Thermochemistry in Chemistry: Chemical reactions take place with the emission of heat or absorption of heat.
The reaction of 2 mole hydrogen gas and 1 mole oxygen gas emits 136.8 Kcal of heat in the formation of 2 mole water in liquid state at 25°C temperature and 1 atmospheric pressure.
2H2(g) + O2(g) → 2H2O(l) ΔH = -136.8 Kcal
A reaction of 1 mole nitrogen gas and 1 mole oxygen gas absorbs 42.0 Kcal of heat in the formation of 25°C temperature and 2 mole nitric oxide gas at 1 atmospheric pressure.
N2(g) + O2(g) → 2NO(g) ΔH = +42.0 Kcal
Thermochemistry in Chemistry
The branch of physicochemical in which the heat changes occurring in chemical reactions are studied is called Thermochemistry.
In chemical equations, the equations are used to represent heat changes as well as heat changes to express heat changes. Such equations are called Thermochemical Equations.
Following are the key features of Thermochemical equations.
(a) The thermochemical equations are balanced, that is, the equation has the same number of both and different atoms.
(b) The physical states of various substances are displayed in thermochemical equations. Solid, liquid and gaseous substances are represented in brackets with their formulas by s, l, and g respectively.
Aqueous solutions are displayed by writing (aq) in brackets with the formula of the substance. Display the various allotropes by writing the name of the epitope in brackets with their formula.
(c) Most chemical reactions are studied at constant pressure. Heat absorbed at constant pressure is called enthalpy change. Therefore, heat changes in chemical reactions are expressed as enthalpy changes (ΔH).
(d) The value of enthalpy change depends on the nature of the reaction. The value of enthalpy change is different for different reactions. The value of enthalpy change also depends on temperature, pressure and volume.
Pressure and volume depend on each other. Therefore, it is necessary to display the temperature and pressure conditions along with the value of enthalpy change. Normally the values of enthalpy change are displayed at 25°C temperature and 1 atmospheric pressure.
Thermodynamic equations can be written in three ways. The reaction of 2 mole hydrogen gas and 1 mole oxygen gas emits 136.8 Kcal of heat in the formation of 2 mole water in a liquid state at 25°C temperatures and 1 atmospheric pressure. This change can be expressed in the following three ways –
2H2(g) + O2(g) → 2H2O (l)+ 136.8 Kcal
2H2(g) + O2(g) – 136.8 Kcal → 2H2O (l)
2H2(g) + O2(g) → 2H2O (l) ΔH = – 136.8 Kcal
The above three equations show that 136.8 Kcal of heat is emitted at constant temperature and pressure due to the action of 2 mole H2 and 1 mole O2 forming 2 mole H2O.
Heat of Reaction
For the following general reaction:
A + B → C + D
Assuming that the enthalpy of A, B, C, and D is HA, HB, HC and HD respectively
ΔH = HC + HD – HA – HB
ΔH = HP – HR
Where HP and HR are the sum of enthalpy of products and reactants respectively.
Enthalpy change is equal to the heat absorbed at constant pressure. Hence the definition of reaction heat can also be given as follows.
The heat absorbed at constant pressure in a reaction is called the Reaction Heat of that reaction.
C(s) + O2(g) → CO2(g) ΔH = – 94 Kcal
The above equation shows that when 1 mole of solid carbon (C2) and 1 mole of gaseous oxygen (O2) is reacted, 1 mole of carbondioxide CO2 is formed. And -94 Kcal heat is absorbed at constant pressure. Hence the reaction heat of this reaction is – 94 Kcal.
C(s) + O2(g) → CO2(g) ΔH = – 94 Kcal
The above equation shows that 2 mole nitric oxide NO gas is formed when 1 mole nitrogen (N2) and 1 mole oxygen (O2) are reacted. And at constant pressure, +42 Kcal heat is absorbed. Hence the reaction heat of this reaction is +42 Kcal.
Reaction heat at constant volume:
Often the value of reaction heat is determined at constant pressure. The value
of the reaction heat can also be determined by the constant volume. Heat
absorbed at a constant volume is called reaction heat at a constant volume.
According to the first law of thermodynamics, the absorbed heat at constant
pressure (QP) equals the enthalpy change (ΔH) and the absorbed heat at constant volume (QV) equals the change in internal energy (ΔE).
Reaction heat = enthalpy change (ΔH)
= absorbed heat at constant pressure (QP)
Reaction heat at constant volume = absorbed heat at constant volume (QV)
= change in internal energy (ΔE)
The reaction at constant volume is carried out in a vessel whose volume is constant in order to find the heat. Reaction heat at constant volume is often determined with the help of a Bomb Calorimeter. It is a device in which the volume of the character used for the reaction is constant and the value of heat absorbed in the reaction is determined with the help of a calorimeter.