Element most stable form table

TABLE 7.6 Examples of the Most Stable Forms of Elements... 

No elements are listed in Table 8.2 because, by definition, the most stable form of any element in its standard state has AH°f = 0 kj. (That is, the enthalpy change for formation of an element from itself is zero.) Defining AH°f as zero for all elements thus establishes a kind of thermochemical "sea level," or reference point, from which all enthalpy changes are measured. 

Values of AG°f at 25°C for some common substances are listed in Table 17.3, and additional values are given in Appendix B. Note that AG°f for an element in its most stable form at 25°C is defined to be zero. Thus, solid graphite has AG°f = 0 kj/mol, but diamond, a less stable form of solid carbon at 25°C, has AG°f = 2.9kJ/mol. As with standard enthalpies of formation, AH°f, a zero value of AG°f for elements in their most stable form establishes a thermochemical "sea level," or reference point, with respect to which the standard free energies of other substances are measured. We can t measure the absolute value of a substance s free energy (as we can the entropy), but that s not a problem because we are interested only in free-energy differences between reactants and products. 

In Example 15-10, we were not given the value of AH for Pb(s). We should know without reference to tables that AH for an element in its most stable form is exactly 0 y/mol. But the element must be in its most stable form. Thus, AH for 02(g) is zero, because ordinary oxygen is gaseous and diatomic. We would not assume that AH would be zero for oxygen atoms, 0(g), or for ozone, 03(g). Similarly, AH is zero for Cl2(g) and for Br2(f), but not for Hrjig). Recall that bromine is one of the few elements that is Uquid at room temperamre and 1 atm pressure. 

Table 7. The free energy offormationfrom elements AG . For the most stable form of the...

Table 6.3 lists the standard enthalpies of formation for a number of elements and compounds. (Eor a more complete list of values, see Appendix 3.) By convention, the standard enthalpy of formation of any element in its most stable form is zero. Take oxygen as an example. Molecular oxygen (O2) is more stable than the other allotropic form of oxygen, ozone (O3), at 1 atm and 25°C. Thus we can write AHfiOf) = 0. but AHf (Os) + 0. Similarly, graphite is a more stable allotropic form of... 

Just as we could not determine the absolute value of G ", we also cannot measure H/". As with G/" we circumvent this problem by assigningH/" a value of zero to all elements in their most stable form at 25"C and 1 atm pressure. In aqueous solution 1 mole/liter of the hydrogen ion, H", in ideal solution (y = 1) also is assigned an H° value of zero. We can determine values of enthalpy of specie based on these assignments and call t se the enthalpy of formation, A/. Similarly to the computations for AG/" values, we can compute the AH/° values of various compounds from th assigned H values of their component elements. A selection of these AH/° values is given in Table 3-1. 

We can calculate AH for this reaction by using Equation 5.31 and data in Table 5.3. Remember to multiply the AHy value for each substance in the reaction by that substance s stoichiometric coefficient. Recall also that AHf = 0 for any element in its most stable form under standard conditions, so AH/[02(g)] = 0 ... 

For a pure substance (element or compound), the standard state is usually the most stable form of the substance at 1 attn and the temperature of interest. In this text (and in most thermodynamic tables), that temperature is usually 25°C (298 K). 

The problem with eqn 1.22 is that we have no way of knowing the absolute enthalpies of the substances. To avoid this problem, we can imagine the reaction as taking place by an indirect route, in which the reactants are first broken down into the elements and then the products are formed from the elements (Fig. 1.24). Specifically, the standard enthalpy of formation, AfH, of a substance is the standard enthalpy (per mole of the substance) for its formation from its elements in their reference states. The reference state of an element is its most stable form under the prevailing conditions (Table 1.7). Don t confuse reference state with standard state the reference state of carbon at 25 C is graphite (not diamond) the standard state of carbon is any specified phase of the element at 1 bar. For example, the standard enthalpy of formation of liquid water (at 25 C, as always in this text) is obtained from the thermochemical equation... 

The concept of an octet of electrons is one of the foundations of chemical bonding. In fact, C, N, and O, the three elements that occur most frequently in organic and biological molecules, rarely stray from the pattern of octets. Nevertheless, an octet of electrons does not guarantee that an inner atom is in its most stable configuration. In particular, elements that occupy the third and higher rows of the periodic table and have more than four valence electrons may be most stable with more than an octet of electrons. Atoms of these elements have valence d orbitals, which allow them to accommodate more than eight electrons. In the third row, phosphoms, with five valence electrons, can form as many as five bonds. Sulfur, with six valence electrons, can form six bonds, and chlorine, with seven valence electrons, can form as many as seven bonds. 

Stars of mass greater than 1.4 solar masses have thermonuclear reactions that generate heavier elements (see Table 4.3) and ultimately stars of approximately 20 solar masses are capable of generating the most stable nucleus by fusion processes, Fe. The formation of Fe terminates all fusion processes within the star. Heavier elements must be formed in other processes, usually by neutron capture. The ejection of neutrons during a supernova allows neutron capture events to increase the number of neutrons in an atomic nucleus. Two variations on this process result in the production of all elements above Fe. A summary of nucleosynthesis processes is summarised in Table 4.4. Slow neutron capture - the s-process - occurs during the collapse of the Fe core of heavy stars and produces some higher mass elements, however fast or rapid neutron capture - the r-process - occurs during the supernova event and is responsible for the production of the majority of heavy nuclei. 

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