Activation energy

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The activation energy in chemistry and biology is the energy you need a system before you can start a process. Activation energy is often used to denote the minimum energy required for a given chemical reaction to occur. For a reaction to occur between two molecules, they must collide in the correct orientation and possess a minimal amount of energy. As the molecules get closer, their electron clouds repel each other. This requires energy (activation energy) and comes from the heat of the system, that is to say from the translational, vibrational energy, etc. of each molecule.

Summary

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  • 1 Activation Energy
    • 1 Activated complex
    • 2 Energy
  • 2 Reaction Kinetics
  • 3 Speed ​​of reaction
  • 4 Reaction order
  • 5 Factors that affect the speed of reactions
    • 1 Temperature
    • 2 Physical State of the Reagents
    • 3 Presence of a catalyst
    • 4 Concentration of reagents
    • 5 Pressure
    • 6 Light
  • 6 Source
  • 7 External link

Activation energy

In 1888, the Swedish chemist Svante Arrhenius suggested that molecules must possess a minimal amount of energy to react. That energy comes from the kinetic energy of the colliding molecules. The kinetic energy is used to cause the reactions, but if the molecules move very slowly, the molecules will only bounce when they collide with other molecules and the reaction does not happen. For molecules to react, they must have a total kinetic energy that is equal to or greater than a certain minimum value of energy called activation energy (Ea). A collision with Ea energy or greater, makes the atoms of the molecules reach the transition state. But for the reaction to take place it is also necessary that the molecules are oriented correctly. The constant of the speed of a reaction (k) also depends on the temperature since the kinetic energy depends on it. .

Activated complex

Activated Complex

It is an intermediate structure between the reactants and the products, with intermediate links between the two reactants and the two products.

The activation energy of the reaction corresponds to the energy necessary for the reaction to take place with less energy from the reagents. The lower the activation energy of a reaction, the higher the speed of the reaction.

A reaction is called exothermic when it provides the medium with a higher energy than necessary to reach the activated complex.

When a reaction is endothermic, it provides the environment with lower energy than necessary to reach the activated complex.

Catalysts are substances that decrease the activation energy for a given reaction, without altering its ΔH. The catalysts are not altered during the reactions. In autocatalysis, one of the products of the reaction acts as a catalyst, at the beginning of the reaction it is slow with the formation of this, the speed gradually increases. In homogeneous catalysis , catalyst and reagents are in the same phase. In heterogeneous catalysis, catalyst and reagents are in different phases. Enzymes are catalysts that act in biological reactions and are generally quite specific and have optimal acting temperatures in the environment of 37º.

Energy

A particular example is the one that occurs in the combustion of a substance. By themselves, the fuel and the oxidizer do not produce a fire, a first input of energy is necessary to initiate self-sustained combustion. A small amount of input heat may suffice for a combustion to be triggered, making the input heat energy times the activation energy and therefore the activation energy is sometimes called the source of cosmos. According to the origin of this first contribution of energy, we classify it as:

  • Chemical: Exothermic chemical energy gives off heat, which can be used as an ignition source.
  • Electric: The passage of an electric current or a spark produces heat.
  • Nuclear: Nuclear fusion and fission produce heat.
  • Mechanical: By compression or friction, the mechanical force of two bodies can produce heat.

The occurrence of a chemical reaction is necessarily related to the contact between reactive molecules and to a minimum necessary energy. This minimum energy for the reaction event is called as activation energy.

If the energy is sufficient, the repulsion is overcome and the molecules get close enough for a rearrangement of the bonds of the molecules to take place. The Arrhenius equation provides the quantitative basis for the relationship between the activation energy and the rate at which the reaction occurs. The study of reaction rates is called chemical kinetics.

Reaction Kinetics

The object of chemical kinetics is to measure the rates of chemical reactions and to find equations that relate the rate of a reaction to experimental variables. It is experimentally found that the speed of a reaction depends largely on the temperature and concentrations of the species involved in the reaction.

In simple reactions, only the concentration of the reagents affects the reaction rate, but in more complex reactions the speed may also depend on the concentration of one or more products. The presence of a catalyst also affects the reaction rate; in this case you can increase your speed. From studying the speed of a reaction and its dependence on all these factors, much can be known about the detailed steps to go from reagents to products.

Reactions can be kinetically classified into homogeneous and heterogeneous. The first occurs in one phase and the second occurs in more than one phase. The heterogeneous reaction depends on the surface area of ​​either the vessel wall or a solid catalyst. Homogeneous reactions are discussed in this chapter.

Rapid reaction

Speed ​​is the speed of things. Something fast is fast and so. The speed (or speed) of reaction is made up of the speed of formation and the speed of decomposition. This speed is not constant and depends on several factors, such as the concentration of the reagents, the presence of a catalyst, the reaction temperature and the physical state of the reagents. One of the most important factors is the concentration of the reagents. The more particles there are in a volume, the more collisions there are between the particles per unit time. At the beginning, when the concentration of reagents is greater, the probability of collisions between the molecules is also greater, and the speed is greater. As the reaction progresses, as the concentration of the reagents decreases, the probability of collision decreases and with it the speed of the reaction. The measurement of the speed of reaction implies the measurement of the concentration of one of the reagents or products over time, that is, to measure the speed of a reaction we need to measure, either the amount of reagent that disappears per unit of time , or the amount of product that appears per unit of time. The reaction speed is measured in concentration / time units, that is, in (mol / l) / s, that is, moles / (l · s). or the amount of product that appears per unit of time. The reaction speed is measured in concentration / time units, that is, in (mol / l) / s, that is, moles / (l · s). or the amount of product that appears per unit of time. The reaction speed is measured in concentration / time units, that is, in (mol / l) / s, that is, moles / (l · s).

 

Reaction order

For each reaction, an equation can be formulated, which describes how many reagent particles react with each other, to form a quantity of product particles. For a reaction of the form:

2A + B + C + D ——— E

This means that two particles A collide with a particle B, a particle C and a particle D to form the product E.

However, the probability that five particles collide at the same time and with sufficient energy is slim.

More likely it is that two or three particles collide and form an intermediate product, this intermediate product collides with the other particles and forms other intermediate products until forming product E, here is an example:

2A —— A 2

A 2 + B + C ——- A 2 BC

A 2 BC + D ——– E

The decomposition of the main reaction into so-called elemental reactions and the analysis of these shows us exactly how this reaction occurs. Using experimental methods or by premises, the dependence of the speed of the elemental reactions on the concentrations of components A, B, C and D can be determined. The order of reaction is defined as the sum of the exponents of the concentrations in the law of speed of reaction. This is also called the total order of reaction, since the order depends on the reagent being analyzed. The order of the reactions is determined experimentally. Assuming that the reaction speed of the first elemental reaction has a quadratic dependence on the concentration of reagent A, this means that this reaction is second order with respect to reagent A. The total order of this reaction is also second, since there are no other reagents. Assuming that the reaction rate of the second elemental reaction has a linear dependence on the concentration of reagent A2 , linear with the concentration of the reactant B and no dependency with C. Then it is the reaction of first order in relation to A2, of first order in relation to B and of zero order in relation to component C. The total order is second. Assuming that the reaction speed of the third elemental reaction has a linear dependence with the concentration of A 2 BC, but none with the concentration of D, then it is the reaction of the first order in relation to A 2 BC and of zero order in relation to a D. The total order of the reaction is first. For a hypothetical reaction of the form:

aA + bB —— gG + hH

reaction speed is defined as:

r = k [A] a [B] b

(Reagent concentrations are raised to their corresponding kinetic coefficient only in the case where the reaction is elemental). Where the square brackets denote the concentration of each of the species; “r” denotes the rate of reaction and “k” is the rate constant. The speed of chemical reactions covers very wide time scales. For example, an explosion can occur in less than a second; cooking a food can take minutes or hours

Factors that affect the speed of reactions

There are several factors that affect the speed of a chemical reaction: the concentration of the reagents, the temperature, the existence of catalysts and the contact surface of both the reagents and the catalyst. Catalysts can increase or decrease the reaction rate.

Temperature

As a general rule, the reaction speed increases with temperature because increasing it increases the kinetic energy of the molecules. With higher kinetic energy, the molecules move faster and collide more frequently and with more energy. The behavior of the speed constant or kinetic coefficient against temperature = lnA – (Ea / R) (1 / T2 – 1 / T1) This linearized equation is very useful and can be described through the Arrhenius equation K = Aexp (- EA / RT) where K is the constant of the speed, A is the frequency factor, EA is the necessary activation energy and T is the temperature, when linearizing it we have that the Neperian logarithm of the speed constant is inversely proportional to temperature, as follows: ln (k1 / k2) when calculating activation energy experimentally, since the slope of the line obtained by graphing the aforementioned law is: -EA / R, by doing a simple clearance, this activation energy is easily obtained, taking into account that the value of the universal gas constant is 1.987cal / K mole. For a good number of chemical reactions, the speed doubles approximately every ten degrees Celsius

Physical State of the Reagents

If reactants interact in different phases in a reaction, their contact area is less and their speed is also less. On the other hand, if the contact area is greater, the speed is greater. As the reagents are in different phases, new kinetic factors appear to be analyzed. The part of the chemical reaction, that is, the transport speed must be studied, since in most cases these are much slower than the intrinsic speed of the reaction and it is the transport stages that determine the kinetics of the process . There is no doubt that a larger contact area reduces the resistance to transport, but the diffusivity of the reagent in the medium, and its solubility, are also very important, since this is the limit of the reagent concentration, and is determined by the balance between phases.

Presence of a catalyst

Catalysts increase or decrease the speed of a reaction without transforming. They tend to worsen the selectivity of the process, increasing the obtaining of unwanted products. The form of action of the same is modifying the reaction mechanism, using elemental steps with more or less activation energy. There are homogeneous catalysts, which are in the same phase as the reactants (for example, iron III in the decomposition of hydrogen peroxide) and heterogeneous catalysts, which are in different phases (for example, the platinum mesh in the reactions of hydrogenation). Catalysts can also slow down reactions, not just speed them up, in this case they are often known as retarders or inhibitors, which impede production.

Reagent concentration

Most reactions are faster in the presence of a catalyst and the more concentrated the reagents are, the greater the frequency of collision.

r = K [A] m [B] m

Obtaining an equation that can be used to predict the dependence of reaction speed on reagent concentrations is one of the basic objectives of chemical kinetics. That equation, which is determined empirically, is called the speed equation. In this way, if we consider the hypothetical reaction again, the reaction speed “r” can be expressed as The terms in square brackets are the molarities of the reactants and the exponents m and n are coefficients that, except in the case of an elementary stage, do not have to be related to the stoichiometric coefficient of each of the reagents. The values ​​of these exponents are known as the order of reaction. There are cases in which the reaction speed is not a function of the concentration,

Pressure

In a chemical reaction, if there is a higher pressure in the system, it will vary the kinetic energy of the molecules. So if there is a higher pressure, the kinetic energy of the particles will increase and the reaction will become faster; as in gases, which with increasing pressure also increases the movement of its particles and, therefore, the reaction speed is greater.

Light

Light is a form of energy. Some reactions, when illuminated, occur more quickly, as occurs in the case of the reaction between chlorine and hydrogen. In general, light pulls electrons from some atoms to form ions, greatly increasing the reaction rate.

 

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