Acid-Base Equilibrium: Recognize Conjugate Acid-Base Pairs
Question
Decide if each of the following are a conjugate acid-base pair:
- NH3 (aq), OH– (aq)
- H2CO3 (aq), HCO3– (aq)
Show/Hide Answer
- No
- Yes
Refer to Section 6.1: Brønsted-Lowry Acids and Bases (1).
Strategy Map
Do you need a little help to get started?
Check out the strategy map.
Show/Hide Strategy Map
| Strategy Map Steps |
|---|
1. Identify the difference between the 2 chemical species.
Show/Hide HintFor it to be a conjugate acid-base pair, they should only differ by 1 hydrogen. |
2. If they differ by 1 hydrogen, identify which species of the 2 listed has more hydrogens.
Show/Hide HintThis may be the acid. |
3. Compare the potential acid to the other species:
Show/Hide HintFor these to be a conjugate acid-base pair, the base should have 1 less hydrogen and 1 more negative charge than its conjugate acid. If both answers are yes, they are an acid-base conjugate pair. |
Solution
Do you want to see the steps to reach the answer?
Check out this solution.
Show/Hide Solution
1. NH3 (aq), OH– (aq)
Answer: No
2. H2CO3 (aq), HCO3– (aq)
Answer: Yes
H2CO3 is the acid, and HCO3– is the conjugate base. The acid will have the extra hydrogen and the base will have 1 less hydrogen and 1 more negative charge (from losing the hydrogen).
Guided Solution
Do you want more help?
The guided solution below will give you the reasoning for each step to get your answer, with reminders and hints.
Show/Hide Guided Solution
| Guided Solution Ideas |
|---|
This question is a theory problem that tests your ability to identify conjugate acid-base pairs. You must be able to tell if the pair are conjugates as well as identify which is the acid and which is the base.
Show/Hide Resource
|
What is a Brønsted-Lowry acid?
Think Don’t Forget!A proton donor during an acid-base reaction. What is a Brønsted-Lowry base? Think Don’t Forget!A proton acceptor during an acid-base reaction. |
Recall what a Brønsted-Lowry conjugate acid-base pair is.
Show/Hide Don’t Forget!
|
How do you identify acid-base conjugate pairs in an acid-base chemical reaction?
Show/Hide Watch Out!
|
| Complete Solution |
|---|
| 1. NH3 (aq), OH– (aq)
Answer: No |
| 2. H2CO3 (aq), HCO3– (aq)
Answer: Yes H2CO3 is the acid and HCO3– is the conjugate base. The acid will have the extra hydrogen and the base will have 1 less hydrogen and 1 more negative charge (from losing the hydrogen). |
Check Your Work
Summary of what we would expect based on the related chemistry theory
Show/Hide Watch Out!
Look at the structures you compared and check hydrogens in each species as well as compare charges.
Does your answer make chemical sense?
Show/Hide Answer
In part 1, the structures are quite different, so it cannot be a conjugate pair.
In part 2, the 2 structures only differ by 1 hydrogen and 1 charge unit. H2CO3 is the acid and HCO3–, which has 1 less hydrogen and 1 more negative charge, is the conjugate base. The acid has an extra hydrogen, which it is able to donate; the base has 1 less hydrogen and a higher negative charge, so it is able to accept the proton.
Show/Hide Don’t Forget!
A species that can donate a proton (a hydrogen ion) to another species in an acid-base reaction is called a Brønsted-Lowry acid. The species that accepts the proton is called a Brønsted-Lowry base.
The species remaining after a Brønsted-Lowry acid has lost a proton is the conjugate base of the acid. The species formed when a Brønsted-Lowry base gains a proton is the conjugate acid of the base. Thus, an acid-base reaction occurs when a proton is transferred from an acid to a base, with formation of the conjugate base of the reactant acid and formation of the conjugate acid of the reactant base.
Refer to Section 6.1: Brønsted-Lowry Acids and Bases (1).
PASS Attribution
- Question 6.E.4(c,d) from LibreTexts TRU: Fundamentals and Principles of Chemistry (CHEM 1510 and CHEM 1520) (2) is used under a CC BY 4.0 license.
References
1. OpenStax. 6.1 Brønsted-Lowry Acids and Bases. In TRU: Fundamentals and Principles of Chemistry (CHEM 1510 and CHEM 1520) LibreTexts, 2022. https://chem.libretexts.org/Courses/Thompson_Rivers_University/TRU%3A_Fundamentals_and_Principles_of_Chemistry_(CHEM_1510_and_CHEM_1520)/06%3A_Acid-Base_Equilibrium/6.02%3A_pH_and_pOH.
2. OpenStax. 6.E: Acid-Base Equilibrium (Exercises). In TRU: Fundamentals and Principles of Chemistry (CHEM 1510 and CHEM 1520); LibreTexts, 2022. https://chem.libretexts.org/Courses/Thompson_Rivers_University/TRU%3A_Fundamentals_and_Principles_of_Chemistry_(CHEM_1510_and_CHEM_1520).
