Periodicity: Chemistry 9701 (Cambridge International AS & A Level)

Table 1 gives data for three consecutive Period 3 elements.

Table 1

(a) Describe and explain the trend in atomic radius from Na to Al, in terms of nuclear charge and electron shielding. [2]

(b) $\text{Na}^+$, $\text{Mg}^{2+}$ and $\text{Al}^{3+}$ all have the same electron configuration.

(i) State this electron configuration. [1]

(ii) Explain why the ionic radius decreases from $\text{Na}^+$ to $\text{Al}^{3+}$, even though the three ions have the same number of electrons. [2]

(c) Table 2 gives the melting points of the Period 3 elements.

Table 2

Explain, in terms of structure and bonding:

(i) why the melting point rises from Na to Al, [2]

(ii) why silicon has by far the highest melting point of any element in the period, [2]

(iii) why the melting point falls sharply after silicon and stays low from $\text{P}_4$ through to Ar. [2]

(d) State whether electrical conductivity generally rises or falls from Na to Al. [1]

Across Period 3, the melting points of the elements rise steadily from sodium to a very high maximum, then fall sharply to low values for the remaining elements.

Which element gives this maximum melting point, and what is the correct reason for it?

A technician burns small samples of sodium, magnesium and silicon, each in an excess of dry oxygen gas. She then reacts fresh samples of sodium and magnesium separately with an excess of dry chlorine gas.

(a) Construct balanced symbol equations, including state symbols, for the reactions of sodium and of magnesium with oxygen. [2]

(b) Silicon reacts with oxygen only when heated strongly, forming solid silicon(IV) oxide. Construct the balanced symbol equation, including state symbols, for this reaction. [1]

(c) Suggest, in terms of structure and bonding, why sodium and magnesium react with oxygen readily at room temperature, whereas silicon must be heated strongly before it reacts. [2]

(d) Construct balanced symbol equations, including state symbols, for the reactions of sodium and of magnesium with excess chlorine gas. [2]

(e) State one observation the technician would make as magnesium burns in chlorine gas. [1]

Solid samples of magnesium oxide, aluminium oxide and silicon(IV) oxide are each tested separately with two reagents: dilute sulfuric acid, and hot, concentrated potassium hydroxide solution. Table 1 summarises whether a reaction is observed with each reagent.

Table 1

(a) Use Table 1 to classify each of the three oxides as basic, amphoteric or acidic, and justify your classification of $\text{Al}_2\text{O}_3$ in terms of its reactions. [3]

(b) Construct the balanced equation, including state symbols, for the reaction of MgO with dilute sulfuric acid. [1]

(c) Aluminium oxide reacts with hot, concentrated potassium hydroxide solution to form a solution containing potassium aluminate, $\text{KAlO}_2$, and water. Construct the balanced equation, including state symbols, for this reaction. [1]

(d) Aluminium oxide also reacts with dilute sulfuric acid to form aluminium sulfate solution. Construct the balanced equation, including state symbols, for this reaction. [2]

(e) Silicon(IV) oxide does not react with dilute sulfuric acid, even though it does react with hot, concentrated potassium hydroxide solution. Suggest, in terms of structure and bonding, why $\text{SiO}_2$ is so much less reactive than $\text{Al}_2\text{O}_3$ and MgO towards dilute acid. [2]

(f) Predict the approximate pH (to the nearest whole number) of the mixture formed when an excess of solid MgO is shaken with distilled water at room temperature, and explain your reasoning. [2]

Separate solid samples of sodium chloride, magnesium chloride, aluminium chloride and silicon tetrachloride are each added to an excess of distilled water at room temperature.

Which chloride produces a resulting solution with a pH closest to $7$?

A student reacts small samples of sodium and magnesium separately with water under different conditions.

(a) Sodium reacts vigorously with cold water. Construct the balanced symbol equation, including state symbols, for this reaction. [2]

(b) When a piece of magnesium ribbon is placed in cold water, only a very slow reaction is observed, with a few small bubbles of gas forming after several minutes. State the name of this gas and construct the balanced symbol equation, including state symbols, for the slow reaction between magnesium and cold water. [2]

(c) When magnesium ribbon is instead heated so that it burns in a stream of steam, it reacts vigorously to form a white solid and the same gas as in (b). Construct the balanced symbol equation, including state symbols, for this reaction with steam. [2]

(d) Suggest why sodium reacts vigorously with cold water at room temperature, whereas magnesium reacts only very slowly with cold water under the same conditions. [2]

Separate solid samples of sodium oxide, $\text{Na}_2\text{O}$, and sulfur trioxide, $\text{SO}_3$, are each added to an excess of distilled water at room temperature until no further reaction occurs.

Which row correctly gives the approximate pH of the resulting solution in each case?

Aluminium chloride and silicon tetrachloride are both chlorides of Period 3 elements, but each reacts differently when added to an excess of water.

(a) Aluminium chloride reacts with water to form a precipitate of aluminium hydroxide and hydrogen chloride. Construct the balanced symbol equation, including state symbols, for this reaction. [2]

(b) Silicon tetrachloride reacts with water to form solid silicon(IV) oxide and hydrogen chloride. Construct the balanced symbol equation, including state symbols, for this reaction. [2]

(c) Carbon tetrachloride, $\text{CCl}_4$, is also a simple molecular tetrachloride, yet unlike $\text{SiCl}_4$, it does not hydrolyse in water at all. Suggest, in terms of atomic structure, why $\text{SiCl}_4$ hydrolyses readily but $\text{CCl}_4$ does not. [3]

(d) State, with a brief reason, which of the two solutions formed in (a) and (b), from $\text{AlCl}_3$ or from $\text{SiCl}_4$, has the lower pH. [1]

An unfamiliar Period 3 element, J, forms a chloride that is a colourless, volatile liquid at room temperature. When added to water, this chloride reacts immediately, producing steamy white fumes of hydrogen chloride and leaving a strongly acidic solution.

Using periodic trends across Period 3, which statement about element J is most likely to be correct?

Phosphorus, $\text{P}_4$, and sulfur, $\text{S}_8$, are both burned separately in an excess of dry oxygen, forming phosphorus(V) oxide and sulfur trioxide respectively. Fresh samples of each solid oxide are then shaken separately with an excess of distilled water, and finally with an excess of aqueous sodium hydroxide.

(a) Construct balanced symbol equations, including state symbols, for the combustion of $\text{P}_4$ and of $\text{S}_8$ in an excess of dry oxygen. [2]

(b) Phosphorus(V) oxide reacts vigorously with an excess of water to form phosphoric(V) acid, $\text{H}_3\text{PO}_4$. Construct the balanced symbol equation, including state symbols, for this reaction. [2]

(c) State the type of structure and bonding present in solid phosphorus(V) oxide and solid sulfur trioxide, and explain why this structure allows both oxides to react rapidly with water despite consisting of covalently bonded molecules. [1]

(d) Sulfur trioxide also reacts with an excess of aqueous sodium hydroxide. Construct the balanced symbol equation, including state symbols, for this reaction. [2]

(e) Magnesium oxide reacts with dilute acid but not with aqueous sodium hydroxide. Using this contrast, explain, in terms of structure and bonding, why phosphorus(V) oxide and sulfur trioxide are classified as acidic oxides whereas magnesium oxide is classified as a basic oxide. [2]