Download atoms, molecules and stoichiometry

1
ATOMS, MOLECULES AND
STOICHIOMETRY
Matter is anything that has mass and occupies space. Everything around
us consists of matter.
Matter is classified into two main groups, elements and compounds.
Elements are substances that cannot be broken down into simpler
substances. They contain only one type of atom. The particles in elements
can be atoms or molecules. For example, sodium and magnesium contain
atoms while oxygen and nitrogen contain molecules.
Compounds are substances that contain more than one type of atoms
combined chemically. The particles in compounds can be molecules or
ions. For example, water and carbon dioxide contain molecules while
sodium chloride and magnesium oxide contain ions.
Subatomic particles were discovered during the 1800s. We will concentrate
only on the fundamental subatomic particles, which are the protons,
neutrons and electrons.
We will also study how atoms combine in simple ratios, that is, the
stoichiometry of reactions.
Concept Map
Atoms
Subatomic
particles
Proton number and
nucleon number
Relative atomic
mass based on 12C
Relative charge
and relative mass
Stable and
unstable isotopes
Mass spectrometry
• Mole and Avogadro’s
constant
• Mole concept of
gases and solutions
Mass spectrum
Behaviour in
electric field and
magnetic field
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1 Atoms, Molecules and Stoichiometry
3
Solution
1 mol of 12C has a mass of 12 g.
1 mol of 12C contains 6.02 × 1023 atoms.
12
= 1.99 × 10–23 g
⬖ Mass of 1 atom of 12C =
6.02 × 1023
Effect of an Electric Field on Subatomic Particles
1
When a beam of protons, neutrons and electrons is passed through
an electric field, the observations are shown in Figure 1.1.
−
beam of protons,
electrons and
neutrons
LINK TO STPM
2003/P2/Q1
protons
α
neutrons
β
Figure 1.1
+
electrons
2
The positive protons are deflected towards the negative plate. The
negative electrons are deflected towards the positive plate while
the neutrons, being electrically neutral, are undeflected.
3
The lighter electrons are deflected more than the heavier protons,
that is, ␣ < ␤.
Effect of a Magnetic Field on Subatomic Particles
1
When a beam of protons, neutrons and electrons is passed through
an electric field, the observations are shown in Figure 1.2.
TAKE NOTE!
North
pole of
magnet
The deflections of the protons
and electrons in a magnetic
field can be determined by
using Fleming’s left hand
rule.
protons
beam of protons,
electrons and
neutrons
neutrons
electrons
South
pole of
magnet
Figure 1.2
Left Hand Rule
direction
of force
magnetic
field
direction
of current
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8
ACE AHEAD Chemistry First Term
TAKE NOTE!
Radioactive disintegration
is a first order reaction. The
half-life of a radioisotope is
independent of the amount
of the radioactive substance
present and is not influenced
by catalyst or changes in
temperature and pressure.
14 The stability of radioisotopes is measured by its half-life. The
shorter the half-life, the less stable the radioisotope.
15 The half-lives of some radioisotopes are given in Table 1.6.
Radioisotope
Iodine-131
Carbon-14
Uranium-238
Radium-226
Half-life
8.14 days
5760 years
4.51 × 109 years
1602 years
Table 1.6
CHEM FILE
Radium was discovered in
1898 by Marie S. Curie
and her husband Pierre.
Radium has 33 isotopes, with
nucleon numbers 202 to
234 and all are unstable.
226
Ra has the longest half-life
of 1602 years.
CHEM FILE
When a tree is alive, C-14
is continually disintegrating,
and it is continually being
replaced by photosynthesis.
However, when the tree dies,
photosynthesis does not take
place. Only disintegration
is occurring and the C-14
concentration in the tree
begins to decrease. By
measuring the C-14 level in
the tree, it is possible to
tell how long the tree has
been dead. This is called
carbon dating.
EXAMPLE 1.4
Uranium-235 decays by emitting an alpha particle.
Determine the proton number and the nucleon number of the isotope
formed.
Solution
Proton number = 92 – 2 = 90
Nucleon number = 235 – 4 = 231
235
231
4
92U 90Th + 2He
REFRESHER
1 The fundamental particles in an atom are protons, neutrons and
electrons.
2 Protons are positively charged, electrons are negatively charged
and neutrons are neutral.
3 Protons and neutrons are found in the nucleus while electrons
move around the nucleus in specific energy levels.
4 A neutral atom has an equal number of protons and electrons.
5 A positive ion is formed when an atom or molecule loses electrons.
A negative ion is formed when an atom or molecule receives
electrons.
6 Isotopes are atoms of an element that have the same number of
protons but different number of neutrons.
7 Unstable isotopes decay spontaneously.
Checkpoint 1.1
1 Determine the number of subatomic particles of the following species.
(a) 21D2168O
(b) 188O2−
(c) [147N168O3]−
2 A beam of 1H+ and 16O– ions is passed into an electric field as shown in the diagram below.
+
beam of
1 +
H and
16 −
O ions
path X
x
y
−
path Y
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24
ACE AHEAD Chemistry First Term
Focus on Exam 1
OBJECTIVE QUESTIONS
Species
Number of
electrons
Number of
neutrons
C−
6
8
A
14
6
B
18
8
O2−
10
10
C
24
11
Na+
10
11
D
37
17
Cl2+
19
20
2 An atom P has a nucleon number of 48.
P forms P2+ ion which is isoelectronic with
40
20Ar atom. Which of the following shows
the correct number of protons, electrons
and neutrons in an atom of P?
A
B
C
D
Protons
Electrons
Neutrons
20
22
22
22
20
20
22
22
28
26
26
22
Clone STPM 2004/P1/Q8
3 Which ion has the same number of protons
and neutrons but has more electrons than
protons?
A 2H+
C [16O2H]–
2
+
16
B [ H3 O]
D [32S18O4]2–
109
4 Silver has two isotopes, 107
47Ag and 47 Ag. If
the relative atomic mass of silver is 107.87,
what is the percentage abundance of the
heavier isotope?
A 35.7%
C 56.5%
B 43.5%
D 59.2%
A
B
C
D
The relative atomic mass of sulphur is 32 g.
A sulphur atom has 32 electrons.
16 g of sulphur contain 6.02 × 1023 atoms.
There are 16 protons and 16 neutrons in
the nucleus of a sulphur atom.
6 The mass ratio of one atom of an isotope
of Q to one atom of carbon-12 is 1.50.
What is the relative isotopic mass of the
isotope of Q?
A 8.0
C 15.0
B 10.0
D 18.0
7 The function of the magnetic field in a
mass spectrometer is to
A vaporise a solid sample
B produce positive ions
C separate ions with different masses
D separate ions from neutral molecules
8 Which species will be deflected the most in
a mass spectrometer?
A 21H+
C 126 C+
B 73Li+
D 147N2+
9 The nitrogen dioxide molecule, NO2,
consists of 14N, 16O and 18O isotopes. How
many molecular peaks would there be in the
mass spectrum of a sample of NO2?
A1
C3
B2
D4
10 The mass spectrum of a compound showing
some major peaks is shown below.
relative intensity
1 Which combination shows the correct
number of electrons and neutrons of the
respective species?
12
32
16
5 The symbol of a sulphur atom is S. Which
statement is true regarding sulphur?
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14
28
m/e
Which compound will give the above mass
spectrum?
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1 Atoms, Molecules and Stoichiometry
[Relative atomic mass: H, 1; C, 12; N, 14;
O, 16]
A N2
C CO
D NO
B C2H4
% abundance
% abundance
11 An element Y occurs naturally as 65Y and
67
Y in the ratio of 2 : 1. Based on this
information, which graph could represent
the mass spectrum of Y2?
A
C
m/e
65
m/e
67
130
134
% abundance
D
% abundance
B
132
m/e
130
132
134
[Avogadro’s constant = 6.02 × 1023 mol–1]
44
A 44 × 6.02 × 1023 C
6.02 × 1023
23
1
D
B 6.02 × 10
44 × 6.02 × 1023
44
16 The formula of a type of polystyrene
is Br3C6H3(C8H8)n. One sample of the
polystyrene contains 10.46% bromine.
What is the value of n?
[Relative atomic mass: H, 1; C, 12; Br, 80]
A3
C 19
B 12
D 22
17 0.85 g of magnesium powder is reacted
with 2.0 mol dm–3 hydrochloric acid. What
is the volume of hydrochloric acid required
for complete reaction?
[Relative atomic mass: Mg, 24.3]
A 17.5 cm3
C 42.5 cm3
3
B 35.0 cm
D 70.0 cm3
m/e
130
132
134
90
12 The relative atomic mass of Sr based on
the 16O standard is 89.936. Based on the 12C
standard, the relative atomic mass of 16O is
15.995. What is the relative atomic mass of
90
Sr based on the 12C standard?
A 89.908
C 89.964
B 89.931
D 90.000
13 The compound NCl3 is formed from 14N,
35
Cl and 37Cl isotopes. The relative abundance
of 35Cl to 37Cl is 3:1. Which statement about
the mass spectrum of NCl3 is true?
A The base peak corresponds to N+ ion.
B The m/e value for the last peak is 123.
C The number of peaks for NCl3+ ion is 4.
D The relative abundance of N35Cl3+ ion to
N37Cl3+ ion is 3:1.
STPM 2010/P1/Q1
14 0.1 dm3 of argon contains x atoms. How
many atoms are there in 0.2 dm3 of
hydrogen gas under the same conditions?
Ax
C 2x
2
Bx
D 4x
15 The relative molecular mass of carbon
dioxide is 44. What is the mass, in g, of a
carbon dioxide molecule?
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25
A
B
C
D
1 only 1 and 2 only 2 and 3 only 1, 2 and 3
18 Which statement(s) is/are true about the
isotopes 168 X and 188Y ?
1 They are isotopes of the same element.
2 The 168 X isotope has more neutrons per
atom than the 188Y isotope.
3 They have the same density.
19 Which statement(s) is/are true of the
28
30
nuclides of silicon, 14Si and 14Si?
1 The mass spectrum shows two peaks.
28
30
2 14SiCl4 and 14SiCl4 molecules have different
shapes.
28
3 Isotope 14Si is more reactive than isotope
30
14Si.
STPM 2010/P1/Q41
20 Which statement(s) is/are true about the
isotope ZAX?
1 Its relative isotopic mass is A g.
2 Its relative isotopic mass is based on the
12
C standard.
3 The isotope ZAX and its ion ZAX 2+ have the
same relative mass.
21 A sample of bromine gas contains 79Br and
81
Br isotopes. Which mass/charge ratio(s)
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