chem 4/5/6

• Describe how a student can obtain pure dry crystals from salt solution

  Heat solution

  Evaporate some water

  Leave/ cool ( the solution to crystallise

  Pour off excess liquid or filter ( to obtain crystal)

  Place in oven/pat w paper towel
• Define Isomers


  isomers have the same molecular formula but different structural formula
• Ethane and ethene both react with bromine

  Describe differences

  Ethane needs UV light to react

  Ethane produces bromoethane

  Reaction w ethene involves breaking Carbon carbon double bond

  Ethane reaction is substitution

  Ethene is addition

  Ethene produces dibromoethane
• What is an isotope?

  Atoms of the same element that have different numbers of neutrons
• Discuss differences between diamond and graphite(6 marks)

  1-Diamond has a tetrahedral structure.

  2-Graphite has a hexagonal structure.

  3-Diamond does not conduct electricity graphite does.

  4-Diamond has no delocalised electrons graphite does.

  5.In graphite the layers can slide over each other.In diamond strong carbon carbon bonds need to be broken

  6-Diamond is hard/graphite is soft
• Explain why c60 fullerene has lower melting point than diamond and graphite

  1- C60 fullerene has weak intermolecular forces between molecules

  2-Little energy to overcome forces

  3- diamond and graphite have strong covalent bonds between atoms

  4 large amount of energy to overcome forces
• Physical test for pure water

  Melts at 0 ºC and boils at 100 ºC
• Discuss advantages and disadvantages of using ammonia and ammonium sulfate as fertilisers

  Nh3 higher percentage of nitrogen

  Nh3 difficult for plants to absorb/ has to be dissolved in water first

  Nh3 will cause soil to become alkaline

  NH3 and Nh4SO4 souble so runoff /wastage of fertiliser
• Explain different ways in which magnesium and magnesium chloride conduct Electricity

  Magnesium has delocalised electrons

  Electrons can move

  Ions are free to move when magnesium chloride is molten or dissolved in solution
• Discuss environmental problems of disposing polymers

  Polymers remain in landfill indefinitely

  They are inert

  Burning produces greenhouse gases and toxic gases

  Which increases climate change /global warming
• Why does lithium oxide have high melting point

  Strong electrostatic attraction between oppositely charged ions

  Which require a lot of energy to overcome
• Definition of covalent bond

  Electrostatic attraction between nuclei and shared pair of electrons
• As you go down group 7

  boiling point increases

  Intermolecular forces get stronger

  As number of electrons /electron shells increase

  So more energy needed to separate the molecules/break intermolecular forces
• Why is graphite soft and conducts electricity 5marks

  Structure in layers

  Weak forces between layers of atoms

  Which can slide over each other

  Carbon makes 3 covalent bonds

  One delocalised electron per atom

  Delocalised electrons move [conduct e]
• Describe how student can obtain dry hydrated crystals from magnesium nitrate

  Fliter off excess magnesium

  Heat solution (crystals start to form)

  Leave solution to cool

  Pour off excess water

  Leave crystal to dry
• Flame tests - Lithium Ions

  Li⁺ = crimson flame
• Flame tests - Calcium Ions

  Ca²⁺ = orange-red flame
• Flame tests - Sodium Ions

  Na⁺ = canary yellow flame
• Flame tests - Potassium Ions

  K⁺ = lilac flame
• Flame tests - Copper ions

  Cu²⁺ = blue-green flame
• Copper with sodium hydroxide

  Blue precipitate
• Iron2+with sodium hydroxide


  Green
• Iron 3+ and sodium hydroxide


  Red-brown
• Ammonia gas


  Turns damp red litmus paper blue
• Chlorine gas test


  Bleaches damp blue litmus paper white
• Carbonates test


  hcl + carbonate give off co2 and cloudy limewater
• Sulfate test


  add HCL and barium chloride solution, white ppt of barium sulphate forms.
• Chloride test


  acidify with dilute nitric acid, then add aqueous silver nitrate forms white precipitate of sodium chloride
• Bromide test


  acidify with dilute nitric acid, then add aqueous silver nitrate forms cream precipitate of sliver bromide
• Iodide test


  acidify with dilute nitric acid, then add aqueous silver nitrate forms yellow precipitate of slive iodide
• Nitrates are


  soluble
• Common chlorides


  Soluble (except silver chloride and lead chloride)
• Common sulfates


  Soluble (except lead, barium and calcium sulfate)
• Common carbonates and hydroxides

  Insoluble (except for sodium, potassium and ammonium ones)
• Test for ammonium ions

  ~ Ammonia gas is alkaline so use a damp piece of red litmus paper

  ~ Add a few drops of aqueous sodium hydroxide and warm the mixture

  ~ Litmus paper turns blue then there is ammonia gas produced
• Describe how crude oil is separated into fractions

  Produces smaller chain molecules

  Smaller chain molecules have more demand

  Smaller chain molecules burn more cleanly = used to make petrol

  Crude oil has surplus of long chain molecules

  Produces alkenes

  Alkenes used to make alcohol/ polymers
• Flame test


  Clean platinum wire using HCI

  dip into substance you want to test
• describe how fractional distillation produces fuel oil from crude oil

  -heat crude oil

  -gas rises to top of the column

  -column hotter at the bottom/cooler at the top

  -fractions condense when the temperature is lower than boiling point

  -heavier fractions are collected near bottom
• student want to obtain dry sample of soluble salt sodium sulfate from dilute sulfuric acid (5marks)

  -Add (25cm3)Na OH to pipette(to transfer to conical flask)

  -place acid in Burette

  add a named indicator to the flask

  acid acid until the indicator changes colour
• student wants to obtain dry sample of insoluble salt barium sulfate from dilute sulfuric acid

  barium nitrate or (any soluble salt with barium)

  mix together

  filter

  wash barium sulfate with deionised water

  pat with paper towels/leave in warm oven