3.2.2 Group 2, the alkaline earth metals Flashcards
What is group 2?
Elements in group 2 are called the alkaline earth metals. This is because their oxides and hydroxides are alkaline.
Group 2 are similar in many ways to group 1 but are less reactive.
Group 2 elements are s-block elements.
What is the trend in atomic radius down group 2?
Atomic radius increases down the group.
As you go down the group, the atoms have more shells of electrons making the atom bigger.
What is the trend in melting point going down group 2?
Melting points decrease down the group. The metallic bonding weakens as the atomic size increases. The distance between the positive ions and delocalised electrons increases. Therefore the electrostatic attractive forces between the positive ions and the delocalised electrons weaken.
What is the trend in 1st ionisation energy down group 2?
The outermost electrons are held more weakly because they are successively further from the nucleus in additional shells.
In addition, the outer shell electrons become more shielded from the attraction of the nucleus by the repulsive force of inner shell electrons.
What is the trend in reactivity down group 2?
Reactivity of group 2 metals increases down the group.
What are the group 2 reactions with water?
Magnesium reacts in steam to produce magnesium oxide and hydrogen. The Mg would burn with a bright white flame. The MgO appears as a white powder.
Mg(s) + H2O(g) → MgO(s) + H2(g)
Mg will also react with warm water, giving a different magnesium hydroxide product.
Mg + 2H2O → Mg(OH)2 + H2
This is a much slower reaction than the reaction with steam and there is no flame.
The other group 2 metals will react with cold water with increasing vigour down the group to form hydroxides.
Ca + 2H2O(l) → Ca(OH)2(aq) + H2(g)
Sr + 2H2O(l) → Sr(OH)2(aq) + H2(g)
Ва + 2H2O(l)→ Ва(ОН)2(aq) + Н2(g)
The hydroxides produced make the water alkaline (if they are soluble in water).
You would observe:
• fizzing (more vigorous down group)
• the metal dissolving (faster down group)
• the solution heating up (more down group)
• with calcium a white precipitate appearing (less precipitate forms down group with other metals)
Why is titanium useful and how is it extracted?
Titanium is a very useful metal because it is abundant, has a low density and is corrosion resistant - it is used for making strong, light alloys for use in aircraft.
Titanium is extracted by reaction with a more reactive metal (e.g. Magnesium).
Why can titanium not be extracted with carbon or via electrolysis?
Titanium cannot be extracted with carbon because titanium carbide (TiC) is formed rather than titanium.
Titanium cannot be extracted by electrolysis because it has to be very pure.
How is magnesium used to extract titanium?
- TiO2 (solid) is converted to TiCl4 (liquid) at 900°C:
TiO2 + 2Cl2 + 2C → TiCl4 + 2CO
- The TiCl4 is purified by fractional distillation in an argon atmosphere.
- The Ti is extracted by Mg in an argon atmosphere at 500°C
TiCl4 + 2Mg → Ti + 2MgCl2
(TiO2 is converted to TiCl4 as it can be purified by fractional distillation, TiCl4 being molecular (liquid at room temperature) rather than ionic like TiO2 (solid at room temperature))
Why is titanium expensive?
- The expensive cost of the magnesium
- This is a batch process which makes it expensive because the process is slower (having to fill up and empty reactors takes time) and requires more labour and the energy is lost when the reactor is cooled down after stopping
- The process is also expensive due to the argon, and the need to remove moisture (because TiCl4 is susceptible to hydrolysis).
- High temperatures required in both steps
(This all makes titanium expensive even though it is a relatively abundant metal. It is only therefore used to a limited amount even though it has useful properties.)
What is a use of calcium oxide?
Calcium oxide can be used to remove SO2 from the waste gases from furnaces (e.g. coal fired power stations) by flue gas desulfurisation. The gases pass through a scrubber containing basic calcium oxide which reacts with the acidic sulfur dioxide in a neutralisation reaction.
SO2 + CaO → CaSO3
calcium sulfite
The calcium sulfite which is formed can be used to make calcium sulfate for plasterboard.
What is the trend in solubility of the group 2 hydroxides?
Group 2 hydroxides become more soluble down the group.
All Group 2 hydroxides when not soluble appear as white precipitates.
Mg(OH)2 = Least soluble
Ba(OH)2 = Most soluble
Describe the solubility and uses of magnesium hydroxide.
Magnesium hydroxide - insoluble in water.
Simplest ionic equation for formation of Mg(OH)2(s):
Mg2+(aq) + 2OH^-(aq) → Mg(OH)2(s)
Forms a suspension in water, rather than a solution.
Magnesium hydroxide is used in medicine (in suspension as milk of magnesia) to neutralise excess acid in the stomach and to treat constipation.
Mg(OH)2 + 2HCl → MgCl2 + 2H2O
It is safe to use because it is so weakly alkaline. It is preferable to using calcium carbonate as it will not produce carbon dioxide gas.
Describe the solubility and uses of calcium hydroxide.
Calcium hydroxide - slightly soluble in water.
Will appear as a white precipitate.
A suspension of calcium hydroxide in water will appear more alkaline than magnesium hydroxide as it is more soluble so there will be more hydroxide ions present in solution.
It is used in agriculture to neutralise acidic soils.
An aqueous solution of calcium hydroxide is called lime water and can be used a test for carbon dioxide. The limewater turns cloudy as white calcium carbonate is produced.
Describe the solubility and uses of strontium hydroxide.
Strontium hydroxide - soluble in water.
Describe the solubility and uses of barium hydroxide.
Barium hydroxide - soluble in water.
Would easily dissolve in water.
The hydroxide ions present would make the solution strongly alkaline.
Ba(OH)2(s) + aq → Ba^2+(aq) + 2OH^-(aq)
What is the trend in solubility of the group 2 sulfates?
Group 2 sulfates become less soluble down the group.
MgSO2 = Most soluble
BaSO2 = Least soluble
Write the ionic equations for the formation of the precipitates of group 2 sulfates.
Mg - no precipitate formed
Ca^2+(aq) + SO4^2-(aq) → CaSO4(s)
Sr^2+(aq) + SO4^2-(aq) → SrSO4(s)
Ba^2+(aq) + SO4^2-(aq) → BaSO4(s)
Describe the solubility and uses of magnesium sulfate.
Magnesium sulfate = soluble in water.
Describe the solubility and uses of calcium sulfate.
Calcium sulfate = slightly soluble in water.
Describe the solubility and uses of strontium sulfate.
Strontium sulfate = slightly soluble in water.
Describe the solubility and uses of barium sulfate.
Barium sulfate = insoluble in water.
BaSO4 is used in medicine as a ‘Barium meal’ given to patients who need x-rays of their intestines. The barium absorbs the x-rays and so the gut shows up on the X-ray image.
Even though barium compounds are toxic, it is safe to use here because barium sulfate’s low solubility means it is not absorbed into the blood.
If barium metal is reacted with sulfuric acid it will only react slowly, as the insoluble barium sulfate produced will cover the surface of the metal and act as a barrier to further attack.
Ba + H2SO4 → BaSO4 + H2
The same effect happens to a lesser extent with metals going up the group as the solubility of the sulfates increases.
The same effect does not happen with other acids like hydrochloric or nitric as they form soluble group 2 salts.
How would you test for the presence of sulfate ions?
BaCl solution acidified with hydrochloric acid is used as a reagent to test for sulfate ions.
If acidified barium chloride is added to a solution that contains sulfate ions a white precipitate of barium sulfate forms (since it is insoluble).
Simplest ionic equation:
Ba^2+(aq) + SO4^2-(aq) → BaSO4(s)
The hydrochloric acid is needed to react with carbonate impurities that are often found in salts which could form a white barium carbonate precipitate and so give a false result.
2HCl + NaCO3 → 2NaCl + H2O + CO2
Fizzing due to CO2 would be observed if a carbonate was present.
You could not use sulfuric acid because it contains sulfate ions and so would give a false positive result.
How do you write ionic equations for precipitation reactions?
We usually write ionic equations to show precipitation reactions.
Ionic equations only show the ions that are reacting and leave out spectator ions.
Spectator ions are ions that are:
- not changing state
- not changing oxidation number
Take full equation
Ba(NO3)2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaNO3(aq)
Separate aqueous solutions into ions
Ba^2+(aq) + 2NO3^-(aq) + 2Na^+(aq) + SO4^2-(aq) → BaSO4(s) + 2Na^+(aq) + 2NO3^-(aq)
Cancel out spectator ions leaving the simplest ionic equation
Ba^2+(aq) + SO4^2-(aq) → BaSO4(s)
