8.2 Halogens Flashcards
Where are the halogens located on the preodic table?
Found in group 7
Most reactive, non metallic group
Halogens are found as stable halide ions, so aren’t found in their elemental form
(Cl-, Br- and I-) dissolved in sea water or found combined with sodium or potassium
What are halogens? What are the trends in BP?
Halogens exist as diatomic molecules at room temprature X2 (2 atoms joined together by a single covalent bond)
In solid state, they form lattice structures with simple molecular structures.
As we go down group 7, the atoms get larger the boiling point and melting increases bcz there are more electrons so greater shielding (each successive element has another shell of electrons) therefore stronger london forces/more induced dipole-dipole interactions as the surface area increases.
More energy required to break the intermolecular/london forces
Volatility decreases down the group.
What is the appearence of fluroine F2, chlorine CL2, bromine Br2, I2 and astatine Ar2 at RTP?
Fluorine is a pale yellow gas that is highly reactive
Chlorine is a pale green gas that is reactive and posinous in high concentrations
Bromine is a red brown liquid that gives off posinous fumes
Iodine is a shiny grey black solid sublimes to purple gas
Astatine has never been seen
Volatility refers to how easily an element can evaporate. Going down group 7, the volatility decreases.
What is the electronic configuration of halogens? Why are halogens considered to be oxidising agents?
It has 7 outer electrons
2 electrons in s subshell and 5 in p subshell
Redox reactions occur w halogens
Halogens gain one electron to form 1- halide ion w the electronic configuration of the nearest noble gas.
Cl2 + 2e- —-> 2Cl- chlorine is reduced
Chlorine is an oxidising agent because they undergo a reduction reaction because another species loses electrons to the halogens, oxidising them
Does the reactivity of halogens increase or decrease down the group?
As we go down group 7, the reactivity decreases. This is because the atomic radii increases (each successive element has an additional shell) so the outer electrons are further away from the nucleus. This means there is an increased shielding effect making it harder for the nucleus to pull an electron. Form -1 ions less easily as it is harder to attract an electron (electronegativity decreases- harder to pull an electron towards itself)
Halogen reactivity decreases down the group due to displacement reactions. If the halogen added is more reactive than the halide present then the halogen displaces the halide from the solution and the solution changes colour.
How can we tell apart solutions of iodine and bromine water?
Organic non polar solvent like cyclohexane can be added and the mixture shaken. Non polar halogens dissolve more easily in cyclohexane than water. In cyclohexane the colours are easier to tell apart.
What is the colors of fluorine, chlorine, bromine and iodine in water?
Yellow Pale green Orange Brown Grey
What are the tests for halides?
1.) Add dilute nitric acid to remove any ions that may interfere with the test/prevent false positive
2.) Add silver nitrate solution (AgNO3 (aq)) and a precipitate is formed
A precipitate is formed of the silver halide
Ag+ + X- ——> AgX (s)
Chloride: white precipitate it dissolves in dilute NH3
Bromide: cream precipitate, dissolve in concentrated NH3
Iodide: yellow precipirate, insoluble in concentrated NH3
Because they can look similar we use ammonia as a follow up test. Dilute concentrated ammonia followed by concentrated ammonia is addded to the silver halide solution to identify the halide ion
Why do halogens get less reactive down the group?
Atomic radius increases
Shielding increases
Less nuclear attraction to pull an electron from another species
Reactivity decreases
What are the benefits and risks of chlorine use?
- kills bacteria/acts as a sterilising agent making drinking water potable therefore reducing the chance of diseases like cholera and typhoid
- it prevents the growth of algae, discolouration and eliminates bad smells and taste.
BUT
-chlorine is an extremely toxic gas, being a respiratory irritant in small concentrations and large comcentrations can be fatal - liquid chlorine on skin and eyes can cause chemical burns
- chlorine in water can react with organic hydrocarbons (from decaying vegitation) to produce chlorinated hydrocarbons, which may be causing cancer (carcinogenic)
What are other ethical considerations/alternates?
We could use
- ozone which is a strong oxidising agent so kills microorganisms but it is expensive/not permenant treatment due to its short half life
- ultraviolet- kills micrororgansims by damaging its DNA but would not stop water being contaminated further down the line.
What is a disporportination reaction?
Same element is oxidised and reduced- a redox reaction
How do we produce bleach?
Chlorine gas with dilute, cold aqueous sodium hydroxide we get sodium chlorate solution NaClO (aq)
2NaOH (aq) + Cl2 (g) —-> NaClO (aq) + NaCl (aq) + H20 (l)
0. +1. -1
Sodium chlorate solution has many uses in water treatment, bleaching paper and textiles.
What happens to halogens ability to oxidise down the group?
- less oxidising agents down the group
Bromine (aq) displaces potassium iodide, giving iodine (I2) and potassium bromine solution (KBr)- aq.
Br2 (aq) + 2KI (aq) ——-> 2KBr (aq) + I2 (aq)
Br2 + 2I- —-> 2Br- + I2
0. -1
-1. 0
What is the reaction between chlorine and water?
Cl2 (g) + h20 (l) —-> hcl (aq) + HClO
0. -1. +1
The aqueous chloric acid reacts with water to produxe chlorate ions
HClO + H20 ——> ClO- (aq) + H3O+ (aq)
