The halogens Flashcards
What are the halogens?
Group 17 (7) of the periodic table - the most reactive non-metallic group
What do the halogens occur as on earth?
Stable halide ions (Cl-, Br-, and I-) dissolved in sea water or combined with sodium or potassium as salt deposits
At RTP, what do the halogens exist as?
Diatomic molecules
How do the physical states vary down the group at RTP?
They range from gas at the top to solid at the bottom
What structure do the halogens form in their solid states?
Lattices with simple molecular structures
What is the general trend in boiling point down the group?
Boiling point increases because each atom has more electrons and stronger London forces, therefore requiring more energy to break the intermolecular forces
How many outer-shell electrons do halogens have?
Seven
Which subshells are halogen’s electrons in?
2 are in the outer s subshell and 5 are in the outer p subshell
What is the most common reaction of the halogens?
Redox reactions
What happens to the halogens during redox reactions?
Each halogen atom is reduced, gaining one electron to form a 1- halide ion with the electron configuration of the nearest noble gas
What does the halogen become during a redox reaction?
An oxidising agent
Why do the halogens become oxidising agents during redox reactions?
Another species loses electrons to halogen atoms - it is oxidised
What is the trend in the reactivity of the halogens?
Reactivity decreases down the group
How do you test halide-halide displacement reactions?
A solution of each halogen is added to aqueous solutions of the other halides
What are the results of halide-halide displacement reactions?
If the halogen added is more reactive than the halide present a reaction takes place, the halogen displacing the halide from the solution, and the solution changes colour
What colour are solutions of iodine and bromine in water?
Orange-brown colour, depending on concentration
How do you tell solutions of iodine and bromine apart?
An organic non-polar solvent, e.g. cyclohexane, can be added and the mixture shaken - non-polar halogens dissolve more readily than in water so colours are much easier to tell apart
What colour is iodine in cyclohexane?
Deep violet
What does chlorine react with in halogen-halide displacement reactions?
Both Br- and I-
What does bromine react with in halogen-halide displacement reactions?
I- only
What does iodine react with in halogen-halide displacement reactions?
Nothing
What is fluorine?
A pale yellow gas
Is fluorine reactive?
Yes - it reacts with almost any substance it comes into contact with
Why is astatine extremely rare?
It is radioactive and decays rapidly and the element has never been seen
What is the least reactive halogen?
Astatine
How do halogens react in redox reactions?
By gaining electrons
What is the trend in reactivity down the group?
Reactivity decreases
Why does reactivity decrease down the halogens?
Atomic radius increases, meaning there are more inner shells so shielding also increases, and so there is less nuclear attraction to capture an electron from another species
What is disproportionation?
A redox reaction in which the same element is both oxidised and reduced
Give two examples of disproportionation reactions in the halogens
1) The reaction of chlorine with water
2) The reaction of chlorine with cold, dilute aqueous sodium hydroxide
What is chlorine used for?
Water purification
What happens when small amounts of chlorine are added to water?
A disproportionation reaction occurs where in each chlorine molecule, one atom is oxidised, and the other is reduced
What is the equation for the disproportionation reaction of chlorine with water?
Cl2 (aq) + H2O (l) –> HClO (aq) + HCl (aq)
What type of products are formed in the disproportionation reaction between chlorine and water?
Two acids - chloric (I) acid, HClO, and hydrochloric acid, HCl
What kills the bacteria in water?
Chloric acid (I) and chlorate (I) ions, ClO-
What can chloric (I) acid act as?
A weak bleach
How can chloric acid acting as a bleach be demonstrated?
By adding some indicator solution to a solution of chlorine in water
The indicator first turns red from the presence of two acids, but then the colour disappears as the bleaching action of chloric (I) acid takes effect
What is the reaction of chlorine with water limited by?
Low solubility of chlorine in water
How can the limitation of the reaction be overcome?
By adding dissolved sodium hydroxide
Why does adding sodium hydroxide to a solution of chlorine and water increase the reaction?
Much more chlorine dissolves and another disproportionation reaction occurs
What is the equation for the reaction between chlorine and sodium hydroxide?
Cl2 (aq) + 2NaOH (aq) –> NaClO (aq) + H2O (l)
What does the resulting solution of the disproportionation reaction between chlorine and sodium hydroxide contain?
A large concentration of chlorate (I), ClO-, ions from the sodium chlorate, NaClO, formed
What is this solution used for?
Household bleach
Why is chlorine use risky?
It is an extremely toxic gas - respiratory irritant at small concentrations but can be fatal at large concentrations
What is the risk of adding chlorine to drinking water?
Chlorine can react with organic hydrocarbons, e.g. methane, formed from decaying vegetation to form chlorinated hydrocarbons
Why are chlorinated hydrocarbons dangerous?
They are suspected of causing cancer
What would happen if chlorine wasn’t added to water?
The quality of drinking water would be compromised and diseases such as typhoid and cholera might break out
What does the reaction of aqueous halide ions with aqueous silver ions form?
Precipitates of silver halides
What is the equation for the reaction of halide ions and silver ions?
Ag+ (aq) + X- (aq) –> AgX (s)
What is the reaction between halide ions and silver ions used for?
It forms the basis for a test for the presence of halides
