Thermochemistry
Calorimeter
Heat involved in a chemical reaction is measured in a suitable apparatus is called calorimeter.Bomb Calorimeter
This apparatus was devised by Berthelot in 1881 to measure the heat of combustion of organic compounds. A modified form of the apparatus is shown below is consists of a sealed combustion chamber, called a bomb, containing a weighed quantity of the substance in a dish along with oxygen under about 20 atm. Pressure.The bomb is lowered in water contained in an insulated copper vessel. This vessel is provided with a stirrer and a thermometer reading up to 1/100th of a degree. It is also surrounded by an outer jacket to ensure complete insulation from the atmosphere. The temperature of water is noted before the substance is ignited by an electric current.
After combustion, the rise in temperature of the system is noted on the thermometer and heat of combustion can be calculated from the heat gained by water and the calorimeter.
Bond Energy and Bond Dissociation Energy
Whenever a bond is formed energy is evolved and bond is broken, energy is absorbed. Bond energy may be defined as, The amount of energy released when 1 mole of bonds are formed from isolated gaseous atoms. Whereas Bond Dissociation energy may be defined as, The amount of energy required to break one mole bond present between the atoms of a gaseous molecule.Hess' Law of Constant Heat Summation
Hess Law of Constant Heat Summation is also known as Second Law of Thermochemistry. The law states that the total heat change accompanying a chemical reaction is the same whether the reaction takes place in one step or multiple steps.Example: Carbon dioxide can be formed directly from carbon and also from carbon via carbon monoxide. The heat change involved in both the process are found to be same. Formation of CO2 directly-
C (s) + O2 (g) → CO2 (g) ΔH = – 94 K Cal.
Formation of CO2 Via Carbon Monoxide
C (s) + 1/2 O2 (g) → CO (g) ΔH1 = – 26.4 K Cal.
CO (g) + 1/2 O2 (g) → CO2 (g) ΔH2 = – 67.6 K Cal.
ΔH = ΔH1 + ΔH2
ΔH = –26.4 + (– 67.6) K Cal.
ΔH = – 26.4 – 67.6 K Cal.
ΔH = – 94 K Cal.
Application of Hess Law
Hess' law finds its application in determining the heat of changes for reactions for which experimental determination is not possible. Some important applications are given below1. Calculation of Enthalpy of Formation
2. Calculation of Enthalpy of Allotropic Transformation
3. Calculation of Calorific Value
Enthalpy of Combustion
The enthalpy of combustion of a substance is defined as the heat exchange when 1 mole of substance is completely burnt or oxidized in oxygen.Standard enthalpy of combustion is the enthalpy change when 1 mole of a substance burns under standard state conditions.
For example, the enthalpy of combustion of ethanol, −1366.8 kJ/mol, is the amount of heat produced when one mole of ethanol undergoes complete combustion at 25 °C and 1 atmosphere pressure.
C2H5OH(l) + 3O2(g) ⟶ 2CO2 + 3H2O(l) ΔH∘298 = −1366.8kJ.
Enthalpy of Neutralization
Neutralisation is the reaction between an acid and a base to form a salt and water. During neutralisation reaction, hydrogen ions from acid react with hydroxide ions from alkali to form water.H+(aq) + OH–(aq) → H2O(aq)
The heat of neutralisation is the heat produced when one mole of water is formed from the reaction between an acid and an alkali.
Enthalpy of neutralization is always constant for a strong acid and a strong base. This is because all strong acids and strong bases are completely ionized in dilute solution.
When an acid and alkali react, heat is given out. So, Neutralisation is an exothermic reaction and always produces heat. Therefore, enthalpy of neutralisation, ΔH is always negative.
Heat of Reaction
Heat of reaction may be defined as the difference in enthalpy or heat content, ∆H, between the products and the reactants.If heat is evolved the reaction is said to be exothermic and when heat is absorbed the reaction is an endothermic.
Heat of Reaction at Constant Pressure
The difference between the sum of enthalpies of products and the sum of enthalpies of reactants at a given temperature and constant pressure is called the heat of reaction at the constant pressure at a given temperature. It is denoted by ΔH.ΔH = ∑ΔHProducts – ∑ΔHReactants
ΔH is negative for exothermic reaction and positive for an endothermic reaction.
Heat of Reaction at Constant Volume
The difference between the sum of internal energies of products and the sum of internal energies of reactants at a given temperature and constant volume is called the heat of reaction at the constant volume at a given temperature. It is denoted by ΔE.ΔE = ΔE = ∑ΔEProducts – ∑ΔEReactants
Factors Affecting Enthalpy of Reaction
The various factors on which enthalpy of reaction depend are given belowPhysical State of Reactants and Products
The enthalpy of reaction changes with change in physical state because as the physical state changes, extent of heat is evolved.
Quantity of Reactants
The change in enthalpy of reaction depends upon the quantity of reactants used. When the number of moles of reactants are doubled, the enthalpy change also becomes double.
Allotropic Modification
For elements existing in different allotropic modifications, the heat of reaction is different if different allotropic form is involved in reaction.
Temperature and Pressure
The enthalpy of reaction depends upon the temperature and pressure of reaction. Therefore, the values are generally expressed under standard conditions of temperature (298K) and pressure(1 atm.)
