The experimental discovery that almost all chemical reactions either absorb or release heat led to the idea that all substances contain heat. Consequently, the heat of a reaction is the difference in the heat contents of the products and reactants:
Δ H = H products – H reactants
Throughout this book, the Greek letter delta Δ will be used to symbolize change. Chemists use the term enthalpy for the heat content of a substance or the heat of a reaction, so the H in the previous equation means enthalpy. The equation states that the change in enthalpy during a reaction equals the enthalpy of the products minus the enthalpy of the reactants. You can consider enthalpy to be chemical energy that is commonly manifested as heat.
Use the decomposition of ammonium nitrate as an example of an enthalpy calculation. The reaction is
and the enthalpies of the three compounds are given in Table 1.
Notice that the enthalpies can be either positive or negative. In general, compounds that release heat when they are formed from their elements have a negative enthalpy, and substances that require heat for their formation have a positive enthalpy. The enthalpy of the decomposition reaction can be calculated as follows:
Observe the doubling of the enthalpy of H 2O (–36 kJ/mole) because this compound has a stoichiometric coefficient of 2 in the reaction. The overall enthalpy of the reaction is –36 kilojoules, which means that the decomposition of 1 mole of ammonium nitrate releases 36 kJ of heat. The release of heat means that this is an exothermic reaction. The sign of the enthalpy of the reaction reveals the direction of heat flow. See Table 2.
If you reverse the previous reaction,
the sign of the enthalpy of the reaction is reversed:
Δ H = +36 kJ
The reversed reaction is, therefore, endothermic. It would require the addition of 36 kcal of energy in order to cause the nitrous oxide and water vapor to react to form 1 mole of ammonium nitrate.
The calculations on the ammonium nitrate reaction demonstrate the immense value of tables that list the enthalpies for various substances. The values at 25°C and 1 atm are called standard enthalpies. For elements, the standard enthalpy is defined as 0. For compounds, the values are called standard enthalpies of formation because the compounds are considered to be formed from elements in their standard state.
Table 3 gives a few values that will be used in subsequent examples and problems. The symbol for standard enthalpies of formation is , where the superscript denotes standard and the subscript denotes formation. Look up both elemental sulfur and nitrogen to see that the standard enthalpies for elements are 0. Then find the pairs of values for H 2O and CCl 4 (carbon tetrachloride) to learn that the enthalpy depends on the state of matter.
Use the data in Table 3 to calculate the enthalpy change of the following reaction:
The difference in the heats of formation of the products is given by:
The enthalpy of the reaction is –1124 kilojoules, meaning that the oxidation of 2 moles of hydrogen sulfide yields or releases 1124 kJ of heat. This reaction is exothermic.
Use Table 3 for the next two problems.
- Calculate the enthalpy change for the following reaction and classify it as exothermic or endothermic.
- Calculate the quantity of heat released when 100 grams of calcium oxide react with liquid water to form Ca(OH) 2 ( s).