3.2.2- Group 2, the alkaline earth metals (PAPER 1) Flashcards
What is the trend in atomic radius down Group 2 alkaline earth metals?
Atomic radius increases from Be to Ba.
Going down the group, the elements have more electron shells so higher principle energy levels are filled, whose orbitals are further from the nucleus. Effective nuclear charge remains constant as number of protons and shielding electrons increases.
What is the trend in ionisation energies down Group 2 alkaline earth metals?
Ionisation energy decreases down Group 2.
Each element down Group 2 has an extra electron shell compared to element above, extra inner shells shield the outer electrons from attraction from nucleus. Extra shell means outer electrons are further from nucleus, so easier to remove.
What is the trend in melting point down Group 2 alkaline earth metals?
Melting points generally decrease down Group 2.
Group 2 elements have metallic structure- positive ions in crystal structure surrounded by delocalised electrons.
Metal ions get bigger going down but n of delocalised electrons per atom doesn’t change (always 2) and charge of ions is always 2+
Larger atomic radius, further the delocalised electrons so easier to remove.
Which metal deviates from the melting point trend?
Mg- lower than expected due to variation of how atoms pack in metallic crystal.
What occurs when Group 2 metals react with water?
They form the metal hydroxide (alkali) and hydrogen gas.
Reactivity increases down the group as it easier to remove the 2 electrons as distance and shielding between outer electrons and nucleus increases.
What is reduced and what is oxidised when strontium reacts with water?
Sr (s) + 2H20 (l) —-> Sr(OH)2 (aq) + H2 (g)
Oxidation state of strontium increased from 0 as element to +2, it has been oxidised. Sr has lost 2 electrons.
Oxidation state of hydrogen decreased from +1 in water to 0 as element, it has been reduced.
What is the difference between Mg reacting with steam and cold water?
2 EQUATIONS
Mg with steam is a faster reaction, the Mg burns with a white flame, producing MgO and H2 gas.
Mg (s) + H2O (g) —-> MgO (s) + H2 (g)
Mg with cold water is a slower reaction, H2 gas bubbles form on the Mg ribbon but the Mg(OH)2 produced is almost insoluble in water preventing further reaction.
Mg (s) + 2H2O (l) —–> Mg (OH)2 (s) + H2 (g)
What is the difference between a metal reacting with cold water and reacting with steam?
When a metal reacts with cold water, the metal hydroxide and hydrogen gas is produced.
When a metal reacts with steam, the metal oxide and hydrogen gas is produced as metal hydroxides thermally decompose to oxide and water.
What is the reaction between a metal and oxygen- eg Ca?
Forms metal oxide
2Ca (s) + O2 (g)—-> 2CaO (s)
What is the reaction between a metal and acid- eg Mg and HCl?
Forms salt and hydrogen
2HCl (l) + Mg (s) —-> MgCl2 (s) + H2 (g)
Why can titanium not be reacted with carbon?
EQUATION
Titanium is a common metal in earth’s crust but it cannot be extracted by reacting the oxide with carbon:
metal oxide + carbon—> carbon dioxide + metal
This is because the metal reacts with the carbon to form titanium carbide (TiC) which makes the metal brittle.
How is titanium extracted? (using Mg)
EQUATION
Titanium oxide reacted with chlorine and carbon (coke) to form TiCl4 and CO.
TiO2(s)+C(s)+2Cl2(g)→TiCl4(s)+CO2(g)
Titanium chloride reduced to titanium by a reaction with magnesium.
TiCl4 (s) + 2Mg (s) —-> 2MgCl2 (s) + Ti (s)
Mg acts as reducing agent, expensive due to Mg and batch process.
What is the trend in solubility of Group 2 hydroxides down the group? (Mg- Ba)
The solubility of the metal hydroxide increases down the group as more of the hydroxide dissolves, the greater the concentration of hydroxide ions (OH-) in the solution formed. they all form white solids.
How soluble are: Mg(OH)2, Ca(OH)2, Sr(OH)2, Ba(OH)2 ?
Mg(OH)2= sparingly soluble
Ca(OH)2= slightly soluble
Sr(OH)2= soluble
Ba(OH)2 = soluble
What is Mg(OH)2 used for?
Used in medicine as a laxative and to relieve acid indigestion- as it can neutralise excess stomach acid.
What happens to the pH as solubility of Group 2 hydroxides increases?
The pH of the solutions formed increases as the more hydroxide that dissolves, the greater the concentration of hydroxide (OH-) ions in the solution formed.
What is Ca(OH)2 used for?
Slaked lime- Ca(OH)2 is used in agriculture to neutralise acidic soils by raising their pH.
What is the trend in solubility of Group 2 sulphates going down the group? (Mg-Ba)
Solubility decreases: opposite of the trend for solubility of Group 2 hydroxides.
How soluble are: Mg(SO)4, Ca(SO)4, Sr(SO)4, Ba(SO)4?
Mg(SO)4= soluble
Ca(SO)4= slightly soluble
Sr(SO)4= insoluble
Ba(SO)4= insoluble
What is Ba(SO)4 used for?
It is used in ‘barium meals’ in medicine- swallows suspension of barium sulfate which coats the tissues, making them show up on an X-ray. It is insoluble so not absorbed by body/ toxic salts not dissolved
What are CaO (lime) and CaCO3 (limestone) used for?
Remove CO2 from flue gases
CaO undergoes flue gas de-sulphurisation. Can be further oxidised to gypsum.
CaCO3 can also be used and undergoes a neutralisation reaction.
This is important as SO2 leads to acid rain which causes problems for asthmatics.
How to test for sulphate ions?
BaCl2 solution acidified with hydrochloric acid ( to decompose any carbonate as this would form white precipitate of barium carbonate) is used as a reagent to test for sulphate ions.
if a sulphate is present a white PPT of barium sulphate is formed.
Why is BaCl2 solution used to test for sulphate ions and why it is acidified?
When BaCl2 is added to a solution containing sulphate ions, it reacts with the sulphate ions to form a white precipitate (BaSO4).
The BaCl2 solution is acidified using a few drops HCl- chloride ions react with carbonate ions and remove them to prevent the formation of precipitates of metal ions present in the solution and to neutralise any carbonate or hydroxide ions present in the solution.
