A Closer Look at Acids

For most people, all acids are liquids. This is most probably caused by the fact that the common acids in eveiyday life, such as hydrochloric acid or battery acid are indeed liquids, and nitric acid or phosphoric acid, which one may remember from school, are also liquids. 

However, a careful look in a supermarket will reveal, perhaps to the customer s surprise, white powders labeled citric acid and tartaric acid . In pharmacies, ascorbic acid is often sold as the pure active ingredient of vitamin C, and it is also a white powder. Can t chemists make up their minds  

actually, the case is much worse. Experienced chemists ordinarily mean different things when they use the word acid . The concept of acid has three different, but commonly used definitions in science. After all this, it may not be too surprising that a fourth one will be discussed first. 

The first scientific definition in this new field was described by a Nobel laureate, Swedish chemist Svante Arrhenius (1859-1927). His theory, which is now called the Arrhenius acid-base concept, states that acids are substances that produce hydrogen ions in an aqueous solution. Hydrogen ions (H+) do not exist in solutions because they are always attached to at least one water molecule- often written as hydroxonium ion (H3O+). More detailed studies show that H3O+ ions are not very common, either, and hydrogen ions are often attached to more than one water molecules. But this is only a matter of notation and different chemists use H+, H3O+, Hj02 or H,03+ to mean essentially the same thing. 

The Arrhenius model is still widely used, and substances that are called acid are usually acids in the Arrhenius sense. Since citric acid, for example, produces hydrogen ions in solution, it is therefore an acid. Inddenlally, it also tastes sour and is edible at the same time. The same is true for tartaric add. The most commonly consumed form of tartaric add is wine, but it is mixed with alcohol there, which may have a significant health efled. In the Arrhenius concept, a substance can be called acid or base without further specifications (bases decrease the concentration of hydrogen ions). In the remaining two theories, strictly speaking, a substaiKe cannot be called acid or base. All that can be said is that a certain substance behaves as an add (or base) in a certain chemical reaction with another reactarrt. 

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Proton Transfer A Closer Look at Acid-Base Reactions... 

Another important type of equihbrium, which we will study in the second half of the chapter, involves the dissolution and precipitation of shghtly soluble substances. These processes are examples of heterogeneous equilibria that is, they pertain to reactions in which the components are in more than one phase. But first we will conclude our discussion of acid-base equilibria by considering buffer solutions and taking a closer look at acid-base titrations. 

The same order of reactivity is also found for reactions such as acid-catalyzed hydrolysis a closer look at the stmcture of these vinyl ether homologues suggests the reason. 

Among the solution reactions considered in Chapter 4 were those between acids and bases. In this chapter, we take a closer look at the properties of acidic and basic water solutions. In particular, we examine—... 

Studying the sequences of farnesylated proteins indicated that all lipidated proteins bear a cysteine residue near the C-terminus revealing the CAAX-motif, where C is a cysteine, A stands for an aliphatic amino acid, and X can be any amino acid. Database searches resulted in more prenylated proteins, all bearing the CAAX-motif, in systems from lower eukaryotes to mammals. A closer look at the mature proteins revealed that prenylation was only the first step of processing of the CAAX-motif-encoded proteins. After transfer of the isoprene unit, the last three amino acids are cleaved proteolytically by an endoprotease and the C-terminal cysteine is carboxymethylated by a methyltransferase. ... 

We now take a closer look at the first stage of fatty acid oxidation, beginning with the simple case of a saturated fatty acyl chain with an even number of carbons, then turning to the slightly more complicated cases of unsaturated and odd-number chains. We also consider the regulation of fatty acid oxidation, the j8-oxidative processes as they occur in organelles other than mitochondria, and, finally, two less-general modes of fatty acid catabolism, a oxidation and [Pg.637]

This chapter categorizes the amino acids into families based on the origin of their carbon skeleton. Is this an absolute pattern Take a closer look at the information in this chapter to produce an answer. 

What is it that makes an acid an acid and a base a base We first raised those questions in Section 4.5, and we now take a closer look at some of the concepts that chemists have developed to describe the chemical behavior of acids and bases. We ll also apply the principles of chemical equilibrium discussed in Chapter 13 to determine the concentrations of the substances present in aqueous solutions of acids and bases. An enormous amount of chemistry can be understood in terms of acid-base reactions, perhaps the most important reaction type in all of chemistry. 

Any substance which, once dissolved in water, releases one or more protons (= H+ ions) to the water molecules is called an acid. Let us have a closer look at how the gas hydrogen chloride (HC1) dissolves in water (Fig. 3.14). For the HC1 molecules to penetrate the water molecules, they have to destroy the H- bridges between those water molecules. HC1 dissolves very easily in water since it is capable of forming new bridges. After the H atoms of the HC1 molecules have actually formed these new bridges with water molecules, competition arises the O atom of the water wants to bind to the H atom, but for that the H-Cl bonds needs to be broken. In this case the H-bridge to the 0 turns out to be stronger than the H-Cl bond. The H atom moves in the form of H+to the water molecule. 

In this section we will have a closer look at the sensors which are used for measuring substances, the so-called chemical sensors. A very well-known example of such a sensor in a chemical laboratory is the pH-meter used to measure the acidity of solutions. In particular the so-called Lambda-probe, abbreviated l-probe, which is used in cars to optimize the combustion process will be discussed in more detail. 

As we said earlier, nucleic acids are the architects and construction contractors for synthesizing proteins. There are two kinds of nucleic acids. DNA, or deoxyribonucleic acid, is the blueprint for synthesis of proteins. RNA, or ribonucleic acid, is the construction contractor. Messenger RNA reads the instructions for synthesis of a protein encoded on a strand of DNA and carries those instructions to the worksite, where transfer RNA brings the amino acids in for incorporation into the polypeptide chain. Now, let s take a closer look at the structures of DNA and RNA. 

To take a closer look at the area per molecule in the condensed phase of the diblock monolayers, the x-axis of Fig. 4.14a is expanded and shown in Fig. 4.14c. The extrapolated value of the area per molecule and the theoretically expected value for the area are listed in Table 4.5. To calculate the theoretically expected area for the hybrid block copolymers in the condensed phase, it is assumed that the stearate end groups are extended into the air perpendicular to the air-water interface as shown schematically in Fig. 4.14d. This assumption is based on the behavior of pure stearic acid, which forms ordered monolayers with the alkyl chains oriented perpendicular to the air-water interface. The area per molecule for stearic acid with this orientation is 20 A2, [113] Here, the theoretically expected area was calculated by multiplying the area per stearate molecule (20 A2) with the number of stearate groups present at the ends of the dendrimer block. PEO(2k)-S having no dendrimer block but a single... 

A typical gene expression profiling experiment takes place in five separate processes. They are (i) microarray fabrication, (ii) purification and labeling of the target material, (iii) hybridization, (iv) detection and (v) data analysis. The characteristics of each step were briefly discussed in the introduction. A closer look at each of these steps is the object of this section. Here we mainly refer to biochips where the probe is constituted by nucleic acids (DNA microarrays). 

The Br0nsted-Lowry definition of acids and bases does not replace the Arrhenius definition, but extends it. The Bronsted-Lowry definition of acids and bases requires you to take a closer look at the reactants and products of an acid-base reaction. In this case, acids and bases are not easily defined as having hydronium and hydroxide ions. Instead, you are asked to look and see which substance has lost a proton and which has gained the very same proton that was lost. 

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