1. Acid/Base Equilibria Notes Part 1: The 3 Acid/Base Definitions, Hydronium, Conjugate Acid/Base Pairs & their Relative Strengths
March 23, 2018

2. The 3 Acid/Base Definitions
There are 3 definitions of what is considered an acid and what is considered a base. You need to know all three of them
The Arrhenius definition classifies substances as acidic or basic by what ions they produce in solution (only applies to aqueous solutions).

3. The 3 Acid/Base Definitions
2. The Brønsted-Lowry definition classifies substances as acidic or basic by the movement of a proton (a proton is a hydrogen ion, H+) . 3. The Lewis definition classifies substances as acidic or basic depending on the transfer of an electron pair.

4. 1. The Arrhenius Definition
An acid is a substance that, when dissolved in water, increases the concentration of hydrogen ions (H+) in solution.
In other words, an acid is a substance that has a hydrogen that can dissociate in water.
Examples: HCl, H2SO4, or H3AsO4
H3PO4  3H+ + PO4-3

5. 1. The Arrhenius Definition
A base is a substance that, when dissolved in water, increases the concentration of hydroxide ions (OH-) in solution.
In other words, a base is a substance that has a hydroxide that can dissociate in water.
Examples: NaOH, Ba(OH)2, or KOH
Mg(OH)2  Mg+2 + 2OH-1

6. 2. The Brønsted-Lowry Definition
A Brønsted-Lowry acid is a substance (can be a molecule or ion) that donates a proton to another substance; it must have an acidic proton (a proton that is able to be removed).

7. 2. The Brønsted-Lowry Definition
The Brønsted-Lowry base is whatever substacne accepts that proton; it must have a nonbonding pair of electrons in order to create a bond to the proton.
From here on out, instead of writing Brønsted-Lowry out over and over, we’re going to just use the shorthand “BL.”

8. 2. The Brønsted-Lowry Definition
Consider this reaction and identify which reactant is acting as a BL acid and which one is acting as a BL base by comparing the reactants to the products and analyzing what happened to the proton.

9. BL base b/c it accepted a proton from HCl.
BL acid b/c it donated a proton to NH3.
BL base b/c it accepted a proton from HCl.

10. 2. The Brønsted-Lowry Definition
Practice: In each of the following chemical reactions, identify which reactant is acting as a BL acid and which one is acting like a BL base.
CH3CO2H + H2O  H3O+ + CH3CO2 -1

11. CH3COOH + H2O  H3O+ + CH3COO-
HCO H2O  H3O+ + CO3 -2
HCO3-1 + H2O  H2CO3 + OH-

12. 2. The Brønsted-Lowry Definition
In each of the previous examples, you’ll notice that one of the products was a hydronium ion, H3O+.
The hydronium ion is what is produced anytime water accepts a proton from any substance acting as a BL acid.
Although using H3O+ more closely represents reality, it’s more common to just write H+.
H3O+ and H+ represent the same thing!

13. 2. The Brønsted-Lowry Definition
Do you see how water can both accept and give up a proton? Any substance that is capable of that is called amphiprotic.

14. 2. The Brønsted-Lowry Definition
You also use the BL definition of acids & bases to identify conjugate acid/base pairs.
A conjugate acid/base pair is two substances whose chemical formulas differ only in the presence of a hydrogen proton. Example: HNO2 & NO2-1
Anytime something loses a hydrogen it’s charge decreases by one. If it gains a hydrogen it’s charge increases by one.

15. 2. The Brønsted-Lowry Definition
Every BL acid has a conjugate base.
The substance formed when the BL acid has lost a proton.
Every BL base has a conjugate acid.
The substance formed when the BL base has gained a proton.
This is an important skill for the AP test! They like to ask about this!

16. 2. The Brønsted-Lowry Definition
Answers:
ClO4 -1
HS-1
PH3
CO3 -2
What are the conjugate bases for these BL acids?
HClO4
H2S
PH4 +
HCO3-1

17. 2. The Brønsted-Lowry Definition
Answers:
HCN
HSO4 -1
H3O+
H2CO3
What is the conjugate acid of each of these bases?
CN-
SO4 -2
H2O
HCO3-1

18. 2. The Brønsted-Lowry Definition
You might also be asked to write equations using BL acids & bases in which they specify how the substance is supposed to act.
Example: The hydrogen sulfite ion (HSO3 -1) is amphiprotic.
Write an equation for the reaction between HSO3-1 and water in which the ion acts as an acid.
Write an equation for the reaction between HSO3-1 and water in which the ion acts as a base.

19. 2. The Brønsted-Lowry Definition
When you’re instructed to use the ion as an acid, that means that it will lose a proton and its charge will decrease by 1. To complete the equation, use the ion and water as reactants and transfer a proton from the ion to water.
HSO H2O  SO3-2 + H3O+

20. 2. The Brønsted-Lowry Definition
When you’re instructed to use the ion as a base, that means that it will receive a proton and its charge will increase by 1. To complete the equation, use the ion and water as reactants and transfer a proton from water to the ion.
HSO H2O  H2SO3 + OH-

21. 2. The Brønsted-Lowry Definition
In the equations you just wrote, label the reactants as BL acids or bases and then identify their conjugates.
HSO H2O  SO3-2 + H3O+
HSO H2O  H2SO3 + OH-
BL Acid
BL Base
Conjugate acid
Conjugate base
Conjugate base
BL Base
BL Acid
Conjugate acid

22. 2. The Brønsted-Lowry Definition
You need to memorize the 7 strong acids & 8 strong bases.
For a conjugate acid/base pair…
The stronger the acid, the weaker the conjugate base.
The stronger the base, the weaker the conjugate acid.

24. 2. The Brønsted-Lowry Definition
For acids, strength is determined by the percentage of molecules that will dissociate in solution.
Dissociation: the process by which a substance will split up into its component ions in solution.

25. 2. The Brønsted-Lowry Definition
For bases, strength is determined by the percentage of molecules that will protonate in solution.
Protonation: the process by which a substance will accept a proton in solution

26. 2. The Brønsted-Lowry Definition
This is a diagram of the relative strength of some conjugate acid/base pairs.
Look at the conjugate pairs listed at the top.

27. 2. The Brønsted-Lowry Definition
HCl totally dissociates in solution so it’s a strong acid and its conjugate base, Cl-, doesn’t demonstrate any ability to accept a proton so its strength as a base is negligible.

28. 2. The Brønsted-Lowry Definition
The acids at the bottom of the list, such as CH4 or H2, are compounds that contain hydrogen but none of the hydrogens are acidic, meaning they do not dissociate in solution. These compounds show no acidic behavior.

29. 2. The Brønsted-Lowry Definition
Conversely, the conjugate bases at this end of the spectrum are very strong BL bases because they will accept a proton (become protonated) 100% of the time.

30. 2. The Brønsted-Lowry Definition
The Main Idea: the stronger an acid, the weaker its conjugate base. The stronger the base, the weaker its conjugate acid.
A strong acid will totally dissociate in solution so that there are no un-dissociated molecules left.

31. 2. The Brønsted-Lowry Definition
A weak acid will only partially dissociate in solution, resulting in a mixture of dissociated and undissociated molecules.
A substance with negligible abilities as an acid will not dissociate at all in solution and will exist totally as undissociated molecules.

32. 2. The Brønsted-Lowry Definition
This concept of relative strength is important because it governs the way acid base reactions occur.
In every acid/base reaction, the position of equilibrium favors transfer of the proton from the stronger acid to the stronger base in order to produce the weaker acid & weaker base in the reaction.

33. 2. The Brønsted-Lowry Definition
In order to apply this theory you need to look at the reactants and products in a reaction, identify them as BL acids or bases and then compare their relative strengths on the diagram above.
Example: For the reaction listed below…
Does the equilibrium lie to the left or right?
Is Kc greater than or less than one?
Which molecules/ions will be present in the largest quantities in solution?

34. CH3COOH + H2O  H3O+ + CH3COO-
To answer “a” label treat both sides of the equation as reactants. Label each as a BL acid or a BL base.
BL Acid
BL Acid
BL Base
BL Base

35. CH3COOH + H2O  H3O+ + CH3COO-
Then, compare their relative strengths.
Acids: CH3COOH is a weaker acid than H3O+.
Bases: H2O is a weaker base than CH3COO-.
In this reaction, the equilibrium will favor the transfer of the proton from the stronger acid, H3O+, to the stronger base, CH3COO-. The equilibrium therefore lies to the left.
BL Acid
BL Acid
BL Base
BL Base

36. 2. The Brønsted-Lowry Definition
To answer “b”, since the substances on the right are going to react to produce the substances on the left, the [reactant] is going to be greater than the [product] so Kc is going to be less than 1.
The reactants will make up the majority of molecules found in solution with only a few of the molecules listed on the right side of the reaction arrow.