C2 - elements, compounds and mixtures (from PMT)

Question 1

define a pure substance scientifically

Answer 1

a substance is made up of a single element or compound

Question 2

define a relative atomic mass

Answer 2

the average mass of an atom of an element compared to 1/12th the mass of an atom of carbon-12

Question 3

what is the meaning of relative formula mass?

Answer 3

the wighted mean average masses of the formula units compared to 1/12th the mass of an atom of carbon-12

Question 4

why are alloys commonly used instead of pure metals?

Answer 4

alloys can be engineered to have more desirable properties than pure metals. alloys are often harder and stronger than pure metals because the different sized atoms within an alloy distort the layers so they can’t slide over each other as easily.

Question 5

many useful materials are formulations of mixtures. explain what this means.

Answer 5

a formulation is a mixture containing exact quantities of different substances. the quantities have been refined and tested to ensure the material has optimum properties for its purpose.

Question 6

what is an rf value?

Answer 6

rf = retention factor
the rf value is a ratio between the distance travelled by the dissolved substance (the solute) and the distance travelled by the solvent

Question 7

when is gas chromatography used?

Answer 7

gas chromatography is used to separate mixtures of volatile liquids

Question 8

why do elements in the same column have similar properties?

Answer 8

they have the same number of outer shell electrons. this determines how they react.

Question 9

define a covalent bond

Answer 9

a shared pair of electrons between two non-metal atoms, usually in their outer shells

Question 10

define an ionic bond

Answer 10

a bond formed between a positive metal ion and a negative non-metal ion

Question 11

describe the bonding in an ionic compound

Answer 11

ionic bonds form between positive metal ions and negative non-metal ions.
ionic compounds are held together by the electrostatic attraction between these oppositely charged ions.

Question 12

why do ionic compounds have high melting and boiling points’;?

Answer 12

the strong electrostatic forces of attraction between oppositely charged ions require a lot of energy to overcome

Question 13

when do ionic compounds conduct electricity and why?

Answer 13

ionic compounds conduct electricity when molten or aqueous because the ions are free to move to carry charge. when solid, the ions are fixed in the ionic lattice so don’t conduct electricity

Question 14

why do simple molecules have low boiling points despite containing covalent bonds?

Answer 14

to change state, simple molecules need to overcome the intermolecular forces, no the covalent bonds
simple molecules, are held together by weak intermolecular forces which require little energy to overcome

Question 15

why are simple molecules unable to conduct electricity?

Answer 15

they have no overall charge

Question 16

how and why do boiling points of simple molecules change as the size of the molecules increases?

Answer 16

-as the size of the molecule increases, the strength of the intermolecular forces also increases
-larger simple molecules have higher boiling points as more energy is required to overcome the intermolecular forces

Question 17

describe the bonding in giant covalent structures

Answer 17

many strong covalent bonds (shared pair of electrons)

Question 18

why do giant covalent structures have very high melting points?

Answer 18

all of the atoms in the structure are covalently bonded to other atoms.
these strong covalent bonds must be broken to melt the substance which requires a lot of energy meaning the melting point is very high.

Question 19

what type of bonds are found in polymers?

Answer 19

covalent bonds

Question 20

describe the structure and bonding in metals

Answer 20

metallic bonding
giant structure with positive metal ions held in a sea of delocalised electrons

Question 21

why are metals typically very malleable?

Answer 21

the atoms in metals are arranged in uniform rows that can easily slide over one another.
this allows metals to be bent and shaped.

Question 22

why do metals have relatively high melting points?

Answer 22

they have strong metallic bonding. a lot of energy is required to overcome the electrostatic attraction between the positive ions and the negative electrons

Question 23

what is a limitation of dot and cross diagrams?

Answer 23

doesn’t show the 3D arrangement of molecules

Question 24

what is a benefit of using ball and stick models to represent molecules? in what way are they limited?

Answer 24

they show the 3D shape and how atoms are bonded but they don’t model electrons

Question 25

how many covalent bonds can carbon form?

Answer 25

4

Question 26

what are organic compounds?

Answer 26

compounds containing carbon covalently bonded to other atoms

Question 27

why is there such an array of natural and synthetic organic compounds?

Answer 27

because carbon can form families of similar compounds, chains and rings.

Question 28

describe the structure of graphite

Answer 28

-each carbon atom bonded to 3 other carbon atoms
-layers of hexagonal rings of carbon atoms
-one delocalised electron per carbon atom

Question 29

describe the properties of graphite

Answer 29

-soft/slippery because the weak intermolecular forces between layers allow the layers to slide over one another
-electrical conductor because it contains delocalised electrons which are free to carry charge

Question 30

describe the structure of diamond

Answer 30

-each carbon atom, is covalently bonded to 4 other carbon atoms
-no charged particles

Question 31

describe the properties of diamond

Answer 31

-very hard and very high melting point due to strong covalent bonds
-doesn’t conduct electricity because there are no charged particles

Question 32

what is a fullerene?

Answer 32

a molecule made up of carbon atoms, shaped like a closed tube or hollow ball

Question 33

name 2 fullerenes

Answer 33

-graphene
-C60 (buckminsterfullerene)

Question 34

what are the properties and uses of fullerenes?

Answer 34

-they have a large surface area so are useful for trapping catalysts onto their surfaces
-hollow structure makes them useful for capturing substances by forming around the target molecule. useful for targeted drug delivery.

Question 35

why is graphene useful in electronics?

Answer 35

-it is extremely strong
-it has free electrons so can conduct electricity
-it’s only one atom thick (single layer of graphite)

Question 36

do individual atoms have the same physical properties of the substance that contains them? explaiun.

Answer 36

no, physical properties of a substance depend on the bonds it contains as well as the strength and arrangement of these bonds

Question 37

in terms of size, how do nanoparticles compare to atoms?

Answer 37

nanoparticles are approximately 100 times larger than atoms

Question 38

why are nanoparticles more reactive?

Answer 38

they have a very high surface area to volume ratio so there are more reaction sites

Question 39

why might nanotubes be suitable for making electrical circuits for computers?

Answer 39

-they are electrical conductors
-they are very small so take up little space in the computer
-they are lightweight

Question 40

why might nanoparticles be used in sunscreen?

Answer 40

they block UV light without leaving visible white marks on the skin

Question 41

what are some of the risks associated with nanoparticulate materials?

Answer 41

-little research has been conducted so there are many unknown factors
-they may be harmful to health as they could enter the bloodstream and are not easily disposed of by the body.