Chapter 8 - Reactivity trends Flashcards
What are the elements in group 2 also known as?
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.
What is the most common type of reaction of group 2 elements?
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.
What is the redox reaction of group 2 elements with oxygen?
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.
What is the redox reaction with water?
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.
What is the redox reaction with dilute acids?
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.
Why does the reactivity of the redox reactions of group 2 elements increase down the group?
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.
What is the reaction of group 2 oxides with water?
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)
What is the solubility trend of group 2 hydroxides?
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.
How can you show the trend of solubility of group 2 hydroxides through an experiment?
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.
What are some of the uses of group 2 compounds in agriculture?
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).
What are some of the uses of group 2 compounds in medicine?
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)
What are some examples of common indigestion medicine?
Gaviscon
Rennie
What are the elements of group 7 also known as?
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.
What is the trend in boiling points of group 7 elements?
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.
What colour is chlorine, bromine and iodine at RTP?
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.
What is the most common type of reactions of group 7 elements?
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.
What is the halogen-halide displacement reactions?
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.
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?
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.
What is the order of reactivity of the three halogens chlorine, bromine, and iodine?
a. Chlorine is the most reactive.
b. Bromine is in the middle.
c. Iodine is the least reactive.
What is fluorine and astatine?
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.
What is the trend in reactivity of the group 7 elements?
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.
What is a disproportionation reaction?
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.
What is the reaction of chlorine with water?
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.
What is the reaction of chlorine with cold, dilute aqueous sodium hydroxide?
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.
