Question

A strong acid has a weak conjugate base, whereas a weak acid has a relatively strong conjugate base. Explain.

Short answer

A strong acid readily donates a proton and forms a weak conjugate base that is less likely to accept a proton again. Conversely, a weak acid is less likely to donate a proton, and its conjugate base is more likely to accept a proton, making it a relatively strong base. This relationship is fundamental in acid-base chemistry. For example, HCl (a strong acid) forms a weak conjugate base (Cl-), while acetic acid (a weak acid) forms a relatively strong conjugate base (CH3COO-).

Step-by-step solution

1. Define acids and bases

An acid is a substance that donates a proton (H+) when dissolved in water, while a base is a substance that accepts a proton (H+). There are two types of acids and bases: strong and weak. Strong acids and bases dissociate completely when dissolved in water, meaning that they give up or accept protons very readily. Weak acids and bases, on the other hand, do not dissociate completely and therefore have a lesser tendency to give up or accept protons.

2. Define conjugate acid and conjugate base

A conjugate acid is the species formed when a base accepts a proton, and a conjugate base is the species formed when an acid donates a proton. In other words, the conjugate acid is the "partner" that the base forms when it gains a proton, and the conjugate base is the "partner" that the acid forms when it loses a proton.

3. Analyze the relationship between acid strength and conjugate base strength

The strength of an acid is determined by its ability to donate a proton, and the strength of a base is determined by its ability to accept a proton. Thus, when an acid donates a proton, it becomes a weaker base, as it is less likely to accept a proton again. Conversely, when a base accepts a proton, it becomes a weaker acid, as it is less likely to donate a proton again.

4. Use examples to illustrate the concept

Let's consider a strong acid, such as hydrochloric acid (HCl). When HCl donates a proton (H+), it forms its conjugate base, chloride ion (Cl-). HCl is a strong acid, so it donates its proton very readily in water. As a result, Cl- is unlikely to accept a proton again and is considered a weak base. Now, consider a weak acid, such as acetic acid (CH3COOH). When it donates a proton, it forms its conjugate base, acetate ion (CH3COO-). Acetic acid is a weak acid, so it is less likely to donate its proton in water. Consequently, CH3COO- is more likely to accept a proton again and is considered a relatively strong base.

5. Conclusion

In summary, a strong acid has a weak conjugate base because, after donating a proton, the resulting species is less likely to accept a proton again. Similarly, a weak acid has a relatively strong conjugate base because, after donating a proton, the resulting species is more likely to accept a proton again. This relationship between acid strength and conjugate base strength is a fundamental concept in acid-base chemistry.