Acid-Base Equilibrium: Label Acid, Base, Conjugate Acid, Conjugate Base in a Reaction
Question
Identify and label the Brønsted-Lowry acid, its conjugate base, the Brønsted-Lowry base, and its conjugate acid in the following equation:
[latex]\begin{gathered} \mathrm{H}_2\mathrm{S}\,\mathrm{(aq)}&+&\mathrm{NH}_2^-\mathrm{(aq)}&\rightarrow&\mathrm{HS}^-\mathrm{(aq)}&+&\mathrm{NH}_3\mathrm{(aq)} \end{gathered}[/latex]
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- H2S — acid
- HS– — conjugate base (cb)
- NH2– — base
- NH3 — conjugate acid (ca)
[latex]\begin{array}{ccccccc} &\mathrm{H}_2\mathrm{S}\,\mathrm{(aq)}&+&\mathrm{NH}_2^-\mathrm{(aq)}&\rightarrow&\mathrm{HS}^-\mathrm{(aq)}&+&\mathrm{NH}_3\mathrm{(aq)}\\ &\text{acid}&&\text{base}&&\text{cb}&&\text{ca}\\ \end{array}[/latex]
Refer to Section 6.1: Bronsted-Lowry Acids and Bases (1).
Strategy Map
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Check out the strategy map.
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1. Identify which reactant is an acid.
Show/Hide HintA Brønsted-Lowry acid has a proton (hydrogen) in its chemical structure that it is able to donate in an acid-base reaction. |
2. Identify which reactant is a base.
Show/Hide HintA Brønsted-Lowry base has a lone pair in its chemical structure that it is able to accept a proton (hydrogen) in an acid-base reaction. |
3. Identify which product is a conjugate base.
Show/Hide HintThe conjugate base will be structurally related to the molecule you identified in step 2 as the acid; it will have 1 less hydrogen and 1 more negative charge. |
4. Identify which product is a conjugate acid.
Show/Hide HintThe conjugate acid will be structurally related to the molecule you identified in step 2 as the base; it will have 1 more hydrogen and 1 less negative charge. |
Solution
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H2S (acid) and HS− (conjugate base)
- H2S is the acid on the reactant side.
- HS– is its conjugate base on the product side.
NH2– (base) and NH3 (conjugate acid)
- NH2 is the base on the reactant side.
- NH3 is its conjugate acid on the product side.
Answer:
[latex]\begin{array}{ccccccc} &\mathrm{H}_2\mathrm{S}\,\mathrm{(aq)}&+&\mathrm{NH}_2^-\mathrm{(aq)}&\rightarrow&\mathrm{HS}^-\mathrm{(aq)}&+&\mathrm{NH}_3\mathrm{(aq)}\\ &\text{acid}&&\text{base}&&\text{cb}&&\text{ca}\\ \end{array}[/latex]
Guided Solution
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The guided solution below will give you the reasoning for each step to get your answer, with reminders and hints.
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| Guided Solution Ideas |
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This question is a theory type problem that tests your ability to identify conjugate acid-base pairs. You are provided with a reaction containing 2 different conjugate acid-base pairs and must identify which molecules make up which pair.
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Recall what a conjugate acid-base pair is.
Show/Hide Think About This!All Brønsted-Lowry acid-base reactions involve the transfer of protons between acids and bases. The pair should only differ by 1 hydrogen atom (proton) and 1 charge unit. |
What is a Brønsted-Lowry acid?
Show/Hide Think About This!A proton donor during an acid-base reaction. |
What is a Brønsted-Lowry base?
Show/Hide Think About This!A proton acceptor during an acid-base reaction. |
What’s the difference between an acid and a conjugate acid?
Show/Hide Think About This!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 acid you are identifying is on the reactant side of the chemical equation, while the conjugate acid is on the product side of the chemical equation. The species formed when a Brønsted-Lowry base gains a proton is the conjugate acid of the base. |
| Complete Solution |
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[latex]\begin{gathered} \mathrm{H}_2\mathrm{S}\,\mathrm{(aq)}&+&\mathrm{NH}_2^-\mathrm{(aq)}&\rightarrow&\mathrm{HS}^-\mathrm{(aq)}&+&\mathrm{NH}_3\mathrm{(aq)} \end{gathered}[/latex] H2S (acid), HS− (conjugate base, cb) H2S donates a proton (hydrogen atom); its conjugate base (on the product side), HS–, accepts a proton (hydrogen atom). NH2– (base), NH3 (conjugate acid, ca) NH2– accepts a proton (hydrogen atom); its conjugate acid (on the product side), NH3, donates a proton (hydrogen atom). Answer: [latex]\begin{array}{ccccccc} &\mathrm{H}_2\mathrm{S}\,\mathrm{(aq)}&+&\mathrm{NH}_2^-\mathrm{(aq)}&\rightarrow&\mathrm{HS}^-\mathrm{(aq)}&+&\mathrm{NH}_3\mathrm{(aq)}\\ &\text{acid}&&\text{base}&&\text{cb}&&\text{ca}\\ \end{array}[/latex] |
Check Your Work
Summary of what we would expect based on the related chemistry theory.
Check to make sure your acid and base are on the reactant side of the chemical reaction and your conjugate acid and base are on the product side.
The species you have identified as conjugate acid-base pairs (acid and conjugate base or base and conjugate acid) should differ in structure by 1 hydrogen and 1 charge unit. You should be able to follow the proton transfer through the chemical reaction.
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Refer to Section 6.1: Bronsted-Lowry Acids and Bases (1).
Show/Hide Think About This!
A chemical species that can donate a proton (a hydrogen ion) to another compound 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.
Does your answer make chemical sense?
Show/Hide Answer
In the proton transfer reaction, the acid loses the proton and becomes more negatively-charged, resulting in the conjugate base. The base gains the proton and becomes more positively-charged, resulting in its conjugate acid. Each Brønsted-Lowry acid-base reaction has 2 conjugate acid-base pairs.
PASS Attribution
- Question 6.E.6(f) from LibreTexts TRU: Fundamentals and Principles of Chemistry (CHEM 1510 and CHEM 1520) (2) is used under a CC BY-NC-SA 4.0 license.
- Question 6.E.6(f) is question 14.E.1.14: 14.1.9(g) from LibreTexts Chemistry 1e (OpenSTAX) (3), which is under a CC BY 4.0 license.
- Question 14.E.1.14: 14.1.9(g) is question 9(g) from OpenStax Chemistry (4), which is under a CC BY 4.0 license. Access for free at https://openstax.org/books/chemistry/pages/1-introduction.
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/6.01%3A_Brnsted-Lowry_Acids_and_Bases.
2. Thompson Rivers University. TRU: Fundamentals and Principles of Chemistry (CHEM 1510 and CHEM 1520); LibreTexts, 2024. https://chem.libretexts.org/Courses/Thompson_Rivers_University/TRU%3A_Fundamentals_and_Principles_of_Chemistry_(CHEM_1510_and_CHEM_1520).
3. OpenStax. 6.E: Acid–Base Equilibria (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)/06%3A_Acid-Base_Equilibrium/6.E%3A_Acid-Base_Equilibrium_(Exercises).
4. OpenStax. 14.E: Acid-Base Equilibria (Exercises). In Chemistry 1e (OpenSTAX); LibreTexts, 2022. https://chem.libretexts.org/Bookshelves/General_Chemistry/Chemistry_1e_(OpenSTAX)/14%3A_Acid-Base_Equilibria/14.E%3A_Acid-Base_Equilibria_(Exercises).
5. Flowers, P.; Robinson, W. R.; Langley, R.; Theopold, K. Ch. 14 Exercises. In Chemistry; OpenStax, 2015. https://openstax.org/books/chemistry/pages/14-exercises.
