2.7 The periodic table — groups 2 and 7

Students will be assessed on their ability to:

1 Properties down group 2

a. explain the trend in the first ionization energy down group 2b. recall the reaction of the elements in group 2 with oxygen, chlorine and waterc. recall the reactions of the oxides of group 2 elements with water and dilute acid, andtheir hydroxides with dilute acidd. recall the trends in solubility of the hydroxides and sulfates of group 2 elementse. recall the trends in thermal stability of the nitrates and the carbonates of theelements in groups 1 and 2 and explain these in terms of size and charge of the cationsinvolvedf. recall the characteristic flame colours formed by group 1 and 2 compounds andexplain their origin in terms of electron transitionsg. describe and carry out the following:i. experiments to study the thermal decomposition of group 1 and 2 nitrates andcarbonatesii. flame tests on compounds of group 1 and 2 iii. simple acid-base titrations using a range of indicators, acids and alkalis, to calculate

solution concentrations in g dm-3 and mol dm-3 , eg measuring the residual alkali presentafter skinning fruit with potassium hydroxideh. demonstrate an understanding of how to minimise the sources of measurementuncertainty in volumetric analysis and estimate the overall uncertainty in the calculatedresult.

a. explain the trend in the first ionization energy down group 2

Explanation of this trend

Definition: The first ionisation energy is the enthalpy change when one mole ofgaseous atoms loses one mole of electrons to form one mole of gaseousmono-positive ions:

Going down Group 2: Nuclear charge increases. The radius of the atom increases, so the distance between the nucleus and theouter electron increases. There are more filled energy levels between the nucleus and the outer electronthese shield the outer electron from the attraction of the nucleus. The first factor is not as important as the other two, therefore the force ofattraction between the nucleus and outer electron is reduced, and less energy isneeded to remove the outer electron.

3600 965.1 502.8 Ba

4210 1064.2 549.5 Sr

4910 1145 589.7 Ca

7732.6 1450.7 737.7 Mg

14848 1757.1 899.4 Be

3rd 2nd 1st

IEs (kJmol-1)

With regard to successive IEs, the “bigjump” here is between IE(2) and IE(3).

Why?

With regard to successive IEs,the “big jump” here is betweenIE(2) and IE(3).

Why?

Reaction of Groups 2

With O2

#Except Be, whose oxide is amphoteric

b. recall the reaction of the elements in group 2 with oxygen, chlorine and water

2M (s) + O2 (g) 2MO (s)

White solid

The compounds formed are almost completely ionic, except with Be.

The ions formed correspond to the +2 oxidation state, simply because of energetics –more stable compounds are formed with a larger net energy release*.

Group 2 (except Ba) react to form the expected simple, basic oxide.#

The colour of the flame on combustion is dealt with later.

*The energy required to overcome IEs isrecovered in the EAs and the latticeenthalpies.

Ba

Ba (s) + O2 (g) BaO2 (s)

Whitesolid

2Ba (s) + O2 (g) 2BaO (s)

Whitesolid

Both the simple oxide and the peroxide are formed.

REACTIONS OF THE GROUP 2 ELEMENTS WITH WATER

Beryllium has no reaction with water or steam even at red heat.

Magnesium burns in steam to produce magnesium oxide and hydrogen.

Very clean magnesium has a very slight reaction with cold water. The reaction soonstops because the magnesium hydroxide formed is almost insoluble in water andforms a barrier on the magnesium preventing further reaction.

Note: As a general rule, if a metal reacts with cold water, you get the metalhydroxide. If it reacts with steam, the metal oxide is formed. This is because themetal hydroxides thermally decompose (split up on heating) to give the oxide andwater.

Calcium, strontium and barium

These all react with cold water with increasing vigour to give the metal hydroxideand hydrogen. Strontium and barium have reactivities similar to lithium in Group1 of the Periodic Table.

The equation for the reactions of any of these metals would be:

Summary of the trend in reactivity

The reactions become easier as the energy needed to form positive ions falls.This is mainly due to a decrease in ionisation energy as you go down the Group.This leads to lower activation energies, and therefore faster reactions.

Reactions with Cl2

All react on heating to form predominantly ionic chlorides (except Be):Ca(s) + Cl2(g) CaCl2(s)

Na(s) + Cl2(g) NaCl(s)

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MgCl2 shows some covalent character, due to the high charge density of the Mg2+ion. BeCl2 is covalent, and forms a solid polymer via dative covalent bonds to givethe Be an octet of electrons:

Solubility of Group 2 Sulphates and Hydroxides

Ba

Sr

Ca

Mg

Be

HydroxidesSulphates

More soluble

d. recall the trends in solubility of the hydroxides and sulfates of group 2 elements

Sulphates (and carbonates) become less soluble as you go down the Group;hydroxides become more soluble.

Easy to remember, as you know thatBaSO4 is a white ppt. used to identifysulphates,

NB1. The pH of magnesium hydroxide NB2. Limewater

Thermal stability of the s-block carbonates and nitrates

The carbonates of Li, and the Gp2 metals decompose according to the general equation:MCO3 MO (s) + CO2 (g)

You try the Lithium eqn.

The other Gp1 metal carbonates (and barium carbonate) do not decompose at bunsen temperatures.

e. recall the trends in thermal stability of the nitrates and the carbonates of the elements ingroups 1 and 2 and explain these in terms of size and charge of the cations involved

You will need to know: - the trends- the reactions- the explanation

In all cases, for Gp1 or Gp2 carbonates and nitrates, the thermal stability increasesdown the group as the ionic radius of the cation increases, and so its polarisingpower decreases. For the same reason, the thermal stability decreases from Gp1 toGp2 across a period.

The nitrates of Li, and the Gp2 metals decompose according to the general equation:

2M(NO3)2 2MO (s) + 4NO2 (g) + O2(g)

Brown gas is evolvedEg…Again, you try the Lithium eqn.

The other Gp1 metal nitrates decompose to form the nitrites:

2MNO3(s) 2MNO2(s) + O2(g)

Eg…

The same pattern is observed for the s-block nitrates.

The nitrates are white solids,the nitrites yellow.

Explanation of the thermal stability trends

One view is that thermal stability increases as polarising power of the cationdecreases.

Basically, large polarisable anions (CO32-, NO3-) will be more stable with non-polarising cations, but small cations with a large polarising power (highcharge density) will pull the oxygen in the anion towards them, thusfavouring the decomposition of the anion to form the oxide.

The flame test

crimson redstrontium

carmine redlithium

bright orangesodium

lilacpotassium

blue/greencopper

brick redcalcium

light (apple) greenbarium

flame colourmetal

f. recall the characteristic flame colours formed by group 1 and 2 compoundsand explain their origin in terms of electron transitions

The distinctive colours that appear when we heat metals or theircompounds can be used to identify them. You need to learn them…

The energy from the bunsen flame causes electrons to jump up to higher energylevels. When they return to their original “ground” state, they release a certaincharacteristic amount of energy.

Expt: 1.6.2 Flame tests

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Sodium and sodiumcompounds emitalmost monochromaticlight used fordistinguishing opticalisomers.Other uses: Analysingbody fluids,astronomy.

P179-187 Questions

Observation can be qualitative or via a spectrometer. The latter method gives aseries of lines of frequencies that correspond to the differences in the energylevels for a particular atom (ion). See p180. These are called line spectra.

g. describe and carry out the following:i. experiments to study the thermal decomposition of group 1 and 2 nitrates andcarbonatesii. flame tests on compounds of group 1 and 2 iii. simple acid-base titrations using a range of indicators, acids and alkalis, to

calculate solution concentrations in g dm-3 and mol dm-3 , eg measuring theresidual alkali present after skinning fruit with potassium hydroxide

h. demonstrate an understanding of how to minimise the sources of measurementuncertainty in volumetric analysis and estimate the overall uncertainty in thecalculated result.

Group 7: The Halogens

Physical properties

Atomic and ionic radii

What do the halogens look like?What do they look like dissolved in water and organic solvents?

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Not surprisingly, with the addition of anextra shell, the atomic radii increase downthe group.

The ionic radii are much larger than theatomic radii due to the increased repulsionforces of the extra electron in the nowcomplete valence shell.

The only intermolecular forces are weakdispersion forces. These increase as thehalogen molecules increase in size and thegreater number of electrons make themomentary dipoles more significant.

Inorganic chemistry of group 7 (limited to chlorine, bromine and iodine)

a. recall the characteristic physical properties of the elements limited to the appearance of solutions of the elements inwater and hydrocarbon solvents

b. describe and carry out the following chemical reactions of halogens:

i. oxidation reactions with metal and non-metallic elements and ions such as iron(II) and iron(III) ions in solution

ii. disproportionation reactions with cold and hot alkali, eg hot potassium hydroxide with iodine to produce potassiumiodate(V)

c. carry out an iodine/thiosulfate titration, including calculation of the results and evaluation of the proceduresinvolved, eg determination of the purity of potassium iodate(V) by liberation of iodine and titration with standard sodiumthiosulfate solution

d. describe and carry out the following reactions:

i. potassium halides with concentrated sulfuric acid, halogens and silver nitrate solution

ii. silver halides with sunlight and their solubilities in aqueous ammonia solution

iii. hydrogen halides with ammonia and with water (to produce acids)

e. make predictions about fluorine and astatine and their compounds based on the trends in the physical and chemicalproperties of halogens.

Ionisation EnergyLook at your table of the halogens. Sketch a graph of the first ionisationenergies. Explain the pattern.

On your graph, predict where the graphs for IE(2) and IE(3) would be.

On your graph, predict where the graph for Group 0 would be.

Tests for Cl2, Br2 and I2*used to extract bromine from sea water.

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More distinctive colours are seen if an organic solvent is added.

Cl2 – tests make use of the oxidising power of chlorine Bleaches damp litmus paper (damp blue litmus paper momentarily turns redbefore white).

Oxidises Br- and I- to Br2* and I2 respectively.

Write the observations and the colour changes for the latter of these tests.

Br2

Bleaches damp litmus paper, but slowly.Oxidises I-(aq)

I2

No change with litmuspaper Blue/black colour withstarch solution Purple in organicsolvents (in the absenceof O2)

Extra: Amylose in starch is responsible for theformation of a deep blue color in the presenceof iodine. The iodine molecule slips inside ofthe amylose coil.

Iodine - KI Reagent: Iodine is not very solublein water, therefore the iodine reagent is madeby dissolving iodine in water in the presence ofpotassium iodide. This makes a linear triiodideion complex which is soluble. The triiodide ionion slips into the coil of the starch causing anintense blue-black colour.

H-X…The hydrogen halides

HX(g) + H2O(l) H3O+(aq) + X-(aq)

*except H-F

all are c-less gases.

give misty fumes in moist air (why?)

are very water soluble (large enthalpy of hydration of the ionscompensates for the energy required to break the H-X bond).

form strong acids with water*, stronger as the H-X bond becomesweaker (Hydroiodic acid, however, is prone to oxidation).

H-F…again the exception

HX(g) + H2O(l) H3O+(aq) + X-(aq)

H-F(aq) + F-(aq) [F H-F]-(aq)

http://www.haverford.edu/chem/Scarrow/GenChem/acidbase/HFinH2Oanim.html

Forms a weak acid with water due to the greater bond enthalpy…

…and due to the hydrogen bonds formed between the water molecules andthe H-F molecules, and between dissociated fluoride ions and H-Fmolecules:

The production of H3O+(aq) is inhibited, whicheverway you look at it.

Testing for aqueous halide ions

very pale yellow precipitateI-

very pale cream precipitateBr-

white precipitateCl-

no precipitateF-

observationion present

STAGE1

The solution is acidified by adding dilute nitric acid (nitric acid reactswith, and removes, other ions such as carbonate ions that might also givea confusing precipitate with silver nitrate).

2. Silver nitrate solution is then added to give:

IonicEquations:

STAGE 2:Confirmation

precipitate is insoluble in ammonia solution of anyconcentrationAgI

AgBr

precipitate dissolves to give a colourless solutionAgCl

observationoriginal precipitate

Add aqueous ammonia solution:

precipitate is almost unchanged using dilute ammoniasolution, but dissolves in concentrated ammonia solution (or

in XS dilute NH3(aq)) to give a colourless solution

Explanation

Ag+(aq) + Cl-(aq) AgCl(s)

The addition of NH3(aq) results in the followingequilibrium:

Reactions of the silver halides with sunlight...INVESTIGATE!!

The silver halides are only very slightly soluble (see p69). In aqueous solution,rather than the above equations showing precipitation, the following equilibriumis set up:

By Le Chatelier, if we remove Ag+ ions from solution, the top equilibrium willshift to the left and the ppt. Redissolves. This happens readily with chlorideions, less with bromide ions and not at all with iodide ions.

Halide salts with concentrated sulphuric acid

I-

Br-

steamy acidic fumes (of HCl)Cl-

steamy acidic fumes (of HF)F-

observationionpresent

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An alternative test for halide ions is adding concentrated sulphuric acid to thesolid salt. You see:

The concentrated sulphuric acid can act both as an acid and as an oxidisingagent, depending on how easy it is to oxidise the halide ion.With all of the salts, a proton is donated to the halide ion to form the hydrogenhalide gas:

NaCl(s) + H2SO4(l) NaHSO4(s) + HCl(g)

steamy acidic fumes (of HBr) contaminatedwith brown bromine vapour

Some steamy fumes (of HI), but lotsof purple iodine vapour (plus various

red colours in the tube)

Overall ionic eqn:

With Br-

With fluorides and chlorides the reaction stops here, but bromides and iodidesare better reducing agents and redox takes place:

With I-

The brown colour is due to the formation of the tri-iodideion:

I- + I2 I3-

Bad eggs aroma

The sulphuric acid is reduced in 3 different ways. The iodide ions are powerfulenough reducing agents to reduce it:

first to sulphur dioxide (sulphur oxidation state = +4) then to sulphur itself (oxidation state = 0) and all the way to hydrogen sulphide (sulphur oxidation state = -2)

Try the other reduction 1/2 eqns and try to deduce the overall ionic equationsfor the 3 reactions.

Reactions of the Hydrogen Halides:

with Ammonia:

with Water

Redox and Group VII: Positive oxidation states of the halides

Give the oxidation states of chlorine in its common forms and in the above 2 ions:

2NaOH(aq) + Cl2(aq) NaOCl(aq) + NaCl(aq) + H2O(l)

Write the ionic eqn and indicate the oxidation states of the chlorine.

disproportionation reactions with cold and hot alkali, eg hot potassium hydroxide with iodine to produce potassium iodate(V)

Although the syllabus only mentions chlorate(I) and chlorate(V), it is possible that aquestion could refer to the equivalent ions of bromine and iodine.

Everyday bleach – sodium chlorate(I) – is prepared by reacting chlorine waterwith sodium hydroxide solution:

When the same species in a chemical reaction is both oxidised and reduced,we say that disproportionation has taken place.

With chlorine and hot concentrated NaOH solution:

3Cl2(aq) + 6NaOH(aq) 5NaCl(aq) + NaClO3(aq) + 3H2O(l)

Ionic eqn? Oxidation states?

Full eqn? Show the oxidation states.

Chlorine with water (“chlorine water”):

Cl2(g) + H2O HCl(aq) + HClO(aq)

Chlorate(I) ions on heating in solution will disproportionate:

3 OCl- (aq) ClO3-(aq) + 2Cl – (aq)

Full eqn? Show the oxidation states.

Write the ionic equation and consider the effects of adding acid or alkali to themixture.

Redox and Group VII: Iodine with Sodium Thiosulphate

c. carry out an iodine/thiosulfate titration, includingcalculation of the results and evaluation of the proceduresinvolved, eg determination of the purity of potassiumiodate(V) by liberation of iodine and titration with standardsodium thiosulfate solution