![]() The structures of alkanes may also be represented by condensed structural formulas, such as CH 3CH 3 for ethane and CH 3CH 2CH 2CH 2CH 3 for pentane. Why there is a zigzag row of atoms is a topic for Unit 2. As the ball-and-stick and space-filling models of pentane in Figure 1 show, the C atoms do not lie in a completely straight line. However, the Lewis structure represents only some aspects of the molecule, not all. Unbranched acyclic alkanes are often called “straight-chain” alkanes because the carbon atoms can be drawn in a single, straight row in the Lewis structure. Move the slider to see the various representations. Pictured are the Lewis structures, ball-and-stick models, and space-filling models for molecules of methane, ethane, and pentane. The familiar plastics polyethylene, polypropylene, and polystyrene are also hydrocarbons.įigure 1. ![]() Other hydrocarbon fuels are acetylene, diesel fuel, jet fuel, and heating oil. For example, methane (CH 4) is the principal component of natural gas, LP gas is mainly propane (C 3H 8) and butane (C 4H 10). ![]() The simplest example is combustion of methane:ĬH 4( g) + 2O 2( g) → CO 2( g) + 2H 2O( g) Δ r H = −802.3 kJ/molĬombustion of hydrocarbons is highly exothermic so hydrocarbons are excellent fuels. The most important reaction of hydrocarbons is combustion. ![]() Many hydrocarbons are found in plants, animals, and their fossils other hydrocarbons have been prepared in the laboratory. In addition to long chains, hydrocarbon molecules can have chains with branches chains folded back on themselves to form rings and chains, branched chains, or rings that include double or triple bonds. When all other bonds in a chain of carbon atoms are to hydrogen atoms, the molecule is a hydrocarbon. This happens because carbon atoms form strong single bonds to other carbon atoms and because carbon’ has four valence electrons, resulting in four bonds per carbon atom. Carbon is unique among the elements in that carbon atoms can form long chains of carbon-carbon bonds. Many properties of substances that consist of covalently bonded molecules are exemplified by hydrocarbons, compounds that contain only the elements carbon and hydrogen. For now, we consider a single class of covalent molecular substances: hydrocarbons. We will begin to explore this broad range of molecules and properties in Unit 2. Covalent molecular substances do not conduct electricity well as solids or liquids, the solids may be weak and brittle or soft and waxy, and many are insoluble in water. Many covalent molecular substances are liquids or gases: they melt (and some boil) below room temperature or not too far above. The same reasoning applies to melting: covalent bonds between atoms within molecules are not broken, but forces between the molecules must be partially overcome.īecause there are many different kinds of nonmetal atoms that can form covalent bonds, and because molecules can consist of anywhere from two to many thousands of atoms, the range of properties of covalent molecular substances is much broader than for ionic compounds or metals. No covalent bonds are broken during the boiling process and the same molecules are present in the gas phase as were present in the liquid phase. An important point in the activity you just completed is this: unlike ionic compounds, metals, or noble gases, where boiling involves freeing ions or atoms from each other, boiling a covalent molecular substance involves freeing molecules from each other. \)Ī substance made of molecules is called a covalent molecular substance.
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