C2.5 Synthesis
Chemistry Revision
Alkanes and Alkenes
Alkanes
 Chains of carbon atoms with single covalent bonds between ...
C2.5 Synthesis
Chemistry Revision
Cracking hydrocarbons
Cracking
 Splitting up long-chain hydrocarbons into shorter molec...
C2.5 Synthesis
Chemistry Revision
Plastics
Polymerisation
Plastics are long-chain molecules called polymers. Lots of small...
C2.5 Synthesis
Chemistry Revision
Relative Formula Mass
Relative Atomic Mass (Ar)
 A way of saying how “heavy” different ...
C2.5 Synthesis
Empirical formulae and Atom Economy
Empirical Formulae
1) List all the elements in the compound
2) Write th...
C2.5 Synthesis
Chemistry Revision
Theoretical and Percentage yield
Theoretical Yield
1) Write out the balanced equation
2)...
C2.5 Synthesis
Ionic Bonding
 Atoms lose or gain electrons to form charged particles
 These charged particles are called...
C2.5 Synthesis
Chemistry Revision
Ionic compounds
Ionic Compounds
The sodiumatom givesupitsouterelectron,becomingaNa+
(sod...
C2.5 Synthesis
Chemistry Revision
Covalent bonding
Covalent bonds = sharing electrons
Atoms share electrons to make sure t...
C2.5 Synthesis
Chemistry Revision
carbon
Covalent bonds in Carbon
Graphite and Diamond are giant molecular covalent struct...
C2.5 Synthesis
Chemistry Revision
ammonia
The Haber Process
Industrial conditions:
Pressure: 200 atmospheres
Temperature: ...
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Chemistry Revision C2.5 Synthesis

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Transcript of "Chemistry Revision C2.5 Synthesis"

  1. 1. C2.5 Synthesis Chemistry Revision Alkanes and Alkenes Alkanes  Chains of carbon atoms with single covalent bonds between them.  Saturated hydrocarbons because they have NO spare bonds.  They do NOT decolourise bromine water  They do NOT form polymers CH₄ C₂H₆ C₃H₈ C₄H₁₀ Alkenes  Chains of carbon atoms with double bonds between them.  Unsaturated hydrocarbons because they have SPARE bonds.  They DO decolourise bromine water by forming bonds with bromine atoms  They DO form polymers by opening up their double bonds to join up in a long chain. Ethene Propene C₂H₄ C₃H₆ Alcohols 1) Alkenes react with water to make alcohols in a process called hydration. 2) Ethene, made by cracking crude oil, reacts with steam at a high pressure. 3) Phosphoric acid is used as a catalyst. 4) The product is ethanol.
  2. 2. C2.5 Synthesis Chemistry Revision Cracking hydrocarbons Cracking  Splitting up long-chain hydrocarbons into shorter molecules.  A form of thermal decomposition i.e. it breaks down substances into simpler molecules by heating.  Cracking produces alkanes which are used to make plastics. Conditions for cracking  Heat Cracking in the laboratory  Catalyst Heated paraffin vapour cracks as it passes over the heated porcelain (catalyst). Small alkanes collect at the end of the boiling tube. Alkenes collect through water in the glass jar. Long-chain alkanemolecule Alkane + Alkenes
  3. 3. C2.5 Synthesis Chemistry Revision Plastics Polymerisation Plastics are long-chain molecules called polymers. Lots of small molecules called monomers join together to give a polymer. 1) Monomers in polymerisation have a carbon double bond i.e. they are unsaturated. 2) Under high pressure and using a catalyst, the double bonds open up and “join hands”/polymerise to form very long saturated chains. More Plasticky Stuff Strong covalent bonds hold the atoms together in long chains, but bonds between different chains determine the properties of the plastic. Thermoplastic polymers:  Very weak bonds between chains – can slide over each other  Low melting point  Mouldable – can melt and cool into new shapes many times over Thermosetting polymers:  Stronger bonds between chains (crosslinks) – holds firmly together  Doesn’t soften when heated, but will burn  Strong, hard and rigid Additives are used to affect the properties of polymers. Plasticisers are added to polymers to make a more flexible plastic. Preservatives can be added to prolong useful life and appearance. 1 2
  4. 4. C2.5 Synthesis Chemistry Revision Relative Formula Mass Relative Atomic Mass (Ar)  A way of saying how “heavy” different atoms are compared with the mass of an atom of carbon-12.  The relative atomic mass is the mass number of the element to the nearest whole number.  That’s the bigger one btw Relative Formula Mass (Mr)  All the relative atomic masses added together when you have a compound.  That’s how easy it is. I don’t even need a second bullet point. Can’t believe we spent so long learning this, and then the revision guide summed it up on half a page. Damn school and its excess-teaching. MgCl₂ 24 + (35.5 x 2) = 95 C₂H₄(OH)₂ (12 x 2) + (1 x 4) + [(16+1) x 2] = 62 Chemistry Revision
  5. 5. C2.5 Synthesis Empirical formulae and Atom Economy Empirical Formulae 1) List all the elements in the compound 2) Write their experimental masses (g, kg, t, %, etc.) 3) Divide each mass or percentage by the relative atomic mass for that element 4) Change the numbers into a ratio (may need to round to nearest integer) and simplify Fe O Experimental masses 44.8 19.2 Divide by Ar 0.8 (44.8 ÷ 56) 1.2 (19.2 ÷ 16) Change to ratio 8 12 Simplify 2 3 = Fe₂O₃ Atom Economy = % changed to useful products E.g. Calculate atom economy of: CH4 (g) + H2O (g) → 3H2 (g) CH4 + H2O → 3H2 12 + (4 x 1) (2 x 1) + 16 3 x (2 x 1) 34 6 6 ÷ 34 x 100 = 17.6% useful products High atom economy is important for conserving resources, maximising profits and maintaining the environment. Raw materials, which are expensive, will run out and waste has to be disposed of. We try to find a use for the waste products so they are useful by-products rather than useless ones.
  6. 6. C2.5 Synthesis Chemistry Revision Theoretical and Percentage yield Theoretical Yield 1) Write out the balanced equation 2) Work out Mr just for the two bits you want 3) Divide to get the ratio, then multiply to answer the question E.g. What mass of magnesium oxide is produced when 60g of magnesium is burned in the air? Balanced equation: 2Mg + O₂ → 2MgO Relative formula masses: 2 x 24 → 2 x (24 + 16) (no need to do oxygen) 48 80 Divide and multiply: 48g of Mg..................reacts to give.....................80g of MgO (÷48 then x60) 1g of Mg.................... reacts to give.......................1.67g of MgO 60g of Mg.....................reacts to give....................100g of MgO Answer: 60g of magnesium will produce 100g of magnesium oxide. Percentage Yield = overall success; compares theoretical yield and actual yield  Percentage yield is ALWAYS between 0 and 100%  In real life, you NEVER get 100% yield  Products or reactants get lost along the way:  Reversible reactions i.e. not all reactants change into product  Filtration loses liquid/solid in the filter paper  Transferring liquids from one container to another  Unexpected reactions so yield of intended product goes down Chemistry Revision Ionic bonding
  7. 7. C2.5 Synthesis Ionic Bonding  Atoms lose or gain electrons to form charged particles  These charged particles are called ions  Ions are strongly attracted to each other because of opposite charges (+ and -)  Atomswantto have stable outershells,sotheywill lose orgainelectronsinordertohave full shells. Electrolysis o Group 1 & 2 elements (metals) are mostlikelyto loseelectronstoform positive ions (cations). o Group 6 & 7 elements (non-metals) are mostlikelyto gain electronstoform negative ions (anions). Group 1 Group 2 Group 6 Group 7 Metals Non-metals Lose electrons Gain electrons Form positive ions (cations) Form negative ions (anions) Li+ , Na+ , K+ Be2+ , Mg2 , Ca2+ O2- F- , Cl-  Cations (+ve) are attractedto the cathode (-ve)  metal isproducedhere  Anions (-ve) are attractedtothe anode (+ve)  gas is producedhere Whenany of these cationsreact withanions,theyform ionicbonds.Onlyelementsatoppositesidesof the periodic table will formionicbonds,e.g.Na+ andCl- .
  8. 8. C2.5 Synthesis Chemistry Revision Ionic compounds Ionic Compounds The sodiumatom givesupitsouterelectron,becomingaNa+ (sodium) ion.The chlorine atompicksupthe electron, becomingCl- (chloride) ion.Heypresto, sodiumchloride. Giant Ionic Structures  Ionicbondsproduce giant ionicstructures  The ionsform a closelypackedregular lattice arrangement o Theyhave highmelting/boilingpoints due tovery strongionicbonds betweenall ions o They conduct electricity when:  Dissolved:separatedionsare free tomove/carryelectriccurrent  Molten:ionsare free to move/carryelectriccurrent
  9. 9. C2.5 Synthesis Chemistry Revision Covalent bonding Covalent bonds = sharing electrons Atoms share electrons to make sure they both have full outer shells Hydrogen atoms share their one electron Two pairs of electrons shared in a H20 molecule to form single covalent bonds to fill their outer shells. This is a hydrogen molecule. Oxygen atoms share two electrons to form covalent bonds with carbon atoms and form a carbon dioxide. Simple molecular substances Although atoms form very strong covalent bonds to form molecules, the forces of attraction between the molecules (inter-molecular forces) are very weak. This means molecular substances have low melting/boiling points which mean they are liquids or gases at room temperature. There are no ions to conduct electricity.
  10. 10. C2.5 Synthesis Chemistry Revision carbon Covalent bonds in Carbon Graphite and Diamond are giant molecular covalent structures. Fullerenes Carbon can also form ‘nanoparticle’ molecules called fullerenes. Buckminster fullerene was discovered by chance. Fullerenes can be joined together to form nanotubes. They conduct electricity, are very strong and very light. Examples of uses of nanotubes include:  Electrical wires/microchips  Body armour  Sports equipment  Turbine blades Graphite Diamond 3 covalent bonds 4 covalent bonds Layers are held loosely and slide over each other Very rigid structure= very hard Strong covalent bonds = high melting/boiling points Can conductelectricity: only 3 out of 4 of each carbon’s four outer electrons are used in bonds so there are free electrons Cannot conductelectricity because no free electrons
  11. 11. C2.5 Synthesis Chemistry Revision ammonia The Haber Process Industrial conditions: Pressure: 200 atmospheres Temperature: 450°c Catalyst: Iron Dynamic Equilibrium The Haber Process is reversible, which means it occurs in both directions. Not all of the nitrogen and hydrogen will convert to ammonia. The reaction reaches a dynamic equilibrium. 1) Increasing pressure favours the forward reaction (less molecules) and increases percentage yield. a. Too high would make the plant too expensive to build, so 200atm is a compromise. 2) Increasing temperature favours the forward reaction, except because it’s exothermic, it’s actually moving the equilibrium the wrong way (away from the end product; ammonia). a. So there would be greater yield at lower temperatures. b. However, lower temperatures mean slower rate of reaction. 3) Therefore the industrial conditions of the Haber Process are the best compromise between maximum yield and speed of reaction. 4) The iron catalyst makes the reaction go faster, but doesn’t affect percentage yield. Ammonia The ammonia is formed as a gas and is removed. Unused hydrogen and nitrogen are recycled. Ammonia is used to make ammonium nitrate fertiliser, which is extremely peng stuff but can do bad things too. N2 (g) + 3H2 (g) 2NH3 (g) + (heat)

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