Enthalpies for Different Types of Reactions

 ENTHALPIES FOR DIFFERENT TYPES OF REACTIONS

Standard Enthalpy of Combustion

The standard enthalpy of combustion is defined as the amount of heat released when one mole of a substance is completely burnt in excess oxygen under standard conditions, with all products being in their standard states. The standard enthalpy of combustion is denoted by ∆cH° and has units of kJ/mol.

For example, the standard enthalpy of combustion of methane (CH4) is -890.4 kJ/mol, which means that when one mole of methane is burned completely in excess oxygen under standard conditions, it releases 890.4 kJ of heat energy.

The standard enthalpy of combustion can be calculated using Hess's law by combining the enthalpy changes of the combustion reaction with other reactions to get the desired overall reaction. It is a useful measure of the energy content of a substance and is often used in industry to calculate the energy released from the combustion of fuels.

Enthalpy of Atomization

The enthalpy of atomization is the enthalpy change that takes place when one mole of gaseous atoms is formed from the element in its standard state. This process involves breaking all the bonds in one mole of the element in its standard state and converting it to individual gaseous atoms. For example, the enthalpy of atomization of hydrogen is the energy required to break the H-H bond and convert one mole of H2 gas into two moles of gaseous hydrogen atoms (H).

The enthalpy of atomization is a measure of the strength of the bonds between atoms in a molecule. The stronger the bond, the higher the energy required to break it and the higher the enthalpy of atomization. The enthalpy of atomization is an important factor in determining the reactivity of an element or molecule, as it indicates the energy required to break the bonds and form new ones.

The enthalpy of atomization is always an endothermic process, as energy is required to break the bonds. The value of the enthalpy of atomization can be experimentally determined by measuring the energy required to break the bonds and form the gaseous atoms, or it can be calculated theoretically using quantum mechanics calculations.

Bond Enthalpy

Bond enthalpy, also known as bond dissociation energy, is the amount of energy required to break one mole of a specific chemical bond in a gaseous molecule under standard conditions (25°C and 1 atm pressure) into its component atoms. It is usually represented by the symbol ΔH°(bond).

Bond enthalpies are positive values because energy is required to break a bond. The higher the bond enthalpy, the stronger the bond between the two atoms.

Bond enthalpies can be used to calculate the enthalpy change of a reaction involving the breaking and forming of bonds. The enthalpy change of a reaction can be calculated by subtracting the sum of the bond enthalpies of the bonds broken from the sum of the bond enthalpies of the bonds formed.

For example, consider the reaction:

CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l)

 

The bonds broken are four C-H bonds and two O=O bonds, and the bonds formed are two C=O bonds and four O-H bonds. The enthalpy change of the reaction can be calculated as follows:

 

ΔH°(rxn) = ΣΔH°(bonds broken) - ΣΔH°(bonds formed) ΔH°(rxn) = (4 x ΔH°(C-H)) + (2 x ΔH°(O=O)) - (2 x

ΔH°(C=O)) - (4 x ΔH°(O-H))

 

By looking up the bond enthalpies for the various bonds involved in the reaction, we can substitute in the values and calculate the enthalpy change of the reaction.

Lattice Enthalpy

Lattice enthalpy is the enthalpy change that occurs when one mole of a solid ionic compound is formed from its gaseous ions under standard conditions. It is also known as lattice energy or lattice dissociation energy.

When ionic compounds form, their constituent ions combine to form a three-dimensional structure called a crystal lattice. This process releases energy, which is the lattice enthalpy. The magnitude of the lattice enthalpy depends on the charges of the ions, their sizes, and the arrangement of the ions in the lattice.

For example, the lattice enthalpy of NaCl is the energy required to separate one mole of solid NaCl into its gaseous ions, Na+ and Cl-. This is an endothermic process and requires energy input. The lattice enthalpy is defined as negative because energy is released when the solid NaCl is formed from its gaseous ions.

Lattice enthalpy is an important concept in understanding the properties of ionic compounds, such as their melting and boiling points, solubility, and stability. It is also used to calculate other thermodynamic quantities, such as the Born-Haber cycle and the enthalpy of solution.

Born Haber Cycle

The Born-Haber cycle is a theoretical model that relates the lattice energy of an ionic solid to the enthalpy change of formation of the solid from its constituent elements. It is named after two scientists, Max Born and Fritz Haber, who developed the concept in the early 20th century.

The Born-Haber cycle involves a series of hypothetical steps that ultimately lead to the formation of an ionic solid from its constituent elements. These steps include:

  1. Formation of gaseous metal atoms: This step involves the sublimation of the metal, which requires an input of energy.
  2. Formation of gaseous non-metal atoms: This step involves the dissociation of diatomic non-metal molecules, such as Cl2 or O2, which also requires an input of energy.
  3. Ionization of the metal atoms: This step involves the removal of one or more electrons from the metal atoms, which requires an input of energy.
  4. Electron affinity of the non-metal atoms: This step involves the addition of one or more electrons to the non-metal atoms, which releases energy.
  5. Formation of gaseous metal ions and non-metal ions: This step involves the combination of the metal ions and non-metal ions to form the ionic solid. The lattice energy, which is the energy required to separate the ions in the solid, is released in this step.

The overall enthalpy change for the formation of the ionic solid can be calculated by summing the enthalpy changes for each step in the cycle. The cycle is designed in such a way that the overall enthalpy change is equal to the enthalpy of formation of the solid from its constituent elements.

The Born-Haber cycle can also be used to calculate the lattice energy of an ionic solid if the enthalpy of formation and other relevant enthalpy changes are known. This is because the lattice energy is equal in magnitude but opposite in sign to the enthalpy of formation, and so can be calculated by rearranging the cycle to solve for the lattice energy.

 lattice enthalpy of Na+Cl(s) by following steps given below

 

Enthalpy of Solution

Enthalpy of solution is the amount of heat absorbed or released when a solute is dissolved in a solvent to form a solution at constant pressure and temperature.

The enthalpy of solution can be either endothermic (positive) or exothermic (negative), depending on whether heat is absorbed or released during the process of dissolution.

The enthalpy change (∆H) of the solution process can be determined experimentally by measuring the temperature change that occurs when the solute is dissolved in the solvent.

The equation for the enthalpy change of solution can be written as:

∆H = H(solution) - [H(solute) + H(solvent)]

where H(solution) is the enthalpy of the solution, H(solute) is the enthalpy of the solute, and H(solvent) is the enthalpy of the solvent.

If the enthalpy of the solution is negative, it means that heat is released during the process of dissolution, and the solution process is exothermic. If the enthalpy of the solution is positive, it means that heat is absorbed during the process of dissolution, and the solution process is endothermic.

The enthalpy of solution can be used to calculate the solubility of a solute in a solvent, and can also be used to predict the behavior of solutions under different conditions.

Enthalpy of Dilution

Enthalpy of dilution is the change in enthalpy when a solute is dissolved in a solvent to form a solution of lower concentration. This process can either be exothermic or endothermic depending on the nature of the solute and solvent.

The enthalpy of dilution can be calculated using the equation:

ΔHdil = ΔHsoln - ΔHmix

Where ΔHdil is the enthalpy of dilution, ΔHsoln is the enthalpy change when the solute dissolves in the solvent, and ΔHmix is the enthalpy change when the solute and solvent are mixed together.

If the solute-solvent interactions are stronger than the solute-solute and solvent-solvent interactions, then the enthalpy of dilution will be negative, indicating an exothermic process. On the other hand, if the solute-solute and solvent-solvent interactions are stronger than the solute-solvent interactions, then the enthalpy of dilution will be positive, indicating an endothermic process.

Enthalpy of dilution is an important concept in many chemical and industrial processes, such as the preparation of solutions for chemical reactions, the purification of solvents, and the production of pharmaceuticals.

CALORIMETER

Calorimetry is a technique used to measure the heat exchange between a system and its surroundings. This technique is used to determine the change in internal energy (∆U) or enthalpy (∆H) of a chemical reaction. Calorimetry involves the use of a calorimeter, which is a device designed to isolate the reaction from its surroundings and measure the heat released or absorbed by the reaction.

There are two main types of calorimeters: constant pressure calorimeters and constant volume calorimeters.

A constant pressure calorimeter, also known as a bomb calorimeter, is used to measure the heat of combustion reactions. The reaction takes place in a bomb calorimeter, which is a sealed container surrounded by a water jacket. The temperature of the water is measured before and after the reaction, and the change in temperature is used to calculate the heat of combustion. Since the reaction takes place at constant volume, the heat measured is the internal energy change (∆U) of the reaction.

A constant volume calorimeter, also known as a coffee cup calorimeter, is used to measure the heat of reactions that take place in solution. The reaction takes place in a container surrounded by a water jacket. The temperature of the water is measured before and after the reaction, and the change in temperature is used to calculate the heat of reaction. Since the reaction takes place at constant pressure, the heat measured is the enthalpy change (∆H) of the reaction.

Calorimetry is an important tool in thermodynamics and is used to measure the energy changes associated with chemical reactions, phase changes, and other processes.