SCHEME ANSWER MID YEAR EXAMINATION CHEMISTRY PAPER 2 2012 QUESTION
1
a
2
a
MARKS
i ii iii iv
Temperature at which a solid changes to liquid 83˚C Solid and liquid Heat energy absorbed Heat is used to overcome the forces of attraction between the molecules/particles x-axis and y-axis labeled with unit smooth curve and vertical part marked with all the temperature involved
1m 1m 1m 1m 1m 1m 1m
i ii
sublimation Iodine-131 To treat thyroid patients // diagnosis of thyroid glands C at least three rows closely packed to each other not overlapping with each other eg:
1m 1m 1m @ 1m
b
c
ANSWER
1m
-
Able to draw sulphur sulphur 3 C at least 6 particles groups of particles touching each other
eg:
1m
-
C at leat three sulphur particles particles not touching each other
eg:
1m
1
QUESTION
ANSWER
b
-
MARKS
x-axis and y-axis labeled with unit smooth curve and vertical part marked with 119 º C
1m 1m
eg : temperature / º C
119
time / min c
i ii iii
3
a b
i
c
ii i Ii
d
4
i ii iii
a b
i ii
c d
i ii iii
e f
Reddish-brown vapour spreads throughout both gas jars after few hours Diffusion in gas Bromine gas is made up of tiny and discrete particles This particles moves randomly into empty spaces between the air particles Diffuse in all direction in air
1m 1m 1m 1m 1m 1m
Number of electrons = 18 Number of neutrons = 22 Nucleon Number is the sum of number of protons and number of neutrons in the nucleus of an atom 40 2.1 3 protons and 4 neutrons are located in nucleus of atom 3 electrons located at the shells
1m 1m 1m
W and Y Isotopes have same number of valence electron Y (C-14) can be used to estimate the age of fossils
1m 1m 1m
Chemical formula is a representation of chemical substance using letters for atoms and subscript number to show number of atom for each element in a compound Hydrochloric acid and zinc metal 2HC + 2Zn → 2ZnC +H2 Air in the combustion tube must be removed before heating process by flow the hydrogen gas continuously Mass of copper = 31.86g – 30.24g = 1.62g -1 Mole of copper used = 1.62g / 64gmol = 0.025 mol Mass of oxygen = 32.26g – 31.86g = 0.4g Mole of oxygen = o.4g / 16gmol -1 = 0.025 mol Mole of copper : mole of oxygen 0.025 mol : 0.025 mol 1:1 Empirical formula of copper (II) oxide is CuO Lead (II) oxide // Iron (II) oxide // Tin (II) oxide Burning of metal in excess hydrogen
1m
2
1m 1m 1m 1m
1m 1m 1m
1m 1m
1m 1m 1m
QUESTION
5
a
i ii
b
c
6
7
iii i ii
i ii
a b c
a
i ii
b c d
ANSWER
MARKS
Elements are arranged in increasing atomic number from left to right/horizontally. Period 4 Group 1 M E The nuclei attraction towards the valence electron is weaker in E Thus, it is easier for E to lose electron/release electron to form positively charged ion J Group 18 elements are chemically unreactive / inert chemically
1m 1m 1m 1m 1m 1m 1m
2.8.2 Q and T Forces of attraction between nucleus and valence electron in atom S is higher than those in atom R Thus, atomic radius of atom S is less than atomic radius of atom R
1m 1m 1m
Sodium Hydroxide Hydrogen X Burning splint Red to blue Blue to blue/colorless 2 Na + 2 H2O → 2 NOH + H2
1m 1m
1m 1m 1m
1m
1m @ 1m 1m SECTION B
8
a b
i ii
Aluminium ion = 2.8 Oxygen ion = 2.8 Noble gas This is because all elements in group 18 are stable and chemically unreactive The physical properties of elements in group 18: When down the group: Atomic radius increase
Melting and boiling point increase
Low density increase Cannot conduct electricity and heat
When proton number increase, number of electron increase. Thus more shells are needed to fill in the electrons. When atomic size increase, forces of attraction between atoms is stronger. Thus more energy needed to overcome these forces. Atomic mass increase faster than the increase in atomic size Absence of mobile electrons since noble gases are stable
1m + 1m
1m + 1m
1m + 1m 1m + 1m 1m 1m
Colourless gas at RTP Insoluble in water Any relevant answers are accepted. iii
1m 1m 1m 1m
Elements in group 18 and its use. Helium Fills airships and weather balloons. Neon Advertising lights, Television tubes, airport landing bulb Argon Fill in light bulbs, provide inert atmosphere for welding at high temperature Krypton Used in laser to repair the retina of the eye, fill in photographic flash lamps Xenon Bubble chambers in atomic energy reactors. Radon Treatment in cancer 3
1m + 1m 1m + 1m 1m + 1m 1m + 1m 1m + 1m 1m + 1m
QUESTION
9
a
i
ii
iii iv
b
ANSWER
MARKS
P1 Relative atomic mass of an element is the number of times the mass of an atom is greater than 1/12 of the mass of a carbon-12 atom. P2 The relative mass of a carbon-12 atom is 12.000 P1 One mol is the quantity of a substance which contains the number of particles similar to the one found in 12.000 of carbon-12. P2 The number of particles forms a constant number called the Avogadro constant, 23 NA, which is 6.02x10 P1 Molar mass is the mass of a substance with 1 mol of particles P2 For example, molar mass of aluminium, Al, is 27g P1 Molar volume of a gas is the volume of 1 mol of gas under certain conditions 3 P2 1 mol of any gas occupies 22.4dm at standard temperature and pressure (s.t.p). 3 At room conditions, the volume of 1 mol of any gas is 24dm . - No of moles of oxygen = 16/2(16) = 0.5 mol 3 Volume of oxygen = 0.5 x 24 = 12 dm - No of moles of carbon dioxide = 22/[12+2(16)] = 0.5 mol 3 Volume of carbon dioxide = 0.5 x 24 = 12 dm The volume of the two gases are the same because at the same temperature and atmospheric pressure, all gases occupy the same amount of volume
1m 1m 1m 1m 1m 1m 1m 1m 1m 1m 1m 1m
c Elements
Carbon
Hydrogen
Nitrogen
Oxygen
Mass
0.48g
0.05g
0.28g
0.16g
No of moles
0.48/12 =0.04
0.05/1=0.05
0.28/14=0.02
0.16/16=0.01
Simplest ratio
0.04/0.01=4
0.05/0.01=5
0.02/0.01=2
0.01/0.01=1
Empirical Formula: C4H5 N2O Molar mass of caffeine = 194 Molecular formula = (Empirical Formula)n [4(12)+5(1)+2(14)+(16)]n = 194 97n = 194 n=2 Therefore, molecular formula of caffeine = C 8H10 N4O2
4m 1m 1m
1m 1m
SECTION C
10
a
b
Empirical formula is the formula that shows the simplest ratio of atoms of each element in a compound
1m
Molecular formula is the formula that shows the actual number of atoms of each element in a compound
1m
Example: empirical formula of ethane, CH 2 but the molecular formula is C 2H4 (any suitable answer)
1m
i element Percentage/mass Number of moles Ratio of moles
ii
carbon 40.00 40/12 = 3.33 1
hydrogen 6.66 6.66/1 = 6.66 2
oxygen 53.33 53.33/16 = 3.33 1
1m 1m
Empirical formula = CH2O
1m
n(CH2O) = 180 n(12+1x2 + 16) = 180 n(30) = 180 n = 180/30 n=6
1m
molecular formula = C6H12O6
1m
4
QUESTION
c
i ii
d
11
ANSWER
Substance P. Substance P has boiling point higher than melting point of naphthalene. Naphthalene is flammable and easily explodes when directly burned. From time t1 and t2, the naphthalene exists as liquid. The arrangement of its particles packed closely together but not in an orderly arrangement. As the temperature falls, the naphthalene molecules lose heat energy. Their movement slows down and they move closer to each other. At t2, the substance still in liquid state. The particles have very little energy and begin to move closer towards one another as it starts to freeze into solid. At t2 to t3, the naphthalene changes it state to solid form. Particles start to rearrange and packed closely together in an orderly manner. Naphthalene exists as both solid and liquid. The temperature remains constant until all the liquid changes to solid. This is because the energy released is the same as the energy lost to the surroundings during cooling. This constant temperature is the freezing point. At t3, all the liquid freezes into solid. The particles are now closely packed in an orderly manner. At t3 to t4, the naphthalene is now in solid state. The temperature falls again until it reaches room temperature. This because it lose heat energy and vibrate slower.
a
MARKS
1m 1m 1m 1m 1m 1m 1m 1m 1m 1m 1m 1m 1m 1m 1m 1m 1m 1m 2m
Antoine Lavoisier He differentiated metal and non metal He classified elements into 4 groups He wrote the first list which contained 33 elements Johann W. Dobereiner He classified three elements with similar characteristics into groups known as triads He suggested the existence of the relationship between chemical characteristics with the mass of element atoms John Newlands He arranged all elements known at that time according to the order of increasing atomic mass Characteristics which are similar repeats at every eighth element (law of octaves) Lothar Meyer He arranged 56 elements into one Periodic Table based on periodic characteristics such as volume He discovered that elements with chemical characteristics which are similar reside on positions of same levels on the graph curve plotted by him Dmitri Mendeleev He arranged elements according to the order of increasing atomic mass He left empty spaces for elements which have yet to be discovered at that time Elements with similar chemical characteristics are grouped together
2m
2m
2m
2m
b Periodic Table proposed by Antoine Lavoisier He wrote the first list which contained 33 elements He classified elements into 4 groups
Modern Periodic Table Elements are arranged in order of increasing proton number Elements with the same chemical properties are placed in the same 5
2m 2m
QUESTION
ANSWER
He differentiated metal and non-metal
c
group The horizontal rows are called periods
P1 No P2 Helium has proton number of 2 and an electron arrangement of 2 P3 Due to the fact that its first shell (the only shell) is filled with 2 electrons, helium is not chemically reactive (inert) P4 Thus, helium is placed in Group 18 in the Periodic Table and not in Group 2
6
MARKS
2m
1m 1m 1m 1m
