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[化學 Chem] HKDSE Chemistry Question Review (2018-05-14)



HKDSE Chemistry Question Review (2018-05-14)

[隱藏]
1. N2O5 can be made by dehydration of dilute Nitric Acid with P2O5 as catalyst.

(a) Write down the chemical equation for the reaction.
(b) Draw its Lewis structure.

Answer

(a) 2 HNO3(aq) ---> H2O(l) + N2O5 (s)
(b)
A Good Analyzer Must Be A Good Collector.

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2. Outline the method of preparing the following gases and write down the concerned chemical equation:

(a) O2
(b) N2 (Hint: Try balancing the atoms on both sides)

Answer

(a) Heating Group (I) nitrates

2KNO3 (aq) ---> 2KNO2(aq) + O2(g)

(b) Heating NH4NO2

NH4NO2 (s) --> N2(g) + 2 H2O(l)






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3. Study the slide carefully:



(a) Is the forward reaction exothermic or endothermic?
(b) What will happen if the reaction mixture of NO2 and N2O4 is compressed?

Answer

(a) Endothermic. Breaking bonds in N2O4 requires mechanical energy input.
(b) The reaction mixture will be pale. According to Le Chareter's Principle, increase in pressure will favor the backward reaction because the number of moles of N2O4 must increase in order to meet the equilibrium.

Note: By ideal gas equation PV=nRT, Volume of NO2 must decrease.

[ 本帖最後由 doraemonserv 於 2018-5-15 12:01 AM 編輯 ]




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4. Both N2O can O2 can burn in air.

(a) Draw the Lewis structure of N2O.
(b) How to distinguish N2O(g) and O2(g) ?

Answer

(a)
(b) Thermal decomposition of N2O(g) with catalyst will produce a large amount of inert N2(g) to quench the burning process of a glowing splint.

2N2O (g) ----> 2N2(g) + O2(g)






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[隱藏]
5. Consider the reaction below:

N2O3(l) + 2 NaOH(aq) --> 2NaNO2(aq) + N2O(g)

(a) Draw the Lewis structure of N2O3
(b) Is the reaction neutralization? Explain briefly.

Answer

(a)
(b) No. The neutralization process is non-redox, but the reaction above involves changes of oxidation numbers in Nitrogen atoms, and NaNO2 is still basic while N2O as an acid anhydride is formed instead of H2O does.
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6. Balance the following chemical equations:

(a) NH3 + O2 ---> NO + H2O
(b) NO2 + H2O --> HNO3 + NO

Answer


(a) 4NH3 + 5O2 ---> 4NO + 6H2O
(b) 3NO2 + H2O --> 2HNO3 + NO






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7. Write down the products for the following reactions:

(a) Zinc + dilute HNO3
(b) Copper + concentrated HNO3
(c) Sulphur + concentrated HNO3

Answer

(a) Zn(NO3)2 + NO + H2O
(b) Cu(NO3)2 + NO2 + H2O
(c) H2SO4 + NO2 + H2O






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8. CO(g) and H2(g) are active ingredient in town gas.

(a) Given C6H14(g). Write down the balanced chemical equation for the steam reforming with catalyst in high pressure.
(b) Given CH4(g). Write ditto.

Answer

(a) C6H14 + 6H2O ---> 6CO + 13H2
(b) CH4 + H2O ---> CO + 3H2




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9. The chemical equation for an explosion is given below.

2NH4NO3(s) ---> 4H2O(l) + 2N2(g) + O2(g)

NO2 and NO are NOT found in the products. Given an account for it.

Answer

Recall the following reaction

2NO2 ---> 2NO + O2

Then we have:

2 NO <---> N2O2

The forward reaction is exothermic, which will lead to cleaving of N-H bond, then N2O4 will react with H2 formed from hydrogen free radical:

H2 (g) + N2O2(g) ---> H2O (g) + N2O (g)

The reduction driven by high temperature will go further:

H2(g) + N2O(g) ----> H2O (g) + N2(g)

That's why NO and NO2 are all depleted in explosion.
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[隱藏]
10. For the following catalyzed endothermic reactions, explain why no NO2 is formed.

(a) NH3 + O2 ---> N2 + H2O
(b) CuO + NH3 --> Cu + H2O + N2
(c) NH3 + 5O2 ---> NO + H2O (using red-hot Platinum)

Answer

Hydrogen atoms/free-radicals as intermediate lying on the surface of reactants and catalysts will further reduce NO2(g), and even NO(g) if negative catalyst is applied.
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11. An experiment is set up to collect NH3(g)

(a)
Given Ca(OH)2(s) and NH4Cl(s) as reactant. What are the products?
(b) How to purify NH3(g) collected?

Answer

(a) CaCl2(s), H2O(l) and NH3(g)
(b) Use quicklime. It will absorb impurities CO2 and dry the products.
A Good Analyzer Must Be A Good Collector.

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