Download rate of chemical reaction and chemical equilibrium

RATE OF CHEMICAL
REACTION AND
CHEMICAL
EQUILIBRIUM
CHEMICAL REACTIONS
A
chemical change is called a chemical
reaction.
Iron rusts when kept exposed to humid
air for a considerable time.
 Change of milk into curd.
 Burning of a paper.
 Combination of Nitrogen and
Hydrogen to form Ammonia
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SLOW CHEMICAL REACTIONS
The reactions which take place in a few minutes or more
reactants are called slow reactions.
 When Rusting of iron
 Weathering of rocks
 Generally the reactions between covalent
compounds are slow
 When ethyl alcohol and acetic acid are heated in
the presence of a little concentrated sulphuric
acid(catalyst), the reaction takes place in a few
minutes and an ester called ethyl acetate is
formed. This is a slow reaction.
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FAST CHEMICAL REACTIONS
 The
reaction which takes place
immediately on mixing the reactants are
called fast reactions.
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Reaction between an Acid and a Base

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

2Mg(s) + O2(g)  2 MgO(s)
RATE OF A CHEMICAL
REACTION
 The
rate of a chemical reaction means how
fast the reaction is taking place by indicating
how much of a reactant is consumed, or how
much of a product is formed in a given time.

Therefore Reaction rate =
Change in concentration of a
reactant or product/ time taken
UNIT OF RATE OF
REACTION
 The concentration is usually
expressed in molarity M
( the number of moles of substance
dissolved per litre of the solution)
and time in seconds
 Then unit of the rate of reaction is
-1
-1
-1
is moleL /s or molesL s
REACTION RATE
When acidified hydrogen peroxide (H2O2) is
added to a solution of potassium iodide (KI) ,
iodine is liberated
 H2O2(aq)+2 KI (aq) + H2SO4(aq) 2H2O(I)+
I2(aq)+ K2SO4(aq)
 Here the concentration of iodine is zero initially.
With the passage of time, it increases and the
reaction solution becomes brownish.
Concentration of iodine can be measured at
different intervals of time by titration against
REACTION RATE
If the concentration of iodine rises from 0
to 10-5mol L-1 in 10 seconds, we write
∆ [ I2]
10-5mol L-1
 Reaction rate = ----------- = -------------∆t
10s
= 10-6 mol L-1s-1
 Here, symbol ∆ represents a change and
[I2], molar concentration of iodine.

AVERAGE RATE

The rate of a reaction which is obtained by measuring
the change in concentration of a reactant or product over
a considerable period of time then it is called average
reaction rate.
Suppose iodine( I2) is being evolved in a reaction
as a product and we measure the change in its
concentration over a time, say 10 seconds, so in
this case
 ∆ [I2] / ∆ t will give us an average reaction rate.
 So Average rate of reaction = ∆ [I2]/ ∆ t

INSTANTANEOUS RATE
 The
rate of a reaction at a particular instant of
time is called instantaneous reaction rate.
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It is the accurate estimate of reaction rate at any
moment.
In order to determine the rate at a particular instant,
one should make the time interval almost zero. i.e
d[I2]
Instantaneous rate = -------------dt
Here d[I2] denotes very small change in the conc. of I2
and dt denotes very small change in time
RATE OF REACTION

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One can think of expressing the rate in terms of
change in concentration of H2O2 and KI is expressed
as,
∆[H2O2]
1 ∆[KI]
Rate of reaction = ------------------ = - --- ---------
∆t

2 ∆t
Here negative sign indicates that concentration of
H2O2 and KI will decrease with time. Here the rate of
consumption of KI is twice the rate of consumption
of H2O2 therefore, in order to make the two rates
equal we divide it by two (stoichiometric coefficient)
and write
½( ∆[KI]/ ∆ t)
RATE OF REACTION
 Cisplatin,
Pt(NH3)2Cl2 is used in
chemotherapy of cancer. It reacts with
water and releases chloride ion bound to
central platinum metal. The reaction is
represented as Pt(NH3)2Cl2 + H2O 
Pt(NH3)2Cl+ + Cl Here the conc. of cisplatin decreases with
lapse of time but conc. of Cl increases.
REACTION RATE CURVES
Reaction Rate Graphs -
Conc.
of
reactants
(mol L-1)
Conc of
products
(mol L-1)
Time (s) 
Here conc. of reactant
changes with time
Time (s) 
Here the conc. Of Product
changes with time
QUESTIONS
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Give an example of slow reaction.
Give an example of fast reaction.
Which reactions are slow and which are fast
Rusting of iron, Burning of Magnesium wire, setting
of cement, Formation of coal, formation of sodium
oxide
Fill in the blanks
Reactions between covalent compounds are usually ----Reactions between ionic compounds are very ------
QUESTIONS
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Define the rate of a chemical reaction ( in term products and reactants)
What is meant by average rate of reaction.
What is meant by instantaneous reaction rate.
Why do we put a minus sign in the formula for the rate of a reaction in terms of the
change in conc. of a reactant but no such sign in case of product.
Write the unit for expressing i) the concentration of an aqueous solution ii) rate of a
chemical reaction
Define molarity ?
Draw a graph to show the variation of conc. Of i) a reactant with time ii) a product
with time.
Write the chemical name and formula of Cisplatin. For what purpose is it used.
A solution contains 0.021 mole of a substance in 3.0 L of the solution. What is its
molarity ?
T he formula of H2O2 has been put within a square bracket as [H2O2]. What does it
represent.
Consider the reaction
2K
ENERGY CHANGES IN A
CHEMICAL REACTION

Most of the chemical reactions are accompanied
by energy changes. The energy is either released
or absorbed during a chemical reaction. In some
reactions, energy is given in form of heat and in
some cases, in form of light.
EXOTHERMIC REACTION
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Exothemic reaction – reactions which release
heat
Ex- all combustion reactions.
CH4(g) + 2O2(g)
CO2(g) +2H2O(l)
+energy(890.4 kj mol-1)
2AL(s) + Fe2O3(s)
Al2O3(s) + 2 Fe(l)+ heat
N2 (g) + 3H2 (g)
Fe
2NH3 (g) + heat
ENDOTHERMIC REACTION
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endothermic reactions- reactions which absorb
heat.
Ba(OH)2.8H2O(s) + 2NH4SCN(s)+ heat
Ba(SCN)2(aq) + 2NH3(aq) + 10H2O(l)
N2 (g) + O2(g) + heat -- 2 NO ( g )
PHOTOCHEMICAL
REACTION
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Photochemical reactions – Reactions that take place in
presence of light. For example, photosynthesis of
plants. Photographic films which are coated with silver
bromide (very small amount of silver iodide) undergo
chemical reaction when exposed to sunlight.
CH4 (g) + Cl2 (g)  CH3Cl (g) + HCl(g)
6CO2(g) + 6H2O (l)  C6H12O6(aq) + 6O2(g)
2AgBr (s)  2Ag (s) + Br2(l)
FACTORS AFFECTING THE
RATE OF REACTION
The following factors affect the rate of reaction.
These are
1) Effect of Concentration
2) Effect of Temperature
3) Effect of Catalyst
4) Effect of light
EFFECT OF CONCENTRATION
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Increase in the concentration of reactants increases
the number of molecules of reactants. There will be
more collisions between the reactant molecules and
the rate of reaction will increase.
When nitrogen oxide (NO) reacts with oxygen (
Which is a homogenous system)
2NO (g) + O2 (g)  2 NO2 (g) , the reaction rate
doubles when oxygen concentration doubles. But rate
quadruples when the concentration of nitrogen
monoxide is doubled.
EFFECT OF TEMPERATURE
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On increasing the temperature of reactants, the
energy of the molecules increases. Due to increased
energy the frequency of collision increases and finally
the rate of reaction increases.
CaCO3 (s) + 2 HCl (aq)  CaCl2 (aq) + CO2 (g) +
H2O (l) . In this reaction the evolution of CO2 is slow
but if we increase the temperature the evolution of
gas is more rapid indicating that the rate of reaction
increases on increasing the temperature.
EFFECT OF CATALYST
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A catalyst is a substance which increases the rate of a
chemical reaction but there is no change in the catalyst
itself. In the presence of a catalyst a reaction takes
place at a faster rate and at lower temperature.
Iron is used as a catalyst in the manufacture of
ammonia. Iron catalyst increases the rate of reaction
between nitrogen and hydrogen to form ammonia.
N2 (g) + 3H2(g)
2 NH3 (g) + heat
Similarly , the reaction of SO2 and O2 gives SO3 in
presence of nitrogen monoxide (NO), which acts as a
NO (g)
catalyst. 2 SO2 (g) + O2 (g)
2SO3 ( g)
EFFECT OF LIGHT
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Photosynthesis and photography both involve
light sensitive reactions.
In our daily life, a coloured cloth gradually fades
when dried in sunlight.
Dyes of the clothes undergo chemical change
and fade gradually.
Plants prepare food in presence of sunlight.
REVERSIBLE REACTIONS

The reaction in which the product can react
under any conditions to give back reactants
are called reversible reactions or a reaction
which takes place in a forward as well as
reverse direction both, is called reversible
reaction. It is represented by putting a double
arrow between the reactants and products.
One arrow pointing towards the products
and other towards reactants.
Examples of reversible reactions

2H2 + O2
2H2O

N2 + O2
2NO
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CaCO3
CaO + CO2

N2 + 3H2
2NH3
IRREVERSIBLE REACTIONS
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Those chemical reactions in which the products
cannot recombine to give back reactants are
called irreversible reactions.
2 Na(s) + 2 H2O (l)  2 NaOH ( aq) + H2 (g)
Here the products, sodium hydroxide and
hydrogen can’t recombine to give back the
reactants sodium and water.
REVERSIBLE PROCESS AND
CHEMICAL EQUILIBRIUM
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The formation of ammonia is
a reversible process. we say that nitrogen and
hydrogen react under appropriate conditions
and give ammonia,
then this statement is expressed in the form of
an equation as
N2 (g) + 3 H2 (g) → 2 NH3 (g)
The reaction is taking place in forward direction
and we, therefore, say it is a ‘forward reaction’.
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We can also write decomposition of NH3 as,
2NH3 (g) → N2 (g) + 3H2 (g)
which is, in fact, a reverse of the forward
reaction
and takes place in opposite direction, and
therefore, we call this reaction as ‘reverse
reaction’. We represent forward and reverse
reactions together as
N2 (g) + 3H2 (g) 2 NH3 (g)
Such reactions never go to completion if
performed in a close container.
Rate of Reaction 
N2 (g) + 3 H2 (g)  2 NH3 (g)
(Rate of formation of ammonia
decreases with time)
Chemical Equilibrium
Rate of decomposition of
ammonia increases with time )
2NH3 (g)  N2 (g) + 3H2( g )
O
Time increases 
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For a reversible chemical reaction, an equilibrium state is
attained when the rate at which a chemical reaction is
proceeding in forward direction equals the rate at
which the reverse reaction is proceeding.
At equilibrium,
Rate of forward reaction= Rate of reverse reaction
In case of ammonia, when reaction is performed at
high pressure and temperature in a close container, at
equilibrium,
Rate of formation of ammonia = Rate of
decomposition of ammonia
DYNAMIC EQUILIBRIUM
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Since the rate of decomposition and rate of
formation is the same at equilibrium, the
concentrations of ammonia, hydrogen and
nitrogen remain constant.
This equilibrium is dynamic in nature and is
therefore, called dynamic equilibrium.
In a close container containing water, rate of
evaporation and rate of condensation of water
molecules become equal at equilibrium.
CHARACTERISTIC FEATURES
OF DYNAMIC EQUILIBRIUM
(i) At equilibrium, macroscopic (observable) properties
such as concentration, density, colour, etc., are
constant under the given condition of temperature,
pressure and initial amount of the substances.
(ii) At equilibrium, microscopic (at molecular level)
processes continue, but they are in balance. This
means that no overall large scale (macroscopic)
changes occur. Rate of forward process (reaction) is
equal to the rate of reverse process, and as a
consequence of this, no net change results.
(iii) The equilibrium can be attained from either
direction, beginning with only the materials on one
side of the change. Such changes are described as
reversible.
(iv) Equilibrium can be achieved in a closed system. In a
closed system, there is no loss or gain of matter to or
from the surroundings.
An open system may allow matter to escape or to enter.
Therefore, in an open system, the equilibrium cannot
be attained.
EQUILIBRIUM CONSTANT
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It has been experimentally found that at a particular
temperature, when equilibrium is attained, the ratio between
concentration of reactants and products becomes constant.
For reaction
N2 (g) + 3 H2 (g) 2 NH3 (g) + 92 kJ
At equilibrium,
Here, concentration of product (ammonia) occurs in
numerator, and those of reactants (hydrogen and nitrogen)
occur in denominator. Each concentration term, [NH3], or
[N2], or [H2], is raised to a power equal to stoichiometric*
coefficient in the balanced equation. K is called equilibrium
constant.
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For a general reaction,
aA+bBcC+dD
at equilibrium, equilibrium constant can be
written as,
In the above expression, square bracket
denotes molar concentration, i.e. concentration
in mol/L. The expression
(1.2) is also known as Law of Chemical
Equilibrium. At a particular temperature,
equilibrium constant has a definite value. When
we express concentration in mol L-1, equilibrium
constant is denoted by Kc.
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Magnitude of equilibrium constant, K, indicates how
far a chemical reaction can go. In other words, it is an
indicator of the extent of a chemical reaction.
Larger the value of K, higher will be the equilibrium
concentration of products on the right hand side of
the reaction. Smaller value of K indicates lower
concentration of the products at equilibrium.
For the reaction at 298 K,
PCl3 (g) + Cl2 (g) _PCl5 (g) ; Kc = 1.9
Low value of equilibrium constant shows that
at equilibrium, the concentration of product (PCl5)
will be low.
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. Similarly, for reaction at
298 K,
N2 (g) + O2 (g) 2 NO (g)
K=c
Very small value of Kc implies that reactants
N2 and O2 will be dominant species in the system
at equilibrium.