The most acidic proton is on the phenol group, so if the compound were to be subjected to a single molar equivalent of strong base, this is the proton that would be donated. Because our goal is understanding dynamic chemical reactivity, we do NOT need to know the specific amount of the protonated and unprotonated forms of a compound. We simply need to know which form is predominate. When the pH of the environment is less than the pKa of the compound, the environment is considered acidic and the compound will exist predominately in its protonated form.
When the pH of the environment is greater than the pKa of the compound, the environment is considered basic and the compound will exist predominately in its deprotonated form.
For example, the pKa of acetic acid is about 5. At a pH of 1, the environment is considered acidic and acetic acid exists predominately in its protonated form.
At pH 8, the environment is considered basic, and acetic acid becomes deprotonated to form acetate CH 3 CO 2 -. 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.
Key Terms dissociation : the process by which compounds split into smaller constituent molecules, usually reversibly. Licenses and Attributions. The square brackets indicate the concentration of respective components. 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.
Chemistry Expert. Helmenstine holds a Ph. She has taught science courses at the high school, college, and graduate levels. Facebook Facebook Twitter Twitter. Updated November 11, A lower pKa value indicates a stronger acid. That is, the lower value indicates the acid more fully dissociates in water. Featured Video. Cite this Article Format. Helmenstine, Anne Marie, Ph. List of the Strong Acids and Key Facts. How to Make a Phosphate Buffer Solution.
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