The most acidic proton is on the phenol group, so if the compound were to be reacted with a single molar equivalent of strong base, this is the proton that would be donated first. As you continue your study of organic chemistry, it will be a very good idea to commit to memory the approximate pK a ranges of some important functional groups, including water, alcohols, phenols, ammonium, thiols, phosphates, carboxylic acids and carbons next to carbonyl groups so-called a-carbons.
These are the groups that you are most likely to see acting as acids or bases in biological organic reactions. If you are asked to say something about the basicity of ammonia NH 3 compared to that of ethoxide ion CH 3 CH 2 O - , for example, the relevant pK a values to consider are 9. From these numbers, you know that ethoxide is the stronger base. Do not make the mistake of using the pK a value of this is the pK a of ammonia acting as an acid , and tells you how basic the NH 2 - ion is very basic!
Using the pK a table, estimate pK a values for the most acidic group on the compounds below, and draw the structure of the conjugate base that results when this group donates a proton. Please determine the Ka for acetic acid. Using a calculator first enter in the value for the pK a 4.
The make the number negative Next, use the inverse log function. Often it is the second function of the LOG button. Steven Farmer Sonoma State University. Objectives After completing this section, you should be able to write the expression for the K a of a weak acid.
Key Terms Make certain that you can define, and use in context, the key terms below. Study Notes Calculations and expressions involving K a and p K a were covered in detail in your first-year general chemistry course. Answer a. The p K a of acetic acid is 4. The K a of benzoic acid is 6. Based on this equation, Ka expresses how easily the acid releases a proton in other words, its strength as an acid.
Carboxylic acids containing -COOH , such as acetic and lactic acids, normally have a Ka constant of about 10 -3 to 10 Consequently, expressing acidity in terms of the Ka constant alone can be inconvenient and not very intuitive. Therefore, pKa was introduced as an index to express the acidity of weak acids, where pKa is defined as follows. In addition, the smaller the pKa value, the stronger the acid. For example, the pKa value of lactic acid is about 3.
Another important point is the relationship between pH and the pKa of an acid. This relationship is described by the following equation. 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. Unlike K a , percent dissociation varies with the concentration of HA; dilute acids dissociate more than concentrated ones.
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