C1.1
STRUCTURE AND BONDING
Ans
The fundamental particles present in an atom are protons and neutrons
present in the nucleus, and electrons which orbit this nucleus.
Q.2
What are the relative charges and relative masses of the fundamental
particles?
Ans
MASS NO. RELATIVE
CHARGE
Proton
1
+1
Electron
0
-1
Neutron
1
0
Ans In an electric field,
protons being positively charged, are deflected towards the negative plate
(attraction of unlike charges). Similarly,
since electrons are negatively charged they too are deflected, however due to
the attraction of like charges, it is deflected towards the positive plate.
Since neutrons are neutral they are not affected and travel straight
through the field.
(including
The 12C scale as an international standard for comparing relative
atomic masses.)
Q.4
Define the following –
(a)
Atomic
(proton) number
Ans
The atomic Number is the number of protons in an atomic nucleus.
(N.B. Not defined in terms
of electrons.)
(b)
mass number
Ans
The Mass Number is the sum of the
number of protons and neutrons (nucleons) in an atomic nucleus.
(c)
relative atomic mass
Ans
The Relative Atomic Mass is the average mass of the isotopes of an atom
relative to one twelfth the mass of a 12C atom.
(d)
isotopes
Ans
Isotopes are atoms which share the same number of protons but have
differing numbers of neutrons.
(e)
relative
isotopic mass.
(e)
Ans
The Relative Isotopic Mass is the average mass of an isotopic atom
relative to the mass of a 12C atom.
C1.1.3
Elementary
mass spectrometry.
Q.5 How can a relative atomic mass be determined from
relative isotopic masses and relative isotopic abundance data?
Ans
RAM =
REF. PAPER 6-JAN
1998 Q1(b) (iv)
EXAMPLE:
ISOTOPE REL. ISOTOPIC MASS
REL. ABUNDANCE
35Cl
35u
75%
37Cl
37u
25%
RAM
= (75x35) + (25x37) = 35.5u
100
REF; Fig.1.2-
CONTEXT Pg. 3 (Diag.)
The
sample to be investigated is vaporised and fed into an ionisation chamber where
it is bombarded by high speed electrons. This
removes electrons from the sample (ionisation).
Singly charged positive ions are accelerated by an electric field,
deflected by a magnetic field and detected by an ion detector which feeds to a
recorder. A mass spectrum for Cl2
is shown:
REF; Diag.Q3-Pg.
107-ACC
The
mass spectrum identifies all the mass numbers present in the sample.
On ionisation some of the molecules in the sample fragment, whilst others
remain as Cl2 molecules. all
the peaks in the above diagram can be accounted for :
35 - 35Cl isotopic ion
37 - 37Cl isotopic ion
70 - 35Cl2 isotopic molecule
74 - 37Cl2 isotopic molecule
72 - 35Cl37Cl isotopic molecule
The
relative isotopic mass can also be obtained from mass spectra.
Intensity
Mass
C1.1.4
Amount
of substance (the mole), solution
concentrations, molar gas volumes,
Avogadro
constant.
AMOUNT OF SUBSTANCE
The
mole is defined as the amount of substance that contains as many elementary
particles as there are atoms in exactly 0.012kg (12g) of Carbon 12
(12C).
MASS
Gas Volumes
1.
Equal volumes of different gases compared at the same temperature and
pressure contain equal numbers of molecules.
2.
Equal numbers of molecules of different gases compared at the same
temperature and pressure occupy equal volumes.
Hence
the molar amount can be related to the volume which a gas occupies.
At STP (273K/1atm) or at RTP (298K/ 1atm) 1 mole of a gas occupies 22.4
or 24 dm3 respectively.
(a)
REF; Q 5.18/5.19/5.20/5.21 ACC.
From these statements and statement 1 we can also see that the number of
basic particles (molecules/gaseous atoms etc.) in a mole can be calculated.
This is known as Avagadros’ Number (6.02 x 1023 basic
particles in one mole of a substance).
(b)
REF; Q 1.4/1.5/1.6 ACC.
Solution concentration is defined as the amount of solute dissolved in a
given volume of solution.
C=n/V
C= Concentration (Moldm-3)
n= Moles
V= Volume (dm3)
(c)
REF; Q 1.28/1.29/1.30 ACC.
A
standard solution is one of known concentration. We can know the concentration
either from exact preparation (NB. Some
substances are unsuitable for forming standard
solutions) or by analysing it. One
method of analysis is titration.
(d)
Ref: Fig 6.2 Pg32 Uchem
An accurately measured amount of alkali is added to the conical flask with a few drops of a suitable indicator, e.g. methylene blue, (changes from yellow to blue), and acid is run in from the burette until the colour just changes, showing the solution in the conical flask is just neutral.
Q.7 Do the following questions in the same book
2.2 & 2.3
Q.8 Do the following questions in the same book
Page 30: 2.1 & 2.2
C1.1.5
Q.9 Calculations based upon the
results obtained during volumetric analysis of acids or bases,
including back titrations.
Essentially,
in order to calculate the amounts of products which will be formed from known
amounts of reactant, one first must produce a balanced equation representing the
reaction eg.
2
H2 (g) +
O2 ---->
2 H2O
From
this we can deduce that 2 molecules of Hydrogen react with one molecule of
Oxygen to give 2 molecules of water. As
this is true then it is also true to say that since one mole is merely a term
used for a large number of molecules, 2 moles of hydrogen molecules react with
one mole of oxygen molecules to give two moles of water molecules.
We can use this fact and the fact that :
Moles = Mass / RMM
to
calculate the molar amount and hence the reacting mass of any component of the
reaction given sufficient information.
Similarly,
if we work out the reacting molar ratios from the balanced equation, we can work
out the volumes of gases which react and are produced in the reaction since we
know that one mole of any gas at STP occupies a volume of 22.4 dm3.
Q.9(b)Do
Exercises 1.10/11/12/13 on reacting quantities and Exercises 5.10/11/12 on molar
gas volumes in ACC.
C1.1.6
Q.10
Describe the formation of the ionic bond by electron transfer.
Examples should include NaCl, MgCl2, CaO, Al2O3
and LiO2
Ans
Examples should include NaCl, MgCl2, CaO, Al2O3
and LiO2.
Ans
PROPERTIES:
·
Ionic
Compounds conduct electricity when molten or in solution, but not when
solid.
·
When they
conduct electricity, the ionic compound is decomposed at the electrodes.
·
They have high
melting points since the energy given out in forming the lattice (the lattice
enthalpy ) must be supplied.
·
They are, more
often than not, soluble in water.
·
They are
brittle, as, if a knock causes one layer to slide over another, the ion will
encounter ions of similar charge and hence repell and cleave along clean
cleavage planes.
Ans
·
Increasing the
charges on the ions in the lattice has the effect of strengthening the
electrostatic attractions and hence increasing the strength of the ionic bond
and raising the lattice enthalpy.
· Larger ions give smaller lattice enthalpies: The nature of the lattice depends upon the co-ordination number ( which indicates how many of one kind of ion can physically fit around another ion and hence bond with it ). If fewer ions can fit around each other - as with larger ions - the bond cannot be as strong as they cannot approach each other as closely.
Q.13 Give a simple definition of the term ‘lattice enthalpy’?
Ans
The Lattice Enthalpy
is the enthalpy change on forming one mole of an ionic solid from its isolated
gaseous ions.
N.B. Ionic bonding can only take place if the lattice energy is sufficient to make the enthalpy of formation exothermic.
Q.14 Define of the term ‘ionic radius’. (cf Section C2.1.5)
Ans
There is no method of measuring the size of an isolated atom or ion.
However, using x-ray diffraction, one can obtain a value for the
internuclear distance between two ions in an ionic salt.
If we measure the distances for a number of electrovalent compounds by
x-ray diffraction, we find that a characteristic ionic radius can be assigned to
each ion, so that the distance between the nuclei, is the sum of the two ionic
radii.
rc = radius of cation
rA = radius of anion
C1.1.7
Q.15 Explain how a covalent bond can be described as ‘electron sharing’.
Examples should include H2, Cl2, N2, H2S,
CH4, CCl4
Ans In covalent bonding electrons are shared, in pairs,
between two bonded atoms. In a
single covalent bond between two atoms, one electron from each is held in common
by both.
Examples
should include H2, Cl2, N2, H2S, CH4,
CCl4
Ref. Pgs. 102-109, Context
Q.16 Describe the characteristic properties of covalent compounds.
Ans
They mostly have weak intermolecular bonds so that most are gases or
liquids at RTP. They do not conduct electricity in general, whatever their
state. They tend not to
dissolve in or mix with water, unless they have relatively polar bonds (e.g.
sugar).
Macromolecular
Simple Molecular
Very hard, (exc. Graphite)
Soft
Very high m.p.s and b.p.s
Low m.p.s and b.p.s
Non-conductors, (exc. Graphite and Si)
Non-conductors
Insoluble in polar & non-polar solvents
Soluble in both
Q.17 Give examples of single and multiple bonds, but NOT delocalisation.
REF. PGS. 106-107 U.CHEM.
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Chemistry Department: Loreto College, Coleraine.
