Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

LJH
Chapter 1 Atoms, Molecules and Stoichiometry
1
...
67x 10-27

1

+1

Electron, e

9
...
67x 10-27

1

0

The mass of an atom is very small
...

1 a
...
u
...
66x 10-27kg

All atoms of an element have the same proton number
...


Nucleon number of an atom is the sum of the protons and neutrons in the atom
...


LJH
- Effect of an electric field on subatomic particles
...

OR
The positive proton deflects towards negative plate whereas negative electrons deflect
towards positive plate
...


-Effect of a magnetic field on subatomic particles

For magnetic field, deflection of the subatomic particles determined by Fleming’s Left Hand
Rule
...

The larger the m/e value, the smaller the angle of deflection
...


LJH
Ions:
When a chemical reaction take place, only the valence electrons are involved, while the
number of protons and neutrons in an atom unchanged
...
The number of protons is more than number of
electrons
...

Usually only metal elements can form cation
...
The number of electrons is more than the number of
protons
...

Usually only non-metal elements can form anion
...
For example, all the ions
from the table above
...

An isotope is an atom of an element which has the same proton number as other atoms of
the same element but it has different nucleon number
...

Isotopes have different physical properties because they have different number of neutrons
...
Other changes: Melting and boiling
point
...
The percentage occurrence of
each isotope of the element is called its percentage abundance
...
78% , 37Cl is 24
...

Eg:
Element

Stable isotope

Unstable isotope

Hydrogen

Hydrogen-1

Hydrogen-3

Carbon

Carbon-12

Carbon-14

Oxygen

Oxygen-16

Oxygen-15

Stability of isotopes depends on the neutron to proton ratio
...
The corresponding numbers
lie in a region called the stability belt
...

As the proton number increases, the number of neutrons will exceed the number of protons
...


LJH
Isotopes that lie outside the stability belt are unstable
...
They undergo radioactive decay spontaneously
...


There are two types of radioactive decay ie Alpha and Beta decay
...

Alpha decay: The emission of an ∝-particle leads to the formation of an isotope with a
decrease in two units of the proton number and a decrease in four units of the nucleon
number
...

∝-particles are Helium nuclei

Beta decay: The emission of a 𝛽-particle forms an isotope with no change in the nucleon
number and an increase by one unit of the proton number
...

𝛽-particles are fast-moving electrons

The decay will continue until a stable isotope is formed
...
The shorter the half-life, the less
stable the radioisotope
...


LJH
1
...


-Development of a relative atomic mass scale
Initially, hydrogen is chosen as a standard for determining relative atomic mass and was
given 1 unit
...

Then, oxygen standard was introduced
...
Physicists used the relative atomic mass of 16O, the most abundant isotope of
oxygen as the standard
...
Hence, conversion was needed between two scales
...
As it is
abundant in the nature, can be easily identified in a mass spectrometer and it is easy to
handle carbon since it exists as a solid at room temperature and pressure
...
0 units
...
0 units
...
0 units
...


LJH
1
...


The instrument used is called a mass spectrometer as shown in the diagram above
...
All neutral
atoms and molecules will be removed by the vacuum pump
...
A heated
filament or electron gun emits a high speed or energy electrons
...

c) Accelerator (Acceleration) : The ions then move into an electric field (act as an
accelerator) where they are accelerated by negatively charged plates and they
emerge through a slit with almost the same velocity
...

The angle of deflection is inversely proportional to the mass to charge ratio, m/e
...


ii)

Ions with the same charge, those with smaller masses are deflected more
...


LJH
e) Detector (Detection): By varying the magnetic field strength and the accelerating
voltage, the ions are focused on to an ion detector
...


Mass spectrum of magnesium
...
The relative abundance of an ion is shown by the height
of the peak produced
...


LJH
-

Mass Spectrum of Monoatomic Element

Use to determine:
-

The number of isotopes of the element
The relative isotopic mass of each isotope
The percentage abundance of each isotope

Use magnesium as an example,

The relative atomic mass of magnesium
=

(

) (

) (

)

= 24
...
However, the method of calculation is still the same
...


For example,
Mass spectrum of chlorine atoms and molecules

The molecular ions of chlorine will produce peaks with a relative abundance of

9:6:1

LJH

-

Mass spectrum of a compound
For example,
The mass spectrum of methane, CH4 with a relative molecular mass of 16
...

Extra notes:
Determination of the number of carbon atoms in a molecule
Formula:
Relative abundance of (M+1)+ ion
Relative abundance of M+ ion

=

1
...
4Mole Concept and Stoichiometry Calculations
Mole -- One mole of substance is that amount of substance which contains the same
number of particles (atoms, molecules, ions, formulae, electrons) as there are 12C atoms in
12
...

Elements that exist as atoms under room conditions (25°C , 1 atm
...

Elements that exist as diatomic molecules: H2, O2, N2, X2 (X= F, Cl, Br, I)
One mole of particles (atoms, molecules, ions, formulae, electrons) is equal to Avogadro
constant, L/ NA
...
02 x 1023 mol-1
Molar mass of a substance is the mass of one mole of that substance and expressed in
g mol-1
One mol-atom of an element has a mass equal to the relative atomic mass of the element,
Ar in grams
...
0

Mass of 1 mole of hydrogen atoms = 1
...
0

Mass of 1 mole of oxygen atoms = 16
...
5

Mass of 1 mole of copper atoms = 63
...
of mol-atom of an element = (mass of element/g) ÷ ( Ar/ g mol-1) (mol-atom)
No
...
of mol-atom) X 6
...

H2 = 2
...


Mass of 1 mole of hydrogen molecules= 2
...
0= 32
...
0g

C2H5OH= 2(12
...
0+ 16
...
0 = 46
...
0g

No of mol-molecule = (mass of element or covalent compound/g) ÷ (M r/g mol-1)
No of molecules= (No
...
02x1023 mol-1

Ionic compounds exist as cation and anions in which the positive and negative charges are
equal and cancel out
...

AlCl3۰6H2O = 27
...
5)+ 6(2
...
0) = 214
...
00mole of aluminium chloride hexahydrate has a mass of 214
...

Therefore, mass of 0
...
128 x 214
...
91g, which contains
0
...
128=0
...
768mol H2O molecules
...
of moles of compound= (mass of compound/g) ÷ (Relative formula mass/ g mol-1) (mol)

LJH
-

Mole concept and Volume of gas

At a certain temperature and pressure, one mole of a gas occupies a fixed volume, which
is called its molar volume
...
4dm 3 at standard temperature and pressure,
s
...
p, (273K, 1atm)
...
4dm 3 at room temperature and pressure,
r
...
p, (298K, 1atm)
...

For example, a 2
...

Percentage by volume of hydrogen=

x 100% =7
...


LJH
Relationship between Mass-Mole-Volume

LJH
-Mole concept for solutions
Concentration of a substance (solute) in a solution is expressed in units of
mol dm-3

OR

g dm-3

The concentration of a substance in a solution is the number of moles/ mass in grams/
amount of substance dissolved in 1 dm3 of the solution
...
of moles of substance/ mol ÷ Volume of solution/ dm3
Concentration in g dm-3= Mass of substance/g ÷ Volume of solution/ dm 3
Conversion mol dm-3  g dm-3 :
Concentration in g dm-3 = Concentration of substance/ mol dm-3 x (RFM/ Mr/ Ar)

No
...
50g of (NH4)2SO4
...
6H2O are dissolved in dilute sulphuric acid and made up to
250cm3 of solution with distilled water
...
FeSO4
...
50 ÷ 392
...
02168mol
Concentration of the solution= 0
...
0867 mol dm-3
1
...
0 mole Fe2+ and 2
...

Therefore, conc
...
0867 mol dm-3
Conc
...
0867 = 0
...
The process is called a
titration and the solution in the conical flask is said to be titrated with the solution in the
burette
...

Acid-base titration

Consider the reaction between a diprotic acid, H2X and NaOH
...

Let Ma mol dm-3 be the concentration of H2X(aq) and Mb mol dm-3 be the concentration of
NaOH(aq)
...
Use a pipette to pump eg 25cm3 of acid into a conical flask
...
Add a few drops of indicator eg phenolphthalein into the conical flask
...
Record the initial reading of the burette containing alkali
...
Run the alkali into the conical flask until the solution change colour
which is the end point
...
Record the final reading on the burette
...
Repeat the titration process to obtain a constant result
...
50cm3 of a solution of sodium carbonate was diluted to 250cm3
...
0563 mol dm-3
...

Therefore,
Concentration of Na2CO3(aq) in the original solution x 22
...
0563 x 250x10-3
Concentration of Na2CO3(aq) = 0

---