Chemistry (C2- Elements, Compounds and mixtures)

Chemistry (C2- Elements, Compounds and mixtures)

1. Relative formula mass

   • Relative masses- there mass compared to the mass of a proton
     • Relative atomic mass (Ar) is the mean mass of an atom of an element compared to 1/12 the massof a carbon-12 atom
       • An elements relative atomic mass is shown by the elements biggest number on the periodic table
         • Chemical formular tells you how many of each atom is in a unit of a substance
           • Relative formula mass (Mr)- the mean mass of a unit of a substance compared to 1/12 the mass of a Carbon 12 atom
             • To work it out you add the Ar values for each formular together
               • On paper
               • Chemical formula tells you how many of each atoms there are
             • The big number of all the bits in it added together
       • It is the big number on the periodic table

2. Empirical formula

   • It shows the simples whole-number ratio of the atoms of each element in a compound
     • Some are already the simpilest they can be but some can simplify futher
       • E.g. C6H12 would become C1H2
         • On paper

3. Pure and impure substances

   • CGP-26
   • Pure substances only contain one substance
     • If a substance isn't pure it is a mixture
       • Mixtures can be useful too e.g. alloys are a mixture of a metal and another element it can make them better for certain things
         • You can test purity of a substance using melting and boiling point
           • If a substance is impure the melting point will be too low
             • The boiling point will be too high if a substance is impure

4. Filtration and crystallisation

   • It is used to seperate a solution (liquid and a solid)
     • Filteration seperates an insoluble substance in a solid state from substances in a liquid state. It works becuase the paper has tiny microscopic holes that solids can't get through. The solid stays in the filtration paper as residue while the liquid passes through as filtrate
       • Crystallisation: If you heat a solution the solvent will evaporate leaving the solute behind. If you heat the solution too strongly you will be left with a powder. If you heat it slowly you will be left with a crystal.
         • Takes patience as you need to heat the solution gently until it becomes a saturated solution (when no more solute can be dissolved at that temperature) after you leave it to cooldown, as it cools it forms crystals

5. Distillisation

   • It is used to seperate a mixture that contains liquids, there are two types simple and fractional
     • Simple
       • Used to seperate out a solvent from a solution
         • Relies on the solvent having a much lower boiling point
           • When a solution is heated the solvent boils but the other liquid does not, the solvent escapes from the solution in the form of a gas
             • It is then cooled and condensed by a condenser (cold water is flowing through it constantly)
               • Used in the labotary if you wan to purify a solvent
                 • The problem is you can only seperate things with very different boiling points
     • Fractional
       • It seperates two or more substances from a mixture in a liquid state
         • Each substance seperated ny fractional distilation is called a fraction (just part of original mixture)
           • How to do it...
             • 1. Put mixture in flask and attatch fractionating column and condenser
               • 2.Gradually heat the flask, differnt liquids have different boiling points so will evapoate at different temperatures
                 • 3.Liquid with lowest boiling pint will evaporate first, when thermometer temperate reaches boilng temperature the sunstance will reach top of column
                   • 4.Other liquids might start to evaporate but it is colder at the top so they will condense before they reach it and run back into the flask
                     • 5. When the first liquid has been collected raise it up untill you reach the next one
                       • It works becuase bigger molecules have stronger intermolecular forces/ bonds. This means more energy is needed to break the bonds, so they boil at a higher temperature (mainly talking about hydrocarbons)
     • Voletile: evaporates easily
       • Viscous: a liquid which does not flow easily

6. Chromatography

   • It relies on two different chemical phases: a stationary phase that does not move and a mobile phase that does move
     • A phase is a substance in the solid, liquid or gas state
       • In paper chromatography (e.g. used to sperate ink) the stationary phase is the absorbent paper, the mobile phase is the solvent (in liquid state)
         • Thin layer chromatography works the same way but the stationary phase is the thin layer of silica or alumina powder spread over the plate of glass/ plastic
           • Scientists use Rf values to compare the spots on a chromatogram, if two spots have the same Rf value and are the same colour they are likely to be identical- can be used to see if a pure substance is present in a mixture
             • The result of chromatography is called a chromatogram
               • An Rf value is the ratio between the distance traveled by the dissolved substance (solute) and the distance traveled by the solute
                 • Rf = distance traveled by substance (the top of it)/ distance traveled by solvent
                   • Should get a number 0-1
           • 1. Put the solvent into a chromatography tank (1cm depth)- flamable= no naked flames
             • 2. Add a small amount of the sample to the base line, don't damage powder
               • 3. Let solven travel through powder and take out before it reaches to the top
                 • 4.Analyse the patterns of coloured spots- called a chromatogram
                   • Advantages of TLC over paper chromatography: TLC is not floppy or flexible so it goes up in a straight line (it is ridig/ easy to control), In TLC you can recover (collect) the substance after carryign out the chromatography
           • Advantages...
             • Fairly quick
               • More sensitive than paper chromatography so smaller samples can be sued
                 • Samples can be scraped off for futher analysis
   • Gas chromatography
     • The stationary phase is a silica or alumina
       • The mobile stage is the carrier gas (usally an inert gas e.g. nitrogen)
         • It seperates components of a mixture an measure the amount of them
           • Sample turned into a gas when it is injected into the coloumn. gas carrier pushes it through, differnet gasses take different amounts of time to travel through the colomn (depending how strongly they are bonded, de3tecter tell computer as each component leave colomn- produces a chromastogram, each component is a peak plotted against the travel time
             • Used to analyse an unkown substance, one peak per chemical
               • The retention time is differne tfor each chemical (used to identify it), the distace along the x-axis that the peak happens is the retention time - can be looked up to see what chemical it is
                 • Relative areas undner peak show relative amounts of chemical in sample
                   • Advantages
                     • Does not change substance
                       • Can identify all substances in a mixture
                         • It is quick
                           • They are reliable and accurate
                   • Disadvantages
                     • Expert training needed
                       • They are expensive
                         • Machnie is very big and heavy , also it needs an electricity supply

7. Purity

   • Pure substance consists of just one element or compound. Impure contain more than one.
     • To get pure substances can be very difficult as they can get easily contaminate (e.g. by the air)
       • Mixtures (impure substances) can be useful e.g. alloys (mixture of metals)
         • You can determine if something is pure using its melting point ( the temperature at which it changes from a solid to a liquid). The melting point of a substance is a single temperature. If a substance is impure: its melting point is less than that of the pure substance (the grater the difference, the bigger the impurity), It often melts over a range of temperatures
           • To determine the melting point: melt the substance, take the temperature throughout, plot a graph, it changes substance when the temperature dosen't change

8. Metals and non-metals

   • Metals and non-metals have different physical properties (a characteristic that can be observed/ measures) and chemical properties (determined by studying chemical reactions)
     • Metals
       • Shiny apperance
         • Usally high melting and boiling points
           • Soild state at room temperature
             • Malleable (bend without shattering)
               • Ductile (can be pulled into wired)
                 • Good conductors of heat and electricity
                   • High density
                     • Loss electrons to form positive ions
                       • Do not react with each other, they mix to form alloys
                         • When they dissolve in water they produce slkaline solutions
     • Non-metals
       • Dull apperance
         • Usally low melting and boiling points
           • About half are solids at room temperature and half are gasses
             • Brittle (they shatter when hammered)
               • Non-ductile they snap when pulled
                 • Poor conductors of thermal and heat energy (they are insulators)
                   • Gain electrons to form negative ions
                     • React with each other to produce compounds
                       • When they dissolve in water non-metal oxides produce acidic solutions
     • Periodic table arranged in rows and columns in order of increasing atomic number
       • Metals are placed on left-hand side and non-metals on the right, can be seperated by zig-zag boundary
         • Some elements have properties of metals and non-metals
     • Metals and non-metals react with oxygen to form oxides

9. Electronic structures

   • The periodic table is arranged into periods (horizontal rows) and groups (verical column)
     • The outermost occupied shell of an atom is called the outer shell
       • The group number tells you how many electrons it has in its outer shell (group 0 has a full outer shell)
         • The number of numbers, electron shells (e.g.2,8,8,5) equals the period number
           • The sum of the numbers (amount of electrons) equals the atomic number
             • Atomic number = small number, Mass number= big, number of protons= small, number of neutrons= big-small, number of electrons= small

10. Forming ions

    • An ion is an electrically charged partical, formed when an atom loses or gaina an electron (it can be a single atom or groups of atoms)
      • A metal atom loses an electron to form a positive ion
        • Non-metal atoms gains an electron to form a negative ion
          • The number of protons and neurons do not change in an ion
            • e.g. Mg becomes Mg2+ and O becomes O-
              • Ions have full outer shells (can work out the electronic structure from that)
                • You can draw a diagram to represent this, an ion has brackets around the electronic structure diagram and the the + or - sign
                  • Group 1&2 and 6&7 are most likely to form ions
                    • As you go down the group the outer electron shell gets futher away from the nucleus
                      • For group 1 and 2 (metals) that means it is easier to remove the outer electron to form an ion, so elements get more reactive as you go down the group
                        • For group 6 and 7 it means it get harder for the nucleus to attract another electron so as you go down the group it gets less reactive

11. Ionic compounds/ bonds/ giant ionic compounds

    • Take electrons
      • Ionic bonding is the attraction between oppositly charged ions (formed by electron transfer)- starts as electrostatic forces then becomes an ionic compound
        • These strong attractive forces act in all directions in the lattice
          • Occurs between metals and non-metals
            • It is when two atoms take and give electrons to each other in order to gain a full outer shell
              • To work out the compounds for an ionic compound you need to make sure both charges are equal e.g. there are the same number of + as -
                • You can represent this with a dot and cross diagram
                  • Always forms a giant ionic strucutre (type of crystal)
                    • Ionic compounds in the solid solid contain positive and negative ions arranged in a regular way- a giant ionic lattice (gaint= arrangement repeated many times, ionic= conatins ions, lattice= regular pattern)
                      • The ions are held in place by ionic bonds (act in all directions)
                        • Strong electrostatic forces between oppositely charged ions
                          • Use a ball-and-stick model to show it
                            • Ionic compounds have...
                              • High melting and boiling points (many strong bonds)
                                • They are hard and brittle as ions cannot slid over each other
                                  • Does not conduct electricity when solid. It does conduct electricity when melted or dissolved in water. Because ions are fixed in position, in a liquid the ions are free to move and carry a current
                                    • Examples: sodium chloride (NaCl), magnesium oxide (MgO), calcium chloride (CaCl2)

12. Simple molecules

    • Covalent bonding: sharing is caring, a shared pair of electrons, they can be single, doubble or tripple covalent bonds
      • Occur between two or more non-metals
        • By sharing electrons they complete their outer shells
          • They can be modelled with a dot-and-cross diagram (one atoms electrons shown as dots, the other shown as dots)
            • Simple molecules
              • Hydrogen, Oxygen (diatomic elements), Water and Carbon dioxide all exist as simple molecules, all simple molecules only contain a few atoms
                • Covalent bonds involve electrostatic forces of attraction, the forces are between the necleus of the atoms
                  • The covalent bonds between atoms in a simple molecule are strong but the intermolecular forces between molecules are weak
                    • So they have low boiling and melting points
                      • Can be demonstrated with ball-and-stick models or displayed formular
                        • Group number + number of covalent bonds= number of electrons in outer shell
                          • Most are gasses or liquids at room temperature
                            • Don't conduct electricity because they don't have any fee electrons or ions

13. Polymer molecules

    • CGP-23

14. Strucutre of metals

15. Developing the periodic table

    • CGP-16

16. Atomic strucute and the periodic table

17. Carbon

18. Changing states

19. Bulk properties of materials

    • CGP-23

20. Nano particles (GCP-22)

21. States, structures, bonding (CGP-25)

22. Key words

    • Solution: a mixture of a solute and a solvent
      • Solvent:The liquid which does the dissolving
        • Solvent:When a solid breaks up into tiny particles and mixes with the solvent particles
          • Insoluble: something which does not dissolve
            • Soluble: something which dissolves
              • Solute: The solid which is dissolved
                • Pure: a substance which consists of only one element or compound
                  • Impure: amixture of substance which contains more than one element or compound
                    • Filtration: A way of seperating insoluble solids from a liquid
                      • Distillation: Evaporating a solvent followed by condensing it and collecting it

23. Giant covalent structures