(a)(i) State two differences betwecii the... | WAEC SSCE - Chemistry - 2006

Question 1 Report

(a)(i) State two differences betwecii the properties of solids and gases

(ii) What process does each of X, Y and Z represent in the changes shown below?

(b)(i) State Charles' Law (ii) Draw a sketch to graphically illustrate Charles' Law.

(c) 60 cm of hydrogen diffused through a porous membrane in 10 minutes. The same volume of a gas G diffused through the same membrane in 37.4 minutes. Determine the relative molecular mass of G. [ H = I ]

(d)(i) State two assumptions X of the kinetic theory.

(ii) Consider the reaction represented by the Solid of Liquid following equation:

H\(_{2(g)}\) + Cl\(_{2(g)}\) \(\to\) 2HCI\(_{(g)}\)

Use the kinetic theory to explain how the rate of formation of HCI\(_{(g)}\) would be affected by I. increase in temperature; II. decrease in pressure.

(e) Given different examples, mention one metal in each case vihich produces hydrogen on reacting with (i) dilute mineral acid; (ii) cold water; (iii) steam; (iv) hot, concentrated alkali.

Answer

(a)(i) Differences between solids and gases

Solid	Gas
It has a definite shape and a definite volume.	It has no definite shape or volume and fills its container.
Its particles are closely packed and it is not easily compressed.	Its particles are far apart and it is highly compressible.

(a)(ii) X is freezing (solidification); Y is sublimation; Z is condensation (liquefaction).

(b)(i) Charles' Law

At constant pressure, the volume of a fixed mass of gas is directly proportional to its absolute temperature.

Thus, \(V \propto T\), and \(\dfrac{V}{T}=\text{constant}\).

(b)(ii) Graphical illustration of Charles' Law

The straight line passes through the origin. For example, its gradient is \(\dfrac{160-40}{400-100}=0.40\ \text{cm}^3\text{K}^{-1}\), showing that \(V/T\) is constant.

(c) Relative molecular mass of G

By Graham's law, rate of diffusion is inversely proportional to the square root of relative molecular mass:

\[\frac{r_{H_2}}{r_G}=\sqrt{\frac{M_G}{M_{H_2}}}\]

Since equal volumes diffuse,

\[\frac{r_{H_2}}{r_G}=\frac{t_G}{t_{H_2}}=\frac{37.4}{10}=3.74\]

\[3.74=\sqrt{\frac{M_G}{2}}\]

\[M_G=2(3.74)^2=27.98\]

Therefore, the relative molecular mass of G is \(\boxed{28}\).

(d)(i) Assumptions of the kinetic theory of gases

A gas consists of tiny particles which are in continuous rapid random motion.
The particles are very far apart compared with their sizes, and forces of attraction between them are negligible.

(d)(ii) Effect on the rate of formation of HCl

\[H_{2(g)}+Cl_{2(g)}\rightarrow 2HCl_{(g)}\]

Increase in temperature: The molecules gain kinetic energy and move faster. They collide more frequently and with greater energy, so the number of effective collisions increases. Therefore, the rate of formation of HCl increases.
Decrease in pressure: The gas molecules become farther apart. Collision frequency and hence the number of effective collisions decrease. Therefore, the rate of formation of HCl decreases.

(e) Metals which produce hydrogen

With dilute mineral acid: zinc, for example \(Zn+H_2SO_4\rightarrow ZnSO_4+H_2\).
With cold water: sodium, for example \(2Na+2H_2O\rightarrow2NaOH+H_2\).
With steam: iron, for example \(3Fe+4H_2O\rightarrow Fe_3O_4+4H_2\).
With hot concentrated alkali: aluminium, for example \(2Al+2NaOH+2H_2O\rightarrow2NaAlO_2+3H_2\).

Read lesson note on Kinetic Theory Of Matter And Gas Laws (JAMB)

Read lesson note on Separation Of Mixtures And Purification Of Chemical Substances (JAMB)

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