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. The difference in the heats of formation of the products is given by: Use the data in Table 3 to calculate the enthalpy change of the following reaction: 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. Look up both elemental sulfur and nitrogen to see that the standard enthalpies for elements are 0. The symbol for standard enthalpies of formation is, where the superscript denotes standard and the subscript denotes formation. Table 3 gives a few values that will be used in subsequent examples and problems. For compounds, the values are called standard enthalpies of formation because the compounds are considered to be formed from elements in their standard state. For elements, the standard enthalpy is defined as 0. The values at 25☌ and 1 atm are called standard enthalpies. The calculations on the ammonium nitrate reaction demonstrate the immense value of tables that list the enthalpies for various substances. 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 reversed reaction is, therefore, endothermic. The sign of the enthalpy of the reaction is reversed: The sign of the enthalpy of the reaction reveals the direction of heat flow. The release of heat means that this is an exothermic 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. 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 enthalpy of the decomposition reaction can be calculated as follows:
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. Notice that the enthalpies can be either positive or negative. The reaction isĪnd the enthalpies of the three compounds are given in Table 1. Use the decomposition of ammonium nitrate as an example of an enthalpy calculation. You can consider enthalpy to be chemical energy that is commonly manifested as heat. The equation states that the change in enthalpy during a reaction equals the enthalpy of the products minus the enthalpy of the reactants. 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. Throughout this book, the Greek letter delta Δ will be used to symbolize change. Consequently, the heat of a reaction is the difference in the heat contents of the products and reactants: The experimental discovery that almost all chemical reactions either absorb or release heat led to the idea that all substances contain heat. Quiz: Introduction to Oxidation-Reduction Reactions.Introduction to Oxidation-Reduction Reactions.Quiz: Heat Capacities and Transformations.Quiz: Introduction to Organic Compounds.Quiz: Compounds with Additional Elements.
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