AQA AS Chemistry Group 2 - The Alkaline Earths PDF

AS CHEMISTRY 3.2.2 GROUP 2 - THE ALKALINE EARTHS PROPERTIES OF PERIOD 3 ELEMENTS The following trends are the most commonly examined. Learn the patterns and the explanations that go with them. Be prepared to compare two elements and...

AS CHEMISTRY 3.2.2 GROUP 2 - THE ALKALINE EARTHS PROPERTIES OF PERIOD 3 ELEMENTS The following trends are the most commonly examined. Learn the patterns and the explanations that go with them. Be prepared to compare two elements and explain the differences with respect to these trends. ATOMIC RADIUS INCREASE Down a Group 2 there is an increase in atomic radius As you move down the group the number of occupied energy levels increases. This causes the radius of the atoms to increase. 1st IONISATION ENERGY DECREASE Down a Group 2 there is a decrease in 1st I.E. As you move down the group, both shielding and distance of the outer electron from the nucleus increases. This means there is a weaker attraction between the outer electron and the nucleus, so less energy is required to remove it. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.2.2 GROUP 2 - THE ALKALINE EARTHS ATOMIC RADIUS DECREASE Down Group 2 the melting point decreases They all have metallic bonding. The charge on the cation in each group is the same (2+), but the ionic radius increases down the group due to increased shielding. This means that the electrostatic attractions between the cations and the delocalised electrons are weaker. Therefore less energy is required to break them, lowering the melting point. REACTIONS OF GROUP 1/2 WITH WATER Where M = any Group 2 element Group 2 Metal + Water M(s) + 2H2O(l) → 2M(OH)2(s/aq) + H2(g) Be wary! This is a precipitate (s) with Mg and Ca, but Sr & Ba are soluble (aq)! Observations: Solid metals disappears Fizzing / Effervescence. Test for H2(g) = a lit splint gives a squeaky pop. Exothermic Reaction Important! Magnesium reacts very slowly with liquid water. It reacts more readily with steam, however it produces magnesium oxide as a product. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.2.2 GROUP 2 - THE ALKALINE EARTHS GROUP 2 ELEMENTS / COMPOUNDS Magnesium Extraction of titanium from its ore (TiCl4) At high temperatures (700-800oC), magnesium is used to reduce titanium in TiCl4 to obtain pure Ti. It is conducted in an inert argon atmosphere. This is because, at high temperatures, in air, the Mg will react with oxygen to produce MgO. Take note of the state symbols here! 2Mg(l) + TiCl4(g) → 2MgCl2(l) + Ti(s) Mg(OH)2 as an antacid Acid indigestion medicines contain Mg(OH)2. As metal hydroxides are basic, this neutralises stomach acid (HCl) to produce a salt and water. Mg(OH)2 is only slightly soluble, hence why these liquid medicines are very “thick”. Mg(OH)2(l) + 2HCl(aq) → 2MgCl2(aq) + H2O(l) Calcium Ca(OH)2 increases soil pH in agriculture A low pH indicates that the soil is acidic. Ca(OH)2 is basic, so it reacts with any acids present in the soil, which increases the pH. Ca(OH)2(l) + acid(aq) → Ca salt (aq) + H2O(l) CaO / CaCO3 used to removed SO2 from flue gases Flue gases are the waste gases from burning fossil fuels. These often contain sulfur which, when burned, produces SO2(g). This is a major contributor to acid rain so it important to limit the amount in the atmosphere. CaO (s) + SO2(g) → CaSO3 (s) (calcium sulfide) Bad!! CO2 is a CaCO3 (s) + SO2(g) → CaSO3 (s) + CO2(g) greenhouse gas! Barium BaSO4 in medicine BaSO4 is highly insoluble and reflects X-rays. A “barium meal” (drink) is ingested by patients. This allows X-ray imaging of their digestive system to highlight / diagnose any medical issues. AQA www.chemistrycoach.co.uk © scidekick ltd 2024