Module 5 Intro

1. Module 5 Intro

1.8. Page 6

Lesson 1

Module 5—Acids and Bases

Read

The Hydronium Ion

A diagram shows a molecule of water reacting with a hydrogen ion. The water molecule is shown as a single red sphere labeled O and two grey spheres each labeled H. Eight dots appear around the red sphere representing its electrons. Six of these dots are red in colour, and two are grey. In the space between each of the grey spheres and the red sphere are a grey and a red dot. Two pairs of red dots appear around the red sphere. These are labeled exposed, lone electron pairs. The single hydrogen ion is depicted as a grey sphere with no dots surrounding it. A series of dotted lines span the region from one of the lone pairs on the water molecule to the hydrogen ion. These dotted lines are labeled electrostatic attraction to negative region of molecule. The product of the reaction is labeled hydronium ion and is a water molecule with an additional grey sphere. The third grey sphere is located nest to one of the lone electron pairs.

hydronium ion: a hydrated hydrogen ion (proton), H3O+(aq); the entity responsible for the acidic properties in an aqueous solution

You may recall that the position of lone pairs of electrons around the surface of a water molecule contributes to its polarity. The tendency of these lone pairs is to attract positive charges. Free hydrogen ions, which have a positive charge, are naturally drawn by electrostatic forces toward water molecules, resulting in the formation of hydronium ions.

Examine the structure of the hydronium ion shown in the diagram. Note that all of the hydrogen atoms are covalently bonded to the central oxygen atom. The presence of the lone pair results in a trigonal pyramidal structure for the hydronium ion.

A combination of theoretical and experimental data supports the notion of the hydronium ion as the acidic particle.

Arrhenius’s theory was unable to explain the mechanism by which an acid “falls apart” (ionizes) to yield the hydrogen ion. The ionization of acids can be explained as a result of collisions between solutes and water molecules, in which a hydrogen ion is transferred to the water molecule, bonding to one of the lone pairs of electrons.

Formation of Hydronium Ions

A diagram shows colliding water molecules and H CL molecules. The product of the reaction shown is a hydronium ion and a single chloride ion.

The observed acidity of substances like NaHSO4(s) can be explained. A reaction between water and dissociated ions can result in the formation of hydronium ions as is summarized in the two reaction equations:

Dissociation of Solute in Water

NaHSO4(s) → Na+(aq) + HSO4–(aq)

Reaction with Water to Form Hydronium Ion

HSO4–(aq) + H2O(l) → H3O+(aq) + SO42–(aq)
The observed basicity of ammonia, NH3(aq), can be explained. Ammonia does not possess any hydroxide ions. So how does it form a base? The answer is that ammonia can react with water to produce a hydroxide ion.

Reaction of Ammonia

A diagram shows colliding water molecules and ammonia, NH3, molecules. The product of the reaction shown is an ammonium ion and a hydroxide ion. The hydroxide ion appears as a water molecule less one of its hydrogen atoms.

Self-Check

Comparing Theories

SC 5. Prepare a table that lists the acidic and basic particles associated with Arrhenius's theory and its more recent revision. Identify the mechanisms by which these particles are formed for each theory.

Test your knowledge of the material covered in this lesson by completing the following questions.

SC 6. State the similarities and differences between a hydrogen ion and a hydronium ion.

SC 7. Write a balanced chemical equation for each situation stated.

HNO3(aq) reacts with water to form an acidic solution.
HBr(aq) reacts with water to form an acidic solution.
Na2HPO4(s) reacts with water to form an acidic solution.
Na2HPO4(s) reacts with water to form a basic solution.
Ca(OH)2(s) dissociates to form a basic solution.
NaCN(s) reacts with water to form a basic solution.

SC 8. Baking soda (sodium hydrogen carbonate) produces a basic solution when dissolved in water. Explain this result using one of the theories you covered in this lesson.

SC 9. Using the modified Arrhenius theory, write the steps in the neutralization reaction between nitrous acid and aqueous sodium hydroxide.

Check your work.

Self-Check Answers

SC 5.

Theory	Acidic Particle	Basic Particle	Mechanism to Form Particles
Arrhenius's original theory	H+(aq)	OH–(aq)	dissociation (hydroxide) ionization (acids)
Arrhenius's modified theory	H3O+(aq)	OH–(aq)	dissociation (for ionic hydroxides) reaction with water molecules (for acids or bases, explains ionization)

SC 6. A hydrogen ion is simply a positively charged proton, whereas a hydronium ion is a water molecule that has incorporated an extra hydrogen atom. A similarity between the two is that they both have a charge of 1+.

SC 7.

HNO3(aq) + H2O(l) → NO3–(aq) + H3O+(aq)
HBr(aq) + H2O(l) → Br–(aq) + H3O+(aq)
Na2HPO4(s) → 2 Na+(aq) + HPO42–(aq)

HPO42–(aq) + H2O(l) → PO43–(aq) + H3O+(aq)
Na2HPO4(s) → 2 Na+(aq) + HPO42–(aq)

HPO42 –(aq) + H2O(l) → H2PO4–(aq) + OH–(aq)
Ca(OH)2(s) → Ca2+(aq) + 2 OH–(aq)
NaCN(s) → Na+(aq) + CN–(aq)

CN–(aq) + H2O(l) → HCN(aq) + OH–(aq)

SC 8. The first reaction involves dissociation of the solid ionic compound.

NaHCO3(s) → Na+(aq) + HCO3–(aq)

The second reaction involves a reaction between the hydrogen carbonate ion and a water molecule.

HCO3–(aq) + H2O(l) → H2CO3(aq) + OH–(aq)

The products of the second reaction, specifically the hydroxide ion, are consistent with the observed basic properties for the solution.

SC 9. HNO2(aq) + H2O(l) → NO2–(aq) + H3O+(aq)

NaOH(aq) → Na+(aq) + OH–(aq)

H3O+(aq) + OH–(aq) → 2 H2O(l)

Module 5: Lesson 1 Assignment

Retrieve the copy of the Module 5: Lesson 1 Assignment that you saved earlier. Complete the questions in Part 3 of your assignment. Save a copy of your answers to your course folder. Once you have completed all of the questions, submit your work to your teacher.