Which one of the following is not an acid/base reaction?
A. {HNO_3 + HClO_4 → H2NO3^+ + ClO4^-}
B. {2NH_3 → NH_2^– + NH_4^+}
C. {Al(H_2O)_6 ^{3+} + H_2O → Al(H_2O)_5(OH)^{2+} + H_3O^+}
D. {CH_4 + H(SbF_6) → CH_5^+ + SbF_6^–}
E. {FeCl_3 + 6H_2O → Fe(H_2O)_6 ^{3+} + 3Cl^-}
To answer this question we have to know what the definition of an acid and base is according to Bronsted-Lowry.
An acid is any substance that is a proton donor and the proton acceptor is the conjugate base. Thus, searching for an acid and its conjugate base in each of the choices can give us the answer, E.
Here are some characteristics of acid and bases that are very important to remember for the IMAT exam, as mentioned in the free IMAT course on the website.
Acids have long been recognized as a distinctive class of compounds whose aqueous solutions exhibit the following properties:
Acidic solutions have a pH less than 7, with lower pH values corresponding to increasing acidity. Common examples of acids include acetic acid (in vinegar), sulfuric acid (used in car batteries), and tartaric acid (used in baking).
There are three common definitions for acids:
The strength of an acid refers to how readily an acid will lose or donate a proton, oftentimes in solution. A stronger acid more readily ionizes, or dissociates, in a solution than a weaker acid. The six common strong acids are:
Each of these acids ionize essentially 100% in solution. By definition, a strong acid is one that completely dissociates in water; in other words, one mole of the generic strong acid, HA, will yield one mole of H+, one mole of the conjugate base, A−, with none of the unprotonated acid HA remaining in solution. By contrast, however, a weak acid, being less willing to donate its proton, will only partially dissociate in solution. At equilibrium, both the acid and the conjugate base will be present, along with a significant amount of the undissociated species, HA.
Two key factors contribute to overall strength of an acid:
These two factors are actually related. The more polar the molecule, the more the electron density within the molecule will be drawn away from the proton. The greater the partial positive charge on the proton, the weaker the H-A bond will be, and the more readily the proton will dissociate in solution.
Acid strengths are also often discussed in terms of the stability of the conjugate base. Stronger acids have a larger Ka and a more negative pKa than weaker acids.
There are three common definitions of bases:
In water, basic solutions will have a pH between 7-14.
A strong base is the converse of a strong acid; whereas an acid is considered strong if it can readily donate protons, a base is considered strong if it can readily deprotonate (i.e, remove an H+ ion) from other compounds. As with acids, we often talk of basic aqueous solutions in water, and the species being deprotonated is often water itself. The general reaction looks like:
Thus, deprotonated water yields hydroxide ions, which is no surprise. The concentration of hydroxide ions increases as pH increases.
Most alkali metal and some alkaline earth metal hydroxides are strong bases in solution. These include:
The alkali metal hydroxides dissociate completely in solution. The alkaline earth metal hydroxides are less soluble but are still considered to be strong bases.
I can’t understand why E is the answer. I think (Fe(H2O)6)³+ is an acid. So because it’s one of the products, the whole of reaction isn’t an acid/base reaction? Or there is another reason for it? And why B is not the answer? NH3 is a base but in the products we cannot see an acceptor for OH_. Right?
None of the reactants in E have produced an H+. So the reaction is not an acid-base reaction.
On the contrary, we have in:
A.
One H+ is produced by perchlorate acid (HClO4) which protonates nitric acid (HNO3). Adding this H+ to HNO3, we get one more H and one + charge on it resulting to H2NO3+. On the other hand one H+ less in HClO4 leaves no H in it, and a negative net charge is produced in ClO4-.
B.
Acidic and basic properties of a substance is revealed in the reactions. It is not quite pre-defined. Here in this reaction, one of the two NH3 molecules contributes with one H+, protonates the other one, resulting in one H and one + charge imposed on NH3 which gives NH4+. The other NH3 is going to lack one H and one +, resulting in NH2-.
C.
One of the water molecules of Al(H2O)6+ acts as an acid, protonating the other reactant which in this case is a water molecule again, resulting in a H3O+.
When that water molecule has contributed with one H+, what is left in Al(H2O)6+ is a OH- which gives the formula Al(H2O)5(OH)2+ where it has reduced its plus charge by 1 due to one negative charge imposed by the OH-.
D.
H(SbF6) can contribute with one H+, protonating CH4 to CH5+, leaving SbF6-.
In the reactions above, HNO3, One of the two NH3 molecules, non-complexed water molecule (lone H2O), and CH4 are the conjugate bases. Conjugate bases are those which get protonated.
I hope these method gives a clear picture of how to handle these kind of questions.
Oh! So an acid can have lower acidic properties in contrast to another acid and play as a base in the reaction, and we don’t need to find _OH in the products to find out which of them was the acidic substance. Thank you very much. I think I have finally got it.
