Chemical reactions: An overview

Contents

-What is a chemical reaction?

-Why and how does a chemical reaction take place?

-What controls a chemical reaction?

-Pathways of reactions: Kinetics vs thermodynamics

-Enthalpy and internal energy of chemical reactions

-Entropy of chemical reactions

-Free energy of chemical reactions

-Kinetics of chemical reactions

Never think that an engineer has no concern for a chemical reaction. Never think it is the job of a chemist only to understand the chemical reaction. A chemical reaction is the backbone of a chemical process.

?An engineer's job is to make sure that any system has some exergy. That means the system does not reach equilibrium. That means he always thinks about how to get maximum work from a system before it reaches equilibrium. Chemical reactions are no exceptions.

Any chemical reaction has two parts [1] ‘How stable’ and [2] ‘How fast’. The first one is dealt with by thermodynamics. Thermodynamics can tell you only that a reaction should go because the products are more stable (have lower free energy) than the reactants. Thermodynamics has nothing to do with time. The second one is dealt with by kinetics.?Kinetics can tell you how fast the reaction will go but doesn't tell you anything about the final state of things once it gets there. Thermodynamically favorable reactions can be kinetically unfavorable

What is a chemical reaction?

A chemical reaction is a process in which one or more substances, also called reactants, are converted to one or more different substances, known as products. A chemical reaction rearranges the constituent atoms of the reactants to create different substances as products.

Why and how does a chemical reaction take place?

The question might sound simple but the answer is not.

Imagine a simple reaction H2 + Cl2 = 2HCl

Everything on the earth looks for an opportunity to go to the lowest energy stable state to relax. Molecules are not exceptions. A chemical reaction always goes in the direction of lower energy where the reactants molecules can offload their energy. A reaction will always try to go towards a state of lower disorder from a state of more disorder where the products are more stable. If a reaction finds that the products are at a higher level of energy than the reactants, the reaction will not move forward. The reaction will reverse. The transition state from reactants to products is the highest energy point for a chemical reaction, this topmost point of the activation energy curve is the decision point of a chemical reaction for it to choose its direction.

For an engineer, it is a challenge how he prevents or delays a reaction going to a state of more disorder or a state of high entropy towards equilibrium.?Fortunately, he has the blessings of Le Chatelier to guide him out of a crisis.

But this is not everything for a chemical reaction

If you put one H2 and Cl2 molecule together they would like to go to the lower energy state which is HCl. But nothing will happen until you have provided the energy to break H-H and Cl-Cl bonds. The energy that is needed to break the bonds of reactant molecules is the activation energy of the reaction.?

Typically, reaction rates increase with increasing temperature because there is more thermal energy available to reach the activation energy necessary for breaking bonds between atoms. Reactions may proceed in the forward or reverse direction until they go to completion or reach equilibrium. Reactions that proceed in the forward direction to approach equilibrium are often described as spontaneous, requiring no input of free energy to go forward.

What controls a chemical reaction?

This is often debated.

Thermodynamics controls a reaction or kinetic controls a chemical reaction is decided by the composition in a reaction product mixture when competing pathways lead to different products.?The distinction is relevant when product A forms faster than product B because the activation energy for product A is lower than that for product B, yet product B is more stable. In such a case A is the kinetic product and is favored under kinetic control and B is the thermodynamic product and is favored under thermodynamic control. The conditions of the reaction, such as temperature, pressure, or solvent, affect which reaction pathway may be favored, either the kinetically controlled or the thermodynamically controlled one. Note this is only true if the activation energy of the two pathways differs, one pathway always follows a lower Ea (energy of activation).

Pathways of reactions: Kinetics vs thermodynamics

The prevalence of thermodynamic or kinetic control determines the final composition of the product when these competing reaction pathways lead to different products. The reaction conditions as mentioned above influence the selectivity of the reaction - i.e., which pathway is taken.

A reaction can be thermodynamically favorable but still kinetically unfavorable

Because thermodynamics deals with state functions, it can be used to describe the overall properties, behavior, and equilibrium composition of a system. It is not concerned with the particular pathway by which?physical or chemical changes occur.

Energy profile diagram for kinetic versus thermodynamic product pathways.

Although thermodynamics provides a significant constraint on what can occur during a reaction process, it does not describe the detailed steps of what actually occurs on an atomic or a molecular level. In many cases, we will encounter reactions that are strongly favored by thermodynamics but do not occur at a measurable rate. In contrast, we may encounter reactions that are not thermodynamically favored under standard conditions but nonetheless do occur under certain nonstandard conditions.

Example: A simple typical example is a graphite and diamond. They are both allotropes of carbon. Both are different structural forms of the same carbon but exhibit quite different physical properties and chemical behaviors. If you look at graphite, it has lower free energy. Therefore, in nature thermodynamics favors the conversion of diamond into graphite but it does not happen. The question is why? Even though the reaction is thermodynamically favorable, it is slow. The kinetics of the process takes the call. It’s just plain too difficult to get the diamond to break all of its bonds and re-form them in the different, more stable graphite configuration.

The standard state Gibbs free energies of formation of C(graphite) and C(diamond) 2.9 kJ/mol with graphite having less free energy yet the nature favors the formation of diamond over graphite, because of the very high activation energy barrier.

Graphite and diamond activation energy diagram: Look at the very high energy barrier between

[To note: Graphite is thermodynamically more stable than diamond yet carbon forms diamond and not graphite because of a very high energy barrier [activation energy] between diamond and graphite.]?

One interesting fact to point out here is that graphite is the most stable allotrope of carbon, but it is only 2.9 kJ/mol more stable than diamond at 300K and 1 atm. Consequently, it would be reasonable to assume that the inter-conversion between the two would be relatively easy and that diamond would quickly decompose to graphite. In practice, however, graphite has been converted directly to diamond, but only in extreme conditions to the magnitude of 3000K and 125 kbar of pressure. Similarly, the decay of diamond to graphite has a half-life of millions of years. The reasons for this can be explained using thermodynamics. Basically, a very large energy barrier exists, that you can see in the image [ activation energy] between the two allotropes, which means that a lot of energy needs to be put into the system in order to interconvert the two.

First, we will discuss the thermodynamics of chemical reactions and then the kinetics of chemical reactions

Thermodynamics of chemical reaction

Chemical thermodynamics is the study of the interrelation of heat and work with chemical reactions or with physical changes of state within the confines of the laws of thermodynamics.

The thermodynamics of chemical reactions is typically used for the determination of the feasibility or spontaneity of a chemical reaction.

The following state functions are of primary concern in chemical reactions:

Internal energy (U)

Enthalpy (H)

Entropy (S)

Gibbs free energy (G)

Most identities in chemical reactions arise from the application of the first and second laws of thermodynamics, particularly the law of conservation of energy, to these state functions. While all three laws are often referred to in the discussion:

-The energy of the universe is constant.

- In any spontaneous process, there is always an increase in entropy of the universe.

-The entropy of a perfect crystal (well ordered) at 0 Kelvin is zero.

Enthalpy and internal energy of chemical reactions

What is enthalpy?

Enthalpy tells us how much heat [energy] is in a system. Enthalpy is the total energy of a substance.

Enthalpy is the sum of [1] Internal energy and [2] work energy.

H = E + W, H is enthalpy, E is internal energy, and W is work energy, respectively.

Internal energy E is the sum of PE + KE.

Internal energy refers to the microscopic energy inside atoms and molecules. Internal energy is defined as the energy associated with the random, disordered motion of molecules. It is separated in scale from the macroscopic ordered energy associated with moving objects.

Internal energy is a measure of the amount of kinetic and potential energy possessed by particles in a body. Internal energy is all contained energy. Molecules have kinetic energy when particles move within a system and potential energy which holds atoms and molecules together

Enthalpy changes in chemical reactions

Enthalpy [H] is the heat content of a system at constant pressure. The heat that is absorbed or released by a reaction at constant pressure is the same as the enthalpy change and is given the symbol ΔH. Unless otherwise specified, all reactions are assumed to take place at constant pressure.

The change in enthalpy of a reaction is a measure of the differences in enthalpy of the reactants and products. The change in enthalpy is also called the heat of the reaction and is given the symbol ΔH.?The heat of a reaction is the difference between the energy of bond formation (in the products) and bond breaking (in the reactants). ΔH can be negative or positive depending on whether the reaction is exothermic (heat is released, negative sign, -ΔH) or endothermic (heat is absorbed, a positive sign, +ΔH).

ΔH = H products – H reactants

The enthalpy of a system is determined by the energies needed to break chemical bonds and the energies needed to form chemical bonds. Energy needs to be put into the system in order to break chemical bonds – they do not come apart spontaneously in most cases.

To be more precise, the first step in a reaction is the endothermic step of breaking the reactant bonds and the second is the exothermic step of making the product bonds. The energy change of the reaction can be viewed as the sum of these two steps, and results in two possibilities:

When the exothermic step (2) is greater than the endothermic step (1) the reaction is exothermic.

When the endothermic step (1) is greater than the exothermic step (2) the reaction is endothermic.

ΔH (reaction) only depends on the initial (reactant) and final (product) state, because enthalpy is a state function this difference is path independent. What we do know is that the first step required energy and the second released energy. Enthalpy change ΔH is the energy difference between the initial and final states

Enthalpy change on the activation energy diagram

LHS image: Endothermic reaction

RHS image: Exothermic reaction

In both cases of endothermic and exothermic reactions you may see the following:

[1] Activation energy > Energy of reactants and energy of products

[2] In exothermic reactions, the energy of product < energy of reactant, - ?H

[3] In endothermic reactions, the energy of product > energy of the product, + ?H

The entropy of chemical reactions

Definition of entropy

In simple language, heat transfer from hot to cold is related to the tendency in nature for systems to become disordered and for less energy to be available for use as work. The entropy of a system is a measure of its disorder and of the unavailability of energy to do work.

Entropy is a measure of how much energy is not available to do work.

The second law of thermodynamics

According to the 2nd law of thermodynamics, the change in entropy (delta S) is equal to the heat transfer (delta Q) divided by the temperature (T), Q/T. For a given physical process, the entropy of the system and the environment will remain constant if the process can be reversed. If we denote the initial and final states of the system by "i" and "f", Sf = Si (reversible).

Entropy generation in chemical reactions

Chemical reactions tend to proceed in such a way as to increase the total entropy of the system.

This is how a reaction can increase entropy?

-Entropy increases in processes in which solid or liquid reactants form gaseous products. Entropy also increases when solid reactants form liquid products.

-Entropy increases when a substance is broken up into multiple parts. The process of dissolving increases entropy because the solute particles become separated from one another when a solution is formed.

-Entropy increases as temperature increases. An increase in temperature means that the particles of the substance have greater kinetic energy. The faster-moving particles have more disorder than particles that are moving more slowly at a lower temperature.

-Entropy generally increases in reactions in which the total number of product molecules is greater than the total number of reactant molecules.

Important points

Exothermic reactions increase the entropy of the surrounding by the amount Q/T. Q is the heat of reaction. There is a total [system+ surrounding] increase in entropy

Endothermic reactions decrease the entropy of the surrounding by Q/T. Here Q is the heat absorbed by the system from surroundings. In the case of endothermic reactions, there is a total decrease in entropy.??

Free energy of chemical reactions

The free energy of a chemical reaction tells us about the spontaneity of the reaction. It is the most important part of chemical reaction thermodynamics and is often not understood. We would discuss Free energy in detail.?

Free energy is energy available for doing work*. That's "free" as in "available".

Free energy, unlike total energy, is not conserved. In fact, it's just the opposite: free energy always decreases in a closed system.

G = U - TS

where G is the free energy, U is the internal energy, T is the temperature, and S is the entropy. U follows the law of conservation of energy; it is neither created nor destroyed.

The entropy, S, always increases (or at least, does not decrease). That means that G always goes down, at least in a closed system. A closed system is one in which heat and work are permeable to move freely to surroundings and back.?

Everything that happens, happens because it increases the entropy of the universe. Any process that happens on its own in nature happens because it increases the total entropy of the universe. For example, there are many more configurations of gas molecules in the room that give an almost uniform distribution (arrangement) of the air in the room than there are for which all the gas molecules collect in one corner of the room, choking you out and thus gas molecules rearrange themselves across a larger space causing more disorder and thereby increasing entropy.

Two important points

A process is spontaneous if and only if the total entropy of the system and the environment increases during the process.

A process is spontaneous if and only if the free energy of the system decreases during the process.

Free energy change of a system during a process is a measure of the change of entropy of both the system and the environment during the process. Any process which increases the entropy of the universe decreases the free energy of the system.

How does free energy work?

Imagine a reaction A + B = C + D

When A and B break bonds to form C and D depending on the types of the structure A, B, C, and D molecules there is either net generation of heat we call it exothermic reaction and say that the enthalpy change is -?H [ negative enthalpy change] and when there is heat absorption from outside, we call such reactions endothermic and the change + ?H [ positive enthalpy change].

Enthalpy – Entropy relation

Depending on the state of the product molecules as compared to reactant molecules there is always a change in the entropy, for example, if a solid reactant converts to liquid product there is an increase in entropy and if a gas converts to liquid there is a decrease in entropy. Whether it is an open or closed system there is no restriction of heat to exchange with surroundings. A close system is permeable to heat exchange.?Remember the reaction has generated a fixed amount of energy ?H positive or negative

Exothermic reaction [ -?H] and definition of free energy

These reactions increase the entropy of the surroundings by Q/T= ?S. Therefore, the system loses an amount of heat T?S to surrounding from its enthalpy ?H that makes the net energy available to the system which is free energy, dG = ?H - T?S.?Therefore, the dG is free the energy available to do work. In other words, free energy is energy that is available to the system to do work after providing energy for entropy change. When after providing energy for total entropy change dG is -ve the reaction is spontaneous.

Endothermic reaction [ +?H]

These reactions work on borrowed energy from the surrounding. They do not generate energy. If a reaction is endothermic (H positive) and the entropy change S is negative (less disorder), the free energy change dG = ?H – [ -T?S] is always +ve and the reaction is never spontaneous. Their borrowed free energy increases further, we express it as dG+

dG = 0.

When ?H= 0, T?S = 0, The system reaches equilibrium.?

Kinetics of chemical reactions

Chemical kinetics is the study of the rates of chemical reactions. Chemical kinetics studies the factors which influence the rates and explains the rates with respect to the reaction mechanisms of chemical processes.?Chemical kinetics and reactor design are of primary concern in the exploitation of chemical reactions in industrial production. From an economic point of view, it is also a crucial factor in the success or failure of a chemical plant.

Some key factors for kinetics of reactions

[1] Reactant concentrations, usually make the reaction happen at a faster rate if raised through increased collisions per unit time. Some reactions, however, have rates that are independent of reactant concentrations. These are called zero-order reactions.

[2] Surface area available for contact between the reactants, in particular solid ones in heterogeneous systems. Larger surface areas lead to higher reaction rates.

[3] Pressure – increasing the pressure decreases the volume between molecules and therefore increases the frequency of collisions between the molecules.

[4] Activation energy, the amount of energy required to make the reaction start and carry on spontaneously. Higher activation energy implies that the reactants need more energy to start than a reaction with lower activation energy.

[5] Temperature, hastens reactions if raised, since higher temperature increases the energy of the molecules, creating more collisions per unit time.

[6] Catalysts change the pathway of a reaction which in turn increases the speed of a reaction by lowering the activation energy needed for the reaction to take place.?

?Arrhenius established the concepts of activation energy and arrived at a relation between the rate of reaction and temperature based on many physical and chemical reactions. This relation is called the Arrhenius equation.

Arrhenius equation

K is rate constant, A is the pre-exponential factor, Ea is the activation energy, R is the universal gas constant and T is the absolute temperature.

What does the Arrhenius equation tell us?

If we neglect the A factor for the time being. What is " declining “here is not the concentration of a reactant as a function of time, but the magnitude of the rate constant K as a function of the exponent –Ea /RT. What is the significance of this quantity? Recalling that RT is the average kinetic energy, it becomes apparent that the exponent is just the ratio of the activation energy Ea to the average kinetic energy. In other words, -Ea/RT tells us the fraction of the collisions that have enough energy to overcome the activation energy barrier greater than or equal to activation energy Ea to break reactant bonds at temperature T. The larger this ratio, the smaller the rate (hence the negative sign) because of larger Ea. This means that high temperature and low activation energy favour larger rate constants, and thus speed up the reaction. Because these terms occur in an exponent, their effects on the rate are quite substantial.

Role of catalysts on reaction rate

The effect of a catalyst on the rate of a chemical reaction. Four criteria must be satisfied in order for something to be classified as a catalyst.

-Catalysts increase the rate of reaction.

-Catalysts are not consumed by the reaction.

-A small quantity of catalyst should be able to affect the rate of reaction for a large amount of reactant.

-Catalysts do not change the equilibrium constant for the reaction.

Credit: Google

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