Acids and Bases - Online Tutor, Practice Problems & Exam Prep

In biochemistry, Bronsted-Lowry acids donate protons, while bases accept them. Conjugate acids and bases differ by one proton and charge. For example, a carboxylic acid donates a hydrogen to water, forming a conjugate base and a conjugate acid. Water can act as both an acid and a base, demonstrating its amphiprotic nature. Understanding these concepts is crucial for grasping acid-base reactions and their implications in biochemical processes.

Br∅nsted-Lowry Acids & Bases

In this video, we're going to refresh on acids and bases. So, usually in biochemistry, when we're talking about acids and bases, we're really referring to Bronsted Lowry acids and bases. And so recall that a Bronsted Lowry acid is a substance capable of donating a proton, whereas on the other hand, Bronsted Lowry bases are substances capable of accepting a proton. And so recall from your previous chemistry courses that conjugate acids and conjugate bases differ from each other respectively by a gain or loss of 1 proton and one charge. And so conjugate acids have 1 additional hydrogen and one additional charge, and conjugate bases have 1 less hydrogen and one less charge.

So let's take a look at our example below. And before identifying the conjugate acids and bases, it's helpful to identify the parent acids and bases. And so, over here on the reactant side, we have a reactant which is a carboxylic acid, because it has a carboxyl group, and then the other reactant is a water gonna be the parent acid, and the water molecule on the other hand is gonna be the parent base. And again, acids donate hydrogens whereas bases accept them. So, the carboxylic acid has an acidic hydrogen shown in green here that it's going to donate, and the base is going to accept that hydrogen. In general, parent bases are going to have a conjugate acid or turn into a conjugate acid, and parent acids are gonna turn into a conjugate base. And so notice that the conjugate acid of the water molecule has one additional hydrogen and one additional charge, whereas the conjugate base of the parent acid here is going to have one less hydrogen and one less charge.

And so notice here that the conjugate acid of the water molecule has a new bond that's formed here that wasn't present over here. And so when you make a bond, you break a bond. And so really what's happening in this reaction is we've got electrons on the water molecule attacking this hydrogen to form a new bond, and then these electrons here are breaking the bond between the oxygen and the hydrogen, and the electrons are jumping on the oxygen to make it a negative charge. So you can see over here, that's where the negative charge comes from and we've broken this bond. And then over here, we formed a bond and we've gained an additional charge because the lone pair is now being shared with the hydrogen.

And so, in the next part, what I want to emphasize is amphiprotic molecules are acid. So up here, we see that water is acting as a base. And down here in this example, notice that water is actually going to act as an acid. And so, what we've got in this example is that we've got an acidic hydrogen on the water molecule shown in green. And so, water is actually more acidic than ammonia, which is over here. And so the parent acid again is going to have a conjugate base, and the parent base is going to have a conjugate acid. And so here, what's happening is we're breaking a bond here and we're making a bond here. So when you make a bond, you break a bond. So really what's happening is electrons on the nitrogen are attacking the hydrogen to make a new bond here, and then the electrons in here, between this bond are breaking to jump on the oxygen. So here we get a hydroxide ion, which is the conjugate base of water, and then we get this NH4+ ion, which is the conjugate acid of ammonia. And so you can see that this is a good summary of acids and bases and amphiprotic molecules. And in our next video, we're gonna get some practice. So, I'll see you guys in those videos.

Which is the conjugate base of methylamine (CH ₃NH₂)?

• A. CH₃NH^-
• B. CH₃NH₃⁺
• C. CH₂NH₂^-
• D. CH₃NH₂OH^-
• E. None of the above.

Consider the reaction & determine which of the following is a conjugate acid-base pair.

HC₂O₄^-_(aq) + H₂O_(l) H₃O⁺_(aq) + C₂O₄^2-_(aq)

• A. HC₂O₄^- and H₂O
• B. H₂O and C₂O₄^2-
• C. HC₂O₄^- and H₃O⁺
• D. HC₂O₄^- and C₂O₄^2-

Here’s what students ask on this topic:

What is the Bronsted-Lowry definition of acids and bases?

The Bronsted-Lowry definition of acids and bases is a fundamental concept in biochemistry. According to this theory, a Bronsted-Lowry acid is a substance capable of donating a proton (H⁺), while a Bronsted-Lowry base is a substance capable of accepting a proton. This definition is broader than the Arrhenius definition, as it does not require the acid or base to be in aqueous solution. For example, in the reaction between hydrochloric acid (HCl) and water (H₂O), HCl donates a proton to water, making HCl the acid and water the base.

What are conjugate acids and bases?

Conjugate acids and bases are pairs of compounds that differ by one proton and one charge. When an acid donates a proton, it forms its conjugate base, which has one less hydrogen and one less positive charge. Conversely, when a base accepts a proton, it forms its conjugate acid, which has one more hydrogen and one more positive charge. For example, in the reaction between acetic acid (CH₃COOH) and water (H₂O), acetic acid donates a proton to water, forming acetate (CH₃COO^-) as the conjugate base and hydronium ion (H₃O⁺) as the conjugate acid.

How does water act as both an acid and a base?

Water is an amphiprotic molecule, meaning it can act as both an acid and a base. This dual behavior is due to its ability to either donate or accept a proton. For instance, in the reaction with ammonia (NH₃), water donates a proton to form hydroxide (OH^-), acting as an acid. Conversely, in the reaction with hydrochloric acid (HCl), water accepts a proton to form hydronium ion (H₃O⁺), acting as a base. This amphiprotic nature of water is crucial in many biochemical processes.

What is the significance of understanding acids and bases in biochemistry?

Understanding acids and bases is crucial in biochemistry because many biochemical reactions involve proton transfer. Enzyme activity, metabolic pathways, and cellular processes often depend on the pH and the presence of specific acids and bases. For example, the function of enzymes is highly pH-dependent, as the ionization state of amino acid residues in the active site can affect substrate binding and catalysis. Additionally, the buffering capacity of biological systems, which helps maintain pH homeostasis, relies on the principles of acid-base chemistry.

What is an example of a biochemical reaction involving acids and bases?

An example of a biochemical reaction involving acids and bases is the bicarbonate buffering system in blood. This system helps maintain the pH of blood around 7.4. The reaction can be represented as: $H_2\mathrm{CO}{3}_{\leftrightarrow }$. Here, carbonic acid (H₂CO₃) dissociates into water (H₂O) and carbon dioxide (CO₂), which can be exhaled. This equilibrium helps resist changes in blood pH, demonstrating the importance of acid-base chemistry in physiological processes.