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Magazine Chapter 7: Problem 63
Predict if the solutions of the following salts are neutral, acidic or basic: \(\mathrm{NaCl}, \mathrm{KBr}, \mathrm{NaCN}, \mathrm{NH}_{4} \mathrm{NO}_{3}, \mathrm{NaNO}_{2}\) and \(\mathrm{KF}\)
Short Answer
Expert verified
NaCl and KBr are neutral; NaCN, NaNO鈧�, and KF are basic; NH鈧凬O鈧� is acidic.
Step by step solution
01
Identify the Ions
For each salt, identify the cation and the anion.
1. NaCl - cation: Na鈦�, anion: Cl鈦�
2. KBr - cation: K鈦�, anion: Br鈦�
3. NaCN - cation: Na鈦�, anion: CN鈦�
4. NH鈧凬O鈧� - cation: NH鈧勨伜, anion: NO鈧冣伝
5. NaNO鈧� - cation: Na鈦�, anion: NO鈧傗伝
6. KF - cation: K鈦�, anion: F鈦�
02
Determine Cation Reaction
Check if the cation can affect pH.
- Group 1 metal cations (like Na鈦� and K鈦�) do not affect pH.
- NH鈧勨伜 is the conjugate acid of a weak base and makes the solution acidic.
03
Determine Anion Reaction
Check if the anion can affect pH.
- Cl鈦� and Br鈦� are conjugate bases of strong acids and do not affect pH.
- CN鈦�, derived from weak acid HCN, makes the solution basic.
- NO鈧冣伝, from strong acid HNO鈧�, does not affect pH.
- NO鈧傗伝, from HNO鈧� (weak acid), makes the solution basic.
- F鈦�, from HF (weak acid), makes the solution basic.
04
Predict Solution pH
Combine cation and anion effects to determine if the solution is neutral, acidic, or basic.
1. NaCl - Neutral (Na鈦� and Cl鈦� don't affect pH)
2. KBr - Neutral (K鈦� and Br鈦� don't affect pH)
3. NaCN - Basic (Na鈦� neutral, CN鈦� basic)
4. NH鈧凬O鈧� - Acidic (NH鈧勨伜 acidic, NO鈧冣伝 neutral)
5. NaNO鈧� - Basic (Na鈦� neutral, NO鈧傗伝 basic)
6. KF - Basic (K鈦� neutral, F鈦� basic)
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Anion Reaction
Anions are negatively charged ions that can affect the pH of a solution. Whether an anion will make a solution acidic or basic depends on the type of acid it derives from. There are some key points to remember when analyzing anion reactions:
- Anions that are conjugate bases of strong acids, like Cl鈦� (from HCl) and Br鈦� (from HBr), do not affect the pH of a solution. This is because strong acids completely dissociate in water, leaving ions that don't react with water.
- Anions from weak acids, such as CN鈦� from HCN, and NO鈧傗伝 from HNO鈧�, will accept hydrogen ions and thereby increase the hydroxide ion concentration in water. This makes the solution basic.
- Each anion must be evaluated based on its source acid to predict the pH change it might cause in a solution.
Cation Reaction
Cations are positively charged ions that can interact with water either to produce or to release hydrogen ions, affecting the solution鈥檚 pH. Understanding how cations react is essential for predicting the overall pH of a solution.
By examining the nature of each cation, we can better predict the acidity or neutrality of a solution.
- Group 1 metal cations, like Na鈦� and K鈦�, generally have no effect on pH. They are from strong bases that fully dissociate, so they do not participate in further reaction with water.
- Ammonium ion, NH鈧勨伜, is the conjugate acid of the weak base NH鈧� (ammonia). It can donate a hydrogen ion to water, increasing the concentration of hydronium ions \( \text{H}_3\text{O}^+ \), hence making the solution acidic.
By examining the nature of each cation, we can better predict the acidity or neutrality of a solution.
Acidic Solutions
Solutions are termed acidic when they contain an excess of hydrogen ions in water. This can come from either the cation or the anion in a salt.
An understanding of how these ions interact in solution is crucial to predicting the overall pH balance.
- In salts like NH鈧凬O鈧�, the NH鈧勨伜 cation is responsible for making the solution acidic. It reacts with water by donating a hydrogen ion to form hydronium ions, increasing the acidity.
- It's important to check if the salt contains a component from a weak base, as this is an indicator that it may act in an acidic manner when dissolved.
An understanding of how these ions interact in solution is crucial to predicting the overall pH balance.
Basic Solutions
Basic solutions occur when a solution favors hydroxide ions \( \text{OH}^- \). In salt solutions, anions derived from weak acids typically turn the solution basic by reacting with water.
With basic solutions, observing the ion composition from weak acid-derived anions is critical for pH prediction.
- Examples include NaCN, NaNO鈧�, and KF, where the CN鈦�, NO鈧傗伝, and F鈦� anions, respectively, lead to a surplus of hydroxide ions.
- These anions accept hydrogen ions from water, decreasing the concentration of hydronium ions and thus shifting the pH towards basic.
With basic solutions, observing the ion composition from weak acid-derived anions is critical for pH prediction.
Neutral Solutions
Neutral solutions have a balanced concentration of hydrogen and hydroxide ions, similar to pure water. They characteristically emerge from salts where both the cation and anion do not affect pH.
Recognizing when neither component of a salt contributes to pH changes helps in identifying neutral solutions.
- In salts like NaCl and KBr, the cations (Na鈦� and K鈦�) and anions (Cl鈦� and Br鈦�) are derived from strong acids and bases, thereby limiting any reaction with water to alter pH.
- These ions are considered spectators as they do not contribute to the overall acidity or basality of the solution.
Recognizing when neither component of a salt contributes to pH changes helps in identifying neutral solutions.
