Molecular covalent substances

Physical properties of molecular covalent substances. At first glance, the model seems inconsistent with physical properties of covalent substances. Most are gases (such as methane and ammonia), liquids (such as benzene and water), or low-melting solids (such as sulfur and paraffin wax). If covalent bonds are so strong (-200 to 500 kJ/mol), why do covalent substances melt and boil at such low temperatures ... 

To answer this, we ll focus on two different forces (1) strong bonding forces hold the atoms together within the molecule, and (2) weak intermolecular forces act between separate molecules in the sample. It is the weak forces between molecules that account for the physical properties of molecular covalent substances. For example, look what happens when pentane (C5H12) boils (Figure 9.13) weak forces between pentane molecules are overcome, not the strong C—C and C—H bonds within each pentane molecule. 

Molecular covalent substances are soft and low melting because of the weak forces between the molecules. Network covalent solids are hard and high melting because covalent bonds join all the atoms in the sample. 

In molecular covalent compounds, intermolecular forces are very weak in comparison with intramolecular forces. For this reason, most covalent substances with a low molecular mass are gaseous at room temperature. Others, with higher molecular masses may be liquids or solids, though with relatively low melting and boiling points. 

A problem with this method is the fact that mineral oil is a mixture of covalent substances (high-molecular-weight hydrocarbons) and its characteristic absorption spectrum will be found superimposed in the spectrum of the solid analyte, as with the solvents used for liquid solutions discussed previously. However, the spectrum is a simple one (Figure 8.24) and often does not cause a significant problem, especially if the solid is not a hydrocarbon. 

There is an ill-defined boundary between molecular and polymeric covalent substances. It is often possible to recognise discrete molecules in a solid-state structure, but closer scrutiny may reveal intermolecular attractions which are rather stronger than would be consistent with Van der Waals interactions. For example, in crystalline iodine each I atom has as its nearest neighbour another I atom at a distance of 272 pm, a little longer than the I-I distance in the gas-phase molecule (267 pm). However, each I atom has two next-nearest neighbours at 350 and 397 pm. The Van der Waals radius of the I atom is about 215 pm at 430 pm, the optimum balance is struck between the London attraction between two I atoms and their mutual repulsion, in the absence of any other source of bonding. There is therefore some reason to believe that the intermolecular interaction amounts to a degree of polymerisation, and the structure can be viewed as a two-dimensional layer lattice. The shortest I-I distance between layers is 427 pm, consistent with the Van der Waals radius. Elemental iodine behaves in most respects - in its volatility and solubility, for example - as a molecular solid, but it does exhibit incipient metallic properties. 

Molecular formulas apply to covalent substances only. 

Organolithium compounds are among the very few alkali-metal compounds that have properties—solubility in hydrocarbons or other non-polar liquids and high volatility—typical of covalent substances. They are generally liquids or low-melting solids, and molecular association is an important structural feature,26... 

Write full molecular formulae for solids and all covalent substances. 

Staudinger rejected the idea that these substances were organic colloids. He hypothesized that the high molecular weight substances known as polymers were true macromolecules formed by covalent bonds. Staudinger s macromolecular theory stated that polymers consist of long chains in which the individual monomers (or building blocks) are connected with each other by normal covalent bonds. The... 

Aqueous chemical reactions are those that occur in water. Because of its molecular shape and uneven distribution of electrons, water dissolves many ionic and covalent substances. In water, many ionic compounds and a few simple, H-containing covalent compounds, such as HCI, dissociate into ions. Section 4.1)... 

Water dissolves many covalent (molecular) compounds also. Table sugar (sucrose, C12H22O11), beverage (grain) alcohol (ethanol, CH3CH2OH), and automobile antifreeze (ethylene glycol, HOCH2CH2OH) are some familiar examples. All contain their own polar bonds, which interact with the bonds of water. However, most soluble covalent substances do not separate into ions, but remain intact molecules. For example,... 

The molecular formula represents the actual number of atoms in a molecule of a simple covalent substance. The empirical formula and molecular formula may be identical for a molecule or they may be different. The empirical formula may be found by dividing the coefficients in the molecular formula by the highest common factor. 

A general problem of the potentiometric measurement is the low exchange current density of enzymatic substrate redox couples. Therefore additional mediators have to be applied with a high exchange current densitiy and fast electrode kinetics, which can react with the enzymatic products. Most often, these mediators are soluble low molecular weight substances, e.g. hexacyanoferrate. Until now, redox polymers based containing covalently bound osmium complexes [5 ] were only used in combination with oxidases for amperometric sensors. We could show that this redox polymer (E =... 

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