Hence, H 2 is a very weak acid. Write a balanced equation to describe the reaction which occurs when a solution of potassium hydrogen sulfate, KHSO 4 , is mixed with a solution of sodium bicarbonate, NaHCO 3.
Therefore any reaction which occurs must be between the hydrogen sulfate ion, HSO 4 — and the hydrogen carbonate ion, HCO 3 —. The reaction between them is thus either. What reactions will occur when an excess of acetic acid is added to a solution of potassium phosphate, K 3 PO 4? There is a further possibility because HPO 4 2— is itself a base and might accept a second proton. Hence double arrows are used. Solution: HCl is a strong acid. When it donates a proton, a Cl — ion is produced, and so Cl — is the conjugate base.
The strength of an acid — its pKa — is measured by the position of the equilibrium between an acid and its conjugate base, which is a function of their relative thermodynamic stability. What makes the difference in nucleophilocity? When you have a conjugate base of a weak acid, the base is necessarily a strong base.
That is to say it has a greater tendency to capture protons than the conjugate base of a strong acid ie. Judging from the above paragraph alone, one might be tempted to think that a stronger base would make for a stronger nucleophile.
However, the strength of a conjugate base is measured by its unstability the more unstable the conjugate base, the stronger the acid. So when you have a weak acid and therefore a strong conjugate base, you have a very unstable species with an enormous tendency for removing protons in order to neutralize its negative charge. Nucleophily though, is a slightly different thing. It is not as much an ability to neutralize the base itself and its negative charge as an urge to neutralize positive charges by donating electrons.
So, when you have a carbocation, for example, you need a species that attacks the positive charge created by the electron deficit generated by the leaving of the exiting group. Thus, the equation for the reaction between HCl and water can be written as follows.
It isn't surprising that The stronger of a pair of acids should react with the stronger of a pair of bases to form a weaker acid and a weaker base. Once again, the reaction between the acid and water must convert the acid into its conjugate base and the base into its conjugate acid.
But this time, the stronger acid and the stronger base are on the right side of the equation. Determine which is the stronger base, the NH 2 - or OH - ions. Click here to check your answer to Practice Problem 4. The magnitude of K a can also be used to explain why some compounds that qualify as Brnsted acids or bases don't act like acids or bases when they dissolve in water. When the value of K a for an acid is relatively large, the acid reacts with water until essentially all of the acid molecules have been consumed.
As K a becomes smaller, the extent to which the acid reacts with water decreases. As long as K a for the acid is significantly larger than the value of K a for water, the acid will ionize to some extent. As the K a value for the acid approaches the K a for water, the compound becomes more like water in its acidity. Although it is still a Brnsted acid, it is so weak that we may be unable to detect this acidity in aqueous solution. Some potential Brnsted acids are so weak that their K a values are smaller than water's.
Ammonia, for example, has a K a of only 1 x 10 Although NH 3 can be a Brnsted acid, because it has the potential to act as a hydrogen-ion donor, there is no evidence of this acidity when it dissolves in water. The Leveling Effect of Water. All strong acids and bases seem to have the same strength when dissolved in water, regardless of the value of K a.
This phenomenon is known as the leveling effect of water the tendency of water to limit the strength of strong acids and bases. Some of the acetic acid ethanoic acid molecules dissociate in water, producing acetate ions ethanoate ions and protons. Play the game now! For example, ammonia, NH 3 , is a moderately weak base. Cl - has little tendency to gain a proton so it is a weak base, but its conjugate acid, HCl, has an enormous tendency to donate a proton it is a strong acid.
Take the test now! See the tutorial on Acid and Base Definitions.
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