as the reaction proceeds the
concentration of products
increases & the concentration
of reactants decreases
Factors
Affecting
Rate
Collision
Theory
molecules
must collide
successfully
for a reaction
to take place
particles must collide
with a certain minimum
energy (activation
energy) in order to
collide successfully
if activation energy isn't
reached particles will
collide unsucessfully
Surface
Area
increased
surface area
(smaller
particles);
increased no.
of collisions
Concentration
increased
no. of
particles
in a fixed
volume
more
particles
collide;
increased
reaction
rate
rate at the
beginning of
the reaction is
greatest due to
higher
concentration
of reactants
as reactants are
converted into products,
the concentration of
reactants decreases;
no. of successful
collisions decreases
Temperature
measure of the average
kinetic energy of a substance
kinetic energy of molecules
in a reaction varies
most
particles will
have kinetic
energy near
the average
nut a small
percentage
will have
values
significantly
more of less
than the
average
Energy Distribution Graphs
lower temp: curve
moves to the left
& is higher
higher temp:
curve moves to
the right & is lower
photochemical
reactions are ones in
which light is used to
increase the no. of
particles with energy
greater than the
activation energy
e.g. photosynthesis
Catalysts
a substance which
increases the rate
of reaction &
remains chemically
unchanged at the
end of the reaction
lowers the
activation energy
heterogeneous catalysts - in a
different state to the reactants
homogeneous catalysts
in the same state
a catalyst provides an alternate
reaction pathway which results in a
lower activation energy
reactant particles are adsorbed onto
the active sites of the catalyst
an activated complex is formed
the products leave the catalyst , leaving it
free for more reactants to adsorb onto it
Enzymes
are
biological
catalysts
catalyse many
reactions inside
the cells of
animals &
plants
they are
specific
rennin in
cheese
production
Catalytic
Poisoning
if a molecule
other than a
reactant is
preferentially
adsorbed onto
the active sites,
the sites are no
longer available
this
poisoning
reduces
the activity
of the
catalyst
industrial
catalysts
can be
poisoned
by
impurities
in
reactants
can be
removed &
replaced
or sometimes impurities can
be removed from active sites
reacting
with gas
e.g. catalyst in catalytic
cracking can be poisoned
by carbon, the carbon is
removed by burning it off
in a supply of air
the catalyst is
removed,
regenerated &
returned to the
catalyst chamber
lead poisons
catalysts in
catalytic
converters
Patterns
in the
Periodic
Table
Mendeleev
similar
chemical
properties in
groups, in order
of increasing
atomic number,
left gaps for
undiscovered
Across a Period
nuclear charge
increases, same electron
shell is filled up, the
attraction between the
nucleus & the electrons
increases due to
increasing size of the
positive nuclear charge
Down a Group
an additional electron
shell is added, the
attraction of the positive
nuclear charge to the
outer electrons
decreases due to
electron shielding of the
nucleus by the additional
electron shells
Atomic Size
decreases across a period, the larger
size of the nuclear charge the greater
the attraction between it & the electrons
resulting in a greater pull towards the
nucleus & a decrease in atomic size
increases
down a
group due
to the
increase in
occupied
electron
shells
BP, MP & Density
MP & BP increase
across period (groups
I to V) & then drops
MP & BP decrease down group I (decreasing
attraction between atoms) but increase down group
8 as van der Waals attractions increase with size
density increases
down all groups,
increases across
periods until group 3
then decreases
Ionisation Energy
the energy
required to
remove 1 mole
of electrons
from 1 mole of
gaseous atoms
across a period
increases due to
increasing nuclear
charge
down a group, decreases
due to decreasing
attraction between outer
electrons & nucleus
Electronegativity
measure of attraction an
atom involved in a covalent
bond has for the shared
electron in that bond
increases, period:
decreases group
Enthalpy
every substance contains
energy known as enthalpy
during reactions,
reactants are used up &
products made - change in
enthalpy going from
reactants to products
the enthalpy change (/\H) is
the energy difference between
reactants & products
Enthalpy Changes
energy is required to
break the bonds in
reactant molecules
(activation energy)
when reactant molecules
collide they forma highly
energetic arrangement called an
activated complex
the activation energy is
required by molecules to
form an activated complex
this highly energetic
complex loses energy &
can either go on to from
products or fall back into
the original reactnats
energy is released when new
bonds are formed in the products
Potential Energy Diagrams
Endothermic;
energy of
products
greater than
reactants,
therefore
enthalpy
change is
positive (+ve)
Exothermic:
energy of
products less
than reactants,
therefore,
enthalpy change
is negative (-ve)
Using Potential
Energy Diagrams
forward reaction: Ea = highest
energy - starting energy, /\H = end
energy - starting energy
the same but everything from
other side of graph e.g. starting
energy fro forwards is ending
energy for backwards
Catalysts
lowers
activation
energy
therefore
more particles
have energy
required to
overcome
activation
barrier so
more product
is formed
no effect on
enthalpy change
Equations
Combustion
1 mole
of fuel
must be
balanced
Solution
1 mole of
solute
balanced
Neutralisation
1 mole
of water
balanced
Bonding,
Metals
Metallic
Bonding
outer electrons move
freely between
neighbouring atoms
these electrons are
said to be delocalised
this leaves behind
a positive core
the metal is held together by
the strong forces of attraction
between positive cores &
negatively charged
delocalised electrons
Structure
giant
lattice
high melting & boiling
point due to strength of
the metallic bond
strength of bond varies from
metal to metal depending on the
number of electrons that each
atom 'delocalises' & also on how
the atoms pack together
Ionic
Bonding
occurs
between
metals &
non-metals
electrons are transferred from the
metal to the non-metal in order for
them to each have a full outer shell
the ionic bond Is
the electrostatic
force of attraction
between the
positively charged
metal ion & the
negatively
charged
non-metal ion
Ionic
structure
giant lattice
of oppositely
charged ions
high melting & boiling points
because it requires a lot of heat to
break millions of strong ionic bonds
Non-Metals
Covalent
Bonding
sharing electrons in order
to achieve full outer shells
the
electrostatic
attraction
between both
positively
charged nuclei
& the
negatively
charged
shared pair of
electrons
Pure
Covalent
Bond
electrons
shared
equally
between
atoms
Electronegativity value will
give a measure of the
attraction an atom has for
the electrons in the bond
the higher the
electronegativity value
the more the atom will
pull electrons towards
itself
in a pure
covalent bond
the
electronegativity
values are
identical
Polar
Covalent
Bond
electrons are
pulled closer to
one atom giving it
a slightly negative
charge
the other atom then has a slightly positive charge
electronegativity values are different
Polar Molecules
symmetrical - nonpolar
non-symmetrical
- polar
Intermolecular
Forces
Van der
Waals
v. weak,
noble gases
are
monoamatic
so Van der
Waals is the
only type of
bonding
they have
(induced)
temporary
dipoles
electrons spin
around the
nucleus - sides
can have slgithly
positive/negative
charges
Permanent
Dipoles
different
electronegativity
values - 1 much
higher than the
other
Hydrogen
Bonding
strongest type of
intermolecular attraction
occurs when hydrogen is bonded
with nitrogen, oxygen or fluorine
these bonds are highly polar due to
the different electronegativity values
hydrogen bonds are electrostatic forces of attraction between molecules which contain these highly polar bonds
v. high temp. needed to break bonds
Structure
Covalent Network
Elements
giant lattice of
covalently bonded atoms
Diamond
v. high melting point -
strong carbon to
carbon covalent bonds
v. hard - v. strong bonds
non-conductor -
electrons held tightly
between atoms - not
free to move
Graphite
high melting point
- covalent bonding
conducts
electricity -
delocalised
electrons
free to
move
between the
sheets
van der Waals
between sheets -
used a s a dry
lubricant
carbon, silicon
& boron
Covalent
Molecular
Elements
consist of
discrete
molecules
held together
by weak
intermolecular
forces
e.g.
fullerene
Covalent
Network
Compounds
2 main ones:
silicon dioxide
& silicon
carbide
silicon
carbide is
used as
an
abrasive
silicon
dioxide
Properties
Solubility
ionic compounds & polar molecules tend to be
soluble in polar solvents (e.g. water) &
insoluble in non-polar solvents (e.g. hexane)
non-polar substances tend
to be soluble in non-polar
solvents & insoluble in
polar solvents
like
dissolves
like
Melting
& Boiling
Points
bond strength
decreases
down these lists
intramolecular
covalent
network
(diamond)
ionic
(sodium
chloride)
intermolecular
hydrogen
bonding (water)
polar attractions
(hydrogen chloride)
van
der
Waals
(neon)
water freezes, hydrogen
bonding causes molecules to
form an open hexagonal shape
which results in ice being less
dense than water causing it to
float on water
The Mole
the mass of 1
mole of a
substance is its
relative formula
expressed in
grams (GFM)