Acidic solutions hydrogen ions from water

In Figure 18.26, several drops of bromthymol blue indicator solution have been added to 0.1 OM aqueous solutions of the salts ammonium chloride (NH4CI), sodium nitrate (NaNOs), and potassium fluoride (KF). Sodium nitrate turns the indicator green, which means that the solution is neutral. The blue color of the KF solution means that the solution is basic, and the yellow color of the ammonium chloride solution indicates that the solution is acidic. Why are some aqueous salt solutions neutral, some basic, and some acidic Many salts react with water in a process known as salt hydrolysis. In salt hydrolysis, the anions of the dissociated salt accept hydrogen ions from water or the cations of the dissociated salt donate hydrogen ions to water. 

A schematic view of the general glass leaching process is given in Figure 2. There is an exchange of the metal ions with the hydrogen ions from water. This is typical of an acidic solution [4]. 

For each of the following weak acids, find the pH of a solution made from 0.100 mol of the acid and 1.000 kg of water at 298.15 K. Do each calculation twice once assuming that y equals unity, and once using the Davies equation to estimate y . In each case, decide whether hydrogen ions from water must be included ... 

When an alkali metal contacts water, metal atoms donate electrons to water molecules, producing hydrogen gas and a solution of the metal cation (for example, Na ). When a metal such as Ca, Zn, or Fe is treated with a strong aqueous acid, hydronium ions in the acid solution accept electrons from metal atoms, creating cations that then dissolve. We describe these redox reactions in Chapter 4. Zinc metal, for example, reacts with hydrochloric... 

If these two aqueous solutions are mixed, a chemical reaction takes place. The hydrogen ion from the hydrochloric acid combines with the hydroxide ion (OH-) from the sodium hydroxide to form water—the (1) following the formula for water shows that... 

In both equations the position of equilibrium lies well over to the left-hand side, i.e. to the reactant side. When potassium propanoate dissolves in water, the propanoate ions react with the hydrogen ions from the water to form propanoic acid molecules. This effectively removes some of the H+(aq) ions and so there will be fewer hydrogen ions than hydroxide ions present and the solution is alkaline. 

According to the Arrhenius definition of acids and bases, acids are substances that produce hydrogen ions (H+) in solution, and bases are substances that produce hydroxide ions (OH ) in solution. When an acid and a base combine, the hydrogen ions from the acid react with the hydroxide ions from the base to form water—a neutralization reaction. 

This reaction shows that the hydroxide ions come from ammonia pulling a hydrogen away from water resulting in the formation of OH . Therefore, a compound does not have to contain hydroxide to be a base. In addition to this limitation, the Arrhenius definition limits acids and bases to aqueous solutions. 

This reaction is clearly effective in removing hydrogen ions from solution. The acid is presumed to be formed by the removal of a molecule of water from two hydroxyl groups of two molecules of thiosulphuric acid—not from one hydroxyl and one thiol group—in which case its structure may be written as ... 

The term acid is derived from the Latin word acidus meaning sour. Arrhenius (1859-1927) of Sweden introduced the idea of compounds splitting into their constituent ions. Based on this idea, he was able to explain that the concentration of the hydrogen ions in water (aqueous solution) determined the strength of an acid. Arrhenius was awarded the Nobel Prize in 1903 for his work on ionization. Acids are classified as naturally occurring acids and mineral acids. 

A standard hydrogen electrode can easily be built from a platinum foil, coated by platinum black by an electrolytic process, and immersed in a solution of hydrochloric acid containing hydrogen ions of unit activity (a mixture of 1000 g water and 1-184 mol hydrogen chloride can be used in practice). Hydrogen gas at a pressure of 1 atm is passed over the foil. A convenient form of the standard hydrogen electrode is shown on Fig. 1.16. The gas is introduced... 

Sodium hydroxide is an alkali. When added to an acid, the hydroxide ion from the sodium hydroxide combines with the hydrogen ion from the add to form water, and the remaining acid ion combines with the sodium to form a salt. To put it in its simplest terms, the hydrogen ions from an add neutralise the hydroxide ions from an alkali by combining with them to form water molecules. The salt is simply the ions which are left in solution, and to get the solid salt you simply have to evaporate off the water. 

The atmospheric agents, above all acids formed from carbon dioxide and the oxides of nitrogen and sulfur (carbonic acid, nitric acid, and sulfuric acid), titrate the bases of the minerals, that is, the carbonates, the silicates, and the oxides to form solutes. Depending on whether the end point of the titration is exceeded or not, the residual waters contain alkalinity (excess of bases such as HCO3 ) or acidity (excess of acids, especially hydrogen ions). The interaction of acids with minerals can be formulated as follows (we use carbonic acid, H2CO3) ... 

As you have discovered, the properties of acids and bases are determined by the submicroscopic interactions between the acid or base and the solvent water. For example, HCl and HC2H3O2 both interact with water to cause a transfer of hydrogen ions from the acid to water molecules to form hydronium ions. Both solutions turn blue litmus red. Even though these properties are the same for both acids, remember that the conductivity of a IMHCl solution is much greater than that of a IMHC2H3O2 solution. 

The simplification in the calculation is not serious, particularly since equilibrium constants are often not known to a high degree of accuracy (frequently no better than 10%). In the above example, solution of the quadratic equation results in [H+] = 1.26 X 10- M (5% less) and pH = 3.90. This pH is within 0.02 unit of that calculated using the simplification, which is near the limit of accuracy to which pH measurements can be made, and almost certainly as close a calculation as is justified in view of the experimental errors in Ka or Kb values and the fact that we are using concentrations rather than activities in the calculations. In our calculations, we also neglected the contribution of hydrogen ions from the ionization of water (which was obviously justified) this is generally permissible except for very dilute (<10 M) or very weak Ka < 10 ) acids. 

The first method for finding approximate roots to an equation is to modify the equation by making simplifying assumptions. As an example, let us consider an equation for the hydrogen-ion concentration in a solution of a weak acid in which the hydrogen ions from the ionization of water cannot be ignored. We must solve simultaneous equations for the ionization of the weak acid and ionization of water. Later in this chapter, we will derive the equation... 

The metals can be activated toward alkalies by coupling them with other metals, just as they are activated toward water and acids. When zinc is in contact with iron the former reacts rapidly with a warm solution of sodium hydroxide. Hydrogen is evolved from the iron and the zinc passes into solution as sodium zincate, NagZnOg. Tin behaves in a similar way and sodium stannite, NagSnOa, is formed. It should be noted that in these cases the metal that dissolves is the one which forms a hydroxide soluble in sodium hydroxide. When two metals are coupled in neutral or acid solution hydrogen is evolved from the metal having the lower solution pressure— that is, the one lower in the electromotive series. When the solution contains an alkali, this is not always the case. Zinc is above and tin is below iron in the electromotive series, but when activated by iron in the presence of an alkali they each dissolve. The solubility of the hydroxide in alkalies is the determining factor in the reaction. The zinc that passes into solution is not presnt in the form of zinc ions, but as zincate ions, ZnOo— which result from the ionization of the salt formed. 

The cations and anions of salts derive from the reactions of acids with bases. The conjugate bases of weak acids are the anions of many salts, e.g., acetate, carbonate, phosphate. All these anions are themselves weak bases, so that in water they hydrolyze and produce basic solutions. Certain cations are weak acids,e.g., NH4, and in water will hydrolyze to produce acidic solutions. There are no basic cations. All Group I and Group II cations, e.g., Na, Ca, are neutral in aqueous solution. However, salts of the cations AP", Pb " ", Sn " and of the transition metals Fe, etc., form acidic solutions. In these metal salts, water is covalently bound to the central cation. A hydrogen ion from the bound water dissociates and causes aqueous solutions of these salts to be acidic. For example, FefHjO)/ is... 

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