The group 2 elements. Flashcards
What is the half life of radium?
1599 years.
What are the properties of the group 2 elements?
They are metallic solids at room temperature and pressure, they have higher melting points, boiling points and enthalpy changes of atomisation than their group 1 neighbours due to stronger metallic bonding due to the presence of two valence electrons per atom.
Why is beryllium an exception to the group?
It forms a number of covalent compounds.
Why are they less reacted than may be expected?
There is coating of oxide on their surfaces.
How can magnesium be prepared?
It is obtained from sea water - calcium hydroxide solution is added causing Mg(OH)2 to precipitate. This is then treated with HCl to produce MgCL2 and the metal can then be liberated using electrolysis.
How can calcium be obtained?
Minerals such as limestone. The carbonate in the limestone is converted into the oxide by heating (900-1100 degrees) and then the metal is obtained from the reduction of CaO with aluminium.
What are the equations involved in obtaining calcium?
CaCO3 –> CaO + CO2
6CaO + 2Al —> 3CaO . Al2O3 + 3Ca
What colour does magnesium burn with?
A bright white flame.
What happens to the reaction with burning the metals in air down the group, in terms of whether it is exothermic or endothermic?
It becomes more exothermic down the group.
What happens when the oxides are dissolved in water?
They form an alkaline solution of the hydroxides.
What was calcium oxide used for in the 1960s?
To provide a spotlight - limelight light burnt a mixture of oxygen and hydrogen.
What happens when the group 2 metals are reacted with oxygen?
They give the hydroxide and liberate hydrogen.
How does the rate of reaction change down the group with water?
It becomes more vigorous.
What happens when the hydroxides are heated?
They decompose to give the oxides.
What does amphoteric mean?
Can act as either a base or acid.
How do BeX2 compounds differ from the others in group 2?
They form polymeric structures with covalent bonds and bridging halides.
How does BeH2 differ from BeCl2?
BeH2 is electron deficient and the Be-H-Be bridges involve 3-centre 2-electron bonding, whereas BeCL2 is not electron deficient because the bridging chloride donates three electrons.
Why doesn’t MgCl exist?
It is unstable with respect to disproportionation, but stable with respect to the elements.
What are ethynides?
Compounds of group 2 metals and carbon - they contain the (C2)2- ion.
What is the equation for calcium ethynide reacting with water?
CaC2 + 2H2O –> Ca(OH)2 + C2H2.
What happens when the group 2 metals react with nitrogen?
They produce nitrides.
What is the equation for magnesium reacting with nitrogen?
3Mg + N2 —> Mg3N2.
What happens when the nitrides are added to water and write an equation.
They decompose to form the metal hydroxide and ammonia.
Mg3N2 + 6H2O —> 3Mg(OH)2 + 2NH3.
What happens when group 2 carbonates are heated?
They decompose to give the oxides and CO2.
What happens to the decomposition temperature of the carbonates down the group?
It increases.
What is decomposition temperature related to?
The temperature at which the reaction becomes spontaneous - when deltarG becomes negative.
What is the change in the Gibbs equation for the carbonates mostly dependent on?
As entropy is largely independent of the group 2 element it is mostly dependent on differences between the lattice enthalpies of the reactants and products.
What do group 2 nitrates and sulfates decompose to give?
The oxides. (and NO and O2 for the nitrates and SO3 for the sulfates).
What is limestone used for?
A building material.
What happens when limestone is heated?
It loses CO2 to form CaO (quicklime).
What is a typical component of cement?
Ca3SiO5.
What is Portland cement?
Gypsum, tricalcium silicate.
Why are group 2 elements less soluble than group 1?
They have higher lattice enthalpies.
What is barium sulfate used in?
X-ray contrast agent in images of the digestive tract.
Why is the coordination chemistry of group 2 more extensive than group 1?
The ions are smaller and have twice the charge.
How does coordination typically change down the group 2 elements?
It increases due to the increasing size of the ions.
What dictates the nature of the complex formed with aqua ligands?
The size of the ions.
What does the high charge density of Be2+ result in?
Hydrolysis so solutions of Be2+ are acidic.
What are chlorophylls?
Pigment molecules that give leaves their green colour.
What are chlorophylls made up of?
They are magnesium coordination complexes that contain Mg2+ ions lying in the centre of a chlorin ring.
What is strange about beryllium in contrast to other group 1 elements?
It does not react with water, it is amphoteric, compounds of beryllium have much greater covalent character, it is acidic in aqueous solution, and BeO does not react with water unlike other oxides.
Why are these unusual properties of beryllium observed?
There is a high charge density on Be2+ which makes the lattice enthalpies of compounds very high. It also means the free ion is strongly polarising and draws the electrons of neighbouring ions towards itself, giving its bonds a much higher covalent character.
