13 Acid-Base Equilibrium — Predict Whether an Aqueous Salt Solution Will be Acidic, Basic or Neutral
Question
Predict whether an aqueous solution of Li3N is acidic, basic, or neutral.
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An aqueous solution of Li3N will be basic.
Refer to Section 6.5: Hydrolysis of Salt Solutions (1).
Strategy Map
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| Strategy Map Steps |
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| 1. Break the salt into its two ions (the positively charged metal and the negatively charged non-metal). |
| 2. Identify the charges on both parts. |
3. Write out the possible reactions that could occur between each ion and water.
Show/Hide HintRemember that it is possible for no reaction to occur. |
| 4. Identify what products (if any) are produced and how they would impact the solution’s acidity. |
Solution
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[latex]\begin{aligned} \mathrm{Li}_3 \mathrm{~N}(\mathrm{aq}) &\rightarrow 3 \mathrm{Li}^{+}(\mathrm{aq})+\mathrm{N}^{3-}(\mathrm{aq}) \\ \mathrm{Li}^{+}(\mathrm{aq})+\mathrm{H}_2 \mathrm{O}(\ell) &\rightleftharpoons \mathrm{No} \text { Reaction } \\ \mathrm{N}^{3-}(\mathrm{aq})+\mathrm{H}_2 \mathrm{O}(\ell) &\rightleftharpoons \mathrm{NH}^{2-}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq}) \end{aligned}[/latex]
N3- is a base because the lone pair on the nitrogen can accept a proton.
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 problem where you identify if an aqueous solution will be acidic, basic, or neutral based on your knowledge of how its ions will react in water.
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When a salt dissolves in water, it breaks apart into ions, and the resulting solution is either acidic, basic or neutral.
Show/Hide Think About This!To break apart a salt into its ions, you must identify which part acts as the metal (this will have a positive charge) and which part is the nonmetal (this will have a negative charge). |
To predict if the solution is acidic, basic or neutral, you must first break apart the salt into its ions.
Show/Hide Think About This!If you break it apart correctly, the charges of the two parts will add up to the overall charge of the original compound. |
The next step is to analyze how the two parts react with the surrounding water molecules. You can do this by writing out their possible reactions to water, keeping in mind that it is possible for no reaction to occur at all. Recall how to write out ionization reactions.
Show/Hide Think About This!React both parts with a water molecule. There are four possible outcomes:
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Recall that you can predict whether an aqueous solution of a salt is acidic, basic or neutral by looking at the products each ion would produce when reacting with water.
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Note: Do not forget about some important rules. If either of the ions produces a strong acid or base after reaction with water, they will ionize 100% back to reactants, forming a neutral solution! Show/Hide Think About This!Although these rules do not apply to this problem, it is important to watch out for them. |
| Complete Solution |
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| Break apart the salt to identify its two ‘parts’ (i.e., the metal and the non-metal). Lithium has a positive charge of one, and the polyatomic nitrogen anion (azide) has a negative charge of three.
\begin{gathered} |
| When the lithium cation is in water, no reaction occurs; this means this ion does not impact pH and is a neutral ion. The Li+(aq) ion is simply hydrated.
\begin{gathered} |
| The N3- (azide) ion will accept a proton from the water, and the reaction produces hydroxide. This means N3- (aq) acts like a base.
\begin{gathered} |
| If the above reaction does happen, N3- (aq) is a weak base. Producing hydroxide makes the solution basic, and Li3N is a basic salt. |
Check Your Work
Look back to make sure you have correctly split the salt into its cation and ion. Then, verify that your reactions with water produce appropriate products. Water can act as an acid or a base, donating or accepting a proton.
Our N3- (aq) anion acts like a weak base and is able to accept a proton from water, producing hydroxide. We know our pH would be basic, and this salt would form a basic solution.
Does your answer make chemical sense?
Show/Hide Answer
When salts dissolve into water, the salts break apart into hydrated ions (surrounded by water molecules). The pH of the solution is determined by both the salt and if it reacts with water. If the salt is basic, the resulting solution will also be basic.
For this reason, we can predict the pH of a solution by analyzing the salt and the reactions its ions can have with water.
PASS Attribution
- LibreTexts PASS Chemistry Book CHEM 1510/1520 (2).
- Question 6.E.34 from LibreTexts PASS Chemistry Book CHEM 1510/1520 (3) is used under a CC BY 4.0 license.
- Question 6.E.34 is 16.E.11 (Conceptual Problems) from LibreTexts Exercises: Brown et al. (4), which is under a CC BY-NC-SA 4.0 license.
References
1. OpenStax. 6.5 Hydrolysis of Salt Solutions. 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/6.05%3A_Hydrolysis_of_Salt_Solutions.
2. Blackstock, L.; Brewer, S.; Jensen, A. In PASS Chemistry Book CHEM 1510/1520; LibreTexts, 2023. https://chem.libretexts.org/Courses/Thompson_Rivers_University/PASS_Chemistry_Book_CHEM_1510%2F%2F1520.
3. OpenStax. 16.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. Brown; LeMay; Bursten; Murphy; Woodward. 16.E: Acid–Base Equilibria (Exercises). In Exercises: Brown et al. LibreTexts, 2020. https://chem.libretexts.org/Bookshelves/General_Chemistry/Exercises%3A_General_Chemistry/Exercises%3A_Brown_et_al./16.E%3A_AcidBase_Equilibria_(Exercises).
