RATE OF REACTION

RATE OF REACTION

from Femosky110 on 06/12/2020 01:22 PM

Rates Of Reactions And Equilibrium Of Reacting Systems
Chemical equilibrium in a chemical reaction is the condition in which both reactants and products are present at concentrations that have no extra propensity to alter with time. Characteristically, this situation arises when the rate of forward reaction is equal to the rate of backward reaction. The rate of forward and backward reaction is not normally zero at this point but is equal. What this means is that there is no net alterations in the concentrations of the reactant(s) and product(s). A situation like this is referred to as dynamic equilibrium.

 

Reversible reactions
A reversible reaction is a reaction which can proceed at either direction depending on the situations.

If you pass steam over hot iron, you would notice a reaction between the steam and the iron to produce a black, magnetic oxide of iron referred to as triiron tetroxide, Fe3O4

The hydrogen that is manufactured during the reaction is carried away by the stream of steam.

Under various situations, the products of this reaction will as well react together. The Hydrogen that is made to pass over hot triiron tetroxide reduces it to iron. Steam is also manufactured.

At this juncture the steam produced in the reaction is taken away by the stream of hydrogen.

These reactions occur simultaneously either ways and are reversible, but under the conditions usually used, they turn out to be one-way reactions.

Reversible reactions taking place in a closed system
A closed system is a system is a reacting system in which no substances are added to the system or lost from it. Energy can, nevertheless, be transmitted in or out freely.

If we consider the example we've considering above, imagine that iron is being heated in steam in a closed container. The system is being heated up but none of the substances in the reaction can break out because the system is closed. As the amount of triiron tetroxide and hydrogen begin to be formed, they would in turn react again to produce the original iron and steam. Therefore, if you analyzed the mixture after sometime you would notice a situation referred to as a dynamic equilibrium.

In a nutshell, a dynamic equilibrium takes place when a reversible reaction occurs in a closed system. The system remains unchanged except that energy would be added to it. When the reaction is at equilibrium, the amount of all that is available in the mixture remains unchanged even though the reactions are still progressing. This situation arises due to the fact that at this point, the rate of forward reaction is equal to the rate of backward reaction.

If there is any alteration in the conditions to the extent that it alters the relative rates of the forward and backward reactions, the position of equilibrium is shifted to annul the changes in the conditions.

Reaction Rate or rate of a chemical reaction
The rate of a chemical reaction or Reaction Rate is the estimation of the alteration in concentration of the reactants or the alteration in concentration of the products over a unit time.

For an example, during the chemical reaction between products A and B as shown in the equation below, the reactants A and B are used up while the concentration of product AB increases. The reaction rate or rate of the chemical reaction can be measured by estimating how fast the concentration of A or B is being used up, or by how fast the concentration of AB is being formed.

A + B ---- AB

For the purpose of stochiometry, the above equation can be further represented as:

aA + bB ------ cC+ dD

Collision Theory
The collision theory states that gaseous state chemical reactions takes place when two gas molecules smash together with enough kinetic energy. The smallest amount of energy that is needed for a thriving collision, which leads to a successful reaction, is referred to as the activation energy. Consequently, only a fraction of collisions result to successful reactions.

The collision theory is derived from the kinetic theory of gases. Thus we are only dealing with gaseous chemical reactions. Because of this, the ideal gas assumptions are applied. Moreover, we are as well making assumptions that:

1. All the reacting molecules are moving through space in a straight line.

2. All molecules are inflexible spheres.

3. The reactions being discussed only takes place between two molecules.

4. The molecules ought to collide with one another.

In the end, the collision theory of gases offers us the rate constant for bimolecular gaseous reactions; it is equal to the rate of winning collisions. The rate of successful collisions is proportional to the small part of successful collisions multiplied by the general collision frequency.

Collision Frequency
The rate or frequency at which molecules collide is referred to as the collision frequency, Z. The unit of collision frequency is collisions/ unit of time. Given a box of molecules A and B, the collision frequency between molecules A and B is given by:

Successful Collisions
To enable a successful collision to take place, the molecules of the reactant ought to collide with sufficient kinetic energy to break original bonds and be able to form fresh bonds to turn into the molecules of the product. Therefore, it is referred to as the activation energy for the reaction; it is as well normally regarded as the energy barrier.

The portion of collisions that possess sufficient energy to exceed the activation barrier is denoted as:

The tiny proportion of successful collisions is directly proportional to the temperature and inversely proportional to the activation energy of the reaction.

In conclusion
The rate constant of a gaseous reaction is proportional to the product of the collision frequency and the fraction of successful reactions. Just like we mentioned above, enough kinetic energy is necessary for a successful reaction; but they need to appropriately.

The Activation Energy of Chemical Reactions
Only a minute portion of the collisions between reactant molecules change the reactants into the products of the chemical reaction. This can be understood by considering the reaction between ClNO2 and NO.

ClNO2(g) + NO(g) NO2(g) + ClNO(g)

During the process of this reaction, a chlorine atom is transmitted from one nitrogen atom to another. To enable the reaction to occur, the nitrogen atom in NO have to bump with the chlorine atom in ClNO2

The reaction will not take place if the oxygen end of the NO molecule collides with the chlorine atom on ClNo2

It will also not take place if an oxygen atom from among the 2 atoms on ClNO2 collides with the nitrogen atom on NO.

An additional factor that determines if a reaction will take place is the energy the molecules carry or bears the moment they come in contact for collision. It is not every molecule that possesses the same kinetic energy. This is significant for the reason that the kinetic energy molecules bear the moment they bump is the most important basis of the energy that ought to be invested in a reaction to make it to occur.

The total typical free energy for the reaction between ClNO2 and NO is favorable.

ClNO2(g) + NO(g) NO2(g) + ClNO(g) Go = -23.6 kJ/mol
But, prior to the reactants being converted into products, the free energy of the system ought to conquer the activation energy for the reaction.

All molecules have a definite minimum quantity of energy. The energy can either be in the form of Kinetic Energy and/or Potential Energy. When molecules come in contact with one another, the kinetic energy of the molecules can be made use of to elongate, twist, and eventually break bonds, resulting to chemical reactions. If molecules are travelling too unhurriedly with a small kinetic energy, or they collided with an inappropriate direction, they will not lead to reaction and would merely bounce away from each other. Nevertheless, if the molecules are travelling at a swift speed sufficient to cause a successful collision orientation, in a manner that the kinetic energy on collision is higher than the minimum energy barrier, then a reaction will take place. The minimum energy obstruction that ought to be achieved for a chemical reaction to occur is known as the activation energy; Ea. Ea is normally in units of kilojoules per mole.

Activation energy, Enthalpy, Entropy and Gibbs Energy

In Thermodynamics, Gibbs free energy change, ΔG, is defined as:

ΔG=ΔH −TΔS

where

• ΔG = Gibbs free energy change of the reaction

• ΔH = Enthalpy change of the reaction

• ΔS = Entropy change of the reaction.

ΔGo is the Gibbs energy change when the reaction takes place at Standard State (1 atm, 298 K, pH 7).

To calculate a change in Gibbs free energy of the reaction that did not occur at a standard state, the Gibbs free energy equation can be denoted as:

ΔG = ΔGo + RTlnk

where

• ΔG = Gibbs free energy change of the reaction

• ΔGo = standard Gibbs free energy of the reaction

• R = 8.314 J/molK

• k = equilibrium constant

At equilibrium, ΔG=0. The equation above then becomes:

0= ΔGo + RTlnk

Solve for ΔGo

ΔGo = −RTlnk

Dynamic Equilibrium
At dynamic equilibrium, reactants are transformed to products and products are transformed to reactants at an equivalent and constant rate. An example of a reaction in dynamic equilibrium is the dissociation of acetic acid. For instance

CH3COOH ---- H+ + CH3COO-

Static Equilibrium
Static equilibrium is as well referred to as mechanical equilibrium. It takes place when all particles in the reaction are at rest and there is no movement between reactants and products. Static equilibrium can as well be experienced in a steady-state system in a physics-based outlook.

Dynamic forces are not performing on the potential energies of the backward and forward reactions. One example of static equilibrium is graphite changing into diamond. This reaction is well thought-out at static equilibrium after it takes place for the reason that there is no more forces acting upon the reactants (graphite) and products (diamond).

Le Chatelier's Principle and Equilibrium
Equilibrium can be explained through the Le Chatelier's principle. The kinetics of a reaction can alter and the position of equilibrium would depend on the properties of the reactants and products. Put in a simpler way, the equilibrium will swing towards one side or the other relative to the concentration, temperature, pressure, and volume. Le Chatelier's principle is not the same thing as dynamic equilibrium. Although they are comparable, they are of variable concepts. Le Chatelier's principle explains the way equilibrium can alter. Dynamic and static equilibrium explain the way systems in equilibrium behave.

Comparison between Dynamic and Static Equilibrium
A reaction at dynamic equilibrium has the capability to be reversible while a reaction at static equilibrium is irreversible. The equilibrium constant alone cannot determine if a reaction is in static or dynamic equilibrium due to the fact that the equilibrium constants are estimated through the concentrations of products against reactants. A reaction is at dynamic equilibrium if the rate of the forward reaction is the same as the rate of the backward reaction. It is at static equilibrium if the reaction has taken place and there is no forward or backward rate of reaction.