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Grade 10
Periodic 2 Term 3 Revision
1. Fill in the blanks of the following table:
Atomic
Number
13
4
83
56
17
Aluminium, Al
Beryllium, Be
Bismuth, Bi
Barium ion, Ba2+
Chloride ion, Cl-
35
Protons
13
4
83
56
17
Particles per atom
Electrons
Neutrons
13
14
4
5
83
126
81
54
18
19
Mass
Number
27
9
209
137
36
Cl
2. Consider the nucleus 17 .
a) What is its atomic number?
17
b) What is its mass number?
35
c) How many nucleons does it contain?
35 nucleons
d) How many neutrons does it contain?
35 – 17 = 18 neutrons
e) How many electrons would its neutral atom contain?
17 electrons
35
3. Chlorine has two stable isotopes, 17
Cl and 37
17 Cl (assume the mass of each nucleon to be
1.0000 amu). If the average molar mass of chlorine is 35.5, find the percentage of each isotope
in nature.
Given:
and
, average molar mass = 35.5
RTF: % of each isotopes
Assume the % of
35
17
Cl is x
Average molar mass of Cl =
Chlorine is 75%
x
100 - x
(35) +
(37) = 35.5
100
100
x = 75%
and 25%
4. Chlorine in nature consists of 75.4% of chlorine-35 and 24.6% of chlorine-37. Find the average
molar mass of chlorine.
Given: 75.4% Cl-35, 24.5 % Cl-37
R.T.F.: average molar mass of chlorine
Assume 100 moles of chlorine
Average molar mass =
(75.4 × 35) + (24.5 × 37)
= 35.5
100
5. Write the equation for the reaction of potassium with chlorine.
2K(s) + Cl2(g) → 2KCl(s)
6. When heated with oxygen potassium reacts vigorously to form potassium oxide, K2O. Write the
equation for the reaction.
4K(s) + O2(g) → 2K2O(s)
7. Write the equation for the reaction of potassium with water.
2K(s) + H2O(l) → 2K+(aq) + 2OH-(aq)
8. There is a large difference between the energy needed to remove an electron from a neutral,
gaseous sodium atom and a neutral, gaseous neon atom. Explain.
Na(g) + 495kJ → Na+(g) + e–
Ne(g) + 2075 kJ → Ne+(g) + e–
To remove an electron from neon means to disturb a stable electronic configuration which
requires large amounts of energy. To remove an electron from sodium means to form a stable
electronic configuration which requires much less amounts of energy.
9. How many electrons surround the nucleus of a chlorine atom?
17
10. How many electrons surround the nucleus of the next inert gas?
18
11. Consider two chlorine atoms very close together. Each atom of chlorine contributes one electron
to form a pair of electrons that goes around both atoms instead of around a single atom. How
many electrons does each nucleus of chlorine “feel” going around it? How does this number
compare to the number of electrons around the nucleus of the next noble gas? 18, the same.
12. Give, in your own words, an explanation for the fact that the element chlorine at room
temperature exists as diatomic Cl2(g) molecules.
By existing as a pair each atom has the same electronic population as an inert gas. This
makes them very stable.
13. How many electrons are shared by the two chlorine atoms?
2 electrons
14. If a pair of electrons shared between two atoms is called a covalent bond, how many covalent
bonds exist between the two chlorine atoms in Cl2(g)? One covalent bond
15. A salt S, is dissolved in water. When a silver wire is dipped in the solution of S then placed in a
blue flame, the color of the flame turns to bright orange. When a few drops of silver nitrate
solution is added to 2 ml of the solution of S, a white precipitate formed, which slowly turned
into purple then black.
What is S? Give a convincing explanation, using balanced equations.
S is sodium chloride NaCl.
Sodium ions produce an orange flame when placed in a burner.
Chloride ions form a white precipitate with silver ions. Ag + (aq) + Cl -(aq) → AgCl (s)
16. A white solid M was dissolved in aqueous solution Z. To a few ml of Z, a little chlorine water
was added (chlorox + HCl). The solution turned brownish. When 2 ml of CCl4 was added into
the tube and shaken, the CCl4 layer tuned purple-violet in color. When a platinum wire was
dipped into the solution Z and placed in a flame, the color of the flame became lilac-violet.
What is M? Write balanced chemical equations whenever possible.
M is potassium iodide. KI.
Potassium ions produce a purple flame.
Chlorine reacts with iodide ions (colorless in water) producing iodine (brown in water,
purple in CCl4)
KI(s) → K+(aq) + I-(aq)
17.
Cl2(aq) + 2I-(aq) → I2(aq) + 2Cl-(aq)
Write the reaction between potassium and hydrogen.
2K(s) + H2(g) + energy → 2KH(s)
18. Write what happens (using equations whenever possible) when the KH and KBr are placed in
water.
2KH(s) + H2O(l) → 2KOH(aq) + H2(g) + energy
KBr(s) → K+(aq) + Br-(aq)
In the first reaction, hydrogen gas is produced
In the 2nd reaction, ions are formed.
19.
Write the reaction between chlorine and hydrogen. H2(g) + Cl2(g) → 2HCl(g) + energy
20. Write what happens (using equations whenever possible) when KCl and HCl are placed in
water. Show in what way the products are similar.
HCl(g) → H+(aq) + Cl-(aq)
KCl(g) → K+(aq) + Cl-(aq)
The products of both reactions are ions
21. Write equations for the following reaction. In each case from (a) to (g), state what type of
bond(s) is/are formed and what type of compound results.
Equation
bonds
Compound
Sodium with sulfur
2Na(s) + S(s) → Na2S(s)
ionic
Ionic
Magnesium with chlorine
Mg(s) + Cl2(g) → MgCl2(s)
ionic
Ionic
Aluminum with oxygen
4Al(s) + 3O2(s) → 2Al2O3(s)
ionic
Ionic
Silicon with oxygen
Si(s) + O2(g) → SiO2(s)
covalent
Network Solid
Silicon with chlorine
Si(s) + 2Cl2(g) → SiCl4(s)
covalent
Molecular
Sulfur with hydrogen
H2(g) + S(s) → H2S(g)
covalent
Molecular
Argon with hydrogen
No reaction
-----
-----
22. Identify the substance:
a) Colors a flame red.
Lithium, Li+
b) Forms a cream-coloured precipitate with silver nitrate.
Bromide ions, Brc) Blue crystals that become a white powder when heated gently.
Hydrated Copper(II) sulphate, CuSO4
d) A dark-grey shiny element that sublimes into purple fumes. Iodine, I2
e) Its solution turns brownish when Clorox and acid is added to it.
Bromide ions, Br- OR iodide ions, If) Least dense unreactive gas.
He, Helium
g) Least dense reactive gas.
H2, Hydrogen
h) The least dense alkali metal.
Lithium, Li
23. Complete and balance the following equations. If no reaction takes place,
write “no reaction”.
a) KI(aq) + AgNO3(aq) → AgI(s) + KNO3(aq)
b) Br2(l) + KCl(aq) → No reaction
c) H2(g) + Cl2(g) + spark → 2HCl(g)
e) Cl2(g) + 2 LiI(aq) → I2(aq) + 2LiCl(aq)
f ) H2(g) + 2 K(s) → 2KH(s)
g) NaH(s) + H2O(l) → NaOH(aq) + H2(g)
24. Why does every element in group I form ions of charge +1?
By losing one electron they achieve the same electron population as the inert gas before
them.
25. What elements form ions of charge +2?
Group 2 elements
26. What elements form ions of charge +3?
Group 3 elements
27. Why don’t elements of group 4 form ions of charge 4+?
Why don’t they form ions of charge 4–?
Too much energy is needed to remove 4 electrons from an atom.
Too much energy is needed to insert 4 electrons into an atom in order to overcome the
repulsive forces between like charges.
28. Why do elements of groups 6 and 7 form ions of charge –2 and –1 respectively?
By gaining electrons they achieve the same electron population as the inert gas after them.
29. How else do elements of groups 6 and 7 react?
Elements of group 6 react by sharing 2 electrons.
Elements of group 7 react by sharing 1 electron
30. In H2O, why does hydrogen make one bond while oxygen makes two?
O needs two electrons to become like Ne while H needs only one to become like He.
31. How do you explain that oxygen forms O2 molecules?
Each O-atom shares two e- with the other. So, each O-atom “feels” 10 e- around it like Ne.
32. To change the temperature of a particular calorimeter and the water it contains by one degree
requires 5.0 kJ. The complete combustion of 0.70 gram of ethene gas, C2H4(g), in the calorimeter
causes a temperature rise of 5.3°C. Find the heat of combustion per mole of ethene.
Given : m of ethene = 0.70 g, ΔT = 5.3°C, C = 5.0kJ
ΔQ = CΔT = (5.0)(5.3) = 26.5kJ
n of ethane =
R.T.F : ΔH
= 0.025 mole
33. Given: 3C(s) + 2Fe2O3(s) + 462 kJ → 4Fe(s) + 3CO2(g)
Rewrite the equation expressing ΔH as per mole of Fe(s).
C(s) + Fe2O3(s) + 115.5 kJ → Fe(s) +
CO2(g)
34. Given: 3C(s) + 2Fe2O3(s) + 462 kJ → 4Fe(s) + 3CO2(g)
Rewrite the equation expressing ΔH as per mole of C(s).
ΔH =
= 1060kJ
C(s) + Fe2O3(s) + 154 kJ → Fe(s) + CO2(g)
35. Given: ½ H2(g) + ½ Br2 (l) → HBr(g)
ΔH = -36 kJ/mole HBr
Rewrite the equation expressing the coefficients as the smallest whole numbers, and express
the heat of reaction
a) per mole of H2. H2(g) + Br2(g) → 2HBr(g)
ΔH = -72 kJ/mole H2
b) How much energy is liberated when 2 moles of HBr (g) are produced according to the
above equation?
72 kJ are produced when 2 moles of HBr (g) are formed.
36.
Consider the following reaction: CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) ΔH = -819 kJ
Calculate the heat change when 1.60 g of methane is burned in excess oxygen.
Given : mass of CH4 = 1.60 g, ΔH = -819 kJ
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
16g
1.60g
R.T.F: ΔH due to the given mass of methane
ΔH = -819 kJ
819kJ
?kJ
? = 81.9 kJ
37. In forming water vapour from its elements, 242 kJ/mol are given out. In forming carbon dioxide
from its elements, 393kJ are released. In forming ethane (C2H6) from its elements, 84.5kJ are
produced. Find the heat of combustion of ethane to form water vapour and carbon dioxide.
Given: molar heat of formation of CO2 = 393kJ, molar heat of formation of C2H6 = 84.5kJ
molar heat of formation of H2O = 242kJ
RTF: molar heat of combustion of C2H6
1
3 × [H 2(g) + O 2(g) → H 2O(g)
ΔH = -242kJ]
2
2 × [C(s) + O 2(g) → CO 2(g)
ΔH = -393kJ]
-1 × [2C(s) + 3H 2(g) → C2 H 6(g)
7
C2 H6(g) + O2(g) → 2CO2(g) + 3H 2O(g)
2
ΔH = -84.5kJ]
ΔH = 84.5 + 2(-393) + 3(-242) = -1427.5kJ
38. The thermite reaction is a spectacular and highly exothermic reaction. It involves the rection
between Fe2O3 and metallic aluminium. The reaction produces white-hot, molten iron in a few
seconds. Given:
4Al + 3O2 → 2Al2O3
ΔH1 = -3348 kJmol
ΔH2 = -1674 kJmol
4Fe + 3O2 → 2Fe2O3
Determine the amount of heat liberated in the reaction of 1 mole of Fe2O3 with Al.
2Fe2O3 → 4Fe + 3O2
ΔH1 = +1674 kJ/mol
4Al + 3O2 → 2Al2O3
ΔH2 = -3348 kJ/mol
Add the two reactions: 2Fe2O3 + 4Al → 4Fe + 2Al2O3 ΔH = +1674 – 3348 = -1674 kJ/mol
We need the reaction per mole of Fe2O3 with Al ⇒ we need to divide the reaction by 2:
Fe2O3 + 2Al → 2Fe + Al2O3
ΔH = -837 kJ/mol
We need to reverse the second reaction:
39. Fission of uranium gives a variety of fission products, including praseodymium, Pr. In the
equation below:
+
→
+ ____ + 3
+ energy
a) What is the charge on the missing element?
Charge is conserved ⇒ 92 + 0 = 59 + y + 0
⇒
y = 33
⇒
x = 86
b) What is the mass of the missing element?
Mass is conserved ⇒ 235 + 1 = 147 + x + 3
c) Identify the element.
As, arsenic
40. a. Give the complete, non-ionic equation for the reaction between potassium and water.
2K(s) + 2H2O(l) → 2KOH(aq) + H2(g)
b. Give the complete, word equation for the reaction between AgNO3 and NaCl solutions.
Silver nitrate + sodium chloride → silver chloride + sodium nitrate