Types of Heat Change in Chemical Reaction

The heat of reaction or enthalpy change obtained under standard conditions is given the symbol, \( \scriptsize \Delta H^{⦵}\)(superscript [⦵] means standard conditions).

The plimsoll symbol (⦵ or o) is used as a superscript in the notation of thermodynamics to indicate a specific arbitrarily chosen non-zero reference point (“standard state”).

Heat of reaction: This is the heat change when all the substances involved in a reaction have reacted completely. It is the heat content of a reaction i.e. the amount of heat involved in a reaction.

\( \scriptsize \Delta H^{⦵}_ {pr} \: – \: \Delta H^{⦵}_ {r} \)

Standard enthalpy change ΔH^⦵ is a state function. It depends on the initial and final states of the system.

ΔH^⦵ = H₂ – H₁

Enthalpy or heat changes are mostly encountered in formation, neutralisation, combustion, solution, fusion, hydrogenation, hydration, and vaporization. 

1. Heat of Formation \( \left( \scriptsize \Delta H^{⦵}_ f \right) \)

The standard enthalpy [heat] of formation is the heat evolved or absorbed when one mole of substance is formed from its elements under standard conditions. The standard heat of formation could be endothermic or exothermic i.e. positive or negative.

Example:

i. Standard enthalpy of formation of water:

H_2(g) + ½ O_2(g) → H₂O _(g)  ∆H^⦵_f = -285 KJ mol^-1

ii. Standard enthalpy of formation of hydrogen iodide:

½ H_2(g) + ½ I₂  → HI_(g)  ∆H^⦵_f = +26.0 KJ mol^-1

iii. Standard enthalpy of formation of carbon(IV) oxide:

C_(s) + O_2(g) →  CO_2(g)  ∆H^⦵_f = -393 KJ mol^-1

Thus, the more exothermic the standard heat of formation of a compound, the greater its energetic stability. Hence CO₂ is more stable with respect to its elements than H₂O because more heat is liberated during the formation of CO₂ than water.

Enthalpy of formation is zero for elements in their normal state, e.g. carbon, hydrogen, sulphur, iodine, etc. 

Calculation Based on Heat of Formation:

Example 6.6.1:

When 5 g of liquid water was formed by burning hydrogen gas, +65 KJ of heat was given off. Calculate the standard heat of formation of water.

H_2(g) + ½O_2(g) → H₂O_(l)  ∆H^⦵_f  = ? KJ mol^-1

[H = 1, O = 16]     [H₂O = 18]

Solution: