Introduction to Acids and Bases
Introduced| English |
|---|
| conjugate acid/ˈkɒndʒuːɡeɪt ˈæsɪd/ |
| conjugate base/ˈkɒndʒuːɡeɪt beɪs/ |
Partners in a proton swap
- When one molecule gives up a proton, another must catch it.
- The giver and its leftover form a linked pair.
- Even pure water quietly trades protons with itself.
- These partnerships are the heart of the topic.
Conjugate acid-base pairs
- An acid donates a proton and becomes its conjugate base 共轭碱.
- A base accepts a proton and becomes its conjugate acid 共轭酸.
- They differ by exactly one $\text{H}^+$.
A conjugate acid-base pair differs by...
The acid loses one $\text{H}^+$ to become its conjugate base.
Water splits itself
- Water slightly ionizes: $\text{H}_2\text{O} \rightleftharpoons \text{H}^+ + \text{OH}^-$.
- The product $[\text{H}^+][\text{OH}^-] = K_w = 1.0\times10^{-14}$ at 25 degrees.
- This links acidity and basicity.
At 25 degrees, the product $[\text{H}^+][\text{OH}^-]$ equals...
This product is $K_w = 1.0\times10^{-14}$.
Which side wins
- Neutral means $[\text{H}^+] = [\text{OH}^-]$.
- Acidic means more $\text{H}^+$; basic means more $\text{OH}^-$.
- Because of $K_w$, raising one lowers the other.
Acid or base?
Sort each species by whether it donates or accepts a proton.
A neutral solution has...
Equal $\text{H}^+$ and $\text{OH}^-$ means neutral.
Increasing $[\text{H}^+]$ decreases $[\text{OH}^-]$.
Their product is fixed at $K_w$, so they trade off.
If $[\text{H}^+] = 1.0\times10^{-3}$, what is $[\text{OH}^-]$?
- $[\text{OH}^-] = K_w / [\text{H}^+] = 10^{-14}/10^{-3}$.
- $[\text{OH}^-] = 1.0\times10^{-11}\ \text{M}$, so the solution is acidic.
If $[\text{H}^+] = 1.0\times10^{-4}$, find $[\text{OH}^-]$. Enter the exponent (a negative number).
$[\text{OH}^-] = 10^{-14}/10^{-4} = 10^{-10}$.
A strong acid has a ____ conjugate base.
Strong acids give up protons fully, leaving a very weak conjugate base.
A conjugate pair differs by one proton -- $\text{HCl}/\text{Cl}^-$, not $\text{HCl}/\text{OH}^-$. The product $[\text{H}^+][\text{OH}^-]$ always equals $K_w$, so they trade off: more acid means less hydroxide. And a strong acid has a weak conjugate base, and vice versa.
An acid and its conjugate base, or a base and its conjugate acid, differ by one proton. Water self-ionizes so that $[\text{H}^+][\text{OH}^-] = K_w = 10^{-14}$, linking acidity and basicity: raising $[\text{H}^+]$ lowers $[\text{OH}^-]$, and vice versa.