Which Substance Has A Standard Enthalpy Of Formation Of Zero - T-Tees - Your Answers Await, Life's Questions Unraveled

Introduction

The standard enthalpy of formation is a measure of the energy released or consumed when one mole of a substance is created under standard conditions from its pure elements. The symbol of the standard enthalpy of formation is ΔHf.

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Δ = A change in enthalpy
o = A degree signifies that it’s a standard enthalpy change.
f = The f indicates that the substance is formed from its elements

The equation for the standard enthalpy change of formation (originating from Enthalpy’s being a State Function), shown below, is commonly used:

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[Delta H_{reaction}^o = sum {Delta H_{f}^o(products)} – sum {Delta H_{f}^o(Reactants)}]

This equation essentially states that the standard enthalpy change of formation is equal to the sum of the standard enthalpies of formation of the products minus the sum of the standard enthalpies of formation of the reactants.

Carbon naturally exists as graphite and diamond. The enthalpy difference between graphite and diamond is too large for both to have a standard enthalpy of formation of zero. To determine which form is zero, the more stable form of carbon is chosen. This is also the form with the lowest enthalpy, so graphite has a standard enthalpy of formation equal to zero. Table 1 provides sample values of standard enthalpies of formation of various compounds.

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Table 1: Sample Table of Standard Enthalpy of Formation Values. Table T1 is a more comprehensive table. Compound ΔHfo O2(g) 0 kJ/mol C(graphite) 0 kJ/mol CO(g) -110.5 kJ/mol CO2(g) -393.5 kJ/mol H2(g) 0 kJ/mol H2O(g) -241.8 kJ/mol HF(g) -271.1 kJ/mol NO(g) 90.25 kJ/mol NO2(g) 33.18 kJ/mol N2O4(g) 9.16 kJ/mol SO2(g) -296.8 kJ/mol SO3(g) -395.7 kJ/mol

All values have units of kJ/mol and physical conditions of 298.15 K and 1 atm, referred to as the “standard state.” These are the conditions under which values of standard enthalpies of formation are typically given. Note that while the majority of the values of standard enthalpies of formation are exothermic, or negative, there are a few compounds such as NO(g) and N2O4(g) that actually require energy from its surroundings during its formation; these endothermic compounds are generally unstable.

Kirchhoff’s Law describes the enthalpy of a reaction’s variation with temperature changes. In general, enthalpy of any substance increases with temperature, which means both the products and the reactants’ enthalpies increase. The overall enthalpy of the reaction will change if the increase in the enthalpy of products and reactants is different.

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