inorganic chem; group 7 Flashcards
Characteristics of group 7 elements
Fluorine — pale yellow gas
Chlorine — pale green gas
Bromine — brown/orange liquid
Iodine — grey solid
What is the boiling point trend for Group 7?
Increases down the group
Explain the boiling point trend in Group 7?
Increasing size & relative mass — VDWs also increase
What is the physical state of Group 7?
Gas at the top to solid at the bottom
Define electronegativity
It’s the ability for an atom to attract electrons towards itself in a covalent bond
What is the electronegativity trend in Group 7?
Decreases down the group
Explain the electronegativity trend in Group 7?
Atoms are larger
Distance between positive nucleus & e- increases
More shielding
More reactive halogens will … less reactive halide ions
Displace
Displacement reactions of halogens
Oxidation trend in Group 7 & how is it shown?
Halogens are less oxidising as we go down the group — shown by reactions of halogens with halide ions
In what condition would a halogen displace a halide in solution?
If the halide is lower in the periodic table
How is bleach made?
By mixing chlorine & sodium hydroxide, forming a sodium chlorate (I) solution — disproportionation reaction
Uses of sodium chlorate
- Treating water
- Bleaching paper & fabrics
- Cleaning agents (bleach)
Define disproportionation reaction
A disproportionation reaction is one in which a single substance is both oxidised and reduced
Examples of a disproportionation reaction & its equation
Making bleach:
2NaOH (aq) + Cl2 (g) → NaClO (aq) + NaCl (aq) + H2O (l)
Water sterilisation:
H2O (l) + Cl2 (g) → 2H⁺ (aq) + Cl⁻ (aq) + ClO⁻ (aq)
How is water sterilised?
By adding chlorine — produces chlorate (I) ions (ClO-) which kills bacteria
What decomposes chlorinated water & write its question
Sunlight
2H2O (l) + Cl2 (g) → 4H⁺ (aq) + 2Cl⁻ (aq) + O2 (g)
Advantages of chlorinating drinking water
- Destroys microorganisms that cause disease
- Long lasting — reduces bacteria build up further down supply
- Reduces growth of algae
Disadvantages of chlorinating drinking water
- Chlorine gas is toxic — irritates the respiratory system
- Liquid chlorine causes severe chemical burns to the skin
- Chlorine can react with organic compounds, forming chloroalkanes — linked with cancer
What are halides in terms of oxidation & reduction?
Reducing agents — as halide ions lose e- in reactions
Why is I⁻ a more powerful reducing agent than F⁻?
As its outer e- is lost more readily due to the ionic radius increase down the group
What tests prove that halides are reducing agents?
- Reaction with sulfuric acid
- Reaction with silver nitrate solution
Process of testing halide ions with silver nitrate solution
- Add dilute nitric acid (HNO3) then silver nitrate solution (AgNO3)
- Colour of precipitate tells you the halide ion
What is the colour of the propitiates formed by halides reacting with silver nitrate solution?
Cl ions — white
I ions — yellow
Br ions — cream
Why is nitric acid added when testing for halides using silver nitrate?
So it reacts with any anions other than halides (e.g. carbonates) — can give a false result
What is the further test of halide ions with silver nitrate?
Add ammonia (NH3) solution to precipitates:
- Cl — white ppt dissolves in dilute NH3
- Br — cream ppt dissolves in conc NH3
- I — yellow ppt insoluble in conc NH3
Test for Group 2 ions (cations)
- Dip the nichrome wire in conc hydrochloric acid
- Dip into sample
- Place the loop into the blue Bunsen flame & observe the colour
What is the colour shown by Group 2 ions in the flame test?
Calcium (Ca2+) — dark red
Strontium (Sr2+) — red
Barium (Ba2+) — green
Why are flame tests used & how?
Used to identify Group 2 ions in a solid sample
Can be made into solutions & sprayed on the Bunsen too
Disadvantage of using flame tests
Difficult to use if sample is insoluble
What ions are tested with litmus paper?
Ammonium compounds & hydroxides
How are ammonium compounds tested?
- Add NaOH & gently heat
- +ve result: ammonia gas produced
OR
- Use damp red litmus
- Ammonia will dissolve in water & litmus turns blue
How are hydroxides tested?
Are alkaline — so red litmus paper turns blue
Further test needed — as red litmus turns blue for any alkali
What is used to test carbonates & sulfates?
Hydrochloric acid & barium chloride
Process of testing for carbonates
HCl reacts with carbonates to make CO2 gas
When bubbled through limewater it turns cloudy
Process of testing for sulfates
- Add HCl to remove any carbonates — can precipitate out giving false result
- Add barium chloride
- +ve result: white ppt
What is the specific order of tests to prevent false positives?
- Test for carbonates
- Test for sulfates
- Test for halides
