[2022DSE CHEM化學］問答討論區

要

想請問enthalpy change of reaction 係咪一定會講係邊reactant/product
因為例如 2AgNO3 + Zn -> Zn(NO3)2 + 2Ag
呢條個enthalpy change per mole of Zn 同 AgNO3係唔一樣
如果唔會講明邊隻做計量的話, 其實enthalpy change of reaction究竟係咩

2022 dsers 加油

其實個意思係
按照條equation 既stoichiometric ratio 咁樣行一次個reaction
所產生既enthalpy change

你可以參考2012 Paper 1B 8 c ii
入面提到既reaction (2)

如果用返你既例子
2AgNO3 + Zn -> Zn(NO3)2 + 2Ag
就係講緊
當 2 moles AgNO3 react with 1 mole Zn , 產生 1 mole Zn(NO3)2 同 2 mole Ag 既時候, 所產生既enthalpy change

好呀感謝您 因為我見外國有啲題目係per mole of某隻/product reactant 嘅 enthalpy change所以有少少confused

同外國嘅題目無衝突
用你呢個reaction嘅例子就係計緊per mole of Zn帶黎嘅enthalpy change

平時都係要化返做25
除左係講緊關於standard enthalpy change of ____ 個陣 然後佢要你用chemical equation去輔助你個解釋個陣 先會唔洗化
因為呢個時候係限死左有一隻野1 mole

例如話standard enthalpy change of combustion of octane
因為你隻octane限死係1 mole 所以其他reactant/product 就有機會係 mole expressed in 份數

thx

想問add a few drops of lemon juice to sodium carbonate powder 點解唔可以答the solid dissovles?
答案係 colorless gas bubble form

我諗係真正做實驗個陣最明顯嘅observation會係colorless gas bubbles evolve
當然solid dissolve係有發生 但最major見到嘅會係gas evolve

無乜做過實驗 真係憑空想像

我覺得應該係答Chemical changes-related observations，而唔係physical changes-related ones。即係formation of new substances先係observations, chemically say

咁放舊Mg 落acid 度嘅observation 我唔可以寫 Mg dissolves ?

首先Dissolve/dissolution係一個physical change，eg salt (NaCl) can dissolve in water (due to formation ion-dipole interactions)，而Mg (a metal solid）唔會dissolve 喺任何solvents, i.e. Mg is insoluble in water or non-polar solvents，你見到Mg冇咗，係因為Mg REACTS with acid, with formation of H2，所以“Mg dissolves”喺chemistry 角度黎講唔正確。

題目問緊observation
唔係問你有咩chemical change

我相信中一都已經有教observation 有好多樣
睇到 / 聽到 etc
你將table salt dissolve in water 嘅observation 就係table salt dissolve
如果照你邏輯，上面個例子就係冇observable change ?

我所講嘅observations，係強調緊一般大眾理解同chem exam所要求嘅分別，Observations一般理解當然係睇到/聽到 etc，但chem exam marking scheme有時就係唔接受solid dissolve，eg lemon juice+ sodium carbonate powder，所以我會用chemical changes係解理 chem exam所要求嘅observations。

所以NaCl(s) dissolve in water，我係覺得chem exam 唔會咁出題問有咩observations，而係會問anhydrous CuSO4(s) dissolve in water之類。

chem exam marking scheme有時就係唔接受solid dissolve

你肯定 ？

2017 1B 6c 己經接受Cu dissolve

題目係話 add a few drops of lemon juice to sodium carbonate powder
即係好多powder , 但好少solvent
情況同你拎一包鹽, 加2滴水入去 一樣
睇佢有冇dissolve 真係難d 既
但colourless gas bubbles evolved就明顯D囉


其實講observation 真係好多
你見到既marking scheme
又未必等如只有呢個答案先拎到分
再講多個
heat release
你覺得你呢一題答 heat release 拎唔拎到分?

我唔同意呢個說法
dse一直都係接受去講metal dissolve呢樣野
題目重點係個observation 而唔係個reaction
姐係只係講你observe到啲咩
例如我搵個唔識chem嘅人黎個reaction
佢都可以答到舊Mg dissolve左/有gas evolve
係唔需要去諗到底Mg本身自己係咪會dissolve in water呢樣野

知唔知咩係 Chemical changes-related observations