What Must Happen Before a Chemical Reaction Can Begin

Activation Free energy

An activation energy determines the rate of reaction past requiring energy to occur.

Key Takeaways

Ability of activation

The catalytic process

In a state of transition

Chemical reactions, and almost all biochemical reactions, do not occur spontaneously and require an initial input of free energy (called activation energy) to get started.An analysis of both endogenous and exerganic reactions must accept into account activation free energy.The exhaustion of energy occurs as a consequence of exergonic reactions. All the same, their energy-releasing steps need a small input of free energy to occur.The minor amount of energy required for all chemical reactions to occur is chosen the activation energy (or free free energy of activation) and is abbreviated as EA.

.The horizontal axis of this diagram describes the sequence of events over fourth dimension.

Activation Energy in Chemical Reactions

Is it possible that an free energy-releasing, negative G reaction would really need energy to proceed?.There are several steps that must have identify during the reaction.The formation of new chemic bonds occurs during chemical reactions.As one example, when glucose molecules are cleaved downward, bonding between atoms of carbon is broken.The broken bonds release free energy.To force the bonds to break, the molecules must be contorted somewhat.A minor amount of energy is needed to attain this contorted state, chosen the transition country. Information technology is an unstable state with a loftier energy level.

In some cases, cells can couple exergonic reactions with endergonic reactions (Delta ext[G]lt0), allowing them to keep.Energy coupling is the modify from 1 reaction to another that occurs spontaneously.During the endergonic reaction, costless energy released by the exergonic reaction is captivated.The utilise of ATP for energy coupling includes a transmembrane ion pump, which is essential for the function of cells.

Gratis Energy Diagrams

Energy profiles for a given reaction are illustrated past gratuitous free energy diagrams.Depending on whether the reaction is exergonic (*G0), the products will exist at a lower or higher energy level than the reactants.As a result, the activation energy is contained of the reaction’s ΔG.In other words, at a given temperature, activation energy depends on the type of chemical transformation taking place, not on the relative energy country of the reactants and products.

Although the image above discusses activation free energy in the context of an exergonic forward reaction, the same principles employ to the reverse reaction, which must be endergonic.The activation energy for the reverse reaction is much higher than the one for the forward reaction.

Heat Energy

A reaction is typically pushed forward by heat energy generated from the surrounding surroundings.The heat free energy in a chemical reaction increases the frequency and force with which molecules collide past increasing the total bail free energy between reactants and products.A slight shift in atoms and bonds within the molecule as well helps to reach the transition country.Thus, heating up a system will result in chemical reactants in that system reacting more frequently.Force per unit area practical to the system will have the same upshot.In society for a reaction to continue, reactants must absorb enough heat free energy from their surroundings to achieve a transition state.

A reaction’s activation energy determines how rapidly information technology will go along.If the activation energy is high, it ways the chemic reactions will proceed more than slowly.For example, rust on iron illustrates that inherently slow reaction.The high coefficient of friction causes it to work slowly over time.The burning of many fuels, which are strongly exergonic, occurs at a negligible rate unless the activation energy of the fuel is overridden by sufficient estrus from a spark.As they burn, however, the chemic reactions release enough energy to keep the combustion process going, supplying the activation energy for surrounding fuel molecules.

The activation energy for most cellular reactions is too high for heat free energy to overcome efficiently. .Cells capeesh this because it is adept for them.There is considerable energy stored by macromolecules, such as proteins, DNA, and RNA, and their breakdown is exergonic.

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The Arrhenius Equation

The Arrhenius equations link the rate of a chemic reaction to the activation free energy of the reaction:

Ext[k] = ext[A]*/ext[E]/ext[RT]


The Collision Theory

Past appealing to the principle of collision, standoff theory accounts for the qualitative nature of chemical reactions and the charge per unit at which they occur.

Learning Objectives

Standoff theory considers activation free energy, collisions, and molecular orientation

Key Takeaways

Energies of activation

This theory provides a qualitative explanation of chemical reactions and how they occur.The collision theory proposes that molecules must collide in society to react.Analyzing the ordinary reaction mechanism follows this fundamental rule.

In the elementary bimolecular reaction, ext{A} + ext{B}

Molecules A and B need to be in sufficiently close proximity for chemic bonds to burst in order for them to react.This is a standoff.Equally long as A and B are both gases, the frequency of collisions between them will be proportional to the concentration of each gas.Doubling A concentration will double the frequency of A-B collisions, and doubling B concentration will take the same effect.Therefore, according to standoff theory, the rate at which molecules collide volition have a significant impact on the overall reaction rate.

Activation Energy and Temperature

Billiard balls simply collide and bounce off each other.The aforementioned happens when two molecules, A and B, come up into contact: they bounciness off each other, completely unchanged and unaffected.In order for a collision to event in a chemical reaction, A and B must collide with enough energy to cause chemical bonding to break.Chemical bonds between reactants and products are shattered during any chemical reaction.Thus, in lodge to effectively initiate a reaction, the reactants must motility fast enough (with sufficient kinetic energy) so that they collide with enough force to cause bonds to dissolve.The activation energy is the minimum amount of movement required for a chemical reaction to occur when molecules collide.

Gas kinetic energy is straight proportional to temperature as we know from the kinetic theory of gases.The molecules in a gas option up energy and move faster as the temperature increases.Due to this, when molecules collide, the college the probability of them moving with enough activating energy for a reaction to occur.

Molecular Orientation and Effective Collisions

If 2 molecules collide with sufficient activation energy, the collision does not necessarily succeed.In reality, even if molecules are moving with enough energy, collisions are not ever successful.In society to accomplish this, molecules demand to collide in an orientation that allows the electrons to line up with each other, and bonds to intermission and re-form successfully.For case, in the gas-phase reaction of dinitrogen oxide with nitric oxide, the oxygen end of N2O must hit the nitrogen end of NO; if either molecule is not aligned correctly, no reaction occurs.Nevertheless, because molecules in the liquid and gas phases are forever moving randomly, it is always possible for two molecules to collide in just the right way for a reaction to unfold.

Equally a full general dominion, the more disquisitional this orientational requirement, such as it is for larger or more complex molecules, the fewer collisions volition be constructive.A collision that occurs as a result of sufficient free energy and proper orientation is called an effective collision.


For chemical reactions to occur, the collision theory states that the following conditions must exist met:

Fundamental Takeaways


Energy that activates

Reactant Concentrations

Reactions go faster when reactant concentrations increase. .When concentration increases, the number of molecules with the minimum required energy will increment, which therefore increases the rate of the reaction.1 in a million particles may have sufficient activation free energy, causing only 100 particles out of 100 million to be activated.200 million of these particles volition, notwithstanding, react if they are all contained in the aforementioned volume.Doubling the concentration as well doubles the rate of reaction.

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Surface Area

.Due to the fact that only the liquid-solid interface, which is on the surface of the solid, is capable of bumping into each other.Molecular interactions between solid molecules within the solid body cannot occur.The increase in area of the solid will expose more solid molecules to the liquid, which enables a faster reaction.

Consider, for example, a brick with dimensions of 6 x 6 x 2 inches.The exposed surfaces of the brick have a surface expanse of 4 (6 x 2) + 2 (half dozen x half-dozen)=120; ext{cm}^2.Nevertheless, when the brick is divided into nine smaller cubes, each cube has a expanse of 6(two imes ii) = 24 ext[cm]*2, meaning the full surface expanse is 9 imes 24 = 216 ext{cm}^2.

Past dividing a large torso into smaller pieces, the total exposed expanse increases.Thus, since a reaction happens on the surface of a substance, increasing the expanse would increase the amount of substance capable of reacting, and thus increase the rate of the reaction.


For a reaction involving gases, increasing the pressure will increase the rate of reaction. .Thus, increasing pressure increases concentration (n/V), and promotes more frequent collisions between the molecules of gas.


Experiments have shown that increasing the temperature past 10 °C triples or doubles the speed of bacterial reactions between molecules.In general, the activation free energy required to initiate a reaction remains constant as temperature increases.The boilerplate increase in particle kinetic energy caused past the absorbed heat, still, gives a larger percentage of the reactant molecules the minimum energy needed to collide and react.Increased temperature causes molecules involved in the reaction to have college energy levels, and so the rate of reaction increases.The charge per unit of reaction volition likewise decrease as the temperature decreases.

Presence or Absence of a Goad

Catalysts piece of work past lowering the activation energy necessary to initiate a reaction, which increases the reaction’s rate.Equally a catalyst is not damaged during a reaction, you can reuse it.Normally, H2 and O2 practice not combine.In the presence of platinum, which acts equally a catalyst, they combine, causing the reaction to occur rapidly.

Nature of the Reactants

Chemical transformation rates vary dramatically betwixt substances.Different reactions may have different reactivity depending on the structure of the materials involved; for example, how the substances are incorporated into the solid or liquid state.Additionally, the relative bond strength within the molecules of the reactants plays a role.Molecules that are atoms are strongly bonded past covalent bonds, so their reactions will proceed at a slower rate than molecules with weakly bonded atoms.Because the bonds between molecules with potent bonds are more difficult to interruption, it requires more free energy.

The Arrhenius Equation

Temperature-dependent reactions are described by the Arrhenius equation.

Learning Objectives

What are the variables in the Arrhenius equation and what does it mean?

Key Takeaways

The Decline in Expansion

Arrhenius’ equation gives a simple, but shockingly authentic, formula for the temperature dependence of the reaction rate abiding, which determines a chemical reaction’southward rate.Svante Arrhenius first formulated the equation in 1884.After five years, in 1889, the Dutch chemist J.Hendrik.van ’t Hoff provided a physical caption for the miracle.Among the most important relationships in chemistry, the activation free energy equation is derived from the Boltzmann distribution law and the activation free energy concept:

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

For now, ignore the A cistron and focus on the meaning of this equation.This is an awarding of the exponential decay law.”Decomposable” here is non a function of the concentration, just rather of the magnitude of the rate constant.

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What does this number mean?.Yous will notice that RT is the average kinetic energy, and that its exponent is simply the ratio of Ea to the average kinetic energy.A higher ratio indicates a lower rate, which is why the sign in the equation is negative.High temperatures and low activation energies favor large rate constants, increasing the speed of a reaction.

Plotting the Arrhenius Equation in Non-Exponential Form

.As a upshot, separating the exponential and pre-exponential terms in each side results in the following: ext[ln]( ext[yard])=ext[ln]( ext[A])-rac[ ext[Eastward]_[ ext[a]]][ ext[RT]

When plotting the slope of ln(grand) against 1/T we become a direct line with the slope –Ea/R.

It is possible to determine activation energy from the values of 1000 observed at unlike temperatures using this approach.By plotting ln(chiliad) versus i/T, nosotros tin determine the slope required for Ea.

The Pre-Exponential Gene

Arrhenius’ equation contains a pre-exponential factor A. .This fraction is dependent on the magnitude of Ea and the temperature, and can range from zero, where no molecules tin can react, to unity, where all molecules have enough energy to react.

The Arrhenius law reduces to k = A if the fraction is unity.Therefore, A represents the maximum possible charge per unit constant, which is what the charge per unit abiding would be if every collision between molecules resulted in a chemical reaction.Only if either the activation free energy would be cipher or if all molecules had kinetic energy greater than Ea would this happen, which is highly unlikely.”Barrier-less” reactions accept been observed, but these are rare, and even in these cases, molecules must collide with the right orientation to crusade the reaction.When molecules collide in real-life, not every collision volition exist an constructive collision, and the value of ext[e]*[ rac[- ext[E]_ ext[a]][ ext[RT]] volition be less than ane.

Transition Land Theory

Key Takeaways

Transient State Theory

Complex activation

A transition state theory describes a hypothetical “transition state” that occurs betwixt the reactants and the products during a chemical reaction.The species that forms during the transition state is called the activated complex.This theory describes how chemical reactions occur based on standoff theory.With the charge per unit constant of a reaction, TST tin calculate the standard enthalpy of activation, standard entropy of activation, and standard Gibbs energy of activation.The TST is also known equally “activated-complex theory,” “absolute-rate theory,” or “theory of accented reaction rates.”

Postulates of Transition Country Theory

In transition state theory, there is an intermediate state between the state in which a molecule exists as a reactant and the state in which it exists as a production.When the transition state occurs, an activated complex forms, a college-energy species.Three main factors determine whether or not a reaction takes place according to TST.Among them are:

In a sense, this third postulate qualifies what we discussed in our section on collision theory.Collisions involving molecules with sufficient energy and the correct orientation consequence in reactions, according to collision theory.In transition state theory, nonetheless, a successful standoff won’t necessarily upshot in the formation of products, but rather the activation of the complex.Afterwards the activated complex is formed, it can be used to make products or reverted dorsum to reactants.

Applications in Biochemistry

Biochemistry is about often used to model enzyme-catalyzed processes in the body by using transition state theory.The noesis of the possible transition states which can form in a given reaction, together with data on the activation energies of the transition states, tin be used to predict the class of a biochemical reaction, too every bit to make up one’s mind its reaction charge per unit and charge per unit constant.

What Must Happen Before a Chemical Reaction Can Begin

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