17 - Reactivity trends
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What is disproportionation?
When an element is both oxidised and reduced in a single reaction
What are the 2 reactions we need to know to do with disproportionation?
Adding chlorine to water Making household bleach
Give the reaction for adding chlorine to water and explain why disproportionation happens
Cl~2 (aq) + H~2 O (l) –> HClO (aq) + HCl (aq) The chlorine starts with an oxidation state of 0 But it ends up with -1 state in the HCl and +1 state in HClO so it has been oxidised and reduced
Give the reaction for making household bleach and explain why disproportionation happens
Cl~2 (aq) + 2NaOH (aq) –> NaCl (aq) + NaClO (aq) + H~2 O Cl goes to -1 in NaCl and to +1 in NaClO
What is the advantage and disadvantage to adding chlorine to water?
Advantage is that bacteria is killed so water is drinkable Disadvantage is the chlorine is toxic in high quantities
What makes a chemical a bleach?
It has a ClO- ion NaClO and HClO are both bleach
What is the boiling point of group 17 elements like and why?
Low boiling points because the element only have London forces and are simple covalent
What happens to boiling point as you go down group 17 and why?
As you go down, boiling point increases Because there are more electrons so there are more London forces
What happens to reactivity as you go down group 17 and why?
As you go down, the elements get less reactive Because: Radius and shielding increase
What is redox displacement?
Where you have a halide ion and a halogen which is more reactive than the halide and the negative charge goes from the halide onto the other halogen The halogen becomes a halide ion and the halide ion becomes a halogen
In redox displacement What happens when the halide is more reactive than the halogen?
No reaction happens because the halogen isn’t reactive enough to displace the halide
What is the reaction for the redox displacement of chlorine and bromide?
Cl~2 + 2Br- –> 2Cl- + Br~2
What is redox displacement used for?
To find out which halide you have by making it into a halogen and looking at the colour change Redox displacement is different to halide tests
What are the colours of the different halogens in both water and cyclohexane?
Solution in water: Cl~2 is pale green Br~2 is orange I~2 is brown Solution is cyclohexane: Cl~2 is pale green Br~2 is orange I~2 is violet Notice that only the iodine changes colour from water to cyclohexane
How do you do halide tests? Different to redox displacement
Dissolve halide in water Add aqueous silver nitrate Silver halide precipitate is formed and its colour tells us what the halide is
In halide tests What are the precipitate colours for the different halides?
Chloride gives white Bromide gives cream Iodide gives yellow
How do you confirm the halide test result?
Add ammonia since it dissolves each halide precipitate differently Chloride dissolves very easily Bromide dissolves in concentrated NH~3 Iodide isn’t soluble in NH~3
What is the ionic equation for halide tests?
Ag+ (aq) + X- (aq) –> AgX (s)
How do you test for carbonates?
Add acid to produce carbon dioxide Bubble through limewater so the carbon dioxide turns the limewater cloudy
What do you need to know about sulfates to understand sulfate tests?
All sulfates are soluble in water Except for barium sulfate
How do you test for sulfates?
React compound with barium chloride or nitrate BaSO~4 precipitate will form if compound is a sulfate
What is the problem with sulfate tests? What do you have to do because of this?
If the compound is a carbonate The sulfate test result will be positive This means that you have to do a carbonate test first to verify that it isn’t a carbonate Then do the sulfate test
If you are doing halide tests, what do you need to do in the sulfate test differently?
Don’t use BaCl~2 because the chlorine will make the halide test positive automatically
How do you test for NH~4 +?
Add a hydroxide to produce ammonia gas Ammonia gas is the only common alkaline gas So use litmus paper which will turn blue if the ammonia is present