8.2 The Halogens Flashcards
What is the most reactive non metallic group?
Group 17
How are the group 17 elements found in nature?
Stable halide ions dissolved in sea water
Combined with Sodium and Potassium as solid deposits
How do halogens exist at RTP?
Diatomic molecules
Appearance of F2
Pale yellow gas
Appearance of Cl2
Pale green gas
Appearance of Br2
Red brown liquid
Appearance of I2
Shiny grey black solid
Appearance of At2
Never been seen
What happens to boiling point going down group 17?
Boiling point increases
Why does boiling point increase going down group 17?
More electrons
Stronger London forces
More energy required to break the intermolecular forces
Boiling point increases
Outer shell electrons in halogens?
7 electrons
2 in the s sub shell
5 in the p sub shell
Most common reaction for halogens
Redox reactions
What happens to halogens during redox reactions?
Each Halogen atom is reduced, gaining an electron, forming a 1- ion
Acts as an oxidising agent, oxidising another species
What happens to reactivity of halogens going down the group?
Reactivity decreases
What happens during halogen-halide displacement reactions?
A more reactive halogen displaces a less reactive halogen in a compound
Experiment to show reactivity of halogens
Solution of each halogen is added to aqueous solutions of other halides
If halogen added is more reactive than the halide, displacement reaction takes place
Solution changes colour
How can you tell apart bromine and iodine in water?
Organic non polar solvent like cyclohexane can be added and mixture is shaken
Non polar halogens dissolve more readily in cyclohexane than water
Here, the colours are easier to tell apart
Bromine - orange
Iodine - purple
Reactivity of Fluorine
Reacts with almost any substance it comes into contact with
Reactivity of Astatine
Radioactive - decays rapidly
Element has never been seen, but predicted to be most reactive halogen
Explain trend in reactivity going down group 7
Atomic radius increases
More inner shells so electron shielding is increased
Less nuclear attraction to attract an electron from another species
Reactivity decreases going down the group
Which halogen is the strongest oxidising agent?
Fluorine
What is a disproportionation reaction?
A redox reaction where the same element is both oxidised and reduced
Examples of disproportionation reactions
Chlorine and water
Chlorine and cold, dilute Sodium Hydroxide
Why is chlorine used in water treatment?
Reducing prevalence of waterborne diseases by killing harmful bacteria
What happens when chlorine is added to water?
Cl2 + H2O —> HClO + HCl
For each chlorine molecule, one chlorine atom is oxidised, the other reduced
What kills microorganisms in water when chlorine is added to water?
The products Chloric acid and Hydrochloric acid -
Chloric acid also acts as a weak bleach
How to show Chloric acid’s bleaching properties
Add universal indicator
It will turn red, for the presence of acids
Bleaching properties causes colour to disappear
What happens when chlorine reacts with cold, dilute aqueous sodium hydroxide?
Cl2 + 2NaOH —> NaClO + NaCl + H2O
Use of NaClO
Sodium Chlorate
Household bleach
Risk of chlorine
Respiratory irritant and toxic gas in large concentrations
Danger of adding chlorine to drinking water
Can react with organic hydrocarbons like methane to form chlorinated hydrocarbons
These can cause cancer
What would happen if chlorine is not added to drinking water?
Quality of drinking water would be compromised
Could cause typhoid or cholera outbreak
Test for halide ions
Add Silver Nitrate
Aqueous halide ions react with aqueous silver ions to form precipitate of silver halides
Different colour precipitates are formed
In the displacement reactions between halogen and halide ions which is reducing/oxidising agent?
Halogen - gain electrons - oxidising agent
Halide ions - lose electrons - reducing agent
How can the reducing ability of halide ions be shown?
Reactions with H2SO4, which is a strong oxidising agent
Can chloride ions reduce H2SO4?
No - they are not powerful enough
Can Bromide ions reduce H2SO4?
Yes - it’s reduced to SO2
2H+ + H2SO4 + 2Br- —> SO2 + Br2 +H2O
Can Iodide ions reduce H2SO4?
Yes, they are even more powerful and reduce the SO2 formed to S, which is further reduced to H2S
