Diprotic acids contain two ionizable hydrogen atoms per molecule; ionization of such acids occurs in two steps. The first ionization always takes place to a greater extent than the second ionization.
For example, sulfuric acid, a strong acid, ionizes as follows:. This stepwise ionization process occurs for all polyprotic acids. When we make a solution of a weak diprotic acid, we get a solution that contains a mixture of acids. Carbonic acid, H 2 CO 3 , is an example of a weak diprotic acid. The first ionization of carbonic acid yields hydronium ions and bicarbonate ions in small amounts. The ICE tables below display both ionization reactions, starting with a If the first ionization constant of a weak diprotic acid is larger than the second by a factor of at least 20, it is appropriate to treat the first ionization separately and calculate concentrations resulting from it before calculating concentrations of species resulting from subsequent ionization.
Example 5. When we buy soda water carbonated water , we are buying a solution of carbon dioxide in water. To summarize:. As for the ionization of any other weak acid:. An abbreviated table of changes and concentrations shows:. Substituting the equilibrium concentrations into the equilibrium gives us:. Solving the preceding equation making our standard assumptions gives:. To summarize: In part 1 of this example, we found that the H 2 CO 3 in a 0. Check Your Learning 5.
The concentration of H 2 S in a saturated aqueous solution at room temperature is approximately 0. We note that the concentration of the sulfide ion is the same as K a2. This is due to the fact that each subsequent ionization of a polyprotic acid occurs to a lesser degree.
A triprotic acid is an acid that has three ionizable protons that undergo stepwise ionization: Phosphoric acid is a typical example:. As with the diprotic acids, the differences in the ionization constants of these reactions tell us that in each successive step the degree of ionization is significantly weaker. This is a general characteristic of polyprotic acids and successive ionization constants often differ by a factor of about 10 5 to 10 6.
This set of three ionization reactions may appear to make calculations of equilibrium concentrations in a solution of H 3 PO 4 complicated. However, because the successive ionization constants differ by a factor of 10 5 to 10 6 , the calculations can be broken down into a series of parts similar to those for diprotic acids.
Polyprotic bases can accept more than one hydrogen ion in solution. The carbonate ion is an example of a diprotic base , since it can accept up to two protons. Solutions of alkali metal carbonates are quite alkaline, due to the reactions:.
Given the following chemical equilibrium reaction, which of the following expressions correctly describes its equilibrium constant, K :. Notice how in this reaction, the main species PO 4 3- accepts a proton as it reacts with water — this means that it is the base in this reaction and hence, K is more specifically K b. We can already rule out options a and c since they do not include K w in the expression for K.
The reaction indicated starts with a phosphate ion which is non-protonated that is converted into its conjugate monoprotic acid HPO 4 —. PO 4 3- is in its most deprotonated form no protons , so this reaction is K b1 — the first reaction in which PO 4 3- picks up its very first proton.
The empirical formula of the compound is The Solid State. Which of the following has the square planar structure? Chemical Bonding and Molecular Structure. The major product obtained in the photobromination of 2-methyl butane is. Questions from Equilibrium. JEE Advanced Which of the following are Lewis acids? JEE Main Which of the following is a Lewis acid? Chemistry Most Viewed Questions. Identify a molecule which does not exist.
Identify the incorrect match. Which of the following set of molecules will have zero dipole moment?
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