Privacy Policy. Skip to main content. Acids and Bases. Search for:. Strength of Acids Strong Acids In water, strong acids completely dissociate into free protons and their conjugate base. Learning Objectives Calculate pH for solutions of strong acids. Key Takeaways Key Points Strong acids can catalyze chemical reactions. Strong acids are defined by their pKa. The acid must be stronger in aqueous solution than a hydronium ion, so its pKa must be lower than that of a hydronium ion.
Strong acids can be organic or inorganic. Strong acids must be handled carefully because they can cause severe chemical burns. Strong acids are essential for catalyzing some reactions, including the synthesis and hydrolysis of carbonyl compounds. Key Terms carbonyl : a divalent functional group -CO- , characteristic of aldehydes, ketones, carboxylic acids, amides, carboxylic acid anhydrides, carbonyl halides, esters, and others.
Weak Acids A weak acid only partially dissociates in solution. Learning Objectives Solve acid-base equilibrium problems for weak acids. Key Takeaways Key Points The dissociation of weak acids, which are the most popular type of acid, can be calculated mathematically and applied in experimental work.
If the concentration and K a of a weak acid are known, the pH of the entire solution can be calculated. The exact method of calculation varies according to what assumptions and simplifications can be made. Weak acids and weak bases are essential for preparing buffer solutions, which have important experimental uses. Key Terms conjugate acid : the species created when a base accepts a proton conjugate base : the species created after donating a proton.
Learning Objectives Calculate percent dissociation for weak acids from their K a values and a given concentration. Using this principle, you can also use the pKa table to give you the strengths of bases. I call this the inverse pKa table. Find the acid on the pKa table. Find the base on the inverse pKa table. Do the acid base reaction — that is, add a proton to the base and remove a proton from the acid.
Evaluate : Is the new acid stronger or weaker? Is the new base stronger or weaker? So CH 4 is the acid and HO - is the base in this reaction. Doing the proposed acid base reaction, we transfer a proton from CH 4 to HO -. The products of this reaction would therefore be CH 3 - and water. Now we ask the question — how do these compare in strength to our starting acids and bases? Our product is a stronger acid. Our product is a stronger base. We need to go to a weaker acid-base pair see 2, above.
This reaction will go. HCl will clearly act as an acid here, and NH 3 will act as a base. Our acidic product, NH4, has a pKa of 9. Our product is a weaker acid than HCl. Our basic product, Cl - ranks below NH 3 on our inverse pKa scale. Our product is a weaker base than NH3. Conclusion: this reaction is also a go. And, indeed, if you find yourself in a freezing hut with only a bottle of concentrated HCl and aqueous ammonia to keep you company, adding them together will definitely warm up your day.
This is about the only situation in which I would recommend this. How do you deal with a compound that is similar but not on the table? Take hexane, for instance. One final point on the big-picture type view.
Note the pattern. In a similar way, if a compound gives up a proton and becomes a strong base, the base will readily take the proton back again. Effectively, the strong base competes so well for the proton that the compound remains protonated. The compound remains a Bronsted acid rather than ionizing and becoming the strong conjugate base.
It is a weak Bronsted acid. The pKa scale and its effect on conjugate bases. However, the terms "strong" and "weak" are really relative. If something with a pKa of 4 is described as a weak acid, what is something with a pKa of 25? A very, very weak acid? It is certainly a better source of protons than something with a pKa of Is that a very, very, very, very weak acid? How many "verys" are there in a pKa unit? This idea is also true when considering the opposite: a base picking up a proton to form a conjugate acid.
How tightly that conjugate acid holds a proton is related to how strongly the base can remove protons from other acids. The weaker something is as a source of protons, the stronger its conjugate is as a proton sponge. Examples of a strong base and an even stronger one. A more extensive pKa table can be found at Prof. David Evans' site at Harvard. Problem AB9. Find a pKa table. Use it to help you decide which of the following pairs is the most Bronsted acidic in water.
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