bonds
48
what are the properties of a plastic with weak intermolecular forces between chains?
the chains will be free to slide over each other so the plastic will be able to be stretched easily and will have a low melting point
49
what are the properties of a plastic with strong intermolecular forces between chains? (these might be covalent bonds, which are sometimes called cross-links)
these plastics have higher melting points, are rigid and can't be stretched (since the cross-links hold the chains firmly together)
50
what do the properties of materials depend on?
structure and bonding
51
what are the properties of a whole material, rather than the individual atoms called?
the bulk properties
52
what structure do metals have?
metals have a crystal structure
53
what happens to the outer electron(s) of metal atoms?
in metals, the outer electron(s) of each atom can move freely, the atoms become positive ions in a 'sea' of delocalised electrons
54
what is metallic bonding?
the electrostatic attraction between the ions and the electrons which surround them, the attraction acts in all directions
55
which part of the periodic table are metals usually on?
the left-hand side
56
what are some properties of metals?
- most have high melting and boiling points
- high density
- they're strong, but also bendy and malleable (metals have high tensile strength)
- they're good conductors of heat and electricity
- they react with oxygen to form metal oxides
- they can be mixed with other elements to form alloys
57
what properties do non-metals have that are different to those of metals?
- they usually have low melting and boiling points
- when solid, they tend to be hard and brittle
- they have lower densities than metals
- they don't conduct electricity
(there are exceptions e.g. carbon)
58
what makes a substance pure?
pure substances contain only one thing
59
what's the name of a substance which contains more than one compound or different elements that aren't all part of a single compound?
a mixture
60
what are formulations?
formulations are useful mixtures with a precise purpose that are made by following a 'formula', each component is present in a measured quantity and contributes to the formulation's properties
61
what can be used to test purity?
melting and boiling points
62
how can you use melting and boiling points to test purity?
- every pure substance has a specific melting and boiling point so you can test purity by comparing the actual melting or boiling point of a sample to the expected value
or
- heat a solid or liquid sample up, in a mixture the different components will melt or boil at different temperatures, so part of the mixture will melt or boil first while the rest will stay in its original state for longer
63
how can you tell from the melting point if a substance is impure?
if a substance is impure, the melting point will be too low
64
how can you tell from the boiling point if a substance is impure?
if a substance is impure, the boiling point will be too high
65
what do mixtures often melt over?
a range of temperatures
66
what is distillation used for?
to separate mixtures that contain liquids
67
what are two types of distillation?
simple distillation and fractional distillation
68
what is simple distillation used for?
to separate out solutions
69
what is fractional distillation used for?
to separate a mixture of liquids
70
what's the problem with simple distillation?
you can only use it to separate things with very different boiling points
71
how do you use simple distillation to get pure water from seawater?
1. pour your sample of seawater into the distillation flask
2. set up apparatus; connect bottom end of the condenser to a cold tap using rubber tubing, run cold water through the condenser to keep it cool
3. gradually heat the distillation flask, the part of the solution that has the lowest boiling point will evaporate (in this case, that's the water)
4. the water vapour passes into the condenser where it cools and condenses (turns back into a liquid), it then flows into a beaker where its collected
5. eventually you end up with just the salt left in the flask
72
what happens during the fractional distillation of crude oil at a refinery?
1. put the mixture in a flask, attach a fractionating column and condenser above the flask
2. gradually heat the flask, the different liquids have different boiling points so will evaporate at different temperatures
3. 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
4. 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 up before condensing and running back down towards the flask
5. when the first liquid has been collected, raise the temperature until the next one reaches the top
73
what is filtration used for?
to separate an insoluble solid from a liquid
74
what is crystallisation used for?
to separate a soluble solid from a solution
75
how does filtration work?
put some filter paper into a funnel and pour the mixture into it, the liquid part of the mixture runs through the paper, leaving behind a solid residue
76
how does crystallisation work?
1. pour the solution into an evaporating dish and gently heat the solution; some of the solvent (usually this is water) will evaporate and the solution will get more concentrated
2. once some of the solvent has evaporated, or when you see crystals start to form (point of crystallisation), remove the dish from the heat and leave the solution 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 (can use a drying oven or desiccator)
77
what does chromatography use?
two phases
78
hat are the two phases of chromatography?
the mobile phase and the stationary phase
79
what is the mobile phase in chromatography?
where the molecules can move, this is always a liquid or a gas
80
what is the stationary phase in chromatography?
where the molecules can't move, the can be a solid or a really thick liquid
81
how does chromatography work?
1. the components in the mixture separate out as the mobile phase moves over the stationary phase, they all end up in different places in the stationary phase
2. this happens because each of the chemicals in a mixture will spend different amounts of time dissolved in the mobile phase and stuck to the stationary phase
3. how fast a chemical moves through the stationary phase depends on how it 'distributes' itself between the two phases
82
what are the stationary and mobile phases in thin-layer chromatography?
the stationary phase is a thin layer of a solid (e.g. silica gel or aluminium oxide powder) on a glass or plastic plate
the mobile phase is a solvent (e.g. ethanol)
83
how do you set up thin- layer chromatography?
1. draw a line near the bottom of the plate(use a pencil since pencil is insoluble) and put spot of mixture to be separated on the line
2. put some of the solvent into a beaker, dip the bottom the plate (not the spot) into the solvent
3. put a watch glass over the beaker to stop any solvent from evaporating away
4. the solvent will start to move up the plate, when the chemicals in the mixture dissolve the solvent, they will move up the plate too
5. you will see the different chemicals in the sample separate out, forming spots at different places on the plate
6. remove the plate from the beaker before the solvent reaches the top, mark the distance the solvent has moved (the solvent front) in pencil
84
what does the amount of time the molecules spend in each phase depend on?
- how soluble they are in the solvent
- how attracted they are to the stationary phase
85
what does it mean if molecules have a high solubility in the solvent?
molecules with a higher solubility in the solvent (and which are less attracted to the stationary phase) will spend more time in the mobile phase than the stationary phase, so they'll be carried further up the plate
86
what is paper chromatography?
paper chromatography is very similar to thin-layer chromatography but the stationary phase is a sheet of chromatography paper and the mobile phase is a solvent like ethanol
87
what is the result of chromatography analysis called?
a chromatogram
88
how is an Rf value calculated?
distance travelled by solute / distance travelled by solvent
89
what is gas chromatography?
gas chromatography is used to analyse unknown substances as well, if the substances aren't already gases, then they have to be vaporised
- the mobile phase is an unreactive gas such as nitrogen, and the stationary phase is a viscous liquid, like an oil
90
how does gas chromatography work?
1. the unknown mixture is injected into a long tube coated on the inside with the stationary phase
2. the mixture moves along the tube with the mobile phase until it comes out the other end; as in the other chromatography methods, the substances are distributed between the phases- so each substance spends different amounts of time dissolved in the mobile phase and stuck to the stationary phase
3. the time it takes a chemical to travel through the tube is called the retention time
4. the retention time is different for each chemical, that's what's used to identify it
91
what does each peak on a chromatogram graph for gas chromatography represent?
a different chemical
92
what is the distance along the x-axis on a chromatogram graph for gas chromatography?
the retention time
93
what do the relative areas under the peaks on a chromatogram graph for gas chromatography show you?
the relative amounts of each chemical in the sample
94
what will a sample of a pure substance produce on a graph?
a single peak
95
what is the 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
96
what is the empirical formula?
the simplest ratio of atoms
