chem 一堆問題

Simple answers for so many questions; answers could be in details if asked separately.
Don't try to delete this Q. to start over other questions; leave this to be and ask new Qs.

1. white

2. black

3. NO! CaCl2 can't be used to dry NH3 , as CaCl2 reacts & absorbs NH3 to give complex. use CaO .

4. can only memorize all of them in textbook; go & read

5. cracking of long HCs can produce hydrogen and shorter HCs only.

6. minimally; you can think "no"

7. sol. w/ definite, known conc.. to ensure sol. is well-prepared to give expected conc.

8. see Q7. use another std. sol. (which reacts with unknown sol. w/ definite stoichiometry) to titrate against unknown sol.

9. "drying" removes water only in molecular form; "dehydrating" can remove water by withdrawing H & O atoms from target in 2:1 ratio

10. hygroscopic prop. ; usually exothermic when comes contact w/ water

11. false. metals can undergo redox when come contact w/ aq. HCl ; inert metals like Au & Pt don't react w/ those acids.

12. false. rxn. btw metal oxides & acids are v. unlikely to be redox

13. V-shaped

14. all of four listed

15. NaOH can react w/ glass and etch it
2NaOH + SiO2 --> Na2SiO3

16. grease cleaner, to remove oil stains

17. your textbook should hv listed types of rxn which are exo- / endo- thermic; go & read & memorize

18. not necessary to be either. conc. H2SO4 : exo- ; propanone: endo-

19. Ag2O & CO2 ; further heating could give Ag & O2

20. = 2.48


2012-02-26 05:31:32 補充：
by a university's postgraduate student, now doing research in chemistry; have got "A" in CE & A-level. HKDSE is of no match, haha.

2012-02-26 05:36:03 補充：
1. potassium silvery, its oxide isn't.

3. CaCl2 reacts w/ NH3 to give complex, CaCl2(NH3)2 ; it also reacts w/ organic amines. go and read the books.

4. silica gel is not dehydrating.

2012-02-26 05:38:50 補充：
5. cracking of hydrocarbons does NOT produce CO2 as there's no oxygen in reaction chamber.

6. there're in fact hydrogen bonds in between ester (lone pair donor) & water molecules (lone pair acceptor); the bonds are not extensive, so solubility is minimal.

2012-02-26 17:10:20 補充：
true, but i suppose that's due to contamination of peroxide and superoxide.

2012-02-29 08:57:50 補充：
no offend, just in doubt.
1.
how can one be sure that no peroxide is formed from decomposition of nitrate? (especially when NO2 could be an oxidant).
also, nitrogen dioxide is yellow/brown in color. could there be contaminations?

2012-02-29 09:00:41 補充：
6.
generally speaking, the solubility of esters is much lower than their parent alcohols & acids. and the formates are just small.

12.
while neutralization is NOT redox, right?
based on arrhenius acid-base definition, neutralization produce salt & water as ONLY products.

2012-03-06 05:04:19 補充：
primary std. soln. is the std. soln. which is prepared by directly dissolving a known mass of solute in the solvent (typically water).

secondary std. soln. is prepared by making a solution of unknown concentration, then determines its EXACT concentration with other known std. soln. via titration.

2012-03-06 05:06:48 補充：
solutes for pri. std. are stable in pure form or solution form.
solutes for sec. std. are not very stable. they could absorb water (like KOH, H2SO4), absorb CO2 (KOH), or decompose (KMnO4, I2, NaOCl). they need regular standardization to determine the exact conc. from time to time.

我反而想問
primary standard solution又係咩....


** 燒K時會同時有K2O2 KO2 K2O 所以有時隻色係怪怪地
不過K2O suppose to be white *

1.
K2O is pale yellow in color. In thermal decomposition of KNO3, K2O is formed which is NOT contaminated with peroxide or superoxide.

6.
Small esters are fairly soluble in water. For example, at 20°C, the solubility of HCOOCH3 is 30 g / 100 g of water, and that of HCOOC2H5 11 g / 100 g of water.

2012-02-29 05:34:07 補充：
7.
"Dehydrating" means the removal of water of crystallization from a hydrated compound, or removal of H and O atoms in the ratio 2 : 1 to form water from a compound.

12.
Neutralization occurs as a salt and water are formed.

上維基搵到K2O係pale yellow solid喎= ="

1. K2O咩色? orange or white?

Ans : Potassium oxide are neither orange nor white , it is silvery .


2. CuO咩色? dull green or black?

Ans : Copper(II)oxide is black but not dull green.


3. 點解 NH3 is dry by CaCl2? 可以用其他野dry嗎?

Ans : Ammonia is dried by Calcium chloride due to the neutral property of calcium chloride but as alkaline as ammonia , and even acid.


4. drying agents有咩例子. 佢地分別可以dry咩. 洗唔洗死記?

Ans : Drying agents are also called dehydrating agents . they can separated by hygroscopic agents and deliquescent agents . Both of thm can absorb water vapor but deliquescent agents are solid form which will desolve in the water vapor .




2012-02-25 23:59:41 補充：
3. 點解 NH3 is dry by CaCl2? 可以用其他野dry嗎?

Ans : Ammonia is dried by Calcium chloride due to the neutral property of calcium chloride but not as alkaline as ammonia , and even acid.

2012-02-26 00:02:49 補充：
5. cracking 點解可 produce H2?

Ans : Cracking involves the breaking bonds and forming bond of larger hydrocarbon molecules into smaller molecules.

When hydrocarbon molecules are broken bonds , Hydrogen gas will produce .

Other substances such as carbon dioxide will also produce ,

2012-02-26 00:04:42 補充：
6. Ester 溶唔溶響水?

Ans : Ester is organic-soluble but water-insoluble organic compounds because its molucules do not contain any hydrogen bonds with polar water molecules.

2012-02-26 00:16:16 補充：
7.standard solution係咩? 點解 the substance making the standard soln must be pure?

Ans : Standard solutions are solutions which have known accurate concentration .

It is important to make the solution pure in order not to affect the concentration of it.

嘩！洗唔洗問咁多題...
分開問好似比較好！