2.11 - group 2 elements and their compounds

Define s-block element

element with atom of higher energy/outer e- in an s-subshell

Define solubility

maximum mass that can saturate 100g of water at a particular temperature

Group 2 metals properties

reactive metals - shiny solids at RT that tarnish quickly on exposure to air as oxide layer forms

Trend in atomic radius in group 2 metals

atomic radius increases as group 2 descended as outer shell e- further from nucleus and no. of e- increases

Trend in 1st ionisation energy in group 2 metals

1st ionisation energy decreases as group 2 descended as atomic radius increases so less energy required to remove e- from outer shell and shielding increases

Reactivity of group 2 metals

very reactive but less reactive than group 1, reactivity increases as group descended as ionisation energy decreases

Group 2 metal compound colours in states

white if solid, colourless if dissolved in water

Reaction of group 2 metals with oxygen

burn in air/oxygen oxygen to form solid white oxides

Magnesium flame colour

bright white flame

Calcium flame colour

brick red

Strontium flame colour

red

Barium flame colour

Apple green

Reaction of magnesium with water

magnesium turnings react slowly with cold water forming magnesium hydroxide

Why does magnesium’s reaction with water end shortly after reaction starts?

magnesium hydroxide formed almost insoluble in water forming barrier preventing further reaction

Observations of reaction between magnesium and water

few bubbles produced slowly and metal dulls/tarnishes

rest of group 2 metals reaction with water

react with cold water with increasing vigour to give metal hydroxide and hydrogen

Observation of other group 2 metals reaction with water

fizzing/effervescence, metal sinks/rises, heat produced, metal disappears, white solid produced

What happens to precipitation formed going down group 2 in reactions with water?

less precipitation formed going down group as more hydroxide dissolves in water

What do group 2 metals reaction with steam to form?

solid white metal oxides and hydrogen

If you want to measure gas volume formed instead of just capturing it, what do you use?

gas syringe instead of gas jar

Reaction of group 2 metals with dilute acid

more vigorous that water forming metal salt and hydrogen

Observations of group 2 metals reaction with dilute acid

metal disappears, heat produced, fizzing and colourless soln produced

What happens to sulfate’s solubility as group 2 metal goes down group?

sulfate’s solubility decreases down group

How to find relativity solubility of group 2 sulfates?

add soln of sodium sulfate to soln of group 2 ion and observe precipitate

Observations of reaction between group 2 metal oxides and acid

heat produced, white solid disappears and colourless soln forms

Observations of reaction between group 2 metal oxides and water

heat produced, solid expands/crumbles, hisses and colourless soln forms

Why are metal oxides bases?

can accept protons from acids/water

What is magnesium used in?

fireworks, extraction of titanium

What are magnesium alloys used in?

aircraft as light/strong

What is basic magnesium oxide used in?

indigestion remedies to neutralise stomach acid

What is basic calcium carbonate used in?

toothpaste to neutralise tooth acid

What is thermal stability?

stability of molecule when exposed to high temp

What happens to thermal stability to group 2 hydroxides as group descended?

thermal stability increases for group 2 hydroxides as group descended

What happens to M(OH)2 when it decomposes?

M(OH)2 = MO + H2O

What happens to thermal stability to group 2 carbonates as group descended?

thermal stability increases for group 2 carbonates as group descended

What happens to MCO3 when it decomposes?

MCO3 = MO + CO2

What is thermal stability dependant on?

cation size and charge density

What is decomposition based on?

Ability of metal cation to attract/distort charge on neighbouring anion

What happens to polarising power of metal cation as group 2 descended?

polarising power of metal cation decreases down group 2 due to increasing cation size/decreasing charge density so ion less distorted, compound more stable so thermal stability increases

Limestone cycle - calcium carbonate to calcium oxide

thermal decomposition of calcium carbonate

Limestone cycle - calcium oxide to calcium hydroxide

add water to calcium oxide

Limestone cycle - calcium hydroxide to calcium hydroxide solution

dissolve in water

Limestone cycle - calcium hydroxide solution to calcium carbonate

bubble carbon dioxide into limewater

4 stages of limestone cycle

calcium carbonate - calcium oxide - calcium hydroxide - calcium hydroxide solution

Why is cement a mixture of?

calcium oxide, silica(sand) and aluminium oxide

What forms when water added to cement and equation?

calcium silicate forms - Call + SiO2 = CaSiO3(slag)

What does aluminium oxide react with calcium oxide to form and equation ?

calcium aluminate - CaO + Al2O3 = Ca(AlO2)2

What is Al2O3?

amphoteric so reacts with acids or bases to form aluminates

What happens as water evaporates from mixture when forming cement?

as water evaporates, calcium silicate and calcium aluminate crystallise forming solid cement

What is concrete?

cement mixed with gravel and sand to provide mass/aerate

What is reinforced concrete?

concrete with steel rods under tension to provide strength

Solubility equation

solubility = (mass of solute / mass of solvent) x 100

Units of solubility

g/100gH2O

What is solubility dependant on?

lattice enthalpy and hydration enthalpy

Lattice enthalpy

measure of strength of ionic bonds

Hydration enthalpy

energy required when a mole of ions become hydrated

What happens to solubility of hydroxides as group 2 descended?

hydroxide ion small so lattice enthalpy strongly affected by group 2 cation size, lattice enthalpy decreases/hydration enthalpy as cation gets larger. Decrease in lattice enthalpy greater than decrease in hydration enthalpy so solubility increases

What happens to solubility of sulfates as group 2 descended?

sulfate ion large so increasing group 2 cation size has little effect on lattice enthalpy, hydration enthalpy decreases down group as ions get bigger. Decrease in hydration enthalpy greater than decrease in lattice enthalpy so solubility decreases

How does solubility of solid solutes change with increasing temperature?

solubility of solid solutes increase with increasing temp

How does solubility of gaseous solutes change with increasing temperature?

solubility of gaseous solutes decrease with increasing temp