Video Transcript
What is the value of the standard enthalpy of formation of any element in its standard state?
Let’s begin by defining the standard enthalpy of formation. The standard enthalpy of formation is the enthalpy change that occurs when one mole of substance is formed from elements in their standard states and under standard conditions. Standard conditions refer to standard temperature and pressure. While chemists generally take standard temperature to be 25.15 degrees Celsius, standard enthalpy measurements can be made at other temperatures. However, by definition, standard pressure is taken to be one atmosphere or its equivalent. The standard state of an element is the physical state that the element has at standard temperature and pressure. Forming a new substance from two or more elements is a chemical reaction.
In this question, we’ll focus on reactions that form compounds. For example, the elements sodium and chlorine react to form the compound sodium chloride. In this reaction, the elements sodium and chlorine are the reactants and sodium chloride is the product. At standard temperature and pressure, sodium is a solid, chlorine is a gas, and sodium chloride is a solid. Now, we’re ready to write an unbalanced equation to represent the standard enthalpy of formation of sodium chloride. Now, there are a few points to make about what we see in this equation.
First of all, in its standard state as a gas at room temperature, chlorine is composed of diatomic chlorine molecules, which are represented by the chemical formula Cl2. Secondly, the standard enthalpy of formation accounts for exactly one mole of product being formed. So when we balance this equation, the stoichiometric coefficient in front of Cl2 will be one-half. The values of the standard enthalpy of formation for the elements sodium and chlorine in their standard states are zero kilojoules per mole. In fact, all elements in their standard states have a standard enthalpy of formation of zero kilojoules per mole. This is because in a formation reaction, the elements are already in their standard states. So no energy is needed to convert them into their standard states.
However, energy is required for the elements to react and make a compound. The standard enthalpy of formation of a compound can be positive or negative, but it’s always a nonzero value. This is because overall there is a net transfer of energy in or out of a system during a chemical reaction. The value of the standard enthalpy of formation for sodium chloride is negative 411 kilojoules per mole. The negative sign indicates that forming sodium chloride at standard conditions is an exothermic reaction.
In summary, we’ve learned that the standard enthalpy of formation of a compound is nonzero because energy is transferred during the chemical reaction. However, the standard enthalpy of formation for any element in its standard state is equal to zero kilojoules per mole.
