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)