[2022DSE CHEM化學］問答討論區

係我講錯 lead係period 6
down the period, s p gap會越嚟越大
不過dse冇學過orbital

所以就唔講太多s同p嘅嘢
如果唔係學生聽完都明唔到

唔緊要嘅，重點只係Pb入面黎講，甩4粒電子需要嘅能量，遠大於甩2粒電子嘅能量，唔止兩倍嘅關係

如果要解釋點解｜delta S｜> | delta H | ，我都淨係識得從enthalpy入手，但要比較entropy喺唔同compound dissolve時嘅差異我都唔識睇，其實有冇得predict到

如果我冇記錯，marking scheme好似接受temperature變化嘅答案，所以全部都係睇考評局點講

同埋我感覺predict到都好難同enthalpy做到比較

所有salt dissolution 都係：AnBn(s) --> An+(aq) + Bn+(aq)，有兩個關於energy change嘅processes考慮：
I) hydration energy, 產生An+(aq)/Bn+(aq)所放出黎嘅能量（delta H = -ve）
II) lattice energy, break AnBn(s) ionic bind 所需要嘅能量 （delta H = +ve）
Enthalpy change of solution=I+II, 只要最終I+II加埋係-ve，隻salt AnBn(s)就係soluble，entropy changle我就唔太了解當中嘅變化，就咁睇我就覺得每個case差唔多

但呢d理論，好多時都係做比較有用(Q1)，或者有個結果後再去解釋(Q2)，唔會直接問某salt溶唔溶(Q3)，因為以上兩個能量，確實數值唔會知，只係會知大約邊個較高，或者根據結果，直接屈某個數值高過某個 (或者有更advance方法predict數值，歡迎指出)
Q1. Why MgSO4 is more soluble than PbSO4 ?
Q2. Why PbSO4 is insoluble?
Q3. Is PbSO4 soluble or not?

Q1. Resulting hydration energy is higher (more -ve)for Mg2+, as its radius is smaller than Pb2+ and it has higher charge density. (Although ionic bond in MgSO4 is stronger and its lattice energy is higher (more +ve) than PbSO4, energy released by hydration should overcome energy required for lattice breaking and the overal enthalpy change of solution should be more -ve for MgSO4)

仲有，外圍有兩粒6s electrons，所產生嘅inert pair effect (比平時更加難甩)，會用黎解釋點解Hg係唯一liquid metal。
Hg最外圍就係兩粒6s electrons，基本上嗰兩粒唔會點甩，即係metallic bond係好弱，melting point比其他metal低（低過RT），所以係liquid

唔好意思
想問electrolysis of dilute H2SO4, discharge O2
係咪話position of OH- lower than that of SO4 2- in E.C.S.
so OH- will be preferentially discharged.
點解我見有人話OH- ECS 高過SO4-
我有無搞錯d野

如果你要講position
係OH-高過SO4 2-
因為ECS越勁嘅R.A.係越上
不過與其記position 不如直接記OH-係stronger R.A. than SO4 2-

呢個表你只係見到SO2係stronger R.A. than OH- (SO2高過OH-)
而唔係SO4 2-係stronger R.A.
我地利用ECS比較兩隻野個陣係要睇當佢地都係做緊同一樣嘅agent咁樣睇

I) For comparison of RA, position of OH- is higher that of SO42- in ECS, so OH- is preferentially discharged.
II) OH- has a stronger reducing power than SO42-, so OH- is preferentially discharged.

Either one you choose for explanation

如果我電解 conc H2SO4 會點？

ECS其實係黎自standard reduction potential table, 每條redox half equation都會有個potential value，會計到每個electrolytic cell需要幾多voltage電先做到electrolysis，例如electrolysis of diluted H2SO4 (Overall: 2H2O-->O2 + 2H2)，係需要+0.4V (計數方面DSE好似都係out-C)，其實少少電就可以。

如果electrolysis of conc. H2SO4 (~98% H2SO4)，其實入面有好小量水份，0.4V電做electrolysis會分解小小水份，但如果上到2V以上，除咗水會分解(Anode: OH- --> O2)，H2SO4都會oxidized (Anode: 2H2SO4 --> H2S2O8)，最後除咗出oxygen+hydrogen gas，仲會出perdisulphuric acid

第一個唔得
第一個electrolysis of K2SO4 本身K+同SO42-兩樣野都無辦法reduced/oxidized, 實際上係electrolysis of water with K2SO4 electrolyte
第二個應該相反, Conc.MgCl2(aq),
H+ reduced Cl- oxdized
係個electrolytic cell入面HCl無出現過

唔該晒
原來有呢d rule

完全無人講野

有冇chem last week tips ?

過幾日考留名，lvl 4 要集中温邊幾個topic？

得返幾日唔溫得幾多
Aim Lv3-4首先建議主攻 Acids and Bases & Redox & Organic Chem 呢幾個佔分較多嘅題目

其次就可能溫一啲以背書為重嘅topic例如Pattern 其實唔算難或者年年考法都較為相似嘅topic譬如係Chemical equilibrium