Calculations for Enthalpy of Reaction

There is no denying the fact that Chemical reactions can either release energy to their surroundings, exothermic, or energy can be transferred to them from the surroundings, endothermic.

Exothermic reactions are most common, however, an important example of an endothermic reaction is photosynthesis in plants, where the energy supplied is from sunlight.
Law of conservation of energy: Energy cannot be destroyed or created but only transferred from one form to another. The total energy of a system of reacting chemicals and surroundings remains constant.
Enthalpy change is the term used to describe the energy exchange that takes place with the surroundings at a constant pressure and is given the symbol ΔH.
Enthalpy is the total energy content of the reacting materials. It is given the symbol, H.
ΔH = ΔH products - ΔH reactants
The units are kilojoules per mole (kJmol-1)
An exothermic enthalpy change is always given a negative value, as energy is lost to the surroundings.
ΔH = -xkJmol-1
An endothermic enthalpy change is always given a positive value, as the energy is gained by the system from the surroundings.
ΔH = + ykJmol-1.

Standard enthalpy changes: standard conditions

If we are to compare the enthalpy changes of a various reactions we must use standard conditions, such as known temperatures, pressures, amounts and concentrations of reactants or products.
The standard conditions are:
  • A pressure of 100kilopascals (102kPa)
  • A temperature of 298K (25oC)
  • Reactants and products in physical states, normal for the above conditions.
  • A concentration of 1.0mol dm-3 for solutions.
The o sign indicates standard conditions.

Standard enthalpy change of reaction

ΔHor
The standard enthalpy change of reaction is the enthalpy change when the amounts of reactants shown in the equation for the reaction, react under standard conditions to give the products in their standard states.

Standard enthalpy change of formation

ΔHof
The standard enthalpy change of formation is the enthalpy change when one mole of a compound is formed from its elements under standard conditions; both compound and elements are in standard states.

Standard enthalpy change of combustion

ΔHoc
The standard enthalpy change of combustion is the enthalpy change when one mole of an element or compound reacts completely with oxygen under standard conditions.

Bond breaking and bond making

For a chemical reaction to occur bonds must break before new bonds can be made. When bonds break energy is absorbed (endothermic). When bonds form, energy is released (exothermic).
If the energy absorbed whilst making bonds is greater than the energy transferred to the surroundings as bonds are made, then an endothermic reaction will occur.
Whereas, if the energy released on bond formation is greater than that absorbed through breaking bonds then an exothermic reaction is observed.
Bond energy: This is the amount of energy required to break a covalent bond, it indicates the strength of a bond. Values are always quoted as bond energy per mole, E.

Example:

O2(g) → 2O(g)
ΔH = +498kJmol-1
Values are always positive, as they refer to bond breaking which is endothermic. They are average values as a particular bond energy will depend upon the molecule the bond is held within, hence average values are taken.

Comments

Post a Comment

Popular posts from this blog

Irin, a mother of silent ocean

Image
A mother behind the deep sea and the ocean Walking past in the silent world of beauty and heaven,  She knows her work in motion and hurdles; Never hesitating through the rains and the clouds  In moving behind the shadow and perils of earth.  She remains happy by her unusual courage; To speak the truth in capsize haze and relies To switch the darkened cloud of the storm.  She walks past all along in the glowing streams  Underneath the big shades of the banyan tree As if the mother of all the child In the earth And the beams of light not under coercive strength. What she sees and explores in front of her  She is moving in her way to have bliss  Upon the supplication she always does.  She becomes remorse and dilapidated  when She hears the destitute of a motherless baby Her sorrows knows no bounds As if her heart always renders to restore the life of the child.  She is second to none to roll her tears  When sadness, pains and death stirs her.  L
Read more

Home, my sweet home

Home, My Sweet home There is no denying the fact that when our time comes to die, we should  not behave  like those whose hearts are filled with fear of death, so that when their time comes they weep and pray for a little more time to live their lives over again in a different way.  We should sing our death song, and die like a hero going home. The palpable image that comes to our cognizance when the expression ‘Home’ is perceived is of an edifice, chalet or a bed-sitter. A home cannot be just defined by a structure with a collection of rooms inside, for some it can also be their smartphones or even their refugee camp. According to Melody Bettie,” Gratitude unlocks the fullness of life. It turns what we have into enough, and more. It turns denial into acceptance, chaos to order, confusion to clarity. It can turn a meal into a feast, a house into a home, a stranger into a friend”.   A home is customarily distinct and comforting for any person but for realism of pedagogy, childr
Read more

Hooks Law and its application

Hooke's Law Hooke's Law states that, for certain elastic materials, force is proportional to extension, when a sample is stretched. This means that the extension of the sample increases linearly with the amount of force applied. Materials that obey Hooke's law are called Hookean Materials. Springs behave like Hookean Materials.Hooke's law can be written as  F = kx , where  F  is Force,  x  is extension, and k  is the Stiffness Constant of the sample. The stiffness constant describes the stiffness of a material, and is measured in Nm -1  (or Kgs -2 ). F = kx Hooke's law can be demonstrated with the use of Force-Extension graphs. However, no sample follows Hooke's law indefinitely, and there comes a point, called the Limit of Proportionality, where there is no longer a linear relationship between force and extension. After yet more force is applied, the Elastic Limit will be reached. This means that the sample will no longer return to its o
Read more