中五化學/會考試題改編/短答題/一
1
Name:
1.
S.5 Revision (1) Short Question
Which element in Period 3 of the Periodic Table is the strongest oxidizing agent? Explain your answer.
(ans) Chlorine.
2.
It belongs to Group VII. It has the higher ability to gain electron.
The atomic numbers of element X andY are 12 and 17 respectively. Draw an electron diagram for the compound formed
from element X and Y. (Show the outermost shell electrons only)
(ans):
3.
Draw it yourself. (an ionic compound)
Arrange the elements in Group VII of the Periodic Table in decreasing order of melting point. Explain your answer.
(ans) The melting point increases down the group. They are simple molecular structures. Molecules are held together by Van der
Waals’ forces which increase with molecular size.
4.
X3 is the formula of a gas. If Avogadro’s number is L mol –1, how many molecules are there in 96g X3?
(Relative atomic mass : X = 16.0 )
(ans) molar mass of X3 = 48g mol-1 , no. of mole of X3 = 96/48 = 2 mol.
5.
Thus there are 2L molecules of X3.
If there are X molecules in 2 g of carbon dioxide, how many molecules (in term of X) are there in 2 g of helium?
(Relative atomic masses : He = 4.0, C = 12.0, O = 16.0 )
(ans) no. of mole of CO2 = 2/44 = 1/22 mol.
No. of mole of He = 2/4 = 1/2.
no. of He molecule = (1/2) /(1/22) x X = 11X
6.
What volume of 0.5 M sulphuric acid is required to react with excess solid sodium hydrogencarbonate in order to produce
4.8 dm3 carbon dioxide under room temperature and pressure?
(molar volume of gas at room temperature and pressure = 24dm3 )
(ans) 2NaHCO3 + H2SO4 → Na2SO4 + 2H2O + 2CO2 ;
so no. of mole of H2SO4 = 0.2/2 = 0.1mole.
7.
no. of mole of CO2 = 4.8/24 = 0.2 mol
Thus Vol of H 2SO4 = 0.1 / 0.5 = 0.2 dm3
After mixing 150.0cm3 of 3.0 M sodium hydroxide solution with 50.0cm3 of 1.0M sodium hydroxidesolution, what is the
concentration of the resulting solution?
(ans) total no. of mole of NaOH = 3 x 0.150 + 1 x 0.050 = 0.50 mol
concentration of the resulting solution = 0.50 / (0.150+0.050) = 2.5M
8.
A student used a dilute acid to carry out an experiment. He poured the dilute acid to the sink after the experiment. It was
found that the copper tubing connected to the sink was broken afterwards. What acid did the student used? Explain your
answer.
(ans) Dilute nitric acid. Dilute nitric acid is an oxidizing agent that can oxidize copper to copper(II) ion while
dilute hydrochloric acid and dilute sulphuric acid are not oxidizing agents.
中五化學/會考試題改編/短答題/一
9.
2
3
Explain whether the following statements, about 25cm 1M hydrochloric acid and 1 M ethanoic acid, are correct or
not.
(1)
They have the same colour change after addition of the same amount of universal indicator.
(ans) false.
HCl is a strong acid and ethanoic acid is a weak acid. So concentration of H+ in HCl is higher than
than in ethanoic acid. So their pH values are not the same.
(2)
Under the same temperature, they react with marble chips with the same rate.
(ans) False.
HCl is a strong acid and ethanoic acid is a weak acid. So concentration of H+ in HCl is higher than
than in ethanoic acid. Reaction rate increases with concentration of reactant. So the marble chips will react faster
with HCl.
(3)
They need the same number of moles of sodium hydroxide for complete neutralization.
(ans) True. No. of moles of both the strong and weak acid are the same. The amount of sodium of hydroxide
needed to neutralize the acids depends only on the amount of acid, but not the stength of acid.
10.
What acid(s) cannot be used to liberate carbon dioxide from limestone? Explain your answer.
(ans) Sulphuric acid. Since the calcium sulphate formed is insoluble in water. It will form on the surface of the limestone
and no further reaction can occur.
11.
Hydrogen cannot be liberated by reacting zinc with 0.5 M nitric acid. Explain why.
(ans) 0.5 M (dilute) nitric acid is an oxidizing agent that will react with metal to form nitrogen monoxide, not hydrogen.
12.
(a)
Explain why burning fossil fuels causes acid rain.
(ans) Burning fossil fuels with produce SO2. SO2 will dissolve in rain water to form sulphurous acid.
(b)
Explain why acid rain corrodes buildings.
(ans) Some of the ingredients (e.g. calcium carbonate) in buildings will react with acid to form soluble salts.
13.
Calculate the oxidation number of S and N in each pair of compounds below.
A.
(ans)
14.
H2SO4 and
+6
HNO3
+5
B.
SO2
and
+4
HNO2
+3
C.
SCl2
+2
and
NO
+2
D.
NaHS
and
-2
NH3
-3
A student used the following setup to electroplate the copper ring with nickel.
Is the copper ring the cathode or anode? Explain your answer.
(ans) Cathode. It is because Nickel(II) ions will reduce at copper ring. Reduction takes place at cathode.
____________________________________________________________________________________________
15.
The melting point of bromine is quite low because
molecules are held together by weak Van der Waals’ forces.
bromine is substance with simple molecular structure. Its
中五化學/會考試題改編/短答題/一
3
16.
Solid zinc chloride cannot conduct electricity because
17.
Gas X is bubbled into solution Y as shown below:
(a)
the ions in solid state are not mobile.
What change would occur if gas X is chlorine and solution Y is potassium iodide? Explain your answer.
(ans) The colour of Y changes from colourless to brown. It is because I- is oxidized by Cl2 to I2 which is brown in colour.
ionic equation :
(b)
Cl2
+
2I-
─→
2Cl-
+
I2
What change would occur if gas X is carbon dioxide and solution Y is sodium hydroxide? Explain your answer.
(ans) No observable change. Carbon dioxide will be absorbed by sodium hydroxide but without any observable change.
ionic equation :
(c)
CO2
─→
2OH-
+
CO32-
+
H2O
What change would occur if gas X is sulphur dioxide and solution Y is bromine water? Explain your answer.
(ans) The solution will changes from brown (orange) to colourless because Br 2 will be reduced by SO2 to colourless Br-.
ionic equation :
18.
Br2
+
SO2
+
2H2O
State three major uses of sulphur dioxide.
→
4H+
+
SO42-
+
2Br-
To make (1)sulphuric acid / (2)bleaching agent . (3)To preserve
food.
19.
Rain water contain nitric acid after lightening. Describe the formation of nitric acid by writing several equations.
(ans)
N2
+
4NO2
20.
O2
+
─→
2H2O
2NO
+
;
2NO
O2 ─→
+
O2
─→
2NO2
4HNO3
How to separate sodium chloride from a solid mixture containing sodium chloride and ammonium chloride?
(ans) Heat the mixture. Ammonium chloride will sublime and leave the mixture. (iodine, ammonium chloride, dry ice
are common substances that will undergo sublimation)
21.
Draw structural formulae for the different isomers of an alkanol with formula C4H9OH. Name each isomer you draw.
(ans) Draw the isomers yourself.
中五化學/會考試題改編/短答題/一
22.
4
Draw structural formulae for the different isomers of an ester with formula C4H8O2. Name each isomer you draw.
(ans) Draw the isomers yourself.
23.
State three experimental methods that can be used to distinguish ethane from ethene.
(ans) 1. Add acidified potassium permanganate solution to them. Ethene can change it from purple to colourless while
ethane cannot.
2.
while ethane cannot.
24.
Add bromine solution to them in the absence of light. Ethene can change if from brown to colouless
3.
Burn the two gases. Ethene will burn with a sootier flame.
Clothes stained with oil can be cleaned by using detergents. Describe the cleaning action of a detergent.
(ans) The hydrophobic tail of the detergent will dissolve in oil while the hydrophilic head will dissolve in water. After
agitation, oil droplets are formed. Due to the repulsion of the charged head of the detergent on the surface of the oil droplets. They
will not rejoin together. So an emulsion is formed and the oil droplets can be washed away easily.
25. After heating under reflux a mixture of vegetable oil and sodium hydroxide solution for a period of time,
(a)
what product will be formed﹖
glycerol and soap
(b)
How can you know the end of reaction? The two immicible layers of the solution will join together at the end of
the reaction.
(c)
What happens after pouring the reaction mixture to brine solution? Why?
White solid will be formed. It is
because the soap formed will not dissolve in brine solution.
26.
A glass of sweet wine tunrs sour after two day. Why?
The ethanol in wine will be oxidized to ethanoic acid by the
oxygen in air.
27.
If some dilute hydrochloric acid is splashed to the eye of a student accidentally, what should he do at once?
(ans) Wash the eye with plenty of water at once.
28.
The bleaching effect of sulphur dioxide is not permanent. Why?
(ans) Sulphur dioxide bleaches a substance by reduction.
However, oxygen in air will oxidize the substance back to its original colour gradually.
29.
As compare with fresh water, sea water causes more serious rusting of ship. Why?
(ans) The dissolved ions in sea water can increase the rate of rusting.