How does pka relate to acid strength

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. From your answers to the questions above, determine whether acetic acid or benzoic acid is stronger.

Contributors and Attributions Dr. Krista Cunningham. This observation leads to a modified equilibrium constant known as the acidity constant or dissociation constant, K a.

To define the acidity constant that is a scale of acidity, consider the generic acid-base reaction:. The following expression represents the acidity constant for this reaction. Figure 2: Acidity constant for a generic acid dissociation. This relationship focuses on the concentration of hydronium ions in the numerator. Accordingly, an increase in these ions leads to an increasing acidity constant and a stronger acid. In organic acids, typically, the magnitude of K a is spread across several orders.

Hence, the strength of different acids is expressed in terms of p K a values, calculated as the negative logarithm of K a :. Here, the minus sign indicates the inverse relationship between the p K a value and acidity. As elucidated with benzoic acid versus hydrobromic acid, a higher p K a value equals a lower K a value, which indicates a weaker acid. By extension of the above principle, the p K a values can also establish the strength of a base.

Since the dissociation of a base forms a conjugate acid, a stronger conjugate acid corresponds to a weaker base. For instance, consider methanol versus ethylamine. Hence, methanol is a weaker base than ethylamine.

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If you have any questions, please do not hesitate to reach out to our customer success team. Login processing Chapter 5: Acids and Bases. Chapter 1: Covalent Bonding and Structure. Chapter 2: Thermodynamics and Chemical Kinetics. An ICE chart is used to calculate the relevant concentrations.

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. Ka is defined by the following equation. 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.