Chapter 8 - Reactivity trends

Question 1

What are the elements in group 2 also known as?

Answer 1

a. They are known as alkaline earth metals.
b. The name comes from the alkaline properties of the metal hydroxides. The elements are reactive metals and do not occur in their elemental form naturally.
c. On Earth, they are found in stable compounds such as calcium carbonate, CaCO3.

Question 2

What is the most common type of reaction of group 2 elements?

Answer 2

a. Redox reactions are the most common type of reaction of Group 2 elements.
b. Each metal atom is oxidised, losing two electrons to form a 2+ ion with the electron configuration of a noble gas. Another species will gain these two electrons and be reduced.
c. The group 2 element is therefore called a reducing agent because it has reduced another species.

Question 3

What is the redox reaction of group 2 elements with oxygen?

Answer 3

a. The group 2 elements all react with oxygen to form a metal oxide with the general formula MO, made up of M2+ and O2- ions.
b. You will have seen in the laboratory the reaction of magnesium with oxygen in the laboratory. The magnesium burns with a brilliant white light and forms white magnesium oxide.

Question 4

What is the redox reaction with water?

Answer 4

a. The group 2 elements react with water to form an alkaline hydroxide, with the general formula M(OH)2, and hydrogen gas.
b. Water and magnesium react very slowly, but the reaction becomes more and more vigorous with metals further down the group – reactivity increases down the group.

Question 5

What is the redox reaction with dilute acids?

Answer 5

a. All group 2 elements react with dilute acids to form a salt and hydrogen – reactivity increases down the group, so the reaction becomes more vigorous down the group.
b. Magnesium ribbon reacts with dilute hydrochloric acid to give off tiny bubbles of hydrogen.

Question 6

Why does the reactivity of the redox reactions of group 2 elements increase down the group?

Answer 6

a. The atoms of Group 2 elements react by losing electrons to form +2 ions. The formation of +2 ions from gaseous atoms requires the input of two ionisation energies.
b. The ionisation energies decrease down the group because the attraction between the nucleus and the outer electrons decreases as a result of increasing atomic radius and increasing shielding.
c. Although other energy changes take place when Group 2 elements react, the first and second ionisation energies make up most of the energy input. The total energy input from ionisation energies to form 2+ ions decreases down the group. The group 2 elements become more reactive and stronger reducing agents down the group.

Question 7

What is the reaction of group 2 oxides with water?

Answer 7

a. The oxides of group 2 elements react with water, releasing hydroxide ions, OH-, and forming alkaline solutions of the metal hydroxide.
i. i.e. CaO(s) + H2O(l) ==> Ca2+(aq) + 2OH-(aq)
b. The group 2 hydroxides are only slightly soluble in water. When the solution becomes saturated (a solution in which no more solute can be dissolved in the solvent), any further metal and hydroxide ions form a solid precipitate:
i. Ca2+(aq) + 2OH-(aq) ==> Ca(OH)2(s)

Question 8

What is the solubility trend of group 2 hydroxides?

Answer 8

a. The solubility of the hydroxides in water increases down the group as the resulting solutions contain more OH- ions and more alkaline.
i. Mg(OH)2 is only very slightly soluble in water. The solution has a low OH- concentration and has a pH of around 10.
ii. Ba(OH)2 is much more soluble in water. The solution has a greater OH- concentration and has a pH of around 13.
b. So, as you go down group 2, the solubility increases, therefore the pH increases and as a result the alkalinity increases.

Question 9

How can you show the trend of solubility of group 2 hydroxides through an experiment?

Answer 9

a. Add a spatula of each group 2 oxide to water in a test tube.
b. Shake the mixture. On this scale, there is insufficient water to dissolve all of the metal hydroxide that forms. You will have a saturated solution of each metal hydroxide with some white undissolved at the bottom of the test-tube.
c. Measure the pH of each solution. The alkalinity will be seen to increase down the group.

Question 10

What are some of the uses of group 2 compounds in agriculture?

Answer 10

a. Calcium hydroxide, Ca(OH)2, is added to fields as lime by farmers to increase the pH of acidic soils.
b. It is a white lime powder on fields.

c. The calcium hydroxide neutralises acid in the soil, forming neutral water:
i. Ca(OH)2(s) + 2H+ ==> Ca2+(aq) + 2H2O(l).

Question 11

What are some of the uses of group 2 compounds in medicine?

Answer 11

a. Group 2 bases are often used as antacids for treating acid indigestion. Many indigestion tablets use magnesium and calcium carbonates as the main ingredients, whilst ‘milk of magnesia’ is a suspension of white magnesium hydroxide, Mg(OH)2, in water.
b. Magnesium hydroxide is only very slightly soluble in water.

c. The acid in the stomach is mainly hydrochloric acid, therefore neutralisation reactions will occur:
i. Mg(OH)2(s) + 2HCl(aq) ==> MgCl2(aq) + 2H2O(l)

ii. CaCO3(s) + 2HCl(aq) ==> CaCl2(aq) + H2O(l) + CO2(g)

Question 12

What are some examples of common indigestion medicine?

Answer 12

Gaviscon

Rennie

Question 13

What are the elements of group 7 also known as?

Answer 13

a. They are known as halogens and are the most reactive non-metallic group.
b. The elements do not occur in their elemental form in nature.
c. On Earth, the halogens occur as stable halide ions (Cl-, Br-, and I-) dissolved in sea water or combined with sodium or potassium as solid deposits, such as in salt mines containing common salt, NaCl.

Question 14

What is the trend in boiling points of group 7 elements?

Answer 14

a. At room temperature and pressure, all the halogens exist as diatomic molecules, X2.
b. The group contains elements in all three physical states at RTP (room temperature and pressure), changing from gas to liquid to solid down the group.
c. In the solid states the halogens form lattices with simple molecular structures.
d. Down the group, the elements contain more electrons and therefore have stronger London forces. This means that more energy is required to break the intermolecular forces and as such the boiling point increases.

Question 15

What colour is chlorine, bromine and iodine at RTP?

Answer 15

a. Chlorine is a pale green gas at RTP.
b. Bromine liquid is extremely toxic, and vaporises readily at room temperature, as can be seen from the orange gas above the red-brown liquid.
c. Iodine is a solid with grey-black crystals.

Question 16

What is the most common type of reactions of group 7 elements?

Answer 16

a. Each halogen has seven outer-shell electrons, two electrons are in the outer s sub-shell and five in the outer p sub-shell – s2p5.
b. Redox reactions are the most common type of reaction of the halogens. Each halogen atom is reduced, gaining one electron to from a 1- halide ion with the electron configuration of the nearest noble gas.
i. Cl2 + 2e-  2Cl- (chlorine is reduced)
c. Another species loses electrons to halogen atoms – it is oxidised. The halogen is called an oxidising agent because it has oxidised another species.

Question 17

What is the halogen-halide displacement reactions?

Answer 17

a. Displacement reactions of halogens with halide ions can be carried out on a test tube scale. The results of the displacement reactions show that the reactivity of the halogens decreases down the group.
b. A solution of each halogen is added to aqueous solutions of the other halides. For example, a solution of chlorine (Cl2) is added to two aqueous solutions containing bromine (Br-) and iodine (I-) ions.
c. If the halogen added is more reactive than the halide present,
i. a reaction takes place, the halogen displacing the halide from solution.
ii. the solution changes colour.
d. A solution of chlorine (Cl2) reacts with Br- ions to form an orange colour from the formation of Br2. A solution of Chlorine (Cl2) also reacts with I- ions to form a violet colour from I2 formation.
e. A solution of Bromine (Br2) reacts with I- only to form a violet colour from I2 formation.
f. A solution of Iodine (I2) does not react at all and so there is no reaction.

Question 18

How can you tell apart solutions of iodine and bromine, as in water they can appear as a similar orange-brown colour, depending on the concentration?

Answer 18

a. To tell them apart, an organic non-polar solvent such as cyclohexane can be added, and the mixture shaken.
b. The non-polar halogens dissolve more readily in cyclohexane than in water.
c. In cyclohexane their colours are much easier to tell apart, with iodine being a deep violet colour.
d. Bromine remaining an orange colour.
e. Chlorine remaining a pale green colour.

Question 19

What is the order of reactivity of the three halogens chlorine, bromine, and iodine?

Answer 19

a. Chlorine is the most reactive.
b. Bromine is in the middle.
c. Iodine is the least reactive.

Question 20

What is fluorine and astatine?

Answer 20

a. Fluorine is a pale-yellow gas, reacting with almost any substance that it comes in contact with.
b. Astatine is extremely rare because it is radioactive and decays rapidly, and the element has never actually been seen. It is predicted to be the least reactive halogen.

Question 21

What is the trend in reactivity of the group 7 elements?

Answer 21

a. Down the group, the atomic radius increases. More inner shells and so shielding increases. There is less nuclear attraction to capture an electron from another species and the reactivity decreases.
b. In the halogens, fluorine is the strongest oxidising agent, gaining electrons from other species more readily than the other halogens. The halogens become weaker oxidising agents down the group.

Question 22

What is a disproportionation reaction?

Answer 22

a. A disproportionation reaction is a redox reaction in which the same element is both oxidised and reduced.
b. The reaction of chlorine with water and with cold, dilute sodium hydroxide are two examples of disproportionation reactions.

Question 23

What is the reaction of chlorine with water?

Answer 23

a. When small amounts of chlorine are added to water, a disproportionation reaction takes place. For each chlorine molecule, one chlorine atom is oxidised, and the other chlorine atom is reduced:
i. Cl2(aq) + H2O(l) ==> HClO(aq) + HCl(aq)
b. The two products HClO (chloric (I) acid) and HCl (hydrochloric acid) are both acids.
c. The bacteria are killed by chloric (I) acid and chlorate (I) ions, ClO-, rather than by chlorine. Chloric (I) acid also acts as a weak bleach. You can show this by adding some indicator solution to a solution of chlorine in water. The indicator first turns red, from the presence of the two acids. The colour then disappears as the bleaching action of chloric (I) acid takes effect.

Question 24

What is the reaction of chlorine with cold, dilute aqueous sodium hydroxide?

Answer 24

a. The reaction of chlorine with water is limited by the low solubility of chlorine in water.
b. If the water contains dissolved sodium hydroxide, much more chlorine dissolves and another disproportionation reaction takes place:
i. Cl2(aq) + 2NaOH(aq) ==> NaClO(aq) + NaCl(aq) + H2O(l)
c. The resulting solution contains a large concentration of chlorate(I), ClO- ions, from the sodium chlorate(I), NaClO, that is formed. This solution finds a use as household bleach, which is made by reacting chlorine with cold dilute aqueous sodium hydroxide.

Question 25

What are the benefits and risks of chlorine use?

Answer 25

a. Although chlorine is beneficial in ensuring that our water is fit to drink and that bacteria are killed, chlorine is also an extremely toxic gas. Chlorine is a respiratory irritant in small concentrations, and large concentrations can be fatal.
b. Chlorine in drinking water can react with organic hydrocarbons such as methane, formed from decaying vegetation. Chlorinated hydrocarbons are formed, which are suspected of causing cancer. However, the overall risk to health of not adding chlorine to the water supply is far greater than the risk posed by the chlorinated hydrocarbons. The quality of drinking water would be compromised and diseases such as typhoid and cholera might break out.

Question 26

What are the precipitation reactions with aqueous silver ions?

Answer 26

a. Aqueous halide ions react with aqueous silver ions to form precipitates of silver halides:
i. Ag+(aq) + X-(aq) ==> AgX(s).

Question 27

What is the reducing ability of halide ions?

Answer 27

a. In the displacement reactions between halogen and halide ions, the halogen gained electrons and the halide ions lost electrons. So, halogens are oxidising agents and halide ions are reducing agents.
b. The reducing ability of halide ions can be shown by their reactions with sulfuric acid, H2SO4, which is a strong oxidising agent. If a reaction takes place, the halide ions will be oxidised to form the halogen.
c. Chloride ions are not powerful enough to reduce H2SO4.
d. Bromide ions are more powerful and can reduce H2SO4 to sulfur dioxide, SO2.
i. 2H+ + H2SO4 + 2Br- ==> SO2 + Br2 + 2H2O.
e. Iodide ions are even more powerful and reduce the sulfur dioxide formed to sulfur, S, which is reduced further to hydrogen sulphide, H2S.

Question 28

How can you test for carbonate ions, CO32-?

Answer 28

a. In a test tube, add dilute nitric acid to the solid or solution to be tested.
b. If you see bubbles, the unknown compound could be a carbohydrate.
c. To prove that the gas is carbon dioxide:
i. Bubble the gas through limewater – a saturated aqueous solution of calcium hydroxide, Ca(OH)2.
ii. Carbon dioxide reacts to form a fine white precipitate of calcium carbonate, which turns the lime water cloudy (milky):
iii. CO2(g) + Ca(OH)2(aq)  CaCO3(s) + H2O(l)

Question 29

How can you test for sulfate ions?

Answer 29

a. Most sulfates are soluble in water, but barium sulfate, BaSO4, is very insoluble. The formation of a white precipitate of barium sulfate is the basis for the sulfate test.
b. Aqueous barium ions are added to a solution of an unknown compound.
i. Ba2+(aq) + SO42-(aq) ==> BaSO4(s).
c. Usually the Ba2+(aq) ions are added as aqueous barium chloride or barium nitrate. If you intend to carry out a halide test afterwards, use barium nitrate – with barium chloride, you are introducing chloride ions into your solution.

Question 30

What is the basis for the halide test?

Answer 30

a. Add aqueous silver nitrate, AgNO3, to an aqueous solution of a halide.
b. The silver halide precipitates are different colours – silver chloride is white, silver bromide is cream-coloured, and silver iodide is yellow.
c. Add aqueous ammonia to test the solubility of the precipitates. This stage is very useful because the three precipitate colours can be difficult to tell apart.
d. Chloride ions are soluble in dilute ammonia.
e. Bromide ions are soluble in concentrated ammonia.
f. Iodide ions are insoluble in concentrated ammonia.

Question 31

What are barium meals?

Answer 31

a. Barium meals are used to enable doctors to see the outline of the gullet, stomach, and upper small intestine in order to identify abnormalities such as ulcers or tumours.
b. The patient swallows’ water that has been shaken with barium sulfate, the insoluble compound that forms during the sulfate test.
c. The white precipitate coats the inner lining of the gut. An X-ray image is then taken that displays the barium sulfate coating the gut.

Question 32

What is the correct order for testing when asked to analyse an unknown inorganic compound?

Answer 32

“CASH”
Carbonate, CO32-.
Sulfate, SO42-.
Halides - Cl-, Br-, and I-.

Question 33

Why is there a correct order for analysing an unknown compound?

Answer 33

a. In the carbonate test, you add dilute acid and are looking for effervescence from carbon dioxide gas. Neither sulfate nor halide ions produce bubbles with dilute acid. The carbonate test can be carried out without the possibility of an incorrect conclusion. If the test produces no bubbles, then no carbonate is present, and you can proceed to the next test.
b. In the sulfate test, you add a solution containing Ba2+(aq) ions and are looking for a white precipitate of BaSO4(s). Barium carbonate, BaCO3, is white and insoluble in water. So, if you carry out a sulfate test on a carbonate, you will get a white precipitate. Therefore, it is important to carry out the carbonate test first and only proceed to the sulfate test when you know that no carbonate is present.
c. In the halide test, you add a solution containing Ag+(aq) ions, as AgNO3(aq), and are looking for a precipitate. Silver carbonate, Ag2CO3, and silver sulfate, Ag2SO4, are both insoluble in water and will form as precipitates in this test. It is therefore important to carry out the halide test last, after carrying out carbonate and sulfate tests to rule out those possibilities.

Question 34

How would you go about analysing a solution with a mixture of ions?

Answer 34

a. Carbonate test – if you see bubble, continue adding dilute nitric acid until the bubbling stops. All the carbonate ions will then have been removed and there will be none left to react in the next tests. If you intend to test for sulfate or halide ions, it is important to use dilute nitric acid, HNO3, for this test. Sulfuric acid contains sulfate ions and hydrochloric acid contains chloride ions, which will show up in the sulfate and halide tests.
b. To the solution left from the carbonate test, add an excess of Ba(NO3)2(aq). Any sulfate ions present will precipitate out as barium sulfate. Filter the solution to remove the barium sulfate. If you intend to test for halide ions, it is important not to use BaCl2(aq), because the chloride ions will show up in the halide test.
c. To the solution left from the sulfate test, add AgNO3(aq). Any carbonate or sulfate ions initially present have already been removed. Therefore, any precipitate formed must involve halide ions. Add NH3(aq) to confirm which halide you have.

Question 35

How can you test for ammonium ions?

Answer 35

a. Aqueous sodium hydroxide, NaOH, is added to a solution of an ammonium ion.
b. Ammonia gas is produced. You are unlikely to see gas bubbles as ammonia is very soluble in water.
c. The mixture is warmed, and ammonia gas is released.
d. You may be able to smell the ammonia, but it is easy to test the gas with moist pH indicator paper. Ammonia is alkaline and its presence will turn the indicator paper blue.