Acids and Bases

How do acids and bases behave in water?

Acids and bases play an important role in our lives. Numerous biological processes, industrial applications,
and even environmental problems are a function of the acidity or basicity (alkalinity) of aqueous solu-
tions. It is therefore important to understand what makes a substance behave as an acid or a base when
dissolved in water. In this activity, we will explore the physical and chemical properties of acids and bases.

Model 1 – Arrhenius Acids and Bases

1. Refer to Model 1.

   1. What is the common chemical name for Vitamin C?

   2. Is Vitamin C classified as an acid or a base?

2. Examine the properties of the Arrhenius acids in Model 1. List three properties that all Arrhenius

   acids have in common.

3. Examine the chemical formulas for the Arrhenius acids in Model 1. What feature do all the
   Arrhenius acid chemical formulas have in common?

4. Examine the properties of the Arrhenius bases in Model 1. List two properties that all Arrhenius
   bases have in common.

Acids and Bases 1

1. Examine the chemical formulas for the Arrhenius bases in Model 1. What anion do all the
   Arrhenius base chemical formulas have in common?

2. A student dissolved a small amount of baking soda in water and tested it with litmus paper. The
   litmus paper turned blue. Is baking soda likely an acid or a base?

Read This!

In 1903 Svante Arrhenius won the Nobel Prize in Chemistry for defining acids and bases in terms of

the ions produced. An Arrhenius acid is any substance that produces hydrogen ions [or hydronium ions

(H O+) a hydrogen ion attached to a water molecule] when dissolved in water. An Arrhenius base is any
3

substance that produces hydroxide ions when dissolved in water. While the Arrhenius definitions of acids
and bases is useful, it is limited. Johannes Brønsted and Thomas Lowry developed more general defini-
tions for acids and bases using H+ion (proton) transfer as the focus.

Model 2 – Brønsted-Lowry Acids and Bases

Reaction 1
Reaction 2
Reaction 3
Reaction 4

7.

8.

9.

HCl(g) + H2O(l)??H3O+(aq) + Cl–(aq)
(acid) (base)

NH (aq) + HF(aq)??NH+(aq) + F–(aq)
34

NH+(aq) + HO(l)??NH(aq) + HO+(aq)
42 33

(base) (acid)

(acid)

(base)

+ H PO–(aq)??HPO2–(aq) + HF(aq)
244

F–(aq)
(base)

(acid)
Identify the Brønsted-Lowry acids in Model 2.

a.Atoms of which element are present in all of the Brønsted-Lowry acids in Model 2?
b.How can you tell from Reaction 1 that HCl loses an H+ion rather than a hydrogen atom

when the reaction occurs?Hint:Look at the products.
For each acid–base reaction in Model 2, describe the role of the Brønsted-Lowryacidin the H+

ion (proton) transfer that occurs.

For each acid–base reaction in Model 2, describe the role of the Brønsted-Lowrybasein the pro-
ton (H+ion) transfer that occurs.

2

POGILTMActivities for High School Chemistry

1. As you saw in Model 1, all Arrhenius bases in Model 1 have an OH–ion in their chemical
   formulas. Write a balanced chemical reaction for the reaction of HCl(aq) with OH–(aq) to
   illustrate that the hydroxide ion is also a Brønsted-Lowry base.

2. If you reverse Reaction 1 in Model 2, the following reaction is obtained.
   H3O+(aq) + Cl–(aq)??HCl(aq) + H2O(l)
   a.Forthereactionabove,whichreactantisactinglikeaBrønsted-Lowryacid?Howcanyoutell?

   b.Forthereactionabove,whichreactantisactinglikeaBrønsted-Lowrybase?Howcanyoutell?

3. Write the reverse reactions for Reactions 2 and 3 in Model 2. Label the Brønsted-Lowry acid and

base reactants for each reaction.
Reaction 2

Reaction 3

Model 3 – Conjugate Acid–Base Pairs

–

Conjugate Acid

+

1. All acid–base reactions have two conjugate acid–base pairs. One conjugate acid–base pair in the

   reaction in Model 3 is H O+/H O. List the other acid–base pair in the reaction.
   32

2. Why is HCO–considered the “acid” part of the pair in the reaction in Model 3?
   3

3. Why is CO2–considered the “base” part of the pair in the reaction in Model 3?
   3

4. The“Read This!”box before Model 2 calls the transfer of a H+ion a “proton transfer.”
   Explain why “H+ion” and “proton” are synonymous.

–

–

HCO1–(aq)
32 33

+ H O(l)

CO2–(aq)

+ H O1+(aq)

Base

Acids and Bases 3

17.

18.

Examine the charges on the species in the Model 3 reaction. Why does the charge on the carbon-
containing ion change from –1 to –2?

Using the list of substances below, select pairs that are conjugate acids and bases. Enter the pairs
in the tables below. The first acid–base pair has been entered for you. Note that you may use a
substance more than once or not at all.

H PO H O+NO–C H O2–H CO
34322323

H O NH–CO CO2–NH
22233

HC H O PO3–HPO2–OH–H PO–
2324424

15
26
37
4

Write the formula for the conjugate base of each of the following acids.Hint:Be sure to consider
charges.

a.HSO–b.HFc.HS–
3

Write the formula for the conjugate acid of each of the following bases.Hint:Be sure to consider
charges.

a.SO2–b.F–c.HS–
3

For the following reactions, label the acid and base in the reactants, and the conjugate acid and
conjugate base in the products.

HCO–(aq) + NH (aq)??NH+(aq) + CO2–(aq)
3343

HCO–(aq) + HCl(aq)??Cl–(aq) + H CO (aq)
323

Is the role of a conjugate acid in the reverse direction the same as the role of an acid in the for-
ward direction? Explain.