Section 13: Industrial Chem_Activation Energy & Energy Profile_Part 1

為何每個反應都有不同速率？還記得Collision Theory的同學應該很清楚反應源於粒子的有效碰撞 (Effective collision)，而如何達到有效碰撞取決於兩大因素︰

Ø   粒子在碰撞過程獲得足夠K.E.(即Kinetic energy動能，由於中文名太核突版主唔想用)

Ø  粒子碰撞的方向正確

這理論亦以增加有效碰撞的機率來解釋為何增加溫度、濃度、表面面積及加入催化劑能增加反應速率。相信同學都不難掌握此部分，但又能否聯想起Chemical Energetics︰

Energetics這課除了教大家Enthalpy (焓，簡單講就是一隻物質的Potential energy，該物質enthalpy越高即是bond strength越弱，所以亦較不穩定)時就有講解化學反應導致enthalpy change的成因︰粒子反應時先會吸收周圍環境的能量 (即熱能)、將自行的bonding拆除，再組成新bonding來形成生成物 (此過程會放出能量)。至於一個反應會吸熱(endothermic, △H= +ve)還是放熱(exothermic, △H= -ve)就需比較反應物及生成物之間的bond strength︰

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)

Section 13: Industrial Chem_Rate Law

A.  Relationship between Reaction rate & Concentration of reactant(s)

先研究速率及濃度之間的關係，參閱以下例子（簡化起見，此處不用實例）：

Case 1:  A à B (30℃)

Conc. of A (M)	Initial Rate (M/s)
1	0.1
2	0.2
3	0.3
4	0.4
5	0.5

注意研究濃度的影響應使用Initial rate (初速，即是反應剛剛開始時的速率)，因為我們只知反應開始時的濃度，當其隨時間下跌，就難以判斷它和速率的關係。