Back To CourseChemistry: High School
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Nicky has taught a variety of chemistry courses at college level. Nicky has a PhD in Physical Chemistry.
Shelly is an ace chemist. She knows all about salt solutions. She has three beakers. Each one contains a different salt dissolved in water. To me, each beaker looks exactly the same, but Shelley will demonstrate that they are very different. Let's see what she does.
Shelley has tested the pH, or in other words, the acidity, of each solution. For Beaker 1, the litmus paper has come out red, so this is an acidic solution; the pH is less than 7. For Beaker 2, the paper has come out blue, so this is a basic solution; the pH is greater than 7. And for Beaker 3, the paper hasn't changed color at all, so this is a neutral solution; the pH equals 7.
Hmm, that is interesting. Shelley knew that there was a difference in the pH of each salt solution. So how did she know that?
Before we get to figuring out the pH of a salt solution, let's take a small step back and define a few important terms for you. Firstly, this lesson is all about salts. A salt is an ionic solid made up of two ions: a positive cation and a negative anion. The cation is any positive ion other than H+ and the anion is any negative ion other than OH- or O2-. A salt is formed by the reaction of an acid and a base and this is called a neutralization reaction.
You can see the general reaction here where a salt is formed along with water. During a neutralization reaction, partners of the acids and bases swap. You can see that the salt is formed from the acid's sulfate ion and the base's ammonium ions. The two hydrogens and the hydroxides join together to form two moles of water.
You will soon see that the identity of the ions in the salt is our clue to predicting the pH of the salt solution. Each salt formed will vary according to the acid and base that formed it.
Okay, let us briefly think about acids and bases. The definition that works really well for us here is an acid is a molecule that donates hydrogen ions, or H+, and a base accepts hydrogen ions, H+. Acids and bases can also be described as strong or weak. And this has to do with how easily they can give up or accept hydrogen ions. Let me use a football analogy to help you with this.
Imagine that acids are the quarterbacks in a football team whose job is to get rid of the ball (H+). A strong acid, just like an awesome quarterback, delivers the ball well. On the other hand, a weak acid, like a poor quarterback, is often left holding the ball. Now imagine that the base is the wide receiver, whose job is to catch and hold onto the ball (H+). A strong base, like an awesome wide receiver, holds onto the ball. But a weak base, like a poor wide receiver, often drops the ball.
We need to be able to tell the difference between a strong and weak acid and base because of the effect it will have on the salt that is formed.
There are only a few strong acids and bases and these should be learned. Look at the table below.
|Strong Acids||HCl, HNO3, H2SO4, HBr, HI, HClO4|
|Strong Bases||The hydroxides of the Group I metals and Ca(OH)2, Sr(OH)2|
All other acids and bases except for these are considered weak.
Okay, so now let's get back to salts. When a salt is dissolved in water, the ions separate from each other. Hydrolysis may occur if the ions react with the water molecules to produce H3O+ and OH- ions. Do not be confused by H3O+. It means the same thing as H+. We just recognize that H+ cannot hang out alone in water so it brings a water molecule with it. You can see that if the ions hydrolyze water and produce either H3O+ or OH- ions, this will affect the pH of the solution.
To predict whether a salt solution will be acidic, basic, or neutral, we have to consider the effect, if any, of the ions in solution. The key to this is whether the ions come from a strong or weak acid and a strong or weak base. Fortunately, there are some neat guidelines we can follow:
1. Salts that are from strong bases and strong acids DO NOT hydrolyze water and so the pH remains at 7. Our pass is completed and the reaction is totally neutralized.
2. Salts that are from strong bases and weak acids DO hydrolyze water, giving it a pH greater than 7. Here, the wide receiver is trying really hard to get that H+ and dominates the play.
3. Salts of weak bases and strong acids DO hydrolyze water, giving it a pH less than 7. Here the quarterback is trying really hard to get rid of the H+ and dominates the play.
Now, you may be asking what will happen if a weak acid and a weak base get together. Well, just like the ball game, it is difficult to predict the outcome. It depends on how bad the wide receiver is compared to the quarterback. We would need to know more about the relative strengths of the acid and base that react. This you will cover in more advanced chemistry courses.
Okay, now we are armed with our new knowledge, let us see if we can identify which salts are responsible for the pH of Shelley's solution. The salts are lithium fluoride (LiF), potassium iodide (KI), and magnesium chloride (MgCl2). The way we do this is to identify the parent acid and parent base and apply our guidelines:
|Parent Acid||Parent Base||Result|
|HF (weak)||LiOH (strong)||Basic Solution|
|HI (strong)||KOH (strong)||Neutral Solution|
|HCl (strong)||Mg(OH)2 (weak)||Acidic Solution|
So for lithium fluoride, the parent acid is hydrofluoric acid (HF). It is not in our list of 6 strong acids, so it must be weak. The parent base is lithium hydroxide (LiOH). This is in our list of strong bases, so it is strong. Guideline 2 tells us that pH will be greater than 7, so this is basic. This is Beaker 2.
Next, potassium iodide. The parent acid is HI. It is in our list of 6 strong acids, so it is strong. The parent base is potassium hydroxide (KOH). This is in our list of strong bases, so it is also strong. Guideline 1 tells us that the pH will be equal to 7, so this is neutral. This is Beaker 3.
Finally, magnesium chloride. The parent acid is hydrochloric acid (HCl) and it is in our list of 6 strong acids, so it is strong. The parent base is magnesium hydroxide (Mg(OH)2). This is not in our list of strong bases, so it must be weak. Guideline 3 tells us that the pH will be less than 7, so this is acidic. This is Beaker 1.
Shelley, are we right? Yes, we are.
In this lesson, you have learned that a salt is an ionic solid made up of two ions: a positive cation and a negative anion. The cation is any positive ion other than H+ and the anion is any negative ion other than OH- or O2-. A salt is formed by the reaction of an acid and a base in a neutralization reaction.
When a salt is dissolved in water a solution is formed. We use three simple guidelines to decide if a solution will be acidic, basic, or neutral. Salts from strong bases and strong acids are neutral. Salts from strong bases and weak acids are basic. And finally, salts of weak bases and strong acids are acidic.
Once you are finished, you should be able to:
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Back To CourseChemistry: High School
19 chapters | 179 lessons | 1 flashcard set