C2 - Elements, Compunds And Mixtures

Question 1

Definition of ‘pure’ in everyday life

Answer 1

The word ‘pure’ is often used to mean clean or natural

Question 2

Definition of ‘pure’ in chemistry

Answer 2

In chemistry, a substance is pure if it’s completely made up of a single element or compound

Question 3

How to distinguish between pure and impure substances:

Answer 3

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

Question 4

Definition of relative atomic mass

Answer 4

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

Question 5

Definition of relative formula mass

Answer 5

The relative formula mass of a compost is all the relative atomic masses in its formula added together

Question 6

Definition of an empirical formula

Answer 6

An empirical formula of a compound tells you the smallest whole number ratio of atoms in the compound E.g. C2H6 —> CH3

Question 7

Describe a ‘formulation’

Answer 7

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

Question 8

Describe the process of filtration

Answer 8

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

Question 9

Describe the process of crystallisation

Answer 9

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.

Question 10

Describe the process of simple distillation

Answer 10

Simple distillation is used for separating out a liquid from a solution:

1. Pour your sample of seawater into the distillation flask.
2. 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.
3. Gradually heat the distillation flask. The part of the solution that has the lowest boiling point will evaporate - in this case, the water
4. The water vapour passes into the condenser where it cools and condenses. It then flows into the beaker where it is collected
5. Eventually you will end up with just salt in the flask.

Question 11

Describe the process of fractional distillation

Answer 11

Fractional distillation is used to separate a mixture of liquids

Method for fractional distillation of crude oil:

1. Put your mixture into the flask. Attach a fractionating column and condenser above the flask as shown in the picture
2. Gradually heat the flask. The different liquids will all have different boiling points - so they 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 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.

Question 12

Describe thin-layer chromatography

Answer 12

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:

1. 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.
2. Put some of the solvent into a beaker. Dip the bottom of the plate (not the spot) into the solvent
3. Put a watch glass over the speaker to stop any solvent from evaporating away
4. 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.
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.

Question 13

Describe paper chromatography

Answer 13

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)

Question 14

What is the mobile phase?

Answer 14

The mobile phase is where the molecules can move. This is always a liquid or a gas

Question 15

What is the stationary phase?

Answer 15

Where molecules cannot move. This can be a solid or a really thick liquid.

Question 16

Formula for R_f value

Answer 16

R_f = Distance travelled by solute/Distance travelled by solvent

Question 17

How to interpret a chromatogram?

Answer 17

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

Question 18

Describe the properties of metals

Answer 18

High melting points and boiling points

High density

Strong

Malleable

Good conductors of heat and electricity

Question 19

Describe the properties of non-metals

Answer 19

Low melting and boiling points

Weak

Brittle

Lower densities than metals

Weak conductors of heat and electricity

Question 20

Why are metals on the left hand side of the periodic table?

Answer 20

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

Question 21

What does the group number correspond to?

Answer 21

Number of electrons in the outer shell

Question 22

What does the period number correspond to?

Answer 22

Number of electron shells in the atom

Question 23

Describe the structure of ionic compounds

Answer 23

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

Question 24

How do simple molecules join together?

Answer 24

Through covalent bonds

Question 25

Describe the arrangement of chemical bonds in giant covalent structures?

Answer 25

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

Question 26

Describe the structure of polymers

Answer 26

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

Question 27

Describe the structure in metals

Answer 27

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.

Question 28

Describe covalent bonding

Answer 28

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

Question 29

Describe ionic bonding

Answer 29

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.

Question 30

Explain chemical bonding?

Answer 30

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.

Question 31

What are the limitations of a dot and cross diagram?

Answer 31

Fails to illustrate the 3D arrangements of the atoms and electron shells

Doesn’t indicate the relative sizes of the atoms

Question 32

What are the limitations of the ball and stick model?

Answer 32

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

Question 33

What are the limitations of using a two-dimensional representation of a particle?

Answer 33

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

Question 34

What are the limitations of using a three-dimensional representation of a particle?

Answer 34

Only illustrate the outermost layer of the compound

Are difficult and time-consuming to draw

Question 35

How many covalent bonds can carbon form?

Answer 35

Four

Question 36

Why is there a vast array of natural and synthetic organic compounds in chemistry?

Answer 36

Due to the ability of carbon to form families of similar compounds, chains and rings.

Question 37

Properties of diamond:

Answer 37

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

Question 38

Properties of graphite:

Answer 38

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

Question 39

Properties of graphene:

Answer 39

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.

Question 40

Properties of fullerenes:

Answer 40

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.

Question 41

What are the typical dimensions of a nanoparticle?

Answer 41

1-100 nanometres

1nm = 0.000000001m

Question 42

What is the relationship between the surface area to volume ratio and the properties the material has?

Answer 42

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.

Question 43

Describe how the properties of nanoparticles relate to their uses.

Answer 43

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

Question 44

What are the risks associated with nano particles?

Answer 44

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.