2.11 - group 2 elements and their compounds

Define s-block element

element with outer e- in an s-subshell

Define solubility

maximum mass that saturates 100g of water at particular temp

Group 2 metals properties

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

Trend in atomic radius in group 2 metals

On descending, atomic radius increases as outer shell e- further from nucleus and no. of e- increases

Trend in 1st ionisation energy in group 2 metals

As descended, 1st ionisation energy decreases as atomic radius and shielding increases

Reactivity of group 2 metals

less reactive than group 1, reactivity increases as group descended

Group 2 metal compound colours in states

white solid, colourless if dissolved in water

Reaction of group 2 metals with oxygen

burn in air/oxygen to form solid white oxides

Magnesium flame colour

bright white flame

Calcium flame colour

brick red

Strontium flame colour

red

Reaction of magnesium with water

magnesium turnings react slowly with cold water forming MgOH

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

MgOH formed almost insoluble in water forming barrier preventing further reaction

Observations of reaction between magnesium and water

few bubbles produced slowly, metal dulls

rest of group 2 metals reaction with water

react with cold water with increasing vigour to give metal hydroxide and H2

Observation of other group 2 metals reaction with water

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

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

less precipitation formed down group as more OH- dissolves in water

What do group 2 metals reaction with steam to form?

solid white metal oxides and H2

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

gas syringe

Reaction of group 2 metals with dilute acid

more vigorous that water forming metal salt and H2

Observations of group 2 metals reaction with dilute acid

metal disappears, heat, fizzing, colourless soln produced

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

decreases

How to find relativity solubility of group 2 sulfates?

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

Observations of reaction between group 2 metal oxides and acid

heat produced, white solid disappears, colourless soln forms

Observations of reaction between group 2 metal oxides and water

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

Why are metal oxides bases?

accept protons from acids/water

What is magnesium used in?

fireworks

What are magnesium alloys used in?

aircraft

What is basic magnesium oxide used in?

indigestion remedies

What is basic calcium carbonate used in?

toothpaste

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

increases

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

M(OH)2 = MO + H2O

What happens to MCO3 when it decomposes?

MCO3 = MO + CO2

What is thermal stability dependant on?

cation size and charge density

What is polarising power?

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

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

decreases as increasing cation size/decreasing charge density so less distorted, compound more stable and thermal stability increases

Limestone cycle - calcium carbonate to calcium oxide

thermal decomposition of CaCO3

Limestone cycle - calcium oxide to calcium hydroxide

add water to CaO

Limestone cycle - calcium hydroxide to calcium hydroxide solution

dissolve in water

Limestone cycle - calcium hydroxide solution to calcium carbonate

bubble CO2 into limewater

4 stages of limestone cycle

CaCO3 - CaO - CaOH - CaOH solution

Why is cement a mixture of?

CaO, silica, Al2O3

What forms when water added to cement and equation?

calcium silicate forms - CaO + SiO2 = CaSiO3

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

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

What feature does Al2O3 have?

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/sand to provide mass/aerate

What is reinforced concrete?

concrete with steel rods providing strength

Solubility equation

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

Units of solubility

g/100gH2O

What is solubility dependant on?

lattice enthalpy, hydration enthalpy

Lattice enthalpy

measure of ionic bond strength

Hydration enthalpy

energy required when mole of ions become hydrated

What happens to solubility of hydroxides as group 2 descended?

OH- small so lattice enthalpy strongly affected by group 2 cation size, lattice/hydration enthalpy decreases as cation grows. Decrease in lattice enthalpy greater than hydration enthalpy so solubility increases

What happens to solubility of sulfates as group 2 descended?

SO4 2- large so increasing group 2 cation size has little effect on lattice enthalpy, hydration enthalpy decreases down group as ions larger. Decrease in hydration enthalpy greater than lattice enthalpy so solubility decreases

How does solubility of solid solutes change with increasing temperature?

solubility of solid solutes increase with temp

How does solubility of gaseous solutes change with increasing temperature?

solubility of gaseous solutes decrease with temp