s-block - group 2

Question 1

what are the group 2 metals called?

Answer 1

alkaline earth metals

Question 2

what electronic behaviour do we expect from group 2 elements?

Answer 2

we expect them to lose 2 electrons to obtain a noble gas configuration. however as they already have completed s-orbitals, they are unlikely to gain more electrons/have a -ve O.S.

Question 3

what is the structure of group 2 metals?

Answer 3

metallic solids at room temp + pressure

Question 4

how does m.p/b.p change down group 2 + how does it compare to group 1?

Answer 4

alkaline earth metals have higher melting points
m.p/b.p decreases down group 2 - we can see that dominant intermolecular forces are metallic bonds, group 2 metal cations have 2+ charge, so greater charge density and electron dense cloud than group 1

Question 6

why don’t group 2 elements float when reacting with water?

Answer 6

they are more dense than water, unlike some group 1 elements

Question 7

how can group 2 elements be identified?

Answer 7

flame tests

Question 8

what are the characteristic flame test colours for group 2 - Mg, Ca, Sr, Ba

Answer 8

Mg - white
Ca - orange
Sr - red
Ba - green

Question 9

what unique property does Ra have?

Answer 9

it is radioactive, as well as all its isotopes

Question 10

why is group 2 called alkaline earth metals?

Answer 10

many are found in the composition of the earth - Mg and Ca are abundant in the earths crust, Mg in sea water

Question 11

how are group 2 metals made?

Answer 11

they can all be made/extracted from their naturally occurring compounds

Question 12

how do group 2 metals react with air?

Answer 12

all group 2 metals burn in air to give oxides with rock salt NaCl structures, except BeO which has a more covalent structure

Question 13

can any group 2 metals form peroxides?

Answer 13

only Ba, when reacted with excess air at high temperatures
the other group 2 elements aren’t stable enough to form a peroxide
- specific temp used is 600-700C as this is where the peroxide is the most stable oxide of Ba
- this reaction is also reversible - reaction goes, air is pumped out, reaction reversed, this produces a vacuum of pure O2

Question 14

how do group 2 metal oxides react with water?

Answer 14

this forms hydroxides M(OH)2
- Ba is reactive enough to react directly with water to form a hydroxide
- Be hydroxide must be prepared by precipitation

Question 15

how do group 2 hydroxides react in water?

Answer 15

all are basic in water, and solubility increases down group 2
- exception is Be which is amphoteric

Question 16

how do group 2 metals react with halogens?

Answer 16

all group 2 metals react with halogens to give water soluble dihalides MX2, all have ionic lattice structures, except Be which has a covalent polymeric structure

Question 17

Mg can form 2 halides, MgCl and MgCl2 - what reaction is caused by this fact?

Answer 17

MgCl is unstable to dispropotionation of Mg, a reaction which is driven by the very large lattice enthalpy of MgCl2 (much greater than MgCl lattice enthalpy) - this tells us that +1 O.C. of Mg is not favoured, +2 is much preferred

Question 18

are group 1 fluoride compounds more or less water soluble + why?

Answer 18

less soluble - this is because of their higher lattice enthalpies

Question 19

how are group 2 carbonates made?

Answer 19

these form as precipitates from basic solutions of the hydroxide when CO2 is added - clearly group 2 carbonates are insoluble

Question 20

what is 1 real life reaction that forms insoluble group 2 carbonate?

Answer 20

formation of CaCO3 from Ca containing water/hard water causes lime scale

Question 21

what happens when group 2 carbonates are heated?

Answer 21

they tend to lose CO2, this is a good source of clean CO2
- this reaction becomes less favourable going down the group as higher temperatures are needed for this decomposition to happen
- this is because ΔH becomes less strongly +ve for the reaction, as lattice enthalpy of the oxide is much greater than that of the carbonate, this decomposition takes place at much lower temps for group 2 oxides

Question 22

carbide definition

Answer 22

binary compounds of carbon + group 2 metal based on dianion of acetylene (HC≡CH)

Question 23

give 3 uses of CaC2 carbide

Answer 23

• can be prepared in industry from limestone and then reacted with water to give acetylene, this was the basis of organic chemistry until we started getting acetylene from oil/gas
• can be used as a flammable gas to light carbide lamps
• reacts with N2 at high temperatures to form CaCN2, used for fertilisers and N-heterocycles

Question 24

nitride definition

Answer 24

triply protonated ammonia salts - Mg is one of 2 elements that burns in nitrogen to form this, charge dense N ion likes to be with small Mg cation

Question 25

what happens when you heat group 2 oxoanion salts?

Answer 25

they decompose, e.g. into group 2 oxide

Question 26

name + give one use of an oxoanion salt of Ba

Answer 26

BaSO4 - this is an insoluble precipitate and can be used to test for sulphates

Question 27

what makes salts soluble or insoluble?

Answer 27

solubility of salts depends on relative size of anion and cation small cation + small anion = insoluble small cation + large anion = soluble large cation + small anion = soluble large cation + large cation = insoluble this is because of ΔmixG of the salt, if <0 compound is soluble, if >0 compound is insoluble

Question 28

how does coordination number of group 2 complexes change down the group?

Answer 28

coordination number decreases down the group

Question 29

describe the unique relationship between Li and Mg

Answer 29

these two elements have a diagonal relationship Li differs in its reactivity from heavier elements of its group due to its much smaller ionic radii and higher charge density Li unlike the rest of group 1: - is able to burn in N2 - can only form an oxide - is the only carbonate that decomposes and is much less soluble

Question 30

describe the unique relationship between Be and Al

Answer 30

these two elements have a diagonal relationship Be differs in its reactivity from heavier elements of its group due to its much smaller ionic radii and higher charge density Be unlike the rest of group 2: - doesn't react with water - amphoteric hydroxide - much more covalent - its oxide is inert to water

Question 31

organometallic compound definition

Answer 31

compounds of metals with bonds to carbon - more common for the rest of PT but groups 1/2 can form them too

Question 32

which 2 elements are most able to form organometallic compounds + why

Answer 32

Li and Mg as they are much more covalent and less soluble - their organometallic compounds are also very sensitive to air and moisture

Question 33

how are Mg organometallic compounds purposed in industry?

Answer 33

Mg reacts with RX to form grinard reagents, RMgX - these are used often in organic synthesis

Question 34

homoleptic definition

Answer 34

a complex with only 1 type of ligand - homoleptic organomagnesium compounds are made via the schlenk equilibirum