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CHEMISTRY

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balaji saravanan

on 5 May 2015

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Transcript of CHEMISTRY

IGCSE Chem revision
CHEMISTRY
BASICS

Experimental techniques
Atoms, elements and compounds
Stoichiometry & MOLES
Electricity and chemistry
Chemical energetics and Chemical reactions
Acids, bases and salts
The Periodic Table
Metals
Air and Water
Sulfur and Carbonates
Organic chemistry
The particulate nature of matter
Solid
Liquid
Gas
Particles are :
Closely packed together
Arrangement :
Regular
( Lattice )
Movement :
Vibrate constantly in fixed position
Force :
Strong

Shape :
Fixed

(Can't be compressed)
1.
Matter is
made up of tiny particles (
Atoms
&
Molecules
)
2. Particles of Matter are in
constant motion
.
3. Particles of
some
Matter
are held together by very strong electric forces
5.
Each substance
has
unique particles
that are
different from
the
particles
of
other substances

Particles are :
Close together

Arrangement :
Irregular
Movement :
Move around and slide past each other
Force :
Weak
Shape :
Takes shape of container

(Can't be compressed)
Particles are :
Far apart
Arrangement :
Random
Movement :
Able to move freely and collide with other particles and bounce away
(can spread)
Force :
No force
Shape :
Fills container

(Can be compressed)
Kinetic theory
Heating curve of water
Explanation
1.From
-25 to 0 Celsius
the water is
gaining energy
2.The graph goes
steady at 0
Celsius as the atoms are
using the energy to weaken the bonds to change states
from solid to liquid
3.The atoms gain energy
4.The graph straightens out at 100 Celsius because the energy is being used to break the bonds to change states into gas
5.The atoms gain more energy
A
process
in which
particles mix
and
spread

through colliding

with other particles
. The overall result is particles
flow
from a
higher concentrated areas to
a
less concentrated
areas
until
they reach
equilibrium
.
Brownian motion
(diffusion)
The
random movement of particles
suspended in a liquid or gas
,
caused by collisions with molecules of the surrounding medium
. Also called Brownian movement.
Rate of diffusion
The lower it's relative molecular mass the faster it will diffuse
MIXTURES
Contains more than one substance
Not chemically combined, is physically mixed together
Separation Techniques
Filtration
A method of
separating insoluble
substances
from
a
solvent using filter paper
Filter paper is placed inside a funnel
The original
mixture
is placed
in
a
test tube
and
poured

into
the
funnel
.
The

residue

is
trapped in
the
filter paper
and the

filtrate
is

separated
into
another
test tube.
Crystallization
Solute from
a
solution
A
separation method
that allows
to create
crystals

from

the

solution
Heat

the

solution
to evaporate
some of the
water
You can

check

that

the

solution

is

saturated
by using
a
glass rod
and
crystals should form on rod than
you know
solution
is
saturated
If it is

saturated stop heating
and let it
cool
.
Crystals

should start to

form
in the beaker
Separate crystals
by filtering.
And then

wash
with
distilled water
(little only as they are soluble)
Evaporating the solvent
For
some

substances
the
solubility

changes
very
little

with

temperature
(
so

crystallization

doesn't

work
) eg. Salt and water
So
instead
of crystallization you
evaporate
the
liquid
(water) and are
left

with
the
crystals
(salt)
Separating two solids
Chromatography
Paper Chromatography is the
method

of

separating

dyes

in

ink
also can be
used

to

identify
the
substances
and
to

purify

a

substance
,
by separating
it from it's
impurities
Why it works:
The
different

substances

have

different

solubility
in water. The
further
the
dye

travels

across
the
paper
, the
more

soluble

it

is
.
(Blue dye is most soluble)
Pure and impure substances
Do not draw the datum line in pen as it will corrupt the pigment results
(use pencil)
It is important for substances in everyday life such as:
Medical drugs
Flavoring for food
Must make sure it
contains

nothing

harmful
to humans
Distillation
A method to obtain a
solvent from a solution
.
Fractional distillation
This is a method used to separate
2 or more liquids from a solution

How it works:
By
making use of the
different

boiling

points

of

different

liquids
. An
example
of this is to
separate

crude

oil

petrol
and other compounds and
products

of

fermentation
, also
to

separate

air
.

- At
78
degrees Celsius the
ethanol

boils
and
some

water

evaporates
, so the
mixture
of ethanol gas and water vapour
rises

up
the
glass bead column
- The combination of
vapors condense

on
the
beads

making them hot
-When the
beads reach 78
degrees Celsius the
ethanol

vapor

no longer condenses
on them
so

ethanol

condenses
in condenser and
drips

into
a
beaker
, therefore separating the ethanol from the water.
This is a similar type of fractional distillation but has been adapted or it's own specific purpose

Apparatus
Stopwatch
-
Used to measure time passed during an experiment

(time)

Electronic

Scale
-
Used to measure mass of an object

(mass)

Thermometer
-
Used to measure the temperature of substances

(temperature)

Burrette -
Used to dispense a small measurement of liquid for more accurate experiments such as titration
(volume) - most precise

Pipettes
- Used to transfer small amounts of liquid carefully between containers without any spillage
(volume) - quite precise

Measuring

cylinder
-
To measure the volume of a liquid.

More accurate than using a beaker

(volume) - for larger amounts - not as precise


Rf Values
Method to show pigments
AIR
SOLUTIONS
A solution is when the solute dissolves in a solvent
solute + solvent = solution
Eg. Salt dissolves in water therefore it forms a solution.
How it works:
The filter paper only allows very small particles through like water.
Why it works:
A
solvent

can

dissolve
less and
less

solute
,
as
its
temperature

falls

Use locating agents
Let paper
dry
and
spray
locating agent (
ninhydrin
- example of locating agent)
This is done using a flask, a condenser and a beaker. This method is used to purify seawater in less developed countries such as the Middle East.
Why is it important
Pure substances:
No other particles mixed with it
Melts and boils at a set temperature
eg.
Water boils at 100 degrees Celsius and ice melts at 0 degrees Celsius
Impure substances:
Other particles mixed with it
Melts and boils over a range of temperature
Melts at lower temperature than pure substance
Boils at a higher temperature than pure substance
Test:
By
measuring

melting
and
boiling

points

of substance
and
checking with
melting and boiling point of
pure
substance
An
unwanted

substance
,
mixed

with
the
substance

you

want
is called an
IMPURITY
To

help
solute
dissolve
in solvent:
Stir
Heat
Add more solvent
Add catalyst
Volatile liquids :
A
liquid

that
can
easily

evaporate
Atomic structure and the periodic table
Atom =
smallest particle of matter
,
that
we
cannot down further by chemical means
The electrons surround the nucleus
Can
identify
an
atom

by
the
number

of

proton
it has
or
Nucleon

number
or
Proton

number
Number of
electron

=
number of
protons
= Atomic mass - Atomic number
Proton number = number of protons

Nucleon number = Protons + Neutrons
Larger one is always Nucleon number
Isotope
Atoms

of
the
same

element
that
has

same

number
of
protons
and
electrons

but

different

number
of
neutrons
There are 2 types :
Radioactive
non - radioactive
Radioisotope:
Some of the isotopes are
unstable
and hence
decay

giving
out
radiation
Radiation:
Consists
of
rays
and
particles
Can

destroy

cells (radiation sickness)
Patterns :
Electron

shells
- shows
period

number
Number
valency

electrons
- show the
group

number
This is
because

valency

electrons

dictates

how
the
element

reacts
and
each

group
reacts
similarly
All
atoms

want
to achieve
noble gas configuration
:
The
electron

valency
is
8

Because the
group 8 atoms
have a
full outer shell
they are very
unreactive
and
stable
Bonding : structure of matter
Element
: Contains only
1 type of atom
Compound
: Contains
more than 1 type of atom
which is
chemically bonded
together.
Can't separate by physical means
Mixture
: Contains
more than 1 type of atom
which is
not chemically combined
together (just
mixed together physically)
Ions
:
Atoms

carrying

a charge

due
to the
gain or loss of electrons
.
Anion = positive
and
Cation = negative
Metals
Alloy
Is a
mixture of metals

with other elements
When force applied
the
layers
can slide past each other
:
More
ductile
and
malleable
When
a
new atom
is
added
, it
makes
it
harder for the layers to slide past each other
:
That is why
alloys
are
stronger
Bonding
Ions
:
Atoms

carrying

a charge

due
to the
gain or loss of electrons
.
Anion = positive
and
Cation = negative
Signs of a chemical change
1.
One or more

new

chemical

substances
are
formed
2.
Energy
is
given

out
or
taken

in
,
during reaction
3.
Change usually difficult to reverse
( chemical change =
chemical reaction
)
Atoms react to achieve noble gas configuration
IF:
Takes in electron so it is negative
("BAD")
Gives electrons so it is positive
("GOOD")
Ionic bonding
When a
metal donates
(gives) an
electron
to a
non-metal
Covalent bonding
An
ionic compound
(in solid state)
has
a
regular arrangement
(
lattice
)
of alternating positive and negative ions.
The
sharing
of
electrons

between
2 or more
non-metals

to achieve noble gas configuration
Formation of complex covalent molecules
Covalent bonding: macromolecules
Silicon dioxide
has
similar
properties
to diamond as
they
both
have
similar structure
(
tetrahedral
)
Metalic bonding:
Metal + metal, covalent bonds
Metalic bonding :
Lattice

of positive ions in a sea of electrons
Mole concept
Stoichiometry
Atoms combine in a ratio

to give
them a
full outer shell
(
Valency
)
Chemical formula:
Valency:
The number of electrons that an atom loses, gains or shares to form a compound
Writing balanced chemical equations:
Writing formula by balancing charges:
only for ionic comopunds
Take this as a ratio
:
The same numbers can be simplified to 1:1 and in chemistry you don't have to show that


If the numbers are
different than nothing
can be done you just have to keep it that way





Here they are
same
so they are
canceled
off







If
there are
ion
that
have more
than one
element

or
if
the important ions
as stated in the first bubble are there
put
a
bracket
around it and then to the cross
Important Polyatomic ions:
In a
word equation

write
the
oxidation state
of a
transition metal in brackets
Determining the name of a compound:

(Not in syllabus but will help)
If only
two elements
are combined, the name
ends in ‘–ide’.
Exception: ammonia
With covalent bonds, Greek prefixes are used to denote the number of atoms
1 = mono
carbon monoxide
2 = di
carbon dioxide
3 = tri
phosphorus trihydride
4 = tetra
5 = penta
6 = hexa
The only time we
drop
a
prefix
is
if
the
mono
is to appear
at
the
beginning
of the name
If there is an oxide the ‘a’ or ‘o’ of the prefix is lost e.g. carbon monoxide
If a
metal ion
is combined
with
a
polyatomic ion
in a compound and one is oxygen, the name
ends in –ate, except hydroxides

Ionic equation:
[ ] earlier multiplied by 2
It is a bit like simultaneous equations
Relative atomic mass
,
Ar
: The
mass

of
an
atom compared
with the
carbon-12 atom

Relative molecular mass
,
Mr
: the
mass of
a
molecule
,
found by adding the relative atomic masses

of
the
atoms in it
, it is called the
relative formula mass
for an
ionic compound
Moles
:
A

substance
that
contains
the
Avogadro constant
Avagadro constant
:
12 grams of carbon-12 contains
carbon atoms.
So

each mole
contains
1 mole
of
gas
at
room temperature
(r.t.p)
and
1 atm

pressure
occupies a
volume of 24
Worked examples:

atoms
Empirical formula
= shows the
simplest ratio
in which atoms
in compounds are combined

Molecular formula
= shows the
actual

numbers of atoms
that
combine

to form a molecule
Empirical formula:
steps =
1.
Get

moles

2.

Make

ratio
3.
Divide by smallest number
(to
simplify
)
Molecular formula:
steps =
1. Calculate
mass of 1 mole =
empirical mass
(if not given)
2.
Divide Mr by empirical mass = n
3.
Multiply
all the
atoms

in
the
empirical formula by n
Electrolysis:
Liquids
that
contain ions
will
conduct electricity

but

will be decomposed
at the same time
The process of
splitting substances by electricity
is called
electrolysis
Electrodes
that are made of
carbon(graphite) or platinum
are
inert
:
They
carry current

But
they are
not
themselves
changed during electrolysis
Anode = positive electrode
:
It is
connected to
the
positive terminal of
the
power supply
Cathode = negative electrode
:
It is
joined

to
the
negative terminal
of the power supply
The
solution
that the electrodes are in is
called
the
electrolyte
Electrons

carry
the
current

in
the
wire
Ions
carry the current
in
the
electrolyte
Electrolysis using inert electrodes:
carbon (graphite) or platinum
Electrolysis

of molten ionic compounds
always
gives
-
Metal

at

cathode
Non - metal at the anode
General rules for solution electrolysis:
Electroplating
Reduction

of

positive ions
happens at the cathode (
CATions at CAThode
)
Oxidation

of

negative ions
happens at the anode (
ANions at ANode
)
Refining copper:
Electrolysis
is
used to refine
(
purify
)
copper
,
for electrical wiring
The
reason
the we
need purer copper
is because -
Purer copper conducts better
Manufacture of aluminium from pure aluminium oxide in molten cryolite:
Reactivity series:
Bauxite
is
impure

aluminium oxide
Chlorine and sodium hydroxide from the concentrated aqueous sodium chloride:

How it works:
Uses:
To
prevent rust
and to
make it
look more
attractive
Steel
is
coated

with tin
to
make rust proof tin cans for food
Steel taps
are coated with
chromium
, to
make them shiny
Cheap
jewelry
is coated with
silver
to make it
look nicer
Conductors and insulators:
Conductor
-
substances

that

allow electricity to pass
through them
Insulator
- substances that
do not allow
electricity to pass

Can test whether they are conductors by putting them into a circuit
Uses :
Exothermic
reaction:
gives out energy
to the surroundings
examples:
Most chemical reactions
Combustion
Neutralization
Respiration
Endothermic
reaction:
takes in energy
from the surroundings
examples:
Most bond making
Photosynthesis
Melting
Decomposition - carbonate
Bond breaking
is endothermic

Endothermic is always a positive energy(
+ joule
)
Bond making
is exothermic

Exothermic is always a negative energy(
- joules
)
Exo = exit
Endo = enter
Production of energy:
Simple cell:
Consists of
2 different metals
, standing
in
an
electrolyte
The
redox reactions
that take place
provide
a
current
, in a simple cell
Changing the metal:
The
further apart

the 2 metals are

in
the
reactivity series
, the
higher
the
voltage
of the cell
The
more reactive metal gives up electrons
e.g:
potential fuel for the future:
For syllabus =
In fuel cells
,
Hydrogen

reacts

safely
with
oxygen

without combustion
Not in syllabus:
Chemical energetics
Chemical reactions
Rates of reactions
Reversible reactions
Redox
cool->filter->wash->dry

such neat
much highlighter
wow red
very mixture
look stepz
decant
- gradually
pouring
liquid from one container into another, esp.
without disturbing the sediment.
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