AQA AS Chemistry Unit 2-6: Group 2, The Alkaline Earth Metals

The s-block,
Which block of the periodic table are the group 2 elements found in?
Each group 7 metal has the outer shell configuration ns2 , they have two outer shells in the s orbital. This s orbital become further from the nucleus as you go down the group
What is the outer shell configuration of group 2 elements?
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Beryllium, Be
magnesium, Mg
calcium, Ca
strontium, Sr
barium, Ba
Radium, Ra
Name the group 2 elements?
Beryllium- very small atom, not typical of grouo
Radium- very radioactive element, only exists in tiny quantities
Which two elements are not usually included in trends and why?
The radius increases as go down group 2
There are more shells of electrons as go down group, each group 2 element has one more main filled electron shell than the one before, this takes up more space
Having more electrons on the atom means that they repel each other more, forcing the shells further apart, making the atomic radius larger
There are more inner shells of electrons going down group 2 which means there is an increase in shielding. Less nuclear charge reaches the outer shell electrons so they are pulled in less, being further away from the nucleus means a larger atomic radius
What is the trend in the atomic radius of the group 2 elements as you go down the group?
The ionisation energy decreases down group 2.
This is the same for first and second ionisation energy.
As you go down the group the outer shell electrons get further away from the nucleus, due to a larger atomic radius (more main shells of electrons). As the outer shell electrons get further away from the nucleus they feel less attraction from the nucleus meaning it is easier to remove. Also there is increased shielding as there are more inner shells of electrons, this also lessens the nuclear charge felt by outer shell electrons, weakening attraction and making them easier to lose.
Although the positive nuclear charge increases as you go down the group, this is offset by the added inner shell negative electrins
Therefore it takes less energy to remove the valence electron as you go down the group
What is the trend in ionisation energy as you go down group 2?
The group 2 elements get more reactive as you go down the group.
When the group 2 elements react they lose their two outer shell electrons, forming 2+ ions, in order to have Nobel gas structure
M—> M2+ + 2e-
The reactivity of the elements depends on the ease of removing these two electrons- the combined first and second ionisation energy
As you go down the group the ionisation energy decreases, less energy is needed to remove the two electrons, they lose the two electrons to form ions more easily, making them more reactive
What is the trend in reactivity as you go down group 2?
The melting points from calcium to barium decrease – the structure of a metal is positive metal ions surrounded by a sea of delocalised electrons. As you go down the group the atomic radius of the atoms increase, this means the delocalised electrons are further away from the positive nucleus. The attraction between the positive nucleus and negative delocalised electrons drop off with distance, as the electrons get further away due to the larger atomic radius, this means the ions and electrons are more easily separated. The metallic bond is therefore weaker as you go down the group meaning it takes less energy to break it, separating the ions and electrons, giving a lower boiling point as you go down the group.
However magnesium does not follow this trend, it has the lowest melting point, this is because it has a different structure to the other metals- it has a different lattice arrangement of atoms. This affects distance between ions meaning attraction and metallic bond is weaker, decreasing the amount of energy needed to separate ions.
Each group 2 metal has the same number of delocalised electrons per ion and same charges on metal ions so the strength has nothing to do with this, the charge density does decrease due to the larger ions.
What is the trend in melting points as you go down group 2?
The electronegativity decreases as you go down group 2.
The atomic radius gets larger meaning there is a larger distance between the nucleus and bonding pair of electrons, this means there is a weaker attraction between them. Also there are more inner shells of electrons resulting in less nuclear charge being felt by bonding pair of electrons, therefore the attraction would be weaker.
Therefore the elect nets get less good at attracting the bonding pair as you go down the group.
The group 2 metals are very weakly electronegative.
As you go down group bonds between group 2 metal and electronegative non metal element get more ionic, greater difference is electronegativity, bonding pair increasingly attracted away from the group 2 element resulting in it being more likely to take the electrons forming an ion.
What is the trend in electronegativity as you go down group 2?
Magnesium reacts very slowly with water.
The eventual product is slightly soluble Magnesium hydroxide and hydrogen gas. Magnesium is oxidised, hydrogen is reduced.
Mg + 2H2O —> Mg(OH)2 + H2
However magnesium reacts very quickly with steam, forming solid Magnesium oxide and Hydrogen gas, burning with a bright white flame.
Mg + H2O —> MgO + H2
The magnesium oxide forms because magnesium hydroxide decomposes to magnesium oxide at high temperatures, such as the temperature of the steam. magnesium is oxidised, hydrogen is reduced.
Describe the reaction of magnesium with water and the product.
Magnesium hydroxide is also known as milk of magnesia. It is used to treat indigestion. It is weakly alkali so will neutralise excess stomach acid.
This is safe and does not produce CO2 like in neutralisation reaction with carbonates.
What is a use for magnesium hydroxide?
Calcium reacts with water, fizzing occurs as gas is given off and he solution goes cloudy and heats up slightly (exothermic reaction)
Ca + 2H2O—> Ca(OH)2 + H2
Calcium hydroxide is formed. Calcium is oxidised to Ca2+ and Hydrogen in water is reduced going from oxidation state of +1 to 0 in hydrogen gas.
Describe the reaction of calcium with water and the product.
It is called slaked lime and is used to neutralise acid soil, being an alkali, so that plants can grow better, as they can not in a acidic soil- thy do not grow as well as possible in these conditions
What is a use of calcium hydroxide?
Ca(OH)2 solution is limewater, used as a test for carbon dioxide, limewater turns cloudy white solution from colourless solution when carbon dioxide is passed through it. A precipitate calcium carbonate is produced.
What is a use of calcium hydroxide solution?
The reactions are both vigorous (barium more vigorous than strontium) producing fizzing. A colourless solution is formed and gas is evolved. Both mot the metals hydroxides here are soluble
Sr +2H2O—> Sr(OH)2 + H2
Ba +2H2O—> Ba(OH)2 + H2
Strontium and barium are oxidised, hydrogen in water is reduced to hydrogen gas
Describe the reaction of strontium and barium with water and the product.
The reactivity of group 2 elements with water increases as go down group, the reactions become more vigorous
As you go down the group the ionisation energy of the metals decrease, meaning they form ions more easily (losing the outer electrons more easily) , this means they are more reactive going down the group.
Also the reactivity depends on solubility of the product formed – solubility of the metal hydroxides. The magnesium and calcium hydroxides are relatively insoluble in water, this means the product stays on the surface of the metal, preventing it from reacting further, decreasing the rate of reaction.
The strontium and barium hydroxides are soluble in water so dissolve once formed allowing the rest of the metals to react, the rate of reaction increases
What is the trend in the metals’s reactivity with water as you go down group 2?
The group 2 hydroxides increase in solubility as you go down the group. Magnesium hydroxide is the least soluble – sparingly soluble, creating a white suspension. This solution is slightly alkaline as some hydroxide ions are released, PH 9
Calcium hydroxide is sparingly soluble.
Barium hydroxide is the most soluble- it dissolves completely creating an strongly alkaline solutions so OH- ions are released.
Ba(OH)2 + aq —> Ba2+ + 2OH-
Describe the trend in solubility of the group 2 hydroxide as you go down the group.
The solubility of the group 2 sulphates decreases as you go down the group.
Magnesium sulphate is soluble in water, giving colourless solution.
MgSO4 + aq —> Mg2+ + (SO4)2-
Calcium sulphate is sparingly soluble in water, giving a faint white precipitate
Ca2+ + (SO4)2- —> CaSO4
Strontium sulphate is insoluble, giving a thick white precipitate
Sr2+ + (SO4)2- —> SrSO4
Barium sulphate is completely insoluble, giving a thick white precipitate – a suspension is formed and not Ba ions are released
Ba2+ + (SO4)2- —> BaSO4
Describe the trend in solubility of the group 2 sulphates as you go down the group.
Barium sulphate is used as a barium meal to identify abnormalities in the digestive system. It is given to a patient and will make the soft tissue organs which are normally not dense enough to. The extremely insoluble and dense (making it opaque to X-rays) barium sulphate lines the digestive system, making it show up on an x-ray.
Although barium ions are very poisonous, this test in not dangerous as barium sulphate is so insoluble that the barium ions will not be released into the bloodstream.
What is a use in medicine for barium sulphate?
The insolubility of barium sulphate is used in this test. Barium chloride, BaCl2, is added to the solution suspected to contain sulphate ions.
First an acid such as nitric or hydrochloric acid is added to the solution, this gets rid of any ions that may interfere with the result, also producing a white precipitate, such as carbonate (forming barium carbonate), giving a false result for sulphate ions. If carbonate ions are present fizzing is seen as it reacts with the nitric acid as CO2 is produced
Then add the barium chloride
If white precipitate formed= likely sulphate ions are present as insoluble barium sulphate is formed.
Ba2+ + (SO4)2- —> BaSO4
If no white precipitate is formed= sulphate ions are not present
What is a test for sulphate ions? Describe how this test is carried out
Add hydrochloric acid
Is barium sulphate is formed, the white precipitate will not re dissolve, so sulphate ions are present
If precipitate redissolves with fizzing and no smell them carbonate ions ares present CO32-
If precipitate redissolves with no bubbles but metallic smelling gas sulphide ions are present SO3 2-
What test can be performed to check
Sulphuric acid contains sulphate ions and they would react with the barium to form the white precipitate of barium sulphate whether or not sulphate ions were in the solution- false positive result
Why can sulphuric acid not be used in the test?

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