C2 - Elements, compounds and mixtures Flashcards
Relative atomic mass
The mean mass of an atom of an element compared to 1/12 the mass of a Carbon-12 atom
Relative formula mass
The mean mass of a unit of a substance compared to 1/12 the mass of a Carbon-12 atom
Empirical formula (of butane)
Shows the simplest whole-number ratio of the atoms of each element in a compound : C2H5
What does pure mean?
A pure substance is one which consist of just one element or compound
What are mixtures?
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
Melting points of pure and impure substances
Pure substance : single temperature
Impure substance : melting point is less than that of the pure substance and often melts over a range of values
How to determine melting point?
1) Melt it slowly
2) Stir the substance
Produces accurate results (evenly distributed and temperature of WHOLE sample to rise)
What happens when a substance dissolves?
The solute is the substance that dissolves in the solvent to form a solution.
Filtration
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.
Crystallisation
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.
How does distillation (simple and fractional) work?
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)
Chromatography phases
Stationary phase : does not move
Mobile phase : does move
Paper chromatography and TLC (stationary and mobile phases)
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
Gas chromatography
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).
Advantages of TLC over paper
Quicker
More sensitive
Larger variety of stationary phases and solvents to choose
Physical characteristics of metals and non metals
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
When metal oxides dissolve in water, what is the ph of the solution?
Alkaline
When non-metal oxides dissolve in water, what is the ph of the solution?
Acidic solutions
How is electronic structure linked to the periodic table?
- Last number of structure = non IUPAC group number
- Number of numbers = period number
- Sum of numbers = atomic number
How do ionic compounds form?
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.
Structure and bonding of ionic compounds?
1) Giant ionic lattice (regular arrangement)
2) Ionic bonds (act in all directions) - strong electrostatic forces of attraction between oppositely charged ions
Space filling models vs ball and stick model
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.
What are covalent bonds?
They are shared pairs of electrons which form between two non-metal atoms (complete their outer shells).
What are simple molecules?
A molecule that only consists of a few atoms covalently bonded together
Structure and bonding in simple molecules
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
Displayed formula
Advantage : Shows how each atom of each element is bonded in the molecule
Disadvantage : does not show the 3D shape of the molecule
Giant covalent structures
Many non-metal atoms bonded together covalently and arranged in a regular repeating manner (giant covalent lattice)
Chemical formulae of giant molecules
Use the empirical formula of those substances (due to how many non-metal atoms there are present in the molecule). Formula for diamond : C
Polymers
Made up of several smaller molecules called monomers - monomers are simple molecules (consist of a few non-metal atoms covalently bonded together)
Models of polymers
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 ānā number of times
Structure of metals
All in the solid state at room temperature (except for mercury). Packed together in a regular way, forming a giant metallic lattice
What are metallic bonds like?
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.
