A pure substance is the most stable form of the substance at 1 atm and the temperature of interest. In most tables the temperature is 25°C (298 K). The standard state of a substance in aqueous solution, is a concentration of 1.00 M and for a gas the pressure is 1 atm. The standard enthalpy (heat) of reaction is given by ΔH^o_rxn. The nought superscript means standard state.

A formation equation is written for 1 mole of substance formed from its elements. For example the formation of NaCl(s) from its elements is written as:

If the equation was written as below, it would not be a formation equation because there are two moles of NaCl. Formation equations are always written in terms of 1 mole of product.

The standard enthalpy of formation, ΔH^o_f, is the enthalpy change for a formation equation when all substances are in their standard states. The standard enthalpy of formation for the following formation reaction is -411.2 kJ/mol.

Values of ΔH^o_f can be found in tables or in the CRC Handbook of Chemistry and Physics. Click to see a table of thermodynamic quantities. The ΔH^o_f, values for elements in their most stable state are zero. For our reaction, ΔH^o_f, for both Na (s) and Cl₂ are zero as both are elements in their standard states. The value for 1 mole of NaCl is -411.2 kJ/mol.

In this example, we are asked to write a formation equation for liquid ethanol, CH₃CH₂OH. Recall, the formation equation is for 1 mole of a substance formed from its elements. The elements are C (graphite), H₂ (g), and O₂ (g)

Next, balance the equation while keeping in mind there can only be 1 mole of ethanol in the balanced formation reaction. The balanced formation equation is:

Finally, we look up ΔH^o_f for liquid ethanol, and it is -277.7 kJ/mol.

We can calculate ΔH_rxn using ΔH^o_f values. Look up the ΔH^o_f values for each substance in the chemical equation. Sum the product standard enthalpies of formation, multiplying each value by the equation coefficient for that substance. Then sum the reactant standard enthalpies of formation, multiplying each value by the equation coefficient for that substance. Subtract the values for the reactants from the products:

The values of m and n are the coefficients of the products and reactants. For example, let’s calculate ΔH^o_rxn for the following reaction:

2 C₄H₁₀ (g) + 13 O₂ (g) → 8 CO₂ (g) + 10 H₂O (g)

Go to the table to look up the ΔH^o_f values for each reactant and product.

Recall, we subtract the reactants from the products and multiply each ΔH^o_f value by the coefficient in the balanced chemical equation.

For the combustion of two moles of butane, there is 5314 J of heat released. For the combustion of one mole of butane, divide equation coefficients and ΔH^o_rxn by 2, there would be 2657 J of heat released.

Worksheet: Heats of Formation at 25 ℃

Exercises

Exercise 1. Write a formation reaction for K₂CrO₄ (s).

Exercise 2. What is the value of ΔH^o_f for LiOH (s)? Write a formation equation for LiOH (s).

Exercise 3. Calculate ΔH^o_rxn, in kJ for the following reaction. (Table of Thermodynamic Quantities)

Exercise 4. Methylcyclopentane, C₆H₁₂, undergoes combustion according to the following equation:

Use the standard heat of reaction, ΔH^o_rxn, and the ΔH^o_f values from the Table of Thermodynamic Quantities to calculate the standard enthalpy of formation, ΔH^o_f for methylcyclopentane.

Check Solutions/Answers to Exercises

 
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