Reacitvity Trends p Flashcards
Physical properties of group 2
Reasonably high melting and boiling pants
Light metals with LOW densities
Form colourless white compounds
Ionisation energy - group 2
Decreases as you go down the group
More shielding, greater atomic radius
Ionisation energy decreases
MORE REACTIVE
Reducing agents - group2.
Each group2. Element has 2 outer shell electrons
Group 2 metals loose 2 electrons to a 2+ ion, the other species gain and is reduced
Redox reaction with oxygen (group 2)
React vigorously with oxygen to form a metal oxide ( general formula of MO)
2Mg + O2 — 2 Mg O
Redox reactions with water ( group2 )
React with water to form an alkaline hydroxide
General formula M( OH)2
Hydrogen gas formed
Be- doesn’t react
Mg + H20 —- Mg(OH)2 + H2
Redox reactions with dilute acids( group 2)
Form a salt and hydrogen gas
Exception of be
E.g. Mg + 2HCl— Mgcl 2 + H2
Group 2 oxides/hydroxides as bases?
Metals are bases( most are soluble so they’re alkalis too)
The group2 oxides react with H20=- metal hydroxides , which dissolve. Make the solution STRONGLY alkaline - releasing OH- ions
What is an exception for oxides of group2 metals
Magnesium oxide reacts SLOWLY and the hydroxide isn’t very soluble
What happens to the solubility fo hydroxides as you go down the group 2
Solubility INCREASES
Resulting solutions contain more OH- IONS, AND ARE more alkaline
Mg(OH)2 - slightly soluble in h20, the solution has a low OH- concentration and a ph of 10
Ba( OH)2 - more soluble in h20- solution has a greater OH- concentration and a ph - 13
SOLUBILTIY, PH AND ALKLAINITY INCREASES
Group2 compounds( bases) in agriculture
Calcium hydroxide( Ca(OH)2), added to fields as lime my farmers to increase the Ph of acidic soils The calcium hydroxide neutralises acids in the soil forming water
Medicine - group2 compounds as bases
Group 2 bases often used for antacids for relating acid indigestion
Many indigestion tabs use magnetism hydroxidea nd calcium carbonates -
Acid in stomach = Hcl
Mg(OH)2 + 2HCl—– 2H20 + MgCL
The halogens- chlorine
Green gas, Poisionesss in high concentrations
Flurorine
Pale yellow gas
Bromine
Red liquid that goes off dense brown Poisionesss fumes
Iodine
Shiny grey solid - sublimes to purple gas
Trend in melting and boiling point - halogens
More electrons as you descend
Stronger London dispersion forces between the molecules
More energy required to break these intermolecular forces
Boiling point INCREASES
Trends in reactivity
Halogen atoms react by gaining an electon in p subs hell to form 1- ion
- they are REDUCED - oxidise another substance
- DESCENDING the group, atomic radius increases, outer electrons are further from the nucleus, outer shell electrons are also shielded move from the attraction of the positive nucleus because of inner electrons
Harder for large atoms to attract electron = less reactive
Halogen- halide displacement, what happens hen a halogen is added to a halid in a solution
If the halogen is MORE reactive - reaction takes place,
Halogen displaces the halide for the solution
Chlorine displaces both bromine and iodine, but iodine doesn’t displace ether
Chlorine in h20 and organic solvent?
Pale green
Bromine in h20 and organic solvent?
Orange
Iodine in water and organic solvent
Brown in h2o
Purple in organic solvent
Fluorine
Pale yellow gas
Reacts with almost any substance
Astatine
Rare
Radioactive - decays rapidly
Disproportion - halogens
Redox reaction
Same element oxidised an deduced
Reaction of heroine wit h20 and cold, dilute sodium hydroxide
Chlorine + sodium hydroxide
BLEACH
2NaOH+ Cl2 — NaClO + NaCl 2 + H20
Chlorine+ water
Cl2+ h20 – Hcl+ Hcl0
Hcl0+ H20– Clo- + H30+
aqueous caloric acid ionises to make chlorite ions that kill bacteria
Chronicled water- PROS
Kills disease-causing microorganism
Some chlorine remains in H20- prevents reinfect ion further down the supply
Prevents algae growth, eliminating bad smell and taste, removing discolouration caused by organic compiunds
Cons of chlorinated water
Harmful if breathed in ( gas)- respiratory irritant
Liquid form = chemical burn
If reacts with organic compounds- chlorinated hydrocarbons( CANCER CAUSING)
Alternatives to chlorinated water
Purification tablets
UV light- ineffective in cloudy water and won’t stop water being infected further own the line, BUT kills microorganism by DNA damaging
OZONE- Strong oxidising agent, kills microorganisms
Carbonate ion?
CO3 … 2-
Carbonate ion test
Add dilute acid e.g, Hcl to form CO2
Bubble the gas through lime water and it will turn CLOUDY( ca(Oh)2)
Co2 forms a fine white precipitate of calcium carbonate, whihc is what turns the water Cloudy
( carbonate+ acid– co2 and water)
Sulphate ion?
So4 2-
SULFATE ion test
Most Sulphates are soluble in water BUT barium sulphate is insoluble - forms a white precipitate
Add dilute Hcl( ensure carbonate ions are removed ) and barium chloride
Ba 2+. + SO4 2- ——-BaSO4
Halide ions test
Most halides are soluble in h2o but silver halides are INSOLUBLE
Aqueous silver ions react with aqueous halide ions to produce precipitates of silver halides
Add aqueous silver nitrate( Ag No3) to an aqueous solution fo a halide add aqueous ammonia if the colour is hard to distinguish
Chloride , bromine and iodine colours- halide test
Chlorine- white, soluble in dilute NH3
Bromine- cream, soluble in concentrated NH3
Iodine - yellow, insoluble in concentrated NH3
Ammonia
Nh3
Ammonium ions
NH4 +
Ammonium ions test
Ammonia gas ( NH3) is alkaline, so you can check for it suing a damp piece of red litmus paper - turns it red
Add naOH and warm the mixture
NH4 + + OH- —– NH3 + H20
False positives?
Barium carbonate is a white and insoluble in h20, Oslo if you carry out the SULFATE test on a carbonate, youll get a white precipitate too, barium sulphite = same m need to ensure there’s no carbonate first
Silver carbonate and silver sulphate are both insoluble in h20 - form as precipitates in. This test( AgNO3 forming white precipitate)
Order of tests
Carbonate, sulphate then halide
Mixture of ions - describe doing tests and why it works in this order
- CARBONATE test= if you see bubbles, continue adding dilute nitric acid until bubbling stops, all carbonate ions removed
- SULFATE test = add an excess of Ba(NO3)2, any sulphate ins present will precipitate as barium SULFATE- filter solution to remove
- HALIDE test= Add AgNO3, any carbonate or sulphate ions initially present have been removed
