C2 - Elements, compounds and mixtures

Question 1

Relative atomic mass

Answer 1

The mean mass of an atom of an element compared to 1/12 the mass of a Carbon-12 atom

Question 2

Relative formula mass

Answer 2

The mean mass of a unit of a substance compared to 1/12 the mass of a Carbon-12 atom

Question 3

Empirical formula (of butane)

Answer 3

Shows the simplest whole-number ratio of the atoms of each element in a compound : C2H5

Question 4

What does pure mean?

Answer 4

A pure substance is one which consist of just one element or compound

Question 5

What are mixtures?

Answer 5

Impure substances as they contain more than one element or compound - many useful materials are mixtures of different substances : often deliberately chosen to produce the desired properties. An alloy is a mixture of a metal with one or more other elements

Question 6

Melting points of pure and impure substances

Answer 6

Pure substance : single temperature

Impure substance : melting point is less than that of the pure substance and often melts over a range of values

Question 7

How to determine melting point?

Answer 7

1) Melt it slowly
2) Stir the substance
Produces accurate results (evenly distributed and temperature of WHOLE sample to rise)

Question 8

What happens when a substance dissolves?

Answer 8

The solute is the substance that dissolves in the solvent to form a solution.

Question 9

Filtration

Answer 9

Filtration separates an insoluble solid from a liquid - water molecules (liquid molecules) small enough to pass through tiny microscopic holes in filter paper but insoluble solids are too large - therefore remain on paper as residue while the water passes through as the filtrate. The filter paper is fluted to increase surface area for the filtrate to pass through.

Question 10

Crystallisation

Answer 10

If you heat a solution - solvent vaporises while solute stays as residue. If you heat too quickly -> get a powder BUT if you heat gently -> form crystals. Heat the solution gently until it becomes saturated (no more solute can be dissolved at that temperature). As solution cools - solubility of the solute decreases (more crystals form). Dry them by patting with filter paper/putting in oven.

Question 11

How does distillation (simple and fractional) work?

Answer 11

Simple distillation involved separating both the solvent and solute from a solution - heat up the solution and water vapours rise, condense in condensing tube and then drip into end container. Relies on difference in boiling points. Fractional distillation- separates two or more substances from a mixture in the liquid state - uses a fractionating column and once temperature in column reaches its boiling point - it condenses and passes through the column. High SA (continual condensation)

Question 12

Chromatography phases

Answer 12

Stationary phase : does not move

Mobile phase : does move

Question 13

Paper chromatography and TLC (stationary and mobile phases)

Answer 13

Paper chromatography
Stationary phase : absorbent paper
Mobile phase : solvent
TLC
Stationary phase :thin layer of silica or alumina powder spread over glass
Mobile phase : solvent

Question 14

Gas chromatography

Answer 14

Stationary phase : silica or alumina powder packed into metal column
Mobile phase : unreactive carrier gas such as nitrogen
Sample is turned into a gas state and injected into the column - carrier gas pushes along the sample and the different times to travel through the column depend on how well they bond with the stationary phase - detector send a signal to the computer as each component leaves the column (computer produces a chromatogram in which each component is a peak plotted against the travel time).

Question 15

Advantages of TLC over paper

Answer 15

Quicker
More sensitive
Larger variety of stationary phases and solvents to choose

Question 16

Physical characteristics of metals and non metals

Answer 16

Metals : shiny, high melting point, solid at room temp, malleable, ductile and good conductors
Non metals : dull, low melting points, solid/gas, brittle, non-ductile and poor conductors

Question 17

When metal oxides dissolve in water, what is the ph of the solution?

Answer 17

Alkaline

Question 18

When non-metal oxides dissolve in water, what is the ph of the solution?

Answer 18

Acidic solutions

Question 19

How is electronic structure linked to the periodic table?

Answer 19

1. Last number of structure = non IUPAC group number
2. Number of numbers = period number
3. Sum of numbers = atomic number

Question 20

How do ionic compounds form?

Answer 20

When a metal reacts with a non-metal : electrons are transferred from the metal to the non-metal to both create stable electronic structures. Positive metal ions and negative non-metal ions : model the ions in an ionic compound through the dot-and-cross diagram.

Question 21

Structure and bonding of ionic compounds?

Answer 21

1) Giant ionic lattice (regular arrangement)

2) Ionic bonds (act in all directions) - strong electrostatic forces of attraction between oppositely charged ions

Question 22

Space filling models vs ball and stick model

Answer 22

Space filling is 2D although the actual bonds are 3D. Ball and stick models to give a clearer view on the structure and shape of the lattice (also 3D) but the bonds are forces rather than physical sticks and ions are close together.

Question 23

What are covalent bonds?

Answer 23

They are shared pairs of electrons which form between two non-metal atoms (complete their outer shells).

Question 24

What are simple molecules?

Answer 24

A molecule that only consists of a few atoms covalently bonded together

Question 25

Structure and bonding in simple molecules

Answer 25

Covalent forces are between the nucleus of each bonded atom and the shared electrons - covalent bonds between atoms in simple molecules are strong but the intermolecular forces are very weak

Question 26

Displayed formula

Answer 26

Advantage : Shows how each atom of each element is bonded in the molecule
Disadvantage : does not show the 3D shape of the molecule

Question 27

Giant covalent structures

Answer 27

Many non-metal atoms bonded together covalently and arranged in a regular repeating manner (giant covalent lattice)

Question 28

Chemical formulae of giant molecules

Answer 28

Use the empirical formula of those substances (due to how many non-metal atoms there are present in the molecule). Formula for diamond : C

Question 29

Polymers

Answer 29

Made up of several smaller molecules called monomers - monomers are simple molecules (consist of a few non-metal atoms covalently bonded together)

Question 30

Models of polymers

Answer 30

Thermosofteneing - each polymer (wavy line) are tangled together - easy to separate
Thermosetting - each polymer is joining together by strong covalent bonds called cross linking
Repeating units - section of the polymer that repeat themselves “n” number of times

Question 31

Structure of metals

Answer 31

All in the solid state at room temperature (except for mercury). Packed together in a regular way, forming a giant metallic lattice

Question 32

What are metallic bonds like?

Answer 32

In metals themselves - the atoms form positively charged ions and also leave the outer shells of their electrons - forming a sea of delocalised electrons. The attraction between the positively charged metal ions and the delocalised electrons are metallic bonds. They are the strong electrostatic forces of attraction between the delocalised electrons and the closely packed positively charged metal ions. Electrons are free to move therefore they are good conductors of electricity.