Thus, the reaction is exothermic because the bonds in the products are stronger than the bonds in the reactants. As the number of bonds between two atoms increases, the bond grows shorter and stronger. Bond order is the number of electron pairs that hold two atoms together. Single bonds have a bond order of one, and multiple bonds with bond orders of two a double bond and three a triple bond are quite common.
In closely related compounds with bonds between the same kinds of atoms, the bond with the highest bond order is both the shortest and the strongest. In bonds with the same bond order between different atoms, trends are observed that, with few exceptions, result in the strongest single bonds being formed between the smallest atoms.
Tabulated values of average bond energies can be used to calculate the enthalpy change of many chemical reactions. If the bonds in the products are stronger than those in the reactants, the reaction is exothermic and vice versa. The breakage and formation of bonds is similar to a relationship: you can either get married or divorced and it is more favorable to be married. Learning Objectives The define Bond-dissociation energy bond energy To correlate bond strength with bond length To define and used average bond energies.
The Relationship between Bond Order and Bond Energy Triple bonds between like atoms are shorter than double bonds, and because more energy is required to completely break all three bonds than to completely break two, a triple bond is also stronger than a double bond. Huheey, E. Keiter, and R. Keiter, Inorganic Chemistry , 4th ed. Bonds between hydrogen and atoms in the same column of the periodic table decrease in strength as we go down the column.
The reason for this is that the region of space in which electrons are shared between two atoms becomes proportionally smaller as one of the atoms becomes larger part a in Figure 8. Bonds between like atoms usually become weaker as we go down a column important exceptions are noted later.
For example, the C—C single bond is stronger than the Si—Si single bond, which is stronger than the Ge—Ge bond, and so forth. As two bonded atoms become larger, the region between them occupied by bonding electrons becomes proportionally smaller, as illustrated in part b in Figure 8. Noteworthy exceptions are single bonds between the period 2 atoms of groups 15, 16, and 17 i. It is likely that the N—N, O—O, and F—F single bonds are weaker than might be expected due to strong repulsive interactions between lone pairs of electrons on adjacent atoms.
Without these two types of bonds, life as we know it would not exist. Hydrogen bonds provide many of the critical, life-sustaining properties of water and also stabilize the structures of proteins and DNA, the building block of cells.
Because the hydrogen is slightly positive, it will be attracted to neighboring negative charges. Figure 1. This interaction is called a hydrogen bond. This type of bond is common and occurs regularly between water molecules. Individual hydrogen bonds are weak and easily broken; however, they occur in very large numbers in water and in organic polymers, creating a major force in combination. Hydrogen bonds are also responsible for zipping together the DNA double helix.
Like hydrogen bonds, van der Waals interactions are weak attractions or interactions between molecules. They are also called inter-molecular forces. They occur between polar, covalently bound atoms in different molecules. Some of these weak attractions are caused by temporary partial charges formed when electrons move around a nucleus. These weak interactions between molecules are important in biological systems and occur based on physical proximity.
Figure 2. A gas is a physical state of matter where the molecules are far apart and moving very quickly. But, because of the hydrogen bonds, as water molecules come together they stick to one another for a small, but significant amount of time. This slows them down, and holds them closer to one another. They become a liquid; a different state of matter where the molecules are closer and slower than in a gas. Molecular water, therefore is a liquid at room temperature, a fact that is profoundly significant for all living things on this planet.
Everything dissolves in water. Stone, iron, pots, pans, plates, sugar, salt, and coffee beans all dissolve in water. Things which dissolve are called solutes and the liquid in which they dissolve is called a solvent.
The water molecules surround the charged solute; positive hydrogens close to negative charges and negative oxygens close to positive charges on the solute molecule. All this interaction suspends the solute molecule in a sea of water molecules; it disperses and dissolves easily. Electrons in the bonds between identical atoms H-H are shared uniformly, so the electrons spend equal amounts of time around each atomic center. These covalent bonds are non-polar.
0コメント