MADNESS OF THERMOCHEMISTRY!!!

Concept Enthalpy

Enthalpy (symbol H), is a measure of the heat content at a constant pressure.

Enthalpy measured at 25 C (298.15 K) and 1 atm, known as the standard enthalpy.

Heat- The energy transferred between system and surrounding

System- Part of universe whose change we are going to observe

Surrounding- The rest of the universe outside the system

Units of energy 

                                       SI units: Joule (J)

                                      1 J = 1 kg m² s^-2

                                      Unit : Calorie (cal)

                                       1 cal = 4.184 J

Some elements exist as allotropes because they can exist as different forms in the same physical states under the same conditions of temperature and pressure.

The most stable allotrope has the standard enthalpy which is equal to zero. Thus, graphite, oxygen gas and sulphur rhombic have zero standard enthalpy. The other allotropes are less stable and their standard enthalpies are not zero.

                              *Note : The concept of enthalpy of elements is useful in the

                                         determination of standard enthalpies of other substance.

Pure substance (element of compound) most stable form of substance at 1 atm and temperature (usually 25 C)

Standard state of pure substance :

O₂(g)      Na(s)      NaCl(s)     H₂O(l)       Br₂(l)    H₂(g)

Maeatomic for 7 diatomic molecules :

I₂         Br₂       Cl₂       F₂       O₂         N₂        H₂

Exothermic and Endothermic reaction

Exothermic reaction	Endothermic reaction
The system has lost heat to surroundings. Reactants have higher enthalpy enthalpy compared to the products. Enthalpy change is negative. Heat released in exothermic reactions increases the temperature of the container, which becomes hot when we touch.	The system absorb heat from the surroundings. Reactants have lower enthalpy compared to the products. Enthalpy change is positive. Heat absorbed in an endothermic reactions causes the temperature of the container to drop. Thus, it feels cold to the touch.
Example :	Example :

A less frequently encountered type of reaction is the thermoneutral reaction. In the case, the enthalpy of the reactants equal to that of the products.

The system does not absorb or release heat. The enthalpy change H, is zero.

The enthalpy change H, depends on the temperature, pressure, amount of substances, and the physical state of both reactants and products. Pressure is very important for reactions involving gases. The amount of substance may be stated as concentration, mass, mole, mole fraction and so on. For a solution, the standard concentration is 1 mol dm^-3. The physical state is solid (s), liquid (l) or gas (g). the physical state for substance depends on the temperature and pressure state.

Example, at 1 atm the physical state of water at 0 C is solid, H₂O (s). At 25 C, its physical state is liquid, H₂O(l), while at temperature exceeding 100 C it is gas, H₂O(g).

Types of enthalpy change (H)

1. Enthalpy of formation

• Also called heat of formation
• Defined as heat change when one mole of a compound is formed from its elements
• Symbol is H_f
• May have negative (exothermic) or positive (endothermic) values.
• Example :

H₂(g) + ½ O₂(g)  H₂O(g)     H_f = -242kJ/mol

Si(s) + 2H₂(g)  SiH₄(g)         H_f = +33kJ/mol

2. Enthalpy of combustion

• Also known as heat of combustion
• Defined as the heat released when one mole of a substance is completely combusted in oxygen gas
• Symbol is H_comb
• Always negative value because it is an exothermic reaction.
• Example :

C(s) + O₂(g)  CO₂(g)    H_comb = -393kJ/mol

3. Enthalpy of atomisation

• Also called as heat of atomisation
• Defined as heat absorbed when one mole of gaseous atom is formed from its element.
• Symbol is H_atom
• Positive value because endothermic reaction
• Example :

Na(s)  Na(g)                 H_atom = +108 kJ/mol

½ H₂(g)  H(g)               H_atom = +218 kJ/mol

4. Enthalpy of sublimation

• Also known as heat of sublimation
• Defined as heat absorbed when one mole of substance in solid state sublimed. *change of phase from solid to gas
• Symbol is H_sublim
• Positive value because it is endothermic reaction
• Example :

Na(s)  Na(g)              H_sublim = +108 kJ/mol

5. Enthalpy of neutralisation

• Also called as heat of neutralisation
• Defined as the heat released when one mole of water is formed, when an acid reacts with a base (or alkali)
• Symbol is H_neut
• Negative value because it is exothermic reaction
• Example :

                  HCl(aq) + NaOH(aq)  NaCl(aq) + H₂O(l)                              H_neut = -57.5 kJ/mol

                 CH₃COOH(aq) + NaOH(aq)  CH₃COONa(aq) + H₂O(l)      H_neut = -57.5 kJ/mol

                 NH₄OH(aq) + HCl(aq)  NH₄Cl(aq) + H₂O(l)                         H_neut = -51.5 kJ/mol

6. Enthalpy of hydration

• Also known as heat of hydration
• Defined as the heat released when one mole of the gaseous ion is hydrated in an aqueous solution. *in form of ion but change of phase
• Symbol is H_hydr
• Negative value because it is exothermic reaction
• Example :

Na⁺(g)  Na⁺(aq)             H_hydr = -405 kJ/mol

7. Enthalpy of solution

• Also called heat of solution
• Defined as the heat change when one mole of the substance is dissolved in water to form one mole aqueous solution.
• Standard enthalpy of solution defined as heat change when one mole of substance dissolve in water to form infinite dilute solution.
• Symbol is H_soln
• May have negative (exothermic) or positive (endothermic) values
• Example :

NaCl(s)  Na⁺(aq) + Cl^-(aq)