topic 2: group 2, the alkaline earth metals Flashcards
trend in atomic radius in group 2
INCREASES DOWN GROUP
-Larger atoms down the group as more electron shells are added.
-One extra filled energy level as you go down.
trend in first ionisation energy in group 2
DECREASES DOWN GROUP
-Electron shells are added as you move down the group
-The inner shells also mean that the outer electrons are further away from the nucleus.
-The inner shells shield the outer electrons from the nucleus.
-So there is less attraction between the outer electrons and the nucleus.
-Meaning the outer electrons are easier to remove so there is a lower ionisation energy.
trend in melting + boiling point in group 2
DECREASE DOWN GROUP (generally, ignoring Mg)
-All group 2 metals have GIANT METALLIC STRUCTURES
-Positive ions (2+) in a crystal structure, surrounded by a sea of delocalised electrons from the outer electron shells
-Going down the group, the metal ions get bigger i.e atomic radius increases.
-But the number of delocalised electrons and charge on ions remains the same.
-It is always +2.
-The larger the ionic radius, the further away the delocalised electrons are from the positive nuclei so they feel less attraction.
-The strength of the metallic bonds decreases down the group.
-So it takes less energy to break the bonds.
Magnesium is an anomaly: it has the lowest melting point in group 2 because the crystal structure (arrangement of metallic ions) changes
trends in density in group 2
INCREASES DOWN GROUP
-Mass of element increases down the group.
-Mass increases at a faster rate than the radius increases.
Density = mass / volume
trends in reactivity in group 2
INCREASES DOWN GROUP
-The easier it is to lose electrons, the more reactive the element is
-Atomic radius increases down the group and shielding increases.
-This reduces the attraction between the outer electrons and the nucleus, Meaning it is easier for larger elements to lose electrons.
-So the first and second ionisation energies are lower down the group.
chemical reactions with water in group 2
METAL + WATER → METAL HYDROXIDE + HYDROGEN
-They all sink in water as they are more dense.
-REDOX reactions occur
-Metal is oxidised - goes from 0 (element) to +2 ion in a hydroxide compound.
-Hydrogen is reduces - goes from +1 in water to 0 (element).
-React more readily down the group as ionisation energies decrease.
-The water is the oxidising agent in this reaction.
-The metal hydroxide produced makes alkaline solutions
what happens when magnesium reacts with water?
-Magnesium reacts very SLOWLY with liquid SOLD WATER
-It reacts RAPIDLY with STEAM
-This is because steam provides the reaction with EXTRA ENERGY
When it reacts with steam it forms an ALKALINE OXIDE and hydrogen.
Mg (s) + H2O (g) → MgO (s) + H2 (g)
The MgO produced is a WHITE POWDER
The relative solubilities of the hydroxides of the elements Mg–Ba in water
Hydroxides form when a metal reacts with water.
They are all white solids and some are soluble.
MORE SOLUBLE DOWN THE GROUP
-So more likely to dissolve and dissociate in water down the group.
-Strength of the base increases down the group as they fully dissociate (a substance that can accept hydrogen ions in water and can neutralize an acid).
-When they dissolve they form alkaline solutions, the stronger the base further down the group, the more alkaline (higher pH) the solution will be
solubilities of each hydroxide
Mg(OH)2
Ca(OH)2
Sr(OH)2
Ba(OH)2
INCREASES DOWN GROUP
Mg(OH)2: Least soluble / sparingly soluble
Ca(OH)2: Sparingly soluble
Sr(OH)2: More soluble
Ba(OH)2: Most soluble - dissolves to produce a strongly alkaline solution.
uses of each hydroxide
Mg(OH)2
Ca(OH)2
Mg(OH)2: MEDICINE An antacid - milk of magnesia which is used in indigestion remedies to neutralise stomach acid.
Mg(OH)2 + 2HCl → MgCl2 + 2H2O
Ca(OH)2: AGRICULTURE to neutralise soils - slaked lime neutralises the soils but is not soluble enough to pollute waters.
solubility of sulfates
LESS SOLUBLE DOWN THE GROUP
-So less likely to dissolve/ dissociate in water down the group.
MgSO4
CaSO4
SrSO4
BaSO4
solubilities of sulfates
MgSO4
CaSO4
SrSO4
BaSO4
DECREASES DOWN GROUP
MgSO4: Most soluble
CaSO4: More soluble
SrSO4: Sparingly soluble
BaSO4: Insoluble
uses of sulfates
BaSO4
Barium meals - a form of medical tracer that allows internal organs to be imaged as it is opaque to x-rays. It is highly toxic if it enters the bloodstream however, it is highly insoluble so it will not be absorbed so is safe to use.
Also acidified then used to test for sulfates
testing for sulfates
BaCl2 (barium chloride) is used to test for sulfates because the sulfate ions displace the chloride ions, forming the insoluble SALT BARIUM SULFATE (A WHITE PRECIPITATE).
It needs to be ACIDIFIED first (using hydrochloric acid) to REMOVE ANY CARBONATE IONS as CARBON DIOXIDE or any sulfates that may be in before the reaction. If BARIUM CARBONATE formed then a WHITE PRECIPITATE would form which would result in a FALSE POSITIVE RESULT
Ba2+(aq) + SO4 2-(aq) →BaSO4 (s)
use of magnesium to extraction titanium
-Magnesium is used to extract titanium from its ore.
-The main titanium ore is TiO2 is converted into TiCl4 by heating with chlorine gas.
-Titanium chloride is then purified during fractional distillation.
-It is then reduced by magnesium in a displacement reaction in a furnace at almost 1000oC
-TiO2 + 2C + 2Cl2 →TiCl4 + 2CO
-TiCl4 + 2Mg → Ti + 2MgCl2
use of calcium oxide and carbonate
FLUE GAS DESULFURIZATION
-Burning fossil fuels releases SO2 which pollutes the atmosphere.
-The acidic sulfur dioxide can be removed from flue gases (gases emitted from industrial exhausts and chimneys) by reacting with an alkali (which is WET SCRUBBING).
-Powdered calcium oxide (lime, CaO) or calcium carbonate (limestone, CaCO3) is used for this.
-A slurry is made by mixing CaO or CaCO3 with WATER
-This is sprayed over the flue gases.
-The sulfur dioxide reacts with the alkaline slurry and produces a solid waste product (calcium sulfite).
-CaO(s) + 2H2O(l) + SO2(g) → CaSO3 (s)+ 2H2O(l)
Or
-CaCO3 (s) + 2H2O(l) + SO2(g) →CaSO3(s) + 2H2O(l) + CO2(g)
