Inorganic Chemistry and the Periodic Table

Question 1

Describe and explain the trend in ionisation energy displayed by group 2 metals.

Answer 1

Ionisation energy decreases down group 2 because the number of shells, and so atomic radius, increases. Therefore, shielding also increases. The nuclear charge also increases, but this is outweighed by the other factors, meaning an overall decrease down the group.

Question 2

What is the trend in reactivity down group 2?

Answer 2

There is a general increase in reactivity down the group because the number of shells and shielding increases, so outer electrons are less strongly attracted to the nucleus, so lower ionisation energy (this outweighs increased nuclear charge). Therefore, it is easier for the elements to lose electrons (and react) as you go down the group, causing a general increase in reactivity.

Question 3

What is the reaction of group 2 elements with oxygen?

Answer 3

2M(s) + O2(g) —> 2MO(s)
Metal + oxygen —> metal oxide
Observations: magnesium burns in oxygen with a bright flame and a white solid is formed. This reaction becomes more vigorous as you go down the group. Note: must be heated to start reaction as otherwise metal oxide will coat the metal and prevent further reaction.

Question 4

What is the reaction between group 2 elements and chlorine?

Answer 4

M(s) + Cl2(g) —> MCl2(s)
Metal + chlorine —> metal chloride
Observations: chlorine gas is pale green, and group 2 elects will burn brightly when reacting with chlorine, this becomes more vigorous down the group. Note: reactants must be heated to start the reaction

Question 5

What is the reaction of group 2 elements and water?

Answer 5

M(s) + 2H2O(l) —> M(OH)2(s) + H2(g)
Metal + water —> metal hydroxide + hydrogen
Observations: magnesium reacts with water very slowly and not completely, calcium, strontium and barium react with increasing vigour, which can be seen by an increase in effervescence. Metal hydroxides are more soluble (in water) as you go down the group, so with magnesium and calcium hydroxide, you get a cloudy solution due to the precipitate.

Question 6

What is the reaction of magnesium and steam?

Answer 6

When heated in steam, magnesium reacts rapidly, forming magnesium oxide (a white solid) and hydrogen gas. This reaction is vigorous. Mg(s) + H2O(g) —> MgO(s) + H2(g)

Question 7

What is the reaction between group 2 oxides and water?

Answer 7

MO(s) + H2O(l) —> M(OH)2(aq)
Metal oxide + water —> metal hydroxide
Ionic equation: O2- + H2O —> 2OH-
Observations: the solids react to form a colourless liquid, which is alkaline

Question 8

What is the trend in solubility of the group 2 hydroxides?

Answer 8

The solubility of the group 2 hydroxides increases down the group (i.e. magnesium hydroxide is very insoluble, while barium hydroxide is very soluble in water). Therefore, the maximum alkalinity of the solutions increases down the group.

Question 9

What is milk of magnesia and how does it work?

Answer 9

Milk of magnesia is a suspension of magnesium hydroxide in water which acts as an antacid. This cures indigestion as it neutralises some of the excess hydrochloric acid in the stomach, relieving the indigestion symptoms. Although hydroxide ions attack human tissue, the very low solubility of magnesium hydroxide means that the concentration of OH- ions in the medicine is low and does not pose a health risk.

Question 10

What is the test for carbon dioxide gas?

Answer 10

The test for carbon dioxide is that when bubbled through limewater, carbon dioxide reacts to give a white precipitate, making the solution cloudy. CO2 + Ca(OH)2 —> CaCO3 + H2O

Question 11

What is the reaction of group 2 oxides and hydroxides with acids?

Answer 11

All group 2 oxides and hydroxides react with acids to give salts and water, so these are neutralisation reactions. Observations: a white solid reacts to form a colourless solution. The reaction is exothermic.
E.g. MgO + H2SO4 —> MgSO4 + H2O
Ba(OH)2 + 2HCl —> BaCl2 + 2H2O

Question 12

What is the trend in solubility of the group 2 sulphates?

Answer 12

The trend is that the solubility of the group 2 sulphates decreases down the group (most soluble at the top). Magnesium sulfate is soluble in water, calcium sulfate is slightly soluble and strontium and beryllium sulfate are insoluble.

Question 13

What is the test for sulfate ions in solution?

Answer 13

The presence of sulfate ions in solution is shown by adding a solution containing barium ions (usually barium chloride or nitrate). Any sulfate ions in solution will react with the barium ions to form a white precipitate of barium sulfate. Ba2+ + SO42- —> BaSO4. Other anions could also form a white precipitate with barium ions, so there must be H+ ions present to prevent barium carbonate from forming as a white precipitate (dilute nitric acid or dilute hydrochloric acid is added for this).

Question 14

What is barium sulfate used for in medicine?

Answer 14

Barium sulfate is used in medicine to make soft tissues show up on X-rays. Solution containing barium ions are highly toxic to humans, but because barium sulfate is so insoluble, the barium won’t get into the bloodstream. This is called a ‘barium meal’.

Question 15

What is thermal stability?

Answer 15

Thermal stability is a measure of the extent to which a compound decomposes when heated.

Question 16

What type of thermal decomposition do most group 1 nitrates undergo?

Answer 16

Most group 1 nitrates undergo lesser thermal decomposition (metal nitrate —> metal nitrite + oxygen). The only exception is lithium nitrate, which undergoes greater thermal decomposition (lithium nitrate —> lithium oxide + nitrogen dioxide + oxygen).

Question 17

What type of thermal decomposition do group 2 nitrates undergo?

Answer 17

Group 2 nitrates undergo greater thermal decomposition (metal nitrate —> metal oxide + nitrogen dioxide + oxygen).

Question 18

How can you tell the difference between a lesser and a greater thermal decomposition reaction?

Answer 18

Greater thermal decomposition produces nitrogen dioxide gas, which is brown and highly toxic, while lesser thermal decomposition does not.

Question 19

How do group 1 carbonates thermally decompose?

Answer 19

Lithium carbonate decomposes to lithium oxide and carbon dioxide. No other group 1 carbonates can thermally decompose.

Question 20

How do group 2 carbonates thermally decompose?

Answer 20

Group 2 carbonates thermally decompose to give group 2 oxides and carbon dioxide.

Question 21

Which factors affect thermal stability?

Answer 21

Size of ion, charge of ion, polarising ability.

Question 22

What is the trend in thermal stability of group 2 carbonates?

Answer 22

Thermal stability increases down the group (decomposes less easily). This is because the charge on the ions remains the same throughout the group, but the size of the ions increases, meaning the charge density decreases. This means that the lower ions in the group have less of a polarising effect on the CO32- anion, and so are less able to weaken and district the C-O bond. This means that more energy is needed to decompose the carbonates as you go down the group, meaning thermal stability is increased.

Question 23

Why do group 2 nitrates decompose more readily on heating than group 1 nitrates?

Answer 23

Group 2 ions have higher charges than group 1 ions, and ions in the same period are a similar size, so generally the charge density of group 2 is higher. This makes group 2 ions more polarising, and so when are more able to weaken and distort the N=O bond, meaning less energy is required to thermally decompose group 2 nitrates than group 1 nitrates (group 2 nitrates less thermally stable).

Question 24

Why does lithium nitrate undergo greater thermal decomposition, while the other group 1 nitrates undergo lesser thermal decomposition?

Answer 24

All group 1 ions have the same charge, but lithium is the smallest of them, so has the highest charge density. Lithium ions are able to polarise the NO3- ions sufficiently to undergo greater thermal decomposition, but other group 1 ions can’t weaken the N=O bond enough to do this, so they undergo lesser thermal decomposition.

Question 25

What does a flame test do?

Answer 25

Flame test can be used to identify some group 1 and 2 ions, but not all of them, by the colour they turn a flame when placed in it.

Question 26

How do you carry out a flame test?

Answer 26

Light a Bunsen burner and put it on a blue (but not roaring blue) flame. Add concentrated hydrochloric acid to a beaker and dip a nichrome wire into it. Heat the nichrome wire in the Bunsen burner flame to remove any impurities, then dip in the solid salt you are testing. Hold the wire near the edge of the flame and observe the colour.

Question 27

What are some problems with flame tests?

Answer 27

Many compounds contain small amounts of sodium compounds as impurities, so the intense colour of sodium can mask other colours. Describing colours with words is subjective, and many ions have similar colours (e.g. many ions give a shade of red, so it is difficult to distinguish between them).

Question 28

What colour do lithium ions give in a flame test?

Answer 28

Red

Question 29

What colour do sodium ions give in a flame test?

Answer 29

Yellow

Question 30

What colour do potassium ions give in a flame test?

Answer 30

Lilac

Question 31

What colour do rubidium ions give in a flame test?

Answer 31

Red

Question 32

What colour do caesium ions give in a flame test?

Answer 32

Blue

Question 33

What colour do magnesium ions give in a flame test?

Answer 33

Colourless

Question 34

What colour do calcium ions give in a flame test?

Answer 34

Brick red

Question 35

What colour do strontium ions give in a flame test?

Answer 35

Red

Question 36

What colour do barium ions give in a flame test?

Answer 36

Pale Green

Question 37

Why do some ions change the colour of flames when burned?

Answer 37

Electrons occupy specific orbitals which are at a particular energy level. When heated, some electrons within the ions are ‘excited’ to higher energy levels. However, this is then followed by the return of the electron to its ‘ground’, original state. Electromagnetic radiation is emitted when this happens, some of which has a wavelength which corresponds to a particular colour of the visible light spectrum, giving the flame a different colour. Ions which turn the flame colourless give out a frequency of EM radiation which is not within the visible light spectrum.

Question 38

What is the test for ammonium ions?

Answer 38

Ammonium ions don’t give a colour in a flame test, so the usual test for ammonium ions do to add sodium hydroxide solution to a solution containing ammonium ions and warm the mixture. NH4+ + OH- —> NH3 + H2O. Ammonia gas is released, which has a particular smell, but also turns damp red litmus paper blue(as ammonia is the only common alkaline gas). Alternatively, hydrogen chloride gas react with ammonia to from white fumes of ammonium chloride.

Question 39

What is the trend in melting and boiling temperatures for group 7?

Answer 39

Melting and boiling temperatures increase down the group as the only intermolecular force is the London force (as there are no permanent dipoles). Down the group, the number of electrons per molecule and the size of the electron cloud increases, so the strength of the London forces increases, resulting in increased melting and boiling temperatures. This is why fluorine and chlorine are gases at room temperature, while bromine is a liquid and iodine and astatine are solids.

Question 40

What is the trend in electronegativity down group 7 and why?

Answer 40

Electronegativity decreases down the group as the distance between the nucleus and the bonding pair of electrons increases (atomic radius increases), so less attraction. Shielding also increases, which decreases attraction of bonding electrons. The nuclear charge does increase, which would increase the electronegativity, but this is outweighed by the other factors.

Question 41

What is the trend in reactivity of group 7 elements?

Answer 41

Reactivity decreases down group 7 as the atomic radius and shielding increase, so there is less attraction to the nucleus, so it is harder to gain electrons (which is required for them to react). This outweighs the effect of increased nuclear charge.

Question 42

What happens when halogens react with group 1 and 2 metals?

Answer 42

The reactions are most rigorous between metals at the bottom of groups 1 and 2 and the halogens at the top of group 7. The products are salts - usually white. These reactions involve electron transfers to the halogen, so they are redox reactions where the halogen acts as an oxidising agent. The oxidation number of the halogen decreases from 0 to -1 and of the metal increases from 0 to +1 or +2, depending of group.
E.g. 2Li + Cl2 —> 2LiCl
Ba + Br2 —> BaBr2

Question 43

What do halogen displacement reactions show?

Answer 43

Halogen displacement reactions show the trend in reactivity of the halogens, as a more reactive halogen will displace a less reactive in from its compound in aqueous solution. A reaction is indicated by a colour change, but his can be difficult to identify in water, so the products can be added to an organic solvent, like cyclohexane, and shaken. The halogens are more soluble in the organic solvent than water, so dissolve in the organic upper layer, where the colour can be more easily seen. Chlorine is pale green, bromine is orange and iodine is purple in cyclohexane.

Question 44

What is a disproportionation reaction?

Answer 44

Where the same species is simultaneously oxidised and reduced.

Question 45

What is the equation for the reaction between chlorine and water?

Answer 45

Cl2 + H2O —> HCl + HClO
Chlorine + water —> hydrochloric acid + chloric(I) acid
This is disproportionation as the oxidation number of Cl in Cl2 is 0, in HCl is -1 (reduction) and in HClO is +1 (oxidation). This reaction is used to disinfect/purify water for drinking/use in swimming pools.

Question 46

What is the equation for the reaction of chlorine with cold alkali?

Answer 46

Cl2 + 2NaOH —> NaCl + NaClO + H2O
Chlorine + sodium hydroxide —> sodium chloride + sodium chlorate (I) + water
Oxidation number of chlorine in Cl2 is 0, in NaCl is -1 (reduction) and in NaClO is +1 (oxidation), so this is disproportionation. This reaction is used to make bleach.

Question 47

What is the equation for the reaction of chlorine with hot alkali?

Answer 47

3Cl2 + 6NaOH —> 5NaCl + NaClO3 + 3H2O
Chlorine + sodium hydroxide —> sodium chloride + sodium chlorate (V) + water
Oxidation number of Cl in Cl2 is 0, in NaCl is -1 (reduction) and in NaClO3 is +5 (oxidation), so this is disproportionation.
This reaction is used to make weed killer.

Question 48

Explain the trend in reducing power of the halide ions.

Answer 48

As you go down the group, the reducing power of the halide ions increases. This is because, in order to reduce other species, the ions must themselves be oxidised (so must lose electrons). As you go down the group, the ionic radius and shielding increase, so attraction to the nucleus is weaker. This outweighs the effect of increased nuclear charge and means that its speaker to lose electron down the group. Therefore, reducing power increases. Note, this is the opposite trend to reactivity as that involves the gain of electrons.

Question 49

What is the equation for the reaction between sodium chloride and concentrated sulfuric acid?

Answer 49

NaCl + H2SO4 —> NaHSO4 + HCl
Oxidation number of S is +6 in H2SO4 and +6 in NaHSO4, so the sulfur is reduced by the chloride ions.
Observations: misty fumes (HCl) form

Question 50

What are the equations for the reaction of bromide ions with concentrated sulfuric acid?

Answer 50

NaBr + H2SO4 —> NaHSO4 + HBr
2HBr + H2SO4 —> 2H2O + SO2 +Br2
Oxidation number of S in H2SO4 is +6 and in SO2 is +4, so S is reduced by halide ions.
Observations - misty fumes (HBr), brown fumes (Br2), colourless gas with choking smell (SO2)

Question 51

What are the equations for the reaction of iodide ions with concentrated sulfuric acid?

Answer 51

NaI + H2SO4 —> NaHSO4 + HI
2HI + H2SO4 —> I2 + SO2 + 2H2O
6HI + SO2 —> H2S + 3 I2 + 2H2O
Oxidation number of S decreases from +6 in H2SO4 to -2 in H2S, so S is reduced by iodide ions.
Observations - misty fumes (HI), purple fumes or black solid (iodine), colourless gas with choking smell (SO2), yellow solid (S), colourless gas with rotten egg smell (H2S).

Question 52

How do the reactions of halide ions with concentrated sulfuric acid show the trend in reducing power of the halide ions?

Answer 52

Chloride ions only reduce H2SO4 (+6) to NaHSO4 (+6). Bromide ions reduce S from +6 to +4 (SO2). Iodide ions reduce S all the way to -2 (H2S). This shows the trend in reducing power as iodide> bromide > chloride.