Acids and Bases: Acid/Base Theory Mrs. Crowley’s Chemistry Class

Measuring pH How do we measure pH? Own a pool? Own a spa? How do you check the water quality? Look at my poster. Also…pH meters can do the trick, too:

This is an example of an “equilibrium” Indicators Acid-base indicators: This is usually a solution that you add to a solution that will undergo a chemical reaction at a certain pH. The reaction is called “dissociation.” This type of reaction is an equilibrium. Remember, equilibriums go from reactants to products and products to reactants (back and forth). H2O H+ + OH- This is an example of an “equilibrium” The rate of flow towards products is the same as the rate to reactants.

Check out my poster! There are many different indicators available for chemists. Sometimes, we can combine these indicators to make a broad spectrum of color changes. This is how traditional pH paper is made.

Acid-Base Theories Well, now it’s time to learn what his theory of acids and bases were. Arrhenius’s main contribution to physical chemistry was his theory (1887) that electrolytes, certain substances that dissolve in water to yield a solution that conducts electricity, are separated, or dissociated, into electrically charged particles, or ions, even when there is no current flowing through the solution

Svante Arrhenius Svante said that acids are hydrogen-containing compounds that ionize to yield hydrogen ions (H+) in aqueous solution. He also said that bases are compounds that ionize to yield hydroxide ions (OH-) in aqueous solution.

Acids – further explaination Not all compounds that contain hydrogen are acids! (poor Arrhenius!...he didn’t know this…YET) Also, not all hydrogens in an acid are released as hydrogen ions. Only the hydrogens in very polar bonds are ionizable. EXAMPLE: HCl + H2O  H3O+ + Cl- Highly Polar molecule

Bronsted-Lawry Acids and Bases The next theory on acids and bases came from these two men around the year 1923.

Bronsted-Lowry Theory These guys re-defined an acid as a hydrogen-ion donor. They also re-defined a base as a hydrogen ion acceptor. Let me explain:

Explanation When ammonia (a well known base) combines with water, the ammonia will act as a base. But ammonia doesn’t have an OH! How can it possibly be a base if there is no OH ion? The OH ion will come from the water! Yes…that’s right. The water will donate a hydrogen to the ammonia. Poor Arrehenius thought ammonia’s formula was NH4OH! But the molar mass of ammonia was discovered to be MUCH less than that of NH4OH!

Further Explanation Does that mean that the water is an acid? Well, according to the Bronsted-Lowry theory…YES! Water acts like a Bronsted-Lowry base. HOW? Look at that “double arrow.”

Equilibriums If the reaction between water and ammonia goes in the reverse direction, NH4+ will react with OH- to form NH3 and H2O. acid = hydrogen-ion donor. base = hydrogen ion acceptor.

Conjugate Acids and Bases If the reaction goes in the reverse direction, NH4+ gives up a hydrogen ion. That makes it an Bronsted-Lowry ACID! OH- will accept that hydrogen ion from NH4+. That makes it a Bronsted-Lowry BASE! Since NH4+ is on the right side of the equation, it is called a conjugate acid. Since OH- is on the right side of the equation, it is called a conjugate base.

Definitions Conjugate acids are the particles formed when a base gains a hydrogen ion. A conjugate base is the particle that remains when an acid has donated a hydrogen ion.

Extra Examples

Lewis Acids and Bases The last theory for acids and bases came from Gilbert Lewis (1875-1946).

Lewis Acids-Bases A lewis acid is a substance that can accept a pair of electrons to form a covalent bond. A lewis base is a substance that can donate a pair of electrons to form a covalent bond.

In this reaction, the ammonia is a lewis base. The ammonia will give the extra pair of electrons to the other substance to form an additional bond with it.

RECAP: Arrhenious: Acids make H+ and Bases make OH- Acids have H+ ion in front of formula, bases have OH- ion in back of formula. Brønsted–Lowry: Acids are proton donors and Bases are proton acceptors Conjugate acid/base pairs Lewis: Acids accepts a pair of electrons, Bases donates a pair of electrons

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