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)。
(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)。
(i) Suggest
an experimental method to follow the change in concentration of Br2(aq)
in the reaction mixture Give a reason for your suggestion.
如果純粹找出反應在不同的速率就可以有很多方法，例如根據反應中放出的氣體(CO2)而用syringe量度該反應在closed system (密封系統)不同時間增加了的容量或用Pressure sensor在同樣情況量度不同時間的壓力就可以。但如需量度Br2的濃度就應利用其啡色在反應過程中因濃度減少而變淡來用Colorimetry (色度法)量度它在不同時間的濃度。(可能有同學會想到既然題目沒有要求一定用physical method的話不如用titrimetric method，但此間接方法既複雜又沒physical method般能準確量度濃度，如果題目要求列出實驗程序就更麻煩，故此可以的話就千萬不要用此方法)。
(ii) Suggest
how the initial rate for the disappearance of Br2 (aq) can be found.
Initial
rate即是反應一開始時的速率。
所以只要先畫出conc. time graph，再找出曲線在0秒時Tangent的斜率就行：
上回提及過要推斷order of reaction with
respect to the reactant就一定要用initial rate計算，因為反應物濃度會隨時間減少，速率亦隨之下跌，如果用平均速率就沒有適合的濃度想作比較。只有將Initial rate和 initial concentration作對比才有意義。同學亦可參閱2012 Paper 2 Q1c(i)：
(i)Explain
why ‘initial rate’ is commonly used in the study of the kinetics of a reaction.
Ans:
Initial rate is used because the initial concentrations of reactants are known
(1M)
(iii) Why
is it necessary to keep the concentration of HCO2H (aq) much higher
than that of Br2 (aq)?
閱畢全題大家就會發現此實驗的目的是找出Br2而不是所有反應物的order of reaction，所以看似只需令三個混合物都有相同份量HCO2H就可，但如果HCO2H本身濃度太低而order又不是零的話，initial rate就會同時受兩種反應物影響。
(除反應物比例外，以下資料純粹虛構，只用於解釋此題。)
Time
|
[Br2]
|
[HCO2H]
|
%
change of [HCO2H]
|
0
|
0.01
|
0.01
|
0%
|
10
|
0.0095
|
0.0095
|
5%
|
20
|
0.0092
|
0.0092
|
8%
|
所以用濃度較高的HCO2H就可避免[HCO2H]下跌得太快的問題，從而將對initial rate的影響降至最低：
Time
|
[Br2]
|
[HCO2H]
|
%
change of [HCO2H]
|
0
|
0.01
|
1
|
0%
|
10
|
0.0095
|
0.9995
|
0.05%
|
20
|
0.0092
|
0.9992
|
0.08%
|
所以答題時要解釋HCO2H濃度高令其在反應中(約略)保持不變，成為一個constant。參閱此題之評卷參考：
Rate
= k[HCO2H(aq)]x[Br2(aq)]y (1)
When
[HCO2H(aq)] >> [Br2(aq)], the rate equation becomes
Rate
= k’[Br2(aq)]y (1)
[Br2(aq)]y
is the only factor which affects the reaction rate.
(iv) Deduce
the order of the reaction with respect to Br2 (aq).
解題前還須解釋為何此實驗中每個reaction mixture會加入不同容量的水。這是個聰明的做法，實驗者想在不同reaction mixture中使用不同的[Br2(aq)]來找出其order，但又無理由真的準備三樽shock standard solution做實驗，所以透過在reaction mixture中加入不同比例的Br2(aq)及水，實驗者只需準備一樽standard solution。須注意兩者在不同reaction mixture的容量都要一樣，否則計算上會非常麻煩，亦會影響HCO2H的濃度，變相影響initial rate的準確性。
回到題目，我們可計算Br2在各reaction mixture的濃度、或比較各mixture中Br2的容量比例(既然每次Br2的濃度一樣，而每個reaction
mixture的容量亦一樣，故此該反應物容量和濃度成正比關係。所以無須次次都將Br2容量乘以其濃度再除整個reaction mixture的容量，直接使用容量即可)︰
Run
|
Volume
of 0.010 M Br2 (aq)
|
Mole
ratio of Br2 (aq)
|
Ratio
of [Br2(aq)]
|
Initial
rate for the disappearance of Br2 (aq) / mol dm-3 s-1
|
1
|
2.0
|
1
|
1
|
1.2 x 10-5
|
2
|
4.0
|
2
|
2
|
2.4 x 10-5
|
3
|
8.0
|
4
|
4
|
4.8 x 10-5
|
比較Run 1 & Run 2，initial rate隨[Br2(aq)]增加兩倍；或比較Run 1 & Run 3，initial rate隨[Br2(aq)]增加四倍。所以order of reaction with
respect to Br2就是1：
The
initial rate doubles when [Br2(aq)] is doubled
∴Order
w.r.t. Br2(aq) is 1.
(v) Suggest
how the order of the reaction with respect to HCO2H (aq) can be
determined.
其實做法一樣，只是Br2和HCO2H的角色掉轉，所以兩分入面一分講實驗上有什麼改動、另一分講點搵order w.r.t. HCO2H(aq)：
Repear
the experiment using the same reagents, keeping the volume of Br2(aq)
used constant, and vary the volume of HCO2H(aq) used. Measure the
initial rate in each case. (1)
Compare
the initial rates to obtain the order w.r.t. HCO2H(aq). (1)
3) HKALE/2010/Paper
II/Q.1(a)(iii)(iv)(v)
‘Methanal
clock experiment’ is based on the following reaction:
HCHO
(aq) + SO32-(aq) + H2O(l) à CH2OHSO3-(aq)
+ OH-(aq)
In
a methanol clock experiment, five 20.0 cm3 methanal solutions were
prepared by mixing different volumes of 0.30 mol dm-3 HCHO(aq) and H2O
(l). A small but fixed amount of a solution containing 0.20 mol dm-3
NaHSO3(aq), 0.05 mol dm-3 Na2SO3(aq)
and a few drops of phenolphthalein indicator was added to each of the five methanol
solutions. The time for the first appearance of a pink colour in each run was
recorded.
The
table below lists the experimental data obtained:
Run
|
1
|
2
|
3
|
4
|
5
|
Volume
of 0.30 mol dm-3 HCHO(aq) used / cm3
|
5.0
|
7.5
|
10.0
|
12.5
|
15.0
|
Volume
of H2O(l) used / cm3
|
15.0
|
12.5
|
10.0
|
7.5
|
5.0
|
Time
for the first appearance of the pink colour /s
|
35.7
|
23.3
|
17.9
|
14.1
|
12.0
|
(iii) By plotting a suitable graph, deduce the reaction order with respect to methanol.
(iv) With reference to the above experimental data, identify ONE major error in this experiment.
(v) Can methyl orange be used instead of phenolphthalein to perform methanol clock experiments? Explain.
(iii) By plotting a suitable graph, deduce the reaction order with respect to methanol.
(iv) With reference to the above experimental data, identify ONE major error in this experiment.
(v) Can methyl orange be used instead of phenolphthalein to perform methanol clock experiments? Explain.
(iii) By plotting a suitable graph, deduce the reaction order with respect to methanol.
有咩慘得過個反應物唔係用濃度做單位，就係連Initial rate都改用Time (for the first
appearance of the pink colour)。所以如果大家對Clock reaction認識不深就未必能正確處理以上數據。
Clock
reaction是研究某反應速率或某反應物的order，和以往量度速率的physical/chemical method不同，此方法利用反應生出某一份量的生成物所需時間作為速率的指標。Sulphur clock reaction之前上堂應該學過，準備一張有Ｘ的卡紙再在卡紙上的Beaker/E-flask加入固定容量HCl及Sodium thiosulphate
solution (Na2S2O3)開始反應。一段時間後當生成一定份量的硫磺(打比方說5 g)，實驗者就不能再在上面看到X。
(Source: http://chemicalminds.wikispaces.com/)
反應速率可由生成物增加的份量除以所需時間計出︰
由於在每次實驗中，只要我們看不見Ｘ就能推斷反應已產生了5g 硫磺，所以只需將所需時間轉成分母 (take reciprocal)就可以找出該反應的Initial rate。而在固定容量的原則下增加濃度或溫度，反應速率增加，(5g) 硫磺所需形成的時間就越短。
回到題目，要找出order r.w.t. methanal就應該畫graph of rate of
reaction against [HCHO]，rate of reaction剛剛教過點計啦，而上題亦解釋過反應物容量及其濃度成正比關係，故此可直接以容量取代濃度︰
Run
|
1
|
2
|
3
|
4
|
5
|
Volume
of 0.30 mol dm-3 HCHO(aq) used / cm3
|
5.0
|
7.5
|
10.0
|
12.5
|
15.0
|
Time
for the first appearance of the pink colour (t) /s
|
35.7
|
23.3
|
17.9
|
14.1
|
12.0
|
1/t
/ s-1
|
0.0280
|
0.0429
|
0.0559
|
0.0709
|
0.0833
|
畫完graph大家就會知速率和HCHO容量成正比，所以order w.r.t. methanal是1。
在這題中正確處理數據得一分、畫圖表兩分、解釋圖表俾個order拎埋最後一分。
(iv) With reference to the above experimental
data, identify ONE major error in this experiment.
講到尾都係想講clock reaction有咩問題。Reaction time of
experimenter, variation in temperature of reaction mixture，呢D統統係’minor error’。而同以往量度rate既方法最大分別在於clock reaction係靠肉眼決定兜野幾時轉色，就算係同一個人判斷晒所有轉色結果亦會有相當誤差
Measuring
the time for the appearance of the pink colour / Deteching the colour change by
eye (1)
(v) Can methyl orange be used instead of
phenolphthalein to perform methanol clock experiments? Explain.
雖然呢題不太關rate law事，但都想係度講methyl orange同phenolphthalein轉色的pH值都唔同(Methyl orange轉色的pH為3.1-4.4；phenolphthalein為8.3-10.0)。所以點解係acid-alkali titration有時可以兩隻都用但有時只可以用其中一隻。
如果同學對呢課仲有咩問題可留言/PM我，下次講Energy profile &
Actication energy (Ea)。
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