(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 describes a substance that absorbs moisture (water vapour) from the atmosphere without dissolving in it (it becomes damp but does not form a solution).
(a)(ii) Two differences between physical and chemical change
| Physical change | Chemical change |
|---|
| No new substance is formed | One or more new substances are formed |
| Easily reversible | Usually not easily reversible |
(a)(iii) Charles' law by the kinetic theory
As the temperature of a fixed mass of gas rises, its molecules gain kinetic energy and move faster, striking the walls more frequently and more forcefully. At constant pressure the gas must expand so that the molecules hit a larger area; the volume therefore increases in direct proportion to the absolute temperature.
(b)(i) Increasing order of boiling point
\(CO_2 < CS_2 < NaH\).
\(CO_2\) and \(CS_2\) are both simple molecular substances held by weak van der Waals forces, but \(CS_2\) has larger, heavier molecules and therefore stronger forces than \(CO_2\) (which is a gas). \(NaH\) is an ionic (giant) solid held by strong electrostatic forces, so it has by far the highest boiling point.
(b)(ii) Chlorine with cold dilute sodium hydroxide:
\[Cl_2 + 2NaOH \to NaCl + NaOCl + H_2O\]
(c) Impure magnesium with excess HCl
(i) \[Mg + 2HCl \to MgCl_2 + H_2\]
(ii) Amount of hydrogen: \[n(H_2) = \frac{8.6}{22.4} = 0.384\ \text{mol}\]
I. From the equation \(1\ \text{mol } Mg\) gives \(1\ \text{mol } H_2\), so \(n(Mg) = 0.384\ \text{mol}\): \[\text{mass of pure Mg} = 0.384 \times 24 = 9.21\ \text{g}\]
II. \[\text{percentage purity} = \frac{9.21}{15.0} \times 100 = 61.4\%\]
III. The reaction forms \(0.384\ \text{mol } MgCl_2\), which contains \(0.384\ \text{mol } Mg^{2+}\) and \(0.768\ \text{mol } Cl^-\), a total of \(1.152\ \text{mol}\) of ions: \[\text{number of ions} = 1.152 \times 6.02\times10^{23} = 6.94\times10^{23}\ \text{ions}\]
(a)(i) Hygroscopic describes a substance that absorbs moisture (water vapour) from the atmosphere without dissolving in it (it becomes damp but does not form a solution).
(a)(ii) Two differences between physical and chemical change
| Physical change | Chemical change |
|---|
| No new substance is formed | One or more new substances are formed |
| Easily reversible | Usually not easily reversible |
(a)(iii) Charles' law by the kinetic theory
As the temperature of a fixed mass of gas rises, its molecules gain kinetic energy and move faster, striking the walls more frequently and more forcefully. At constant pressure the gas must expand so that the molecules hit a larger area; the volume therefore increases in direct proportion to the absolute temperature.
(b)(i) Increasing order of boiling point
\(CO_2 < CS_2 < NaH\).
\(CO_2\) and \(CS_2\) are both simple molecular substances held by weak van der Waals forces, but \(CS_2\) has larger, heavier molecules and therefore stronger forces than \(CO_2\) (which is a gas). \(NaH\) is an ionic (giant) solid held by strong electrostatic forces, so it has by far the highest boiling point.
(b)(ii) Chlorine with cold dilute sodium hydroxide:
\[Cl_2 + 2NaOH \to NaCl + NaOCl + H_2O\]
(c) Impure magnesium with excess HCl
(i) \[Mg + 2HCl \to MgCl_2 + H_2\]
(ii) Amount of hydrogen: \[n(H_2) = \frac{8.6}{22.4} = 0.384\ \text{mol}\]
I. From the equation \(1\ \text{mol } Mg\) gives \(1\ \text{mol } H_2\), so \(n(Mg) = 0.384\ \text{mol}\): \[\text{mass of pure Mg} = 0.384 \times 24 = 9.21\ \text{g}\]
II. \[\text{percentage purity} = \frac{9.21}{15.0} \times 100 = 61.4\%\]
III. The reaction forms \(0.384\ \text{mol } MgCl_2\), which contains \(0.384\ \text{mol } Mg^{2+}\) and \(0.768\ \text{mol } Cl^-\), a total of \(1.152\ \text{mol}\) of ions: \[\text{number of ions} = 1.152 \times 6.02\times10^{23} = 6.94\times10^{23}\ \text{ions}\]
