Molecular Polarity 試題分解

1.          DSE/13/1B/2

Both BF₃ and NH₃ exist as simple molecules.

(a)    For each of these molecules, draw its three-dimensional structure.

BF₃

NH­₃

(2 marks)

(b)    For each of these molecules, explain whether or not it is polar.

(2 marks)

(c)    BF₃ reacts with NH₃ to give F₃BNH₃. Describe the bond formation between BF₃ and NH₃.

(2 marks)

Structure & Bonding_試題分析 2

DSE/Sample Paper/1B/7

Complete the table below by

(a) drawing a three-dimensional diagram for the structure of each solid substance (3 marks), and

(b) giving an explanation of whether the solid substance is an electrical conductor. (3 marks)

Solid substance	Three-dimensional diagram for the structure of the solid substance	Explanation of whether the solid substance is an electrical conductor
Diamond
Graphite
Caesium chloride

考評局的DSE Sample paper問題較簡單直接，但介紹這題的目的想一次提醒大家要識畫鑽石、石墨及二氧化矽的巨型共價結構之餘，亦希望大家留意DSE只會考的兩種ionic compounds︰Sodium chloride & Cesium chloride(氯化銫)的巨型離子結構不一樣。

續談Rate Law (2)

2)      HKALE/2007/Paper II/Q.3(a):

(a)    Consider the reaction below:

Br₂ (aq) + HCO₂H (aq) à 2 Br^- (aq) + 2H⁺ (aq) + CO₂ (g)

The table below lists the experimental data obtained at a certain temperature:

Run	Volume used / cm3			Initial rate for the disappearance of Br2 (aq) / mol dm-3 s-1
	0.010 M Br2 (aq)	0.20 M HCO2H (aq)	H2O (l)
1	2.0	10.0	8.0	1.2 x 10-5
2	4.0	10.0	6.0	2.4 x 10-5
3	8.0	10.0	2.0	4.8 x 10-5

(i)    Suggest an experimental method to follow the change in concentration of Br₂(aq) in the reaction mixture Give a reason for your suggestion.
(ii)   Suggest how the initial rate for the disappearance of Br₂ (aq) can be found.
(iii)  Why is it necessary to keep the concentration of HCO₂H (aq) much higher than that of Br₂ (aq)?
(iv)    Deduce the order of the reaction with respect to Br₂ (aq).
(v)    Suggest how the order of the reaction with respect to HCO₂H (aq) can be determined.

留意雖然題目有提供initial rate但沒有提供濃度，同學或可計算各反應物的no. of mole再除以該混合物的總容量(Vol. of Br₂ + Vol. of HCO₂H + Vol. of H₂O)來找出反應物濃度，但實際上還有更簡易的方法。詳閱(iv)。

續談 Rate Law(1)

今次先講解一條有關Rate Law的文憑試試題。

1)      HKDSE/Sample Paper/Paper II/Q.1(a)(i)(ii)(iii)

In acid solution, chlorate ions (ClO₃^-) slowly oxidize chloride ions to chlorine. The following kinetic data are obtained at 25℃:

Run	[ClO3–(aq)] /mol dm-3	[Cl–(aq)] /mol dm-3	[H+(aq)] /mol dm-3	Initial rate /mol dm-3 s-1
1	0.08	0.15	0.20	1.0 x 10-5
2	0.08	0.15	0.40	4.0 x 10-5
3	0.16	0.15	0.40	8.0 x 10-5
4	0.08	0.30	0.20	2.0 x 10-5

(i)   Write the balanced equation for this reaction. (1 mark)
(ii)   Determine the order of the reaction with respect to each reactant. (3 marks)
(iii)   Determine the rate constant at this temperature. (3 marks)