(a)(i) Define the term hygroscopic.
(ii) Give two difference: between a physical change and a chemical change.
(iii) Using the kinetic theory of gases, explain briefly the Charles' law.
(b)(i) Arrange the following compounds in order of increasing boiling points: CS\(_2\); CO\(_2\); NaH. Give reasons for your answer.
(ii) Write a balanced chemical equation to illustrate the reaction of chlorine gas with cold dilute sodium hydroxide.
(c) In a certain reaction, 15.0 g of impure magnesium sample reacted with excess hydrochloric acid liberating 8.6 dm\(^2\) of hydrogen gas at s.t.p.
(i) Write a balanced equation for the reaction.
(ii) Calculate the: I. mass of pure magnesium in the sample; I. percentage purity of the magnesium sample; III. number of ions produced in the reaction. [Mg = 24.0; volume at s.t.p. 22.4 dm\(^{-3}\), Avagadro's constant = 6.02 x 10\(^{23}\)mol\(^{-1}\)]
(a)(i) Hygroscopic
A hygroscopic substance is one which absorbs moisture (water vapour) from the atmosphere without dissolving in it or forming a solution.
(a)(ii) Two differences between physical and chemical changes
| Physical change |
Chemical change |
| No new substance is formed. |
One or more new substances are formed. |
| It is usually easily reversible. |
It is usually not easily reversible. |
(a)(iii) Charles' law using the kinetic theory of gases
At constant pressure, when the temperature of a fixed mass of gas is increased, the gas molecules gain kinetic energy and move faster. The molecules move farther apart, causing the gas to expand. Hence, the volume of the gas increases directly with its absolute temperature.
(b)(i) Order of increasing boiling points
\[
CO_2 < CS_2 < NaH
\]
Both \(CO_2\) and \(CS_2\) are simple molecular substances held together by van der Waals' forces. The forces in \(CS_2\) are stronger because its molecules are larger and more polarizable than those of \(CO_2\). Sodium hydride, \(NaH\), is an ionic compound with strong electrostatic forces of attraction between its ions; therefore, it has the highest boiling point.
(b)(ii) Reaction of chlorine with cold dilute sodium hydroxide
\[
Cl_{2(g)} + 2NaOH_{(aq)} \rightarrow NaCl_{(aq)} + NaOCl_{(aq)} + H_2O_{(l)}
\]
(c)(i) Balanced equation for the reaction
\[
Mg_{(s)} + 2HCl_{(aq)} \rightarrow MgCl_{2(aq)} + H_{2(g)}
\]
(c)(ii)
I. Mass of pure magnesium in the sample
Volume of hydrogen gas evolved \(= 8.6\text{ dm}^3\).
\[
n(H_2)=\frac{8.6}{22.4}=0.384\text{ mol}
\]
From the equation, \(1\) mole of \(Mg\) produces \(1\) mole of \(H_2\).
\[
n(Mg)=0.384\text{ mol}
\]
\[
\text{Mass of pure }Mg = 0.384 \times 24.0 = 9.21\text{ g}
\]
II. Percentage purity of the magnesium sample
\[
\%\,\text{purity}=\frac{\text{mass of pure magnesium}}{\text{mass of impure sample}}\times100
\]
\[
=\frac{9.21}{15.0}\times100=61.4\%
\]
III. Number of ions produced
The \(0.384\) mol of magnesium forms \(0.384\) mol of \(MgCl_2\).
\[
MgCl_2 \rightarrow Mg^{2+} + 2Cl^-
\]
Therefore, \(1\) mole of \(MgCl_2\) produces \(3\) moles of ions.
\[
\text{Moles of total ions}=3 \times 0.384=1.152\text{ mol}
\]
\[
\text{Number of ions}=1.152 \times 6.02\times10^{23}
\]
\[
=6.94\times10^{23}\text{ ions}
\]
