Energy and Chemical Reactions

All chemical reaction carry with him a change in energy, due to the transformation of the substances that participate in it. Energy can manifest itself in various ways:

• Hot
• Internal energy
• Activation energy

Heat in chemical reactions

The molecules of chemical compounds they are formed by links that carry an energy included, which holds the atoms together. When a chemical reaction occurs, the participating molecules undergo the breaking some of these links, which causes a variation in energy. It usually appears as a change in heat.

The hot in chemical reactions it is measured by means of the Enthalpy (H), which is a thermodynamic quantity that describes the thermal changes brought to constant pressure. It is measured in calories per mole (cal / mol), and is calculated for each compound of the reaction, with the following formula:

ΔH = mCpΔT

Where:

ΔH: change in enthalpy of the substance

m: mass of the substance participating in the reaction

Cp: ​​specific heat at constant pressure, of the substance

ΔT: temperature change in the reaction

If they participate in the chemical reaction elements, their enthalpy is considered 0 because no energy has been invested in forming them.

For a complete reaction, the form of which is:

2A + B -> 3C + D

The enthalpy will result from doing a subtraction:

Enthalpy of reaction = Enthalpy of products - Enthalpy of reactants

ΔH_reaction = ΔH (3C + D) - ΔH (2A + B)

Each of the enthalpies will carry the coefficient with which the substance acts in the reaction (the number of moles. For A, in this case, it is 2, and it is going to multiply the value of its enthalpy.

For example, for the propane combustion reaction:

C₃H₈(g) + 5O₂(g) -> 3CO₂(g) + 4H₂O (l)

ΔHC₃H₈ = -24820 cal / mol

ΔHOR₂ = 0 cal / mol

ΔHCO₂ = -94050 cal / mol

ΔHH₂O = -68320 cal / mol

Enthalpy of reaction = Enthalpy of products - Enthalpy of reactants

ΔH_reaction = [3 (-94050 cal / mol) + 4 (-68320 cal / mol)] - [-24820 cal / mol + 5 (0)]

ΔH_reaction = [-282150 + (-273280)] – (-24820)

ΔH_reaction = -555430 + 24820

ΔH_reaction = -530610 cal / mol

Types of chemical reactions according to heat

Chemical reactions are going to be classified into two types according to the heat involved in them:

• Exothermic reactions
• Endothermic reactions

The exothermic reactions are those in which, during the interaction, the substances have released heat. This is the case, for example, of a strong acid that comes into contact with water. The solution warms up. It also occurs in the combustion of hydrocarbons, which release heat in the form of fire, accompanied by carbon dioxide CO₂ and water vapor H₂OR.

The endothermic reactions are those in which, to begin to react, the reactants must receive heat. It is from a certain heat that the products begin to be generated. This is the case, for example, of the generation of nitrogen oxides, for which there must be a large amount of heat in the process for oxygen and nitrogen to unite in a compound.

Internal energy in chemical reactions

The internal energy (U, E) of a substance is the sum of the kinetic and potential energies of all its particles. This magnitude intervenes in the chemical reactions in the enthalpy calculations:

ΔH = ΔU + PΔV

This enthalpy formula is based on the first law of thermodynamics, which is written:

ΔQ = ΔU - ΔW

Where:

Q: heat from a thermodynamic system (which can be a chemical reaction). It is measured in calories per mole, just like enthalpies.

OR: Internal energy of the thermodynamic system.

W: Mechanical work of the thermodynamic system, and is calculated with the product of the pressure and the change in volume (PΔV).

Activation energy in chemical reactions

The activation energy is that amount of energy that will determine the beginning of chemical reactions, as follows:

• If the activation energy is too short, the reaction will be spontaneous, that is, it will start on its own and the reagents will transform just by coming into contact.
• If the activation energy it is low, you will need to add some energy to the reagents for them to begin to interact.
• If the activation energy is high, enough energy will have to be invested for the reaction to take place.
• If the activation energy it is very high, we will have to resort to the so-called catalysts, to make it more accessible.

The catalysts They are chemical substances that do not participate in chemical reactions transforming, but are responsible for accelerating them, decreasing activation energy so that the reactants start to become products.

A spontaneous reaction is, for example, one found in human metabolism: spontaneous decarboxylation of acetoacetate to become acetone, in the way of synthesis of ketone bodies. It does not need enzymes to be carried out.

Chemical equilibrium and LeChatelier's Law

LeChatelier's Law is the one that governs equilibrium in chemical reactions, and it says:

"Any stimulus given to a chemical reaction in equilibrium will make it respond by counteracting it, up to a different point of equilibrium"

LeChatelier's Law can be described according to the variables pressure, volume and concentration:

• Whether increase the pressure to the reaction, it will be directed to where less moles are generated, either towards the reactants or towards the products.
• Whether reduce pressure to the reaction, this will go to where more moles are generated, either towards the reactants or towards the products.
• Whether increase the temperature to the reaction, it will go to where the heat is absorbed (endothermic reaction), either in the direct way (from reactants to products) or in the reverse way (from products to reactants).
• Whether reduce the temperature To the reaction, it will go to where the heat is released (exothermic reaction), either in the direct way (from reactants to products) or in the reverse way (from products to reactants).
• Whether increases the concentration of a reagent, the reaction will be directed to generate more products.
• Whether reduces the concentration of a product, the reaction will be directed to generate more reagents.

Factors that modify the speed of a reaction

The speed of a reaction is the concentration of the reactants (in mol / liter) that is consumed for each unit of time.

There are six factors that influence this speed:

• Concentration
• Pressure
• Temperature
• Contact surface
• Nature of reagents
• Catalysts

The concentration is the amount of reagent for each unit of volume (mol / liter). If an amount is added, the reaction will respond by generating products more quickly.

The Pressure it only affects if the reactants and products are gases. The reaction will respond according to the LeChatelier Law.

The temperature favors reactions depending on whether they are endothermic or exothermic. If it is endothermic, an increase in temperature will speed up the reaction. If it is exothermic, a reduction in temperature will drive it.

The contact surface It helps the reagent particles to be better dispersed among themselves, so that the reaction is accelerated and the products are reached faster.

The nature of reagents, which consists of its molecular structure, determines the rate of the reaction. For example, acids like hydrochloric acid (HCl) are immediately neutralized, even aggressively, by bases like sodium hydroxide (NaOH).

The catalysts They are chemical substances that are not involved in the reaction, but that are responsible for accelerating or delaying the interaction of the reactants. They are marketed in a physical shape that offers a good contact area.

Examples of energy in chemical reactions

The heats of combustion of various chemicals are shown below:

Methane: CH₄ + 2O₂ -> CO₂ + 2H₂OR

ΔH = -212800 cal / mol (Gives off heat, it is exothermic)

Ethane: C₂H₆ + (7/2) O₂ -> 2CO₂ + 3H₂OR

ΔH = -372820 cal / mol (Gives off heat, it is exothermic)

Propane: C₃H₈ + 5O₂ -> 3CO₂ + 4H₂OR

ΔH = -530600 cal / mol (Gives off heat, it is exothermic)

Butane: C₄H₁₀ + (13/2) O₂ -> 4CO₂ + 5H₂OR

ΔH = -687980 cal / mol (Gives off heat, it is exothermic)

Pentane: C₅H₁₂ + 8O₂ -> 5CO₂ + 6H₂OR

ΔH = -845160 cal / mol (Gives off heat, it is exothermic)

Ethylene: C₂H₄ + 3O₂ -> 2CO₂ + 2H₂OR

ΔH = -337230 cal / mol (Gives off heat, it is exothermic)

Acetylene: C₂H₂ + (5/2) O₂ -> 2CO₂ + H₂OR

ΔH = -310620 cal / mol (Gives off heat, it is exothermic)

Benzene: C₆H₆ + (15/2) O₂ -> 6CO₂ + 3H₂OR

ΔH = -787200 cal / mol (Gives off heat, is exothermic)

Toluene: C₇H₈ + 9O₂ -> 7CO₂ + 4H₂OR

ΔH = -934500 cal / mol (Gives off heat, it is exothermic)

Ethanol: C₂H₅OH + 3O₂ -> 2CO₂ + 3H₂OR

ΔH = -326700 cal / mol (Gives off heat, it is exothermic)