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Chemistry - Module 3 - Periodic table - Part 2 - Coggle Diagram
Chemistry - Module 3 - Periodic table - Part 2
Tests for ions
test for carbonates CO3 2-
add dilute acid (e.g. dilute HCl)
CO2 produced if carbonates present
test CO2 with limewater which turns cloudy for a positive test
test for sulfates SO4 2-
add dilute HCl then barium chloride solution
white precipitate of barium sulfate indicates positive test
test for halides (summary)
add nitric acid then silver nitrate solution
chloride gives white precipitate that dissolves in dilute ammonia
bromide gives cream precipitate that dissolves in concentrated ammonia
iodide gives yellow precipitate that is insoluble in ammonia
test for ammonia and ammonium ions
test for ammonia
damp red litmus paper turns blue because ammonia is alkaline
test for ammonium ions
add sodium hydroxide and warm the mixture to release ammonia
ammonia released turns damp red litmus paper blue
group 7 - halogens
properties of the first four halogens
halogens exist as diatomic molecules bonded by single covalent bond
first four halogens properties
fluorine F2, pale yellow gas
Chlorine Cl2, green gas
Bromine Br2, red-brown liquid
Iodine I2, grey solid
trends in physical properties and reactivity
melting and boiling points increase down the group due to stronger london forces from larger atoms
volatility decreases down the group
reactivity (ability to gain an electron) decreases down the group
larger atoms have greater radius and shielding, making electron attraction harder
halogens act as oxidising agents by being reduced to 1- ions
displacement reactions and observation techniques
more reactive halogen displaces less reactive halide from solution
reactions produce colour changes
using an organic solvent like hexane concentrates halogen in a separate layer to make colour easier to see
test for halides with silver nitrate
procedure
add dilute nitric acid to remove interfering ions
add AgNO3 to form silver halide precipitate
identification by precipitate colour and ammonia solubility
chloride: white precipitate, dissolves in dilute ammonia
Bromide: cream precipitate, dissolves in concentrated ammonia
Iodide: yellow precipitate, insoluble in ammonia
group 2 - alkaline earth metals
general properties and electronic structure
group 2 elements have two electrons in outer shell and form 2+ ions to achieve noble gas configuration
ionisation energies decrease down the group due to increasing radius and shielding
reactivity and common reactions
reactivity increases down the group as ionisation energy decreases
common reactions
reaction with water produces metal hydroxide and hydrogen
burning in oxygen forms solid white oxides
reaction with dilute acids produce a salt and hydrogen
oxides, hydroxides and alkalinity
oxides and hydroxides are basic; many hydroxides dissolve in water and are alkalis
oxides react with water to form hydroxides that dissolve and produce OH- ; making solutions strongly alkaline
hydroxides become more soluble and more strongly alkaline down the group
uses
group 2 compounds commonly used to neutralise acids
disproportionation and water treatment
disproportionation example
disproportionation is when same element is both oxidised and reduced
bleach (NaClO) is a typical example
use of chlorine in water treatment
benefits
kills disease-causing microorganisms and leaves residual chlorine to prevent recontamination
prevents algae growth and removes tastes, smells and discolouration from organics
risks and ethical concerns
chlorine gas is harmful and liquid chlorine causes chemical burn
chlorinated hydrocarbons formed can be carcinogenic though risk is small compared to untreated water
ethical concern about lack of choice when water is chlorinated
alternatives to chlorine
ozone
strong oxidiser, effective at killing microorganisms, but expensive and short-lived in water
UV light
kills microorganisms by damaging DNA, but ineffective in cloudy water and does not prevent downstream contamination