Rates of Reactions
Tímea Szentgyörgyi
SEK Budapest International School
1021 Hungary, Budapest,
Hűvösvölgyi u. 131.
Tel.:36-1-394-2968;
Fax: 36-1-200-6615
www.sek.hu; www.sek.net

Chemical kinetics is the
study of the speed with
which a chemical
reaction occurs and the
factors that affect this
speed.

The rate of a reaction is
the speed at which a
reaction happens.

All the molecular level
of life some reactions
go very rapidly and
other take forever.

The rate of reaction, r,
is defined to be the
slope of the
concentration-time
plot for a species
divided by the
stoichiometric
coefficient of that
species.
A+2B=3C
r = -d[A]/dt = -1/2
d[B]/dt= 1/3 d[C]/dt

The reactants have a
negative slope, because
they are being
consumed in the
reaction. Products have
a positive slope,
because they are being
formed in the reaction.
What factors affect the rate of a reaction?

The concentration of
the reactants. The
more concentrated
the faster the rate.
Remember for
gasses, increasing
the pressure simply
increases the
concentration so
that's the same thing.
What factors affect the rate of a reaction?



Temperature. Usually reactions
speed up with increasing
temperature.
Physical state of reactants.
Powders react faster than blocks greater surface area and since the
reaction occurs at the surface we
get a faster rate.
The presence of a catalyst (or
inhibitor). A catalyst speeds up a
reaction, an inhibitor slows it
down.

Reaction with High Rates:

1. FeCl3 + KSCN → Fe( SCN)3 + 3 KCl

2. Faro’s snake

Reaction with Middle Rates:

FeCl3 + Na2S2O3 → [ Fe( S2O3)2 ]-

Halloween reaction / Old Nassau Reaction
The reaction in this
experiment takes place in
several steps [5].

First, sodium
metabisulphite
reacts with water to form
sodium hydrogen sulphite:
Na2S2 O5 + H2O ==>
2NaHSO3

Hydrogen sulphite ions
reduce iodate(V) ions to
iodide ions:
IO3- + 3 HSO3-==>
I- + 3 SO42+3H+


Once the concentration of
iodide ions is large
enough that the solubility
product of HgI2 is
exceeded, orange
mercury(II) iodide solid is
precipitated until all of the
Hg2+ ions are
used up (provided that
there is an excess of Iions).

Hg2+ + 2 I- ==> HgI2
(orange or yellow)




If there are still I- and IO3 - ions in the mixture, the
iodide-iodate reaction
IO3-+5 I- + 6 H+ ==> 3 I2 + 3 H 2O
takes place and the blue starch-iodine complex is
formed,
I2 +starch ==>complex (blue or black)
Reactions with low rates:

1. Pb( CH3COO)2 + 2KI = PbI2 +CH3COOK

2.NiCO3+2 dmg=

3.CoSO4+2NH4SCN=Co(SCN)2 +
[Ni( dmg)2]2+ + CO32(NH4)2 SO4
Thank you for your attention