Molecular Structure of Acids and Bases
Introduced| English |
|---|
| oxoacid/ˈɒksəʊsɪd/ |
What makes an acid strong
- Two acids can look alike but differ wildly in strength.
- The secret is hidden in their bonds and atoms.
- A weaker or more polar bond to hydrogen lets the proton leave.
- A stable leftover ion also helps the proton go.
Bond strength and polarity
- A weaker bond to hydrogen releases the proton more easily.
- A more polar bond also loosens the $\text{H}^+$.
- Both make the acid stronger.
A weaker bond to hydrogen makes an acid...
A weaker H bond releases the proton more easily.
A more polar bond to hydrogen helps release the proton.
Polarity loosens the $\text{H}^+$, aiding ionization.
A stable conjugate base
- The more stable the conjugate base, the stronger the acid.
- Spreading the negative charge out stabilizes it.
- A contented leftover ion means the proton leaves willingly.
A more stable conjugate base means a stronger acid.
A stable conjugate base lets the proton leave more readily.
Spreading out the negative charge makes the conjugate base more ____.
A spread-out charge is more stable, so the acid is stronger.
Two useful trends
- Binary acids (H-X): stronger down a group, as the H-X bond weakens.
- An oxoacid 含氧酸 grows stronger with more oxygen atoms.
- These trends predict relative strengths quickly.
Which is the stronger binary acid?
The weaker H-I bond makes HI stronger than HF.
Adding more oxygen atoms to an oxoacid generally makes it...
More oxygens stabilize the conjugate base, strengthening the acid.
Why is $\text{HI}$ a stronger acid than $\text{HF}$?
- The H-I bond is weaker than the H-F bond.
- So $\text{HI}$ releases its proton more easily, making it stronger.
What makes an acid strong?
Sort each comparison by which structural feature gives the stronger acid.
Down a group, binary-acid strength increases because the H-X bond weakens (bond strength matters more than electronegativity here). More oxygens on an oxoacid pull electron density away, stabilizing the conjugate base and strengthening the acid. A more stable conjugate base always means a stronger acid.
An acid is stronger when its bond to hydrogen is weaker or more polar, and when its conjugate base is more stable. Two trends follow: binary acids strengthen down a group (weaker H-X bond), and an oxoacid strengthens with more oxygen atoms.