AQA A Level Chemistry - Group 2

what is the trend in atomic radius down group 2?

increases

why does atomic radius increase down group 2?

• each element has 1 more electron shell than the previous

• also means more shielding so the outer e- is less attracted to the nucleus and moves away

what is the trend in 1st ionisation energy down group 2?

decreases

why does 1st ionisation energy decrease down group 2?

• atomic radius and shielding significantly increase

• outer e- is further away from the nucleus and also more repelled

• means it is more easy to remove it

what is the trend in melting point down group 2?

general decrease (increase after magnesium but ignore)

why does melting point decrease down group 2?

• ion size increases down group 2

• attraction between ion nucleus and delocalised e- decreases

• less energy is required to overcome it

• no change in number of delocalised electrons as is proportional to group number

how does the reactivity with water change down group 2?

as you go down group 2 all react increasingly readily

what is the equation for the reaction of a group 2 metal with water?

• X (s) + 2H₂O (l) → X(OH)₂ (s) + H₂ (g)

• ionic - write X(OH)₂ as X²⁺ and OH^-

describe how magnesium reacts with ‘water’

• reacts readily with steam

• Mg (s) + H₂O (g) → MgO (s) + H₂ (g)

• forms magnesium oxide as magnesium hydroxide isn’t stable at high temperatures

describe the observations when group 2 metal reacts with water

• effervesence

• white precipitate that decreases in amount down the group

how does hydroxide solubility change down group 2?

• increases down group 2

• alkalinity also decreases down the group due to an increase in the concentration of dissociated OH- ions

what is magnesium hydroxide described as?

sparingly soluble

how can group 2 hydroxide solubility be demonstrated?

• mixing solutions of g2 salts with NaOH

• white precipitate forms that decreases in thickness down the group

  • thick, normal, thin, very thin

how does sulfate solubility change down group 2?

decreases down group 2

how can sulfate solubility be investigated?

• adding sodium sulfate solution

• produces more precipitate as Mg → Ba

  • none, thin, normal, thick

how is barium sulfate described?

insoluble

what can be used to test for sulfate ions?

acidified barium chloride solution (as BaSO4 is insoluble so visible ppt formed)

how is acidified barium chloride solution used to test for sulfate ions?

• acidified to remove any other ions like carbonate ions that would affect the test

• add 1cm³ HCl, 1cm³ BaCl₂ (aq) to 1cm³ unknown

• if thick white precipitate produce, then SO₄^2- ions present

what are the uses of group 2 metals?

• magnesium: used to extract titanium from titanium oxide

• magnesium hydroxide: used as antacid

• calcium hydroxide: used to neutralise acidic soil

• calcium oxide/carbonate: used to remove SO2 from flue gases

• barium sulphate: used as radiocontrast for x-rays

describe how magnesium is used to extract titanium from titanium oxide

• acts as reducing agent

• TiO₂ + 2C + 2Cl₂ → TiCl₄ + 2CO

• TiCl₄ + 2Mg → Ti + 2MgCl₂

• useful as Ti cannot be extracted using carbon as TiC is formed which makes the metal impure and brittle (also has to be carried out in Ar atmosphere for this reason - O2/N2)

describe how magnesium hydroxide is used as an antacid

• used as a suspension in water (as is sparingly soluble)

• used to alleviate constipation and as antacid to neutralise excess acid in the gut

• Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O

describe how calcium hydroxide is used to neutralise acidic soil

• sprayed as a powder onto fields to neutralise acidic soil - raises to 6.5 (known as slaked lime)

• used as low pH in soil restricts growth of nitrogen fixing bacteria and increases Al³⁺ and Mn^+2/7 ion levels to toxic amounts - so neutralised soil removes this

• also improves texture

describe how CaO or CaCO₃ are used to remove SO₂ from flue gases

• CaO + SO_{2 (g)} → CaSO₃

• CaCO₃ + SO_{2 (g)} → CaSO₄ + CO₂

• done in order to prevent acid rain formation

• as flue gases produced as byproduct of fossil fuel combustion

describe how barium sulfate is used in medicine

• used as radiocontrast agent for x-ray imaging of digestive tract (usually)

• used as although Ba²⁺ ions are toxic, BaSO₄ is insoluble and not absorbed into the blood and Ba is good at absorbing x-rays