by Maryam Hussain How the working of a theoretical engine was proposed? The theoretical heat engine was proposed by Sadi Carnot in 1824 . Heat engines are devices that can convert heat into mechanical energy. Like a Steam engine or Motor vehicle, the working principle of all heat engines is the same in which any working substance gas or vapor is brought back to the initial stage by going through different thermodynamic stages of a cyclic process . Carnot suggested that, if a heat engine goes through fixed thermodynamic stages of a specified process, then the efficiency of the heat engine may be 100% possible. Ideally, this engine gives maximum efficiency. No other engine can be more efficient than this. The specified cyclic process suggested by Carnot is called the Carnot cycle. The Carnot engine has four main parts: 1). First, an insulated, non-conducting stand , so that the transfer of energy is not possible . 2). Second, a hot reservoir with infi...
By Maryam Hussain
In total, there are four
requirements for the occurrence of a chemical reaction.
The first requirement is: to
bring the molecules of the reactants in the vicinity of each other.
The next requirement is: the
molecules that have to react with each other, must collide with one
another. That is why we stir liquid-liquid, solid powder-liquid, solid
powder-gas, gas-gas & solid powder-powder reactants.
The next requirement is: the
molecules that have to react with each other, must collide with
sufficient energy to initiate the process of breaking and forming bonds.
This energy is called the energy of activation. That is why we heat or pre-heat
or heat the reactants Or cool or pre-cool the reactants. We also bring the
energy level up to the required level by introducing the reactants with
suitable catalysts with or without heating or cooling.
The next requirement is: the
molecules that have to react with each other, must collide in an
orientation that can lead to the rearrangement of the atoms and the formation
of products. That is why we introduce the reactants in suitable
arrangements e.g. side or cross injection, parallel injection, and counter
injection.
In earlier paragraphs &
blogs, it was considered that a chemical reaction proceeds only in a forward direction
and the reactants were shown on the tail side of arrow, with the arrowhead
towards the right as viewed on the computer screen, and the products on the
right-hand side of arrowhead.
It was also assumed that all of
the reactants were converted to products. However, most of the time,
reactants are not completely converted to products because a reverse
reaction takes place in which products come together and form the reactants;
this reverse directional reaction is called a backward reaction. When a
reaction proceeds in both forward and backward directions, it is said to be
reversible. In a reversible reaction equation, the forward reaction points
the arrowhead toward the right direction and the backward reaction points the
arrowhead toward the left direction.
A + B
⇀ C
+ D Forward
reaction
A + B
↽ C
+ D Backward reaction
A + B
⇌ C
+ D Reversible
reaction
The double arrow generally
signifies that the reaction occurs in both directions and that the forward
reaction does not go to completion. A reversible reaction does not go to
completion and some reactant material remains.
The rate of forward reaction
depends on the concentrations of reactants &, in the case of reversible
reactions, the rate of forward reaction depends additionally on the
concentrations of products. The forward and backward reactions continue just as
they do when a system is away from chemical reaction equilibrium, but when
chemical reaction equilibrium has been reached these forward and backward rates
are equal. As a result, there is no change in the concentration of any of
the species, on any side of the double arrows even though the reactions are
still going on.
What is the rate of reaction?
Raw material for any chemical
reaction is the reactant(s) involved in this reaction. When a product is formed
then at least a reactant is consumed either wholly or partially. When the
reactant is partially consumed over a given period of time, then an equal mass
of product must have formed over the same period of time. When the reactant is
still present then more mass of the product may appear over more duration of
time.
Now let us consider the following
chemical equation to represent a chemical reaction in an unbalanced form:
A ⇀
B
After multiplying the coefficients
(here n and m) with each member of the chemical equation, the equation is
balanced. The balanced equation is:
n. A ⇀
m. B
Therefore, the rate of liquid phase chemical reaction is defined as the number of moles of reactant A that disappeared or were consumed in the given duration of time in a given volume of the reaction mixture.
If
V = Volume of the liquid mixture within which reaction is taking place; it is a mixture or blend of reactants & products. It is usually called a reaction mixture. It is mostly measured in liters.
NA = number of moles of
reactant A
t = time, seconds
- rA =
rate of disappearance or rate of consumption of reactant A
The negative sign with rA
shows a decrease in the number of moles of reactant A with an increase
in time duration.
Then
rate of disappearance or rate of
consumption of reactant A
= ( Change in Number of moles of A ) / ( Change in time ) x 1 /
volume of the reaction mixture
= ( dNA / dt ) . ( 1/ V )
Or
- rA =
( dNA / dt ) . ( 1/ V )
g-moles / ( Sec. liter )
The rate of the same chemical reaction
( n. A ⇀
m. B) may also be defined in terms of product B.
The rate of liquid phase chemical reaction is defined as the number of moles of product B produced in the given duration of time in a given volume of the reaction mixture.
If
V = Volume of the liquid mixture
within which reaction is taking place; liters.
NB = number of moles of
product B
t = time, seconds
rB =
rate of appearance or rate of formation of product B
The positive sign with rB
show an increase in the number of moles of product with an increase in time
duration
Then
rate of appearance or rate of
formation of product B
= ( Change in Number of moles of B ) / ( Change in time ) x 1 /
volume of the reaction mixture
= ( dNB / dt ) . ( 1/ V )
Or
rB = ( dNB / dt ) . ( 1/ V ) g-moles / ( Sec. liter )
To verify that the
calculations or measurements are correct for the same chemical
reaction, equate the two reaction rate or sum up these rates with an
acceptance criterion
- rA + rB = 0
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