回應(AL)

回應

hahayi:
1.咩係methylating agent?
2.如果問題比左個concentration of reactant remain 同埋time of reaction ge表你,係咪一定要試晒order 0,1,2 先可以找到個答案出黎..我次次做都徙了很多時間..
3.幾時用back titration?
4.岩岩AL果題呢,點解CaCl2唔可以用黎dry solution of C6H5COOH?係咪同Cl會replace OH 有關?

化學友:
1. hahayi，methylating agent即係一隻chemical，佢好容易加個 methyl group (CH3)到其他compound身上。 o向AL chem exam CH3X (X= Cl, Br and I)係一隻出名嘅methylating agent。Through SN reaction (nucleophilic substitution)，個methyl group好容易加到其他compound上。例如:
RO- + CH3I --> ROCH3 + I-
alkoxide methyl group
ion is added to alkoxide ion

2. 你唔使太擔心呀，大部份AL exam題目都會明示、暗示叫你draw邊種graph o架。睇o下下面嘅例子:

2006 Paper II 3a (ii)
An experiment is conducted, at 293 K, to study the kinetics of the decomposition of H2O2(aq) in the presence of peroxidase, an enzyme.

2H2O2(aq) --> 2H2O(l) + O2(g)

By plotting a suitable graph, show that the decomposition is first order with respect to H2O2(aq). Hence, calculate the rate constant at 293 K.

2003 Paper II 3a (ii)
An experiment was carried out at 298 K to study the acid-catalysed hydrolysis of sucrose:

C12H22O11(aq) + H2O(l) --> C6H12O6(aq) + C6H12O6(aq)

By plotting a suitable graph, show that the hydrolysis is first order with respect to sucrose.

不過，如果條題目真係冇寫清楚 (好似2004 Paper II 3a(ii))，咁就可能要plot多過一個graph。但係化學友發覺通常plot 1st order就已經係答案。所以你可以plot ln[reactant] aganist time先，如果係直線就唔使plot其他graph啦。

2004 Paper II 3a (ii)
The decomposition of dinitrogen pentoxide in tetrachloromethane can be represented by the following equation:

2N2O5(in CCl4) --> 4NO2(in CCl4) + O2(g)

The table below lists the results obtained in an experiment to study the kinetics of the decomposition of dinitrogen pentoxide at 318 K.

By plotting a suitable graph, determine the rate equation for the decomposition, and hence the rate constant at 318 K.(o個 graph係1st order呀)

3. 考試時通常都唔使你自己用或者describe back titration o架。如果真係考back titration，條題目會清楚寫出o黎，你o吾使worry ar。

4. 化學友覺得唔係啦，因為alkanoic acid同chloride ion係有化學反應o個喎。

hahayi，如果仲有唔明嘅，留言比化學友丫。