Protonic acids strengths

The ability of an acid to lose a proton (acid strength) is experimentally measured by its acid ionization constant the higher the value of this constant the higher is the concentration of H30+ ions, and the stronger the acid. 

If, for a given acid, we wish to increase the acid strength, then we choose a solvent which has a greater affinity for protons than has water. If we add ammonia to a solution of hydrogen chloride in water, the essential equilibrium is... 

When we use any substance as a solvent for a protonic acid, the acidic and basic species produced by dissociation of the solvent molecules determine the limits of acidity or basicity in that solvent. Thus, in water, we cannot have any substance or species more basic than OH or more acidic than H30 in liquid ammonia, the limiting basic entity is NHf, the acidic is NH4. Many common inorganic acids, for example HCl, HNO3, H2SO4 are all equally strong in water because their strengths are levelled to that of the solvent species Only by putting them into a more acidic... 

Acids that are better proton donors than the solvent are leveled to the acid strength of the protonated solvent bases that are better proton acceptors than the solvent are leveled to the base strength of the deprotonated solvent. 

The kinetics of reactions cataly2ed by very strong acids are often compHcated. The exact nature of the proton donor species is often not known, and typically the rate of the catalytic reaction does not have a simple dependence on the total concentration of the acid. However, sometimes there is a simple dependence of the catalytic reaction rate on some empirical measure of the acid strength of the solution, such as the Hammett acidity function Hq, which is a measure of the tendency of the solution to donate a proton to a neutral base. Sometimes the rate is proportional to (—log/ig)- Such a dependence may be expected when the slow step in the catalytic cycle is the donation of a proton by the solution to a neutral reactant, ie, base but it is not easy to predict when such a dependence may be found. 

As might be expected intuitively, there is a relationship between the effectiveness of general acid catalysts and the acid strength of a proton donor as measured by its acic dissociation constant K. This relationship is expressed by the following equation, which is known as the Brensted catalysis law ... 

Many organic reactions involve acid concentrations considerably higher than can be accurately measured on the pH scale, which applies to relatively dilute aqueous solutions. It is not difficult to prepare solutions in which the formal proton concentration is 10 M or more, but these formal concentrations are not a suitable measure of the activity of protons in such solutions. For this reason, it has been necessaiy to develop acidity functions to measure the proton-donating strength of concentrated acidic solutions. The activity of the hydrogen ion (solvated proton) can be related to the extent of protonation of a series of bases by the equilibrium expression for the protonation reaction. 

Neutron-to-proton ratio, 29-30 Newton, 457,635 Newton, Isaac, 136 Nickel hydroxide, 78 Nicotinic acid, 364-365 NIMBY syndrome, 526 Nitric acid acid rain and, 400 acid strength of, 567 commercial use, 76 copper penny dissolving in, 570 production, 570-571... 

From this work the relative efficiencies of Bronsted acids, in the presence of stannic chloride (0.166 M) in promoting hydrogen exchange at 25 °C can be ascertained from the rate coefficients (106Art) as follows HC1 (44) H20 (27) AcOH (1.6) CF3COOH (very small). Thus the ability of the dual acid systems to transfer protons is not simply related to the conventional acid strength of the BrQnsted component. 

From the shapes of the rate versus acid strength graphs obtained for mesit-aldehyde and triisopropylbenzaldehyde it was concluded that although neither compound was specific acid-catalysed, the latter compound showed the nearer tendency to this catalysis at the higher acid concentrations again this may be a manifestation of the greater steric hindrance to protonation by H3 S04 than by H30+. 

The Brpnsted coefficient a represents the sensitivity of the rate to the acid strength of the catalyst. It is a measure of the degree of proton transfer from catalyst to substrate in the transition state. For nearly all reactions where BH+ contains acidic N-H or O-H groups, a is in the range 0-1. 

The proton-donating strength of an acid is measured by its acidity constant the proton-accepting strength of a base is measured by its basicity constant. The smaller the constants, the weaker the respective strengths. The larger the value of pK, the weaker the acid or base. 

In the treatment of weak acids, we found that the percentage deprotonated gave an indication of acid strength. Similarly, when we describe the strengths of weak bases, it is useful to know the percentage protonated, the percentage of base molecules that have been protonated ... 

Acid strength may be defined as the tendency to give up a proton and base strength as the tendency to accept a proton. Acid-base reactions occur because acids are not equally strong. If an acid, say HCI, is placed in contact with the conjugate base of a weaker acid, say acetate ion, the proton will be transferred because the HCI has a greater tendency to lose its proton than acetic acid. That is, the equilibrium... 

The catalyst acidity is determined by the number of acid sites and their acid strength. The total concentration of acid sites, C<, can be obtained from independent TPD measurements. The average acid strength of the sites is characterized by the alkene standard protonation enthalpy,, and is typically determined by regression using reference... 

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