Module 10 – Acid/Base Chemistry

I’m going to take Module 10 from a different direction than the book. I think that it will help make some things a little clearer and spark some “remembering” in your minds. The fact is that you already know quite a bit about Module 10, you just don’t know that you do.

So first, let’s do some reviewing.

REVIEW

Find more information on these by visiting Module 3, Module 4, Module 8, and Module 9

When 2 or more reactants are involved in a chemical reaction, their particles – atoms, ions, and molecules – have to make contact. It’s important, because reactions take place within the valence electrons.

The valence electrons form bonds – some ionic and some covalent – and they create these bonds based on their electronegativity.

But atomic particles need to move around in order to make contact with one another. That’s why solids have a harder time coming together in reactions. Gases and liquids have a much easier time accomplishing this simply because they are farther apart and have more room in which to move.

When a reaction needs to take place between solids and other phase types, it’s easier to do so by dissolving the solid reactants in a liquid.

NOT SO NEW STUFF

Those of you who took Biology last year will remember the following material. If the material is new to you, please take a moment to read through it carefully.

First, let’s review the definition of certain terms:

Solvents, Solutes, and Solutions

solvent: the substance in which the solute is being dissolved in order to make a solution *

solute: the substance being dissovled in order to make a solution *

solution: the result of one or more solutes being dissolved in a solvent *

* These definitions come from your book.

So, in a sentence, the solvent is the liquid, the solute is the solid dissolved in it, and the solution is the combined solvent and solute.

REAL WORLD USE OF THE DEFINITION

Let’s look at the following equation:

3 Na2CO3 (aq)  +    2 C6H8O7 (aq)    →   2  Na3C6H5O7  (aq)  +  3  CO2 (g)    +  3  H2O (l)

It’s easy to become overwhelmed by the equation. There’s a lot to look at, but remember that the best way to look at chemistry is that it’s really about the smaller pieces of the puzzle. Focus on the pieces, and you will quickly see the big picture.

So having said that,

Look at each reactant.

The first reactant is IONIC.

It contains a metal combined with nonmetals.

Look also at the letters aq within parentheses: This means that this reactant is a solution. We have a metal/nonmetal solute within a liquid solvent.

The second reactant is COVALENT.

It contains nonmetals. It also is a solution.

NOW FOR A COMMERCIAL…

Usually a solution is a solid dissolved in pure water.

PURE WATER is not in your tap or even in the bottled water that you buy at the store. That water, as filtered as it may be, still contains certain metals that do not affect you but would tend to change the results of a chemical reaction. These metals in such small quantities in the water still change it from a NONCONDUCTIVE compound to a CONDUCTIVE IONIC COMPOUND.

SO Pure water is:

  • An excellent solvent because large molecules can be suspended within it and smaller molecules can be dissolved within it. (Brownian motion.)
  • It tends to sink at certain temperatures so other lighter molecules are easier to see within it.
  • It tends to be less dense at 0° C, and forms distinctive ice crystals that actually float.
  • It has high surface tension.
  • It has high specific heat and vaporization heat.
  • It is a very polar material.
  • IT DOES NOT CONDUCT ELECTRICITY.

BACK TO OUR REGULARLY SCHEDULED PROGRAM…

Now that we understand what’s happening here, let’s see what the reaction produces:

Look at the products.

We have an ionic compound, carbon dioxide, and water.

You may not recognize it the way it is right now, but it may help to see a picture of the reaction itself:

Alka Seltzer

What you’re looking at here is the chemical reaction that occurs when Alka Seltzer hits the water.

Is the water creating the reaction?

NO. It is merely the way that the atoms of the two reactants COME TOGETHER in the water in order to react!

And the new solution, the bubbling, not so clear liquid that people drink in order to calm an upset stomach, has a new property, too – it CAN conduct electricity.

ELECTROLYTES

Electrolytes are ionic compounds that have solid crystals that already carry existing ions – the best example being salt. The ions separate from each other and enter the solution as more or less independent particles. We call this process DISSOCIATION.

Electrolytes play an important part in biological functions – without them, everything from the chemical balance of the blood to nerve synapses fail to work properly.  As a matter of fact, these electrolytes are so important, that we buy special drinks in order to replace the ions that are lost when we sweat.

Now, most aqueous solutions of covalent compounds can’t conduct electricity. (By the way, you may sometimes see the phrase molecular compound. This is another way of saying covalent compounds.) We call these nonelectrolytes. The particles stay the same when they are dissolved into the solvent. They intermingle with the H2O molecules when their solutions form, but don’t dissociate. You might say that they are SPECTATOR IONS.

IONIC COMPOUNDS AND DISSOCIATION

So, when ionic compounds enter into a solvent like water, the ions separate from one another, move around within the water molecules,  and sometimes recombine with or within the water molecules.

Look at the equation of the dissociation of common table salt.

NaCl (aq) →  Na +  +    Cl - 

Let’s break it apart:

Salt Dissociation

Notice that the sodium and chlorine ions are CHARGED particles. In this reaction the ions are working, more or less, independently of each other.

POLYATOMIC IONS

Alright, now let’s look at a reaction that involves polyatomic ions. These ions tend to remain intact when dissociation occurs.

Na2SO4 (s) →  2 Na+ (aq)  +   SO4 2- (aq)

The sulfate ionic compound really stayed in one piece. That’s why it’s so important for you to have a clear knowledge of common ionic compounds when you are writing chemical equations. Memorizing common ionic compounds will make the rest of the course a lot easier.

THE WONDERFUL MONKEY WRENCH IN THE CHEMICAL MACHINE

Until today, you’ve heard over and over again that only ionic compounds are capable of conducting electricity. The fact is that that’s not quite exactly true. If you remember, I told you that a couple of weeks ago.

Like all things in God’s universe, there are exceptions to this rule.

We call covalent compounds that can carry an electrical charge: ACIDS and the compounds that neutralize them: BASES.

INDICATORS

Long ago, there were very few ways to determine the properties of things around us, so often people touched or tasted things. That may seem like a reasonable idea if you’re talking about baking soda or lemon juice.

It’s an entirely OTHER thing if you’re talking about sulfuric acid or lye. Both are extremely harmful if tasted or touched. Needless-to-say, scientists needed a better way to test the properties of certain chemicals. But how?

You can imagine how nice it was to discover that certain dyes respond to acids and bases. We call these kinds of substances that can indicate acids or bases, what else, indicators.  One very responsive substance is the dye made from some kinds of lichens that is called litmus.

Litmus paper is made by soaking absorbent paper in the liquid dye and allowing it to dry. When blue litmus comes into contact with an acid, it turns red. When red litmus comes into contact with a base it turns blue.

ACIDS

Acids have specific characteristics. Let’s take a look at them.

Acids:

  • taste sour (TASTING a chemical substance may not be the best or safest way to make a determination.)
  • are covalent compounds that can conduct electricity when added to water.
  • are often corrosive when they come in contact with metals.
  • turn blue litmus paper red.

In 1884, a Swedish chemist began developing a series of theories about acids and bases and how they work. His name was Svante Arrhenius. (By the way, Happy Birthday, Svante – February 19th!)

Svante Arrhenius
Svante Arrhenius

He believed that acids dissociate in water to produce charged ions, (H+ions in particular which are called hydrogen ions).

He was partially right.

Two separate chemists – Thomas Lowry from England and J. N. Bronsted from Denmark – came up with the other part of the puzzle as to what makes acids able to conduct electricity.  For one thing, they learned that these Hydrogen ions could not exist in water on their own. They had to combine to form other substances.

But in order to explain their ideas properly, we need to add the other piece they discovered.

BASES

Bases:

  • taste bitter (Again, tasting and touching chemical substances IS NOT the best way to determine their properties.)
  • are slippery to the touch when dissolved in water.
  • are sometimes caustic to the skin and eyes.
  • turn red litmus paper blue.

Arrhenius was missing a very important part of the puzzle: the fact that bases also produce ions: hydroxide ions (OH-)

This is why all acids have similar properties. They produce similar ions. And why bases have similar properties. They produce similar ions.

The acidic and basic properties of the solutes disappear when they are combined. The solution becomes neither acidic nor basic. It becomes a neutral liquid and a salt – AN IONIC SOLUTION THAT CAN CONDUCT ELECTRICITY.

Take a look for yourself:

Hydrochloric acid combined with Sodium Hydroxide is neutralized, and creates a salt and water. This equation is called a neutralization.

HOW IONS IN ACIDS AND BASES WORK

Let’s bring the pieces together.

H+ ions can only exist in water if they are attached to something -

Look at this reaction:

HCl (aq) + H2O (l) → H3O + (aq) + Cl – (aq)

HCL is hydrochloric acid. This is it’s Lewis structure:

Water has this Lewis structure:

Think about the electronegativity of both molecules. Hydrogen has a much lower electronegativity than chlorine. Though they share the electron pair in hydrochloric acid, the chlorine keeps the electron pair most of them time. As soon as the compound is dissolved in water, the hydrogen separates from the chlorine, (remember that in this case the ions work almost independently of each other).

Hydrogen has only 1 proton and 1 electron, because the chlorine is considerably more electronegative, the hydrogen dissociates itself from the chlorine, and leaves behind the chlorine ion. (Cl -)  In doing so, it becomes a Hydrogen ion (H+) But think about it. This hydrogen ion is really nothing more than a proton. That is why this particular ability, the ability to dissociate a hydrogen ion, makes acids proton donors.

But where does that proton (hydrogen ion) go? After all, hydrogen ions cannot exist on their own.

This is where the water molecule comes into play.

As the hydrogen ion comes into contact with the water molecule, it’s attracted to it. Remember that water is a polar compound. One side of it is more electron heavy than the other, and so MORE NEGATIVELY CHARGED on one side. The proton, completely positive, becomes attached to that negative polar end, and so, creates a polyatomic ion: H3O + HYDRONIUM.

This is what we call an ionization reaction – it’s called that because an ion forms where it didn’t exist before.

Whenever you see an H+ ion, remember that there is also going to be an H3O+ ion nearby.

A NEW POINT OF VIEW

So now we can derive a NEW definition for acids.

An acid is a substance that reacts in water to produce hydronium ions, H3O+. It is a proton donor, that carries within it a hydrogen ion (H+).

Sometimes an acid, however, may also have other atoms that can’t transfer to water molecules.

Look at this reaction:

HC2H3O2 (l) + H2O (l) → H3O + (aq) + C2H3O2- (aq)

Only the hydrogen can transfer, so that it produces one hydronium ion and another compound.

DIPROTIC AND TRIPOTIC

Other acids can produce more than 1 hydronium ion, but the process takes more than one step. Like this one:

The reaction takes place very quickly, but it is really two separate reactions. At the end of the reaction, two hydronium ions are produced. This is a diprotic acid. Sometimes, an acid can produce three hydronium ions. This is called a triprotic acid.

BASES CAN PLAY, TOO

We know now that bases can also create ions in water. There are called hydroxide ions (OH-). Since bases yield solutions that contain ions, they are electrolytes, too.

A very common base is ammonia – NH3. It reacts within water like this:

NH3 (aq) + H2O → NH4 + (aq) + OH -(aq)

This ionization reaction produces an OH- ion that wasn’t there before.

There are two kinds of bases:

IONIC AND COVALENT BASES

Ionic bases that contain (OH-) ions. They include metal hydroxides like NaOH or Ca(OH)2. In water, they will dissociate.

Covalent bases (also called molecular bases) don’t start out with a hydroxide ion, but do create them like in the ammonia reaction.

Bases will absorb the proton donation made by the acid. That makes bases proton acceptors. That also makes them easier to spot in chemical reactions.

FREE AGENTS IN THE NFL OF CHEMISTRY

So now we understand that acids are proton donors. Bases are proton acceptors. That gives them the ability to NEUTRALIZE acids. But some substances can do BOTH – donate or accept depending on what its work is in a reaction.

We call this an AMPHIPROTIC substance.

Water is the most common. It can serve as an acid in some reactions, and as a base in others.

NOW, LET’S MAKE THIS EASY…

So now that we know what an acid and what a base are, it’s important to know how to recognize them in a reaction.

Here are some handy tools for doing just that. (Bear in mind that like everything in Chemistry, there are exceptions to these rules, but for the most part, they’ll make your work very much easier.)

RECOGNIZING AN ACID:

1.  I know this is going to sound silly, but let’s face it, sometimes the obvious gets overlooked. If it says “acid” it’s an acid,  as in sulfuric acid.

2.  If the chemical formula looks either like this:

H and another element, like HF

or

HaXbOc where a, b, c are subscripts of some kind and X is another element, like H2CO3.

RECOGNIZING A BASE:

1. If it contains an OH already like in Mg(OH)2

2.  If it accepts the H in the product

3. NH3 (ammonia) is always a base.

4. If it is ionic.

Let’s work a few problems to see this in action.

Look at the following equations. Which are the base and the acid?

1.  HBr (aq) + NH3 (aq) → NH4 + (aq) + Br - (aq)

In this equation, look at HBr. There’s the pattern: an H followed by another element, Br. This is an acid. Check it: If you look at the products, notice that the Br has now become an ion. The H has gone off to become an H+ ion – it’s been donated.

NH3 is always a base. Check it: Notice that it ACCEPTED the H that came off the HBr.

It’s that simple.

2.  H2PO4 (aq) + 3 KOH (aq) → 3 H2O(l) + K3PO4 (aq)

Notice that H2PO4 is the acid. It fits the pattern of an H, an element, and an Oxygen. By the way, it can be in any order. As long as it fits the pattern. Check it: The PO4 is no longer attached to the 2 H’s. This is because the H’s have combined with the OH’s in the KOH in order to become water – H2O

That means that the KOH is obviously the base. Notice that it starts out with an OH.

There’s something else you should notice. The products became a salt (any substance that no longer has any protons (hydrogens) to donate, and water.

THIS IS A NEUTRALIZATION.

By the way, in a diprotic acid like H2PO4, 2 molecules of base are required. if you had a triprotic acid, you would need 3 molecules of base.

3.  H2CO3 + H2O → H3O + + HCO3 -

In this reaction, we have a diprotic acid, H2CO3, but look at the water. What’s it doing?

Well, if I look at the products, notice that it has ADDED or ACCEPTED on an H, another proton, because it’s now a positive ion with another H. THIS IS THE BASE.

It’s really that simple.