[DSE化學2019]同學準備好DSE CHEM未?

closed system只代表物質唔可以進出 無話volume唔可以變
constant pressure一般係指空許個container expand volume
當system內gas volume increase(不論係reaction產生gas定exothermic process令system內既空氣expand)，會有work done by system to surroundings令活塞向外推
最後令system pressure同外界既pressure相等，亦即係樓上提過既唔fix V而fix P
至於你話neutralization既case 因為做實驗時唔會用太多既acid / alkali (因為最後都係計kJ mol-1 用多都無意義) 放出既熱通常唔多 air expand造成既work done好有限 正路都會忽略

standard condition係指environment
但reactant本身都要standard state (即係C要graphite / O要O2)

感謝支持 有咩意見可以同我講
覺得幫到你既share下比frd已經好感激

可以咁講
如果d reaction涉及work done太大(e.g. 火藥爆炸)
咁淨計enthalpy change就好難反映到個overall energy change

yes以市面可買到黎講呢間出版商既chem雞精書/exercise算幾好 起碼d野唔會點錯 亦唔太離地
p.s. 千其唔好買紫色果本X星期雞精 d野亂+錯漏百出 伏到嘔

想問下點樣extract cu from its ore
書就寫heating alone 就得但會考就要carbon reduction

正路係heating with C / CO / H2

但點解精工本書寫
To extract copper from copper pyrite 首先係heat the ore alone to produce copper(I) sulphide跟住再燒佢出Cu
咁到底寫邊個先啱

工業上實際做法好多時同in theory唔同 因考慮成本/效率/產量之類
不過咁樣會令出題上造成混亂 變成好多可能答案
所以考得你正常係由metal oxide開始做extraction
咁就一定跟返carbon / hydrogen reduction
就我自己睇過咁多年公開試題黎講 都無發現唔consistent既情況 (有睇漏請指教 )

退一百步用sulphide (CuS)做extraction
都係加多步heat alone變做CuO先
再carbon reduction

btw其實textbook多好野都唔exam oriented
頭先上堂有題compare boiling point
有學生話考科書講用dimer解

明白晒 真係唔該阿sir

From the perspective of entropy, the first one will be favoured because of less decrease in entropy(disorder)
From the thermodynamic perspective, S=O bond strength should be weaker than O=O bond strength, so it is less likely that 3 S=O will be present in the same molecule,although the additional resonance stabilisation effect may be taken into account for SO3.
If the reaction is a single step reaction, then it is probably kinetically unlikely that 5 molecules from the reactant side will collide with each other to form the desired products.
However, kinetic control and thermodynamic control may help us achieve selectivity between the two pathways by tuning the temperature of interest which affects the reversibility of the reaction.

如果我考緊midterm會咁答，有d嘢無absolute answer，只係得reasonable answer，姐係合理地吹都可以，直到你去做實驗confirm。

利申：大學organic chem /inorganic chem A+ DSE chem 5**

S(s) + O2(g) = SO2(g)
2S(s) + 3O2(g) = 2SO3(g)

Which one is more likely to occur?

From the perspective of entropy, the first one will be favoured because of less decrease in entropy(disorder)
From the thermodynamic perspective, S=O bond strength should be weaker than O=O bond strength, so it is less likely that 3 S=O will be present in the same molecule,although the additional resonance stabilisation effect may be taken into account for SO3.
If the reaction is a single step reaction, then it is probably kinetically unlikely that 5 molecules from the reactant side will collide with each other to form the desired products.
However, kinetic control and thermodynamic control may help us achieve selectivity between the two pathways by tuning the temperature of interest which affects the reversibility of the reaction.

如果我考緊midterm會咁答，有d嘢無absolute answer，只係得reasonable answer，姐係合理地吹都可以，直到你去做實驗confirm。

利申：大學organic chem /inorganic chem A+ DSE chem 5**

呢句真 可以係好多個原因都關事 睇你考緊邊科囉 但邊個factor影響最大真係做過實驗先會知
不過DSE會學既幾乎都係概定事實 possible ans少又係真既

reactant
呢個我自己都求證左一段時間(唔關知識事 關點define事) 亦都同過唔同日校老師/博士朋友s/一個幾出名既同行討論過

結論係 以前9x年 特別係CE時期 會教學生當alkaline hydrolysis係OH- catalyze 然後出黎個RCOOH再neutralization出RCOO-Na+
但其實嚴格黎講係錯的(事實上CE好多簡化左既內容其實背後都係錯)

以下係解釋個mechanism 儘量簡單 量力而為：
睇返個mechanism OH-撞左埋個R1COOR2度之後係甩-OR2再即時protonate返做HOR2同埋R1COO- 個OH-的確係consume左無regenerate返
呢個同acid hydrolysis好唔同 因為acid hydrolysis真係會甩返個H+出黎

而實際上好多大學level既書都改返base hydrolysis做base-promoted / base-induced hydrolysis 亦表明左同acid-catalyzed hydrolysis分割開咁睇

好嚴格黎講 其實加acid做先乎合hydrolysis既基本定義(加H2O & break down)
加alkali其實係涉及ester同OH-之間既反應 用hydrolysis形容唔係太準確
但由於acid / alkali兩個做法上好似 所以約定俗成地兩個都叫做hydrolysis 特別係中學level

而依家DSE年代好樣野好過CE 就係內容上非常政治正確 背後background knowledge一定會啱先出題

事實上DSE 2013 MCQ32都有問過alkaline hydrolysis of amide (mechanism同ester一樣)
答案都表明左alkali唔係catalyst

CE年代係連mechanism呢個概念都無 好直接咁話NaOH act as catalyst

因為acid / alkali hydrolysis主要係研究佢個mechanism 有無做calatyst呢個位反而唔係重點 所以好多書都講得好曖昧 或無特別提呢d野

留意左上有標明base-induced同acid-catalyzed

至於hydrolysis(水解) 故名思義就係involve H2O add to substance (usually cause fragmentation of substance)
基於呢個定義下 base hydrolysis過程中其實係無H2O直接參與

唔會 even AL chem都只係提少少
事實上唔一定要有chiral center都可以optically active
有chiral center都可以optically inactive (e.g. meso)
而不過一個compound如果只有一個chiral center 就一定會optically active
所以DSE寫syllabus就係捉住呢個位去set 講明係consider single chiral center既optical activity

其實24 hrs果本以雞精黎講真係算係咁
但始終雞精好多野一句起兩句止 得d重點
要星星既chem有好多細微位或者special case要知
唔出去補習既 唯有仔細研究pastpaper係最efficient既方法 唔係單做 係清楚理解曬每題ABCD點解啱同錯 最好留意埋exam report同HKEA個seminar講乜