Where are we going with this? This page will assist in developing the ability to classify solutions as acids or bases and describe their characteristic properties and/or compare and contrast the strength of acids and bases in solutions and/or, given the hydronium ion and/or the hydroxide ion concentration, calculate the pH and/or the pOH of a solution. Explain the meanings of these values.
Acid/Base Concepts, Definitions, and Other Word-type Things
Most people reading this have some idea of what an acid is. It is fictionalized to be that stuff in a beaker that eats holes in everything. Usually instantly. And dramatically.
Likewise, most people know the word "base" but… possibly not within the context of chemistry.
Looks like we have some concepts to clear up!
Definitions and Concepts
Acid: A substance that, when dissolved in water, produces H+ ions. More generally, a proton donor when in a solution.
Generally…
- Conduct electricity
- Acidic foods like lemons have a sour taste
- React with bases to neutralize their properties
- React with active metals to liberate hydrogen
Base: A substance that, when dissolved in water, produces OH- ions. More generally, a proton acceptor when in a solution.
Generally…
- Conduct electricity
- Basic foods have a bitter taste
- React with acids to neutralize their properties
Oh, well, now… that certainly cleared things up!
There's a lot in those definitions. First off, we have to think about that H+ ion. That's a hydrogen ion with a positive charge. It's a hydrogen (H) atom missing an electron. That means that the H+ ion is essentially a proton… hence, the "more generally" part of the definitions.
Next, acids and bases are very frequently considered within solutions where their ions are present. This is common, but NOT exclusive.
What happens is this… You have an acid. Let's say you have some HCl (since that's easy to type). So, this is an ionic bond, meaning that the Cl attracts the electron from the H, giving each of the atoms a charge and making them cling to each other (ionic bond).When you put them into water that whole solute, solvent, solution thing happens. So, in solution the H and the Cl keep their changes, but stop clinging to each other. <Complicated explanation about water being a polar molecule and stuff omitted.>
There you go… that H+ ion is just roaming around in the solution, ready to connect to something else. When the something else happens to be a base, you have an acid / base reaction.
But, the whole "what's an acid?" question is illustrated. An acid is a substance in solution that dissociates into an H+ ion and its constituent anion.
The whole "What happens is this…" could be rewritten with NaOH. Put it in water and it dissociates into Na+ and OH-. And then if an acid comes along… magic! (Not actually magic.)
Therefore, a base is a substance in solution that dissociates into an OH- ion and its constituent cation.
Different types of buffers stabilize pH at different levels.
More on that acid / base reaction…
To get more detailed, in 1923, some dudes named J. N. Brønsted and T.M. Lowry extended the definition of the acid - base reaction to include non-aqueous reactions where the protons are transferred.
By extending the concept with the "Brønsted-Lowry definition," reactions need not be in water. For example…
NH3 + HCl --> NH4Cl
satisfies the definition: the H from the HCL jumps over to the NH3, then the Cl bonds ionically to the NH4 "chunk."
Okay, so what?
So, the acid-base reaction involves the transfer of a proton originating (typically) in the form of hydrogen in one of the compounds.
Examples of Acids and Bases
What are we actually talking about? Pretty much a LOT of things!
Household acids and bases…
Lemons, vinegar (and things that have vinegar in them like ketchup), aspirin, coffee, grapefruit, teas, cranberry…stuff, and lots of marinades are acidic. Foods with a sour taste… or "tangy"? are acidic.
Baking soda, bleach, ammonia and some green vegetables are bases. You might correctly guess that antacids are bases! Foods with a bitter taste are basic.
Some "laboratory" acids and bases…
Here are some common acids:
HClH2SO4HNO3HBr
Here are some common bases:
LiOHNaOHKOHCa(OH)2
WAZZAT? It seems like all of the acids start with hydrogen and all of the bases end with OH!
This is true as a general rule. Except…
One exception is water (little surprise there!)
Written as HOH, it could be either and acid or a base. In fact, it is neither! Distilled water is neutral. Although it can behave in some situations as either acid or base, it's safe to consider it neither; to consider it neutral.
Also, HLi (aka LiH) is a base… But…
But, within the realm of common classroom chemistry, the H / OH rule is pretty solid. If it starts with an H you can be pretty sure that it is acidic and if it ends on OH, you can be pretty sure that it is basic. Unless it's water.
Strengths of Acids and Bases
Strong. Weak. There… Some acids/bases are strong. Some are weak.
Not helpful.
A strong acid completely reacts with water so that ALL of its H+ ions separate from the corresponding anion (the other thing in the acid molecule, such as Cl or SO4).
A strong base completely dissociates in water so that ALL of the the OH- ions separate from the corresponding cation (the other thing in the base molecule, like Na or K).
So… weak? Yeah… Intuition works on this!
Weak acids and bases exist where only a fraction of the molecules actually break up into ions.
Because the ions are charged, the more of them in a solution, the more it will conduct electricity. Conducting electricity is a characteristic of being an electrolyte.
So… more logic… Strong acids and bases are also very good electrolytes.
Measuring the Strengths of Acids and Bases
Why does it feel like we are about to see NUMBERS!
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The strength of acids and bases is rated on a scale called the pH scale. A scale that… (sorry about this) ranges from 0 to 14. Not 15? Not 10? Not 100? 0 to 14? Really? Really.
This is NOT the "math" behind it but, as a mnemonic, you can think of it as this: the bigger the number, the more protons it can take in. High numbers means it can accept a lot of protons. High numbers means it is very basic. Low numbers means it is very acidic.
AND 7 is neutral. Water. Water is, in most cases, considered to be neutral.
So… Acids range from 0 to 7, with 0 being strongest. Bases range from 7 to 14 with 14 being strongest.
Buffers
So… You can think of these (though it is only metaphorical) as "pH shock absorbers."
Buffers work to keep the pH… of, for simplicity sake, let's say a solution… stable. If the pH gets too high, they let go of some H+ ions to lower it. If the pH gets too low, they absorb some H+ ions to raise it back up.
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For example, human blood needs to have a pH of around 7.4. There is a buffer in blood that helps do that.
The compound H2CO3 and H+ ions in the blood sort of do a little give and take. If there needs to be more H+ ions, the buffer (the H2CO3) will add one and become H+ + HCO3 -. When there are too many H+ ions, it will take the ion back and return to its H2CO3 form. In this way, the pH of the blood is buffered to around 7.4.
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References
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