Group 7
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Suggest, with reasons, the order of melting points for Br₂, SrCl₂ & ICl.
1) SrCl₂ - strong ionic bonds 2) ICl - dipole-dipole between molecules – weaker than ionic bonds but stronger than VDW 3) Br₂ - VDW forces between molecules – much weaker
Write an equation for the reaction of chlorine with cold water.
Cl₂ + H₂O ⇌ HCl + HClO
Suggest three disadvantages of treating water with chlorine.
• wasteful as most potable water is not used for drinking • some people suffer eye irritation • can react with organic compounds to produce harmful substances
Explain why acid should be added before barium chloride when testing for sulphate ions.
• to remove any carbonate ions • carbonate ions react with barium chloride to form barium carbonate • which precipitates to give a false positive test for sulphate ions
State why sulfuric acid should not be used to acidify barium chloride.
• sulfuric acid contains sulphate ion • which reacts with barium chloride to form white precipitate
Suggest why magnesium chloride wouldn’t be a suitable test for sulphate ions.
• reacts with sulphate ions to form magnesium sulphate • magnesium sulphate is soluble • will not form a precipitate to give any indication of a positive result
Name the four halogens, their states and colour.
fluorine → yellow gas chlorine → green gas bromine → red/brown liquid iodine → black solid
State and explain the trend in electronegativity down group 7.
→ decreases down the group → shielding increases → nucleus is further away from shared pair of e-
State and explain the trend in boiling points down group 7.
→ increases down the group → molecules get bigger & no. of electrons increases → stronger VDW forces hence requires more energy
State the trend in oxidising ability down group 7.
decreases down the group.
State the trend in reducing ability down group 7.
increases down the group
Write the equation for the reaction between sodium chloride and sulphuric acid. State what this type of reaction is called.
NaCl + H₂SO4 → NaHSO4 + HCl [acid base reaction]
Write two equations for the reaction of sodium bromide with sulphuric acid. State what you would observe in each.
acid base reaction: (steamy fumes) NaBr + H₂SO4 → NaHSO4 + HBr redox reaction: (bromine vapour// brown) 2H+ + 2Br- + H₂SO4 → SO₂ (g) +2H₂O + Br₂
Write an equation for the redox reaction between sodium iodide and sulphuric acid. State what each product is observed as.
8H+ + 8I- + H₂SO4 (l) → H₂S (g) + 4H₂O (l) + 4I₂ (s) → H₂S as smell of rotten eggs → iodine as black solid → sulphur as yellow solid (intermediate step)
Describe how to test for halide ions.
1) dissolve in water to form aqueous solution 2) add acid to eliminate carbonate ions 2) add silver nitrate 3) silver halide is formed 4) AgF - no ppt, AgCl - white, AgBr - cream, AgI - yellow
Explain why solutions need to be acidified when testing for halide ions.
1) solution may contain OH- / CO3- ions which can form AgOH/ Ag₂CO3 2) insoluble therefore form ppt to give false +ive 3) acid is added to remove these ions
Describe a follow up test for halide ions using ammonium solution.
1) add dilute ammonia to each silver halide 2) silver chloride will dissolve to give colourless solution 3) add concentrated ammonia 4) silver bromide will dissolve 5) silver iodide is insoluble in all concentrations of ammonia
State two disadvantages of chlorination.
→ toxic at high concentrations → wasteful (not all water is used for drinking)
Write an equation for the reaction of chlorine with water under sunlight.
2Cl₂ (aq) + 2H₂O (l) → 4HCl (aq) + O₂ (aq)
Write an equation for the reaction of chlorine with cold, dilute sodium hydroxide.
Cl₂ (aq)+ 2NaOH (aq) → NaClO (aq) + NaCl (aq)+ H₂O (l)
State one use of NaClO
household bleach to kill bacteria & remove stains
Give a reason why chlorine is not formed when solid potassium chloride reacts with concentrated sulphuric acid.
chlorine ions are weak reducing agents
Explain why reducing ability increases down group 7.
→ electrons are easier to lose from larger ions due to shielding & larger atomic radius → therefore more easily oxidised