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*Chemistry P1 May/June 2003 Fully Explained*

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abdullahaftab

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1) The use of the Data Booklet is relevant to this question.
What is the number of molecules in 500 cm3 of oxygen under room conditions?

A) Step 1: Under room conditions 1 mol of oxygen occupies 24 dm3
x mol of oxygen occupies 0.500 dm3
Amount of oxygen = (0.500 x 1) / 24 = 0.0208 mol


Step 2: There are 6.02 x 1023 molecules in 1 mol of oxygen
There are "y" molecules in 0.0208 mol of oxygen
Number of oxygen molecules = [0.0208 x (6.02 x 1023)] / 1 = 1.25 x 1022


2) In the preparation of soft margarine, glyceryl trieleostearate



is suitably hydrogenated so that, on average, one of its side-chains is converted into the CH3(CH2)4CH=CHCH2CH=CH(CH2)7CO2 residue and two side-chains are converted into the
CH3(CH2)7CH=CH(CH2)7CO2 residue.
How many moles of hydrogen are required to convert one mole of glyceryl trieleostearate into the soft margarine?

B) Step 1: Count the hydrogen atoms in the side-chains.
Upper side-chain and lower side-chain: 2 x (3+6+6+14) = 58
Middle side-chain: 3 + 6 + 6 + 14 = 29

Step 2: Count the hydrogen atoms in each residue
First residue: 3 + 8 + 6 + 14 = 31
Second residue x 2 (because two side-chains are converted): 2 x (3 + 14 + 2 + 14) = 66

Step 3: Calculate how many hydrogen atoms had to be added to each side-chain in order to form the residues.
Upper and lower side-chain residues have 66 - 58 extra hydrogens = 8 extra hydrogens
Middle side-chain residue has 31-29 extra hydrogens = 2 extra hydrogens

Step 4: You know that 10 hydrogen atoms have been added to the original molecule. Each hydrogen molecule contains two hydrogen atoms, so there are 10/2 = 5 hydrogen molecules ie. 5 moles of hydrogen

3) Which isotope of an element in the third period of the Periodic Table contains the same number of neutrons as 3216S?

D) Number of neutrons = relative atomic mass - atomic number (protons)
Neutrons in an atom of S = 32 - 16 = 16
Neutrons in an atom of P = 31 - 15 = 16
3115P


4) The successive ionisation energies, in kJ mol-1 of an element X are given below.


870 1800 3000 3600 5800 7000 13200


What is X?


C) Arsenic and Zirconium have a small number of protons compared to Tellerium and Iodine. Their number of protons is not big enough to be consistent with the high 7th ionization energy.
You're left with either Tellerium or Iodine. You can see the ionization energies of iodine in the Data Booklet, and they're not consistent with the ionization energies above, therefore the answer is Tellerium


5) Which of the following solids has a simple molecular lattice?


D) Magnesium Oxide has a a giant ionic lattice.
Sodium has a metallic structure
Silicon (IV) Oxide has a giant covalent structure
The only chemical with a simple molecular lattice is sulphur
 
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abdullahaftab

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6) Measured values of the pressure, volume and temperature of a known mass of gaseous compound are to be substituted into the equation


pV = nRT


in order to calculate the relative molecular mass, Mr, of the compound.
Which conditions of pressure and temperature would give the most accurate value of Mr ?


C) Ideal gases have negligible size, no intermolecular forces, and are always in the gaseous state. When temperature is low and pressure is high, the intermolecular forces and size become relevant, and there are phase changes, which would make the equation less accurate in calculating the relative molecular mass because the equation is made for ideal gases, not real gases... so the answer is a low pressure and high temperature




7) Gaseous phosphorus pentachloride can be decomposed into gaseous phosphorus trichloride and chlorine by heating. The table below gives the bond energies.




What is the enthalpy change in the decomposition of PCl5 to PCl3 and Cl2?

D) The enthalpy of formation of PCl5 is 330 x 5 = -1650 kJ mol-1 (bond formation is exothermic)
The enthalpy of formation of PCl3 is 330 x 3 = -990 kJmol-1
The enthalpy of formation of Cl2 is 240 x 1 = 240 kJmol-1
In order to be sure that your calculations are correct, you should draw a Hess's diagram:



ΔHfO + (-1650) = -990 + (-240)
ΔHfO = +420 kJ mol-1


8) When ammonia is converted into nitric acid on a commercial scale, the following reactions can occur.
In which reaction does the greatest change in oxidation number of the nitrogen occur?


A) In the first reaction:
Oxidation number of Nitrogen before reaction... x + 3 = 0... x = -3
Oxidation number of Nitrogen after reaction... x -2 = 0... x = +2




9) At the age of 17, in a woodshed in Ohio, Charles Martin Hall discovered the commercial process for the production of aluminium metal by the electrolysis of a mixture of bauxite, Al2O3, and cryolite, Na3AlF6.
What is the main purpose of the cryolite?


C) Aluminium oxide dissolves in cryolite and so the cryolite acts as a flux, reducing the melting point required to extract the aluminium from its ores




10) When 0.20 mol of hydrogen gas and 0.15 mol of iodine gas are heated at 723 K until equilibrium is established, the equilibrium mixture is found to contain 0.26 mol of hydrogen iodide.
The equation for the reaction is as follows.
H2(g) + I2(g) 2HI(g)
What is the correct expression for the equilibrium constant Kc?


C) Step 1:
H2(g) + I2(g) 2HI(g)
Initial moles: 0.20 0.15 0
Equilibrium moles: (0.20 - x) (0.15 - x) 0.26


Step 2: The ratio of hydrogen to hydrogen iodide is 1:2 and from iodine to hydrogen iodide is 1:2. This means that both hydrogen and iodine gave half the number of moles of hydrogen iodide that were formed.
The number of moles (x) that each, hydrogen and iodine gave = 0.26 / 2 = 0.13 mol
Moles of hydrogen = 0.20 - 0.13 = 0.07 mol
Moles of iodine = 0.15 - 0.13 = 0.02 mol


Step 3:
Kc = [products] / [reactants]
= (0.26)2 / (0.07 x 0.02)




11) Why is ethanoic acid a stronger acid in liquid ammonia than in aqueous solution?


A) Ammonium ethanoate is not completely ionised in water and it's not acidic in aqueous solution. Water is more polar than ammonia, so the only choice left is that ammonia is a stronger base than water




12) It is often said that the rate of a typical reaction is roughly doubled by raising the temperature by 10°C.
What explains this observation?


D)
The Boltzmann distribution shows that by increasing temperature, the number of molecules with a certain minimum energy increases (more than the activation energy)





13) A mixture of the oxides of two elements of the third period is dissolved in water. The solution is approximately neutral.
What could be the constituents of the mixture?

C) 6Na2O + P4O10 → 4Na3PO4
Na4PO4 + H2O → Na+ + PO-34
The result is slightly basic i.e. approximately neutral


14) Aluminium chloride catalyses certain reactions by forming carbocations (carbonium ions) with chloroalkanes as shown.
RCl +AlCl3 → R+ + AlCl-4
Which property makes this reaction possible?

C) This is the lewis structure for AlCl3


There are only 6 electrons being shared between aluminium and chlorine atoms. According to the octet rule, the valence shell should be filled with 8 electrons, which means that aluminium has an incomplete octet of electrons. This is why when the chlorine atom (from RCl) is added onto the molecule, so as to fulfill the octet rule by forming a dative bond with the aluminium atom, thus leaving the carbon atom as a carbocation.


15) What are the products of the thermal decomposition of magnesium nitrate?

D) The thermal decomposition of metal nitrates is as follows:
Metal nitrate [heat] → Metal oxide + Nitrogen dioxide + Oxygen
2Mg(NO3)2 [heat] → 2MgO + 4NO2 + O2


16) Chlorine compounds show oxidation states ranging from -1 to +7.
What are the reagent(s) and conditions necessary for the oxidation of elemental chlorine into a compound containing chlorine in the +5 oxidation state?


D) A) AgNO3 + Cl- → AgCl + NO3- and then AgCl + 2NH3 → Ag[(NH3)2]+ + Cl- Oxidation number = -1
B) Cl- + H2SO4 → 2HCl + HSO4- Oxidation number = -1
C) 2NaOH + Cl2 → NaCl + NaClO + H2O Oxidation number = -1 and +1 (disproportionation)
D) 3Cl2 + 6NaOH → 5NaCl + NaClO3 + 3H2O Oxidation number = -1 and +5




17) Which gaseous hydride most readily decomposes into its elements on contact with a hot glass rod?


C) Ammonia and gaseous water molecules are attracted by hydrogen bonding, so A and D are incorrect.
Both hydrogen chloride and hydrogen iodide are attracted by Van der Waal's forces. the H-Cl bond is more polar than the H-I bond because Cl is more electronegative than I, so hydrogen iodide decomposes more readily into its elements (and form an equilibrium... H2 + I2 2HI)


18) Which reagent, when mixed and heated with ammonium sulphate, liberates ammonia?

C) Ammonium compounds are ionic salts (soluble in water). When heated with a base, they give off ammonia (Ammonium compound + base → salt + water + ammonia). The only base in the multiple choice is limewater.
(NH4)2SO4 + Ca(OH)2 → CaSO4 + 2H2O + 2NH3


19) Which pollutant is formed in the internal combustion engine and, if not removed by the catalytic converter, may become involved in the formation of acid rain?


D) The only compound that would end up forming an acid is NO. If not removed catalytically, NO combines with oxygen in the air forming NO2 which then reacts with water forming nitric acid (HNO3)




20) How many structural and cis-trans isomers are there for dichloropropene, C3H4Cl2?
D) 7 isomers
 
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abdullahaftab

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21) The isomers, citric acid and isocitric acid, are intermediates in the Krebs cycle of the oxidation of glucose in living cells.
How many chiral centres does each acid possess?
B) Citric acid (from top to bottom): The first carbon is bonded to 2 hydrogens, so it's not chiral. The second carbon is bonded to the top and bottom chains which are the same, so it's not chiral. The third carbon is bonded to 2 hydrogens, so it's not chiral.
Isocitric acid (from top to bottom): The first C is bonded to 2 hydrogens, so it's not chiral. The second carbon is bonded to the top chain, an H, a CO2H, and the bottom chain, so it is chiral. The third carbon is bonded to the upper part of the molecule, an OH, an H, and a CO2H, so it is chiral.
Conclusion: Citric acid has 0 chiral centres and isocitric acid has 2 chiral centres
22) The compound hex-3-en-1-ol, P, has a strong 'leafy' smell of newly cut grass and is used in perfumery.
P: CH3CH2CH=CHCH2CH2OH
What is produced when P is treated with an excess of hot concentrated acidic KMnO4?
D) P is an alkene and a primary alcohol. Alkenes are oxidized by hot concentrated KMnO4 to carboxylic acids with two carboxylic acid groups. Primary alcohols are oxidized to a carboxylic acid. This means that the products of this oxidation are CH3CH2CO2H (from primary alcohol oxidation) and HO2CCH2CO2H (from alkene oxidation)
23) The complete combustion of alkanes to produce carbon dioxide and water is an important exothermic reaction.
Which line on the graph shows the relationship between the number of carbon atoms in the alkane and the number of moles of oxygen gas needed for complete combustion of the alkane?
B) Write down some equations for the complete combustion of simple alkanes so as to determine how the ratio of carbon atoms to moles of oxygen changes as there are more carbon atoms in the alkane.
CH4 + 2O2 → CO2 + 2H2O
2C2H6 + 7O2 → 4CO2 + 6H2O
C3H8 + 5O2 → 3CO2 + 4H2O
The ratio of carbon atoms to moles of oxygen in each equation respectively is:
1:2, 2:7, 3:5
The number of carbon atoms is directly proportional to the number of moles of oxygen, so the answer has to be B
24) Cyclohexa-1,4-diene is treated with a solution of bromine in tetrachloromethane.
Which product is formed?
B) When an alkene reacts with cold liquid bromine, dibromoalkene is formed. Since the compound above is made up of two alkenes, then four bromine atoms are added on to the molecule. This reaction is an electrophilic addition reaction. Each bromine atom adds on to a carbon engaged in the double bond. The answer is 1,3,4,6 - tetrabromocyclohexane
25) Which reaction is not an electrophilic addition?
D) Reaction D is a nucleophilic addition (CN- is the nucleophile)
26) The reaction scheme outlines the production of one of the monomers of nylon 66 from compound X.
Which compound could be X?
A) When a halogenoalkanes reacts with KCN in ethanol, a nucleophilic substitution reaction takes place, in which the CN- ions substitute the halogen atoms (in this case the bromine atoms from 1,4-dibromobutane)
27) Chlorofluoroalkanes, commonly known as CFCs, undergo homolytic fission by ultraviolet irradiation in the stratosphere.
Which radical could result from this irradiation of CHFClCF2Cl?
C) The weakest bond is the C-Cl bond. The C-H bond is very strong because it is an ionic bond. The C-F bond is strong because fluorine is very electronegative. The C-Cl bond however, is covalent and Cl is not as electronegative as Fluorine, which means that it is a relatively weak bond (relative to the other bonds in the molecule). This is why in the reaction, the most susceptible atom to leave the molecule is chlorine, and so the radical in option "C" is formed
28) In its reaction with sodium, 1 mol of a compound gives 1 mol of H2(g).
Which compound might X be?
D) When an alcohol reacts with sodium an alkoxide ion is formed (O-Na+). When a carboxylic acid reacts with sodium, since it's an acid, a salt is formed (COONa). The only compound that forms one mole of hydrogen is the last one.
CH3CH(OH)CO2H + 2Na → CH2CH(ONa)COONa + H2
29) In the preparation of ethene, ethanol is is added a drop at a time to a heated reagent Y. To purify the ethene it is bubbled through a solution Z and then collected.
What could reagent Y and solution Z be?
C) An alcohol and concentrated sulphuric acid under reflux will produce an alkene which can be purified by dilute sodium hydroxide (base hydrolysis of esters).
30) The product of the reaction between propanone and hydrogen cyanide is hydrolysed under acidic conditions.
What is the formula of the final product?
D) When propanone reacts with hydrogen cyanide, nucleophilic addition takes place and a hydroxynitrile is formed.
CH3COCH3 + HCN → CH3C(OH)CH3CN
2-hydroxybutanenitrile is hydrolysed under acidic conditions to Butanoleic acid
CH3C(OH)CH3CN + H2O + H+ → (CH3)2C(OH)CO2H + NH4
31) Silicon tetrachloride, SiCl4, is a liquid of low boiling point. In the presence of water it decomposes to form silicon (IV) oxide and hydrogen chloride.
What types of bonding occur in SiCl4 (l)?
C) Silicon tetrachloride doesn't have co-ordinate bonding because it follows the octet rule, sharing all of its valence electrons with the chlorine atoms.
Both silicon and chlorine are non-metals, so, it has covalent bonding.
There are instantaneous dipole-induced dipole forces between the molecules (Van der Waals forces)
32) Long-chain alkanes are converted on an industrial scale into alkylsulphates for use as detergents, e.g. sodium lauryl sulphate.
What deductions about the properties of this substance can be made from this structure?
A) The right-hand side of the structure is polar and since water is a dipole, it is attracted to water.
The alkyl chain is non-polar and attracted to other alkyl chains by Van der Waals forces. Since oil is of a similar character to this alkyl chain, the alkyl chain is soluble in oil droplets.
In alkanes, each carbon atom forms a tetrahedral structure (due to sp3 hybridisation), so the C-C-C bond angles are tetrahedral
33) The conversion of graphite into diamond is an endothermic reaction (ΔH = +3 kJ mol-1)
C (graphite) → C (diamond)
Which statements are correct?
A) The reaction is endothermic, which means that diamond has more energy than graphite. The enthalpy change of atomisation is the enthalpy change when one mole of gaseous atoms is formed from one mole of the element in the standard state. Since diamond has more energy than graphite, it requires a smaller enthalpy change of atomisation.
Since the enthalpy change of atomisation is smaller in diamonds, it means that the C-C bonds in diamond are weaker than in graphite because it requires less energy to change into the gaseous state.
Since diamond has more energy than graphite, there is a higher energy requirement to break the C-C bond to form new C=O bonds (in carbon dioxide) in combustion.
34) Which of the following statements are correct for the sequence
of compounds below considered from left to right?
NaF MgO AlN SiC
C) The electronegativity difference decreases between the elements (3.05, 2.13, 1.43, 0.65)
All of the compounds fulfill the octet rule and are isolelectronic.
The compounds become increasingly covalent (starting from ionic)
35) Which statements are reasons why sulphur dioxide is used as a
food preservative?
B) Sulphur dioxide is a reducing agent which prevents oxidation.
Since it's an anti-oxidant, it prevents alcohols from oxidizing to carboxylic acids (prevents sour-tasting acids).
It does smell and is toxic in large quantities.
36) Why is the addition of concentrated sulphuric acid to solid
potassium iodide unsuitable for the preparation of hydrogen
iodide?
C) Iodide ions are strong reducing agents and so they reduce the sulphuric acid, first to sulphur dioxide, then to sulphur and then hydrogen sulphide. Barely any hydrogen iodide is formed because it's displaced by the sulphuric acid.
Iodide ions are reducing agents, and become oxidised to iodine.
The majority of the products of the reaction are sulphur compounds (as explained above)
37) What will always be a characteristic of a compound containing
a single carbon atom with four different groups bonded to it?
B) A chiral centre is an atom bonded to four different groups.
An optical isomer (geometric isomer) occurs when there's a chiral centre.
Chiral carbon atoms DO NOT need to have structural isomers.
38) Chloroethane can be formed from bromoethane in two steps.
Which statements about these steps are correct?
B) Step X is a nucleophilic substitution because the reagent is hot aqueous sodium hydroxide (OH- being the nucleophile).
A chloroalkane cannot be formed by reacting sodium chloride with alcohol. It can be done with phosphorus (III) chloride or phosphorus (V) chloride.
39) When the apparatus below was used with compound Z, a brick-red precipitate formed in the right-hand tube.
Which compound could be Z?
C) Only an aldehyde forms a brick-red precipitate with Fehling's solution. Aldehydes are formed by the oxidation (in acidified dichromate) of primary alcohols. The two primary alcohols are CH3CH2CH2OH and CH3OH
40) Mevalonic acid is an intermediate in the biosynthesis of cholesterol, and is shown below.
Which properties does mevalonic acid have?
B) It only has one chiral carbon (The one in the middle).
It has a carboxylic acid group, so it can be esterified by ethanol. It has an OH group, so it can be esterified by ethanoic acid.
The molecule contains tertiary and primary alcohols, not secondary.
 
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Jay

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Thank u so much for this much of time u have spent to help complete strangers..... U r really good. May Allah bless u....
 
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