🖐️ Before we start,
Make sure you’re familiar with:
- Brønsted-Lowry acids and bases
- Factors that affect the stability of conjugate bases
Challenge 1
Question:
Predict which side of the reaction is favored under equilibrium conditions
Ready? Let’s dive in!
Step 1: Identify the base on either side of the equilibrium
🔍 Hint:
You’ll often deal with Brønsted-Lowry acids and bases in these types of questions. Remember, a Brønsted-Lowry acid donates a proton, while a Brønsted-Lowry base accepts one.
In this example,
Acetic acid donates a proton to form acetate, making it the acid and acetate its conjugate base.
Methoxide accepts a proton to form methanol, making it the base and methanol its conjugate acid.
So, the bases are methoxide and acetate.
💡Quick Tip:
Bases in Brønsted-Lowry acid-base reactions typically carry a negative charge—this helps you quickly spot them.
Step 2: Compare the stability of these bases
Now, let’s determine which base—acetate or methoxide—is more stable. Stability is the key to predicting which side of the reaction is favored.
To evaluate stability, we’ll consider all five factors in this order (Charge, Atom, Resonance, Induction, Orbitals)
Charge
What is the charge on each base?
Both acetate and methoxide carry a negative one (-1) charge.
Since their charges are identical, this factor doesn’t help us determine which is more stable.
Atom
Which atom is holding the negative charge?
In both cases, the negative charge is on oxygen.
Again, this factor does not help differentiate stability here.
Resonance
Is the negative charge delocalized over multiple atoms?
For methoxide, the charge is ‘stuck’ to a single oxygen atom. For acetate, the negative charge is spread over two oxygens.
💡 Conclusion: Resonance stabilization makes acetate the more stable base.
Since we’ve determined stability based on resonance, we don’t need to evaluate the remaining factors (Induction and Orbitals).
Step 3: Equilibrium will favor the more stable base
Here’s the rule of thumb: Equilibrium will favor the side with the more stable base.
If the more stable base is on the reactant side, equilibrium will favor the reactants. On the other hand, if the more stable base is on the product side, equilibrium will favor the products.
In this reaction, the more stable base—acetate—is on the product side. As a result, equilibrium shifts toward the products.
📝 Note:
To visually indicate the direction in which the equilibrium favors either the reactants or the products, you can use one of these two arrows.
The longer arrow points toward the favored side (in this case, the products).
