Chapter 7: Reactions in Aqueous Solutions

Question 1

What are the “driving forces” that pull reactants toward products?

Answer 1

1. Formation of a solid
2. Formation of water
3. Transfer of electrons
4. Formation of a gas

When two or more chemicals are brought together, if any of these things can occur, a chemical change (a reaction) is likely to take place.

Question 2

What happens when a chemical reaction results in the formation of a solid?

Answer 2

The formation of a solid is a process called precipitation. The solid that forms is called a precipitate, and the reaction is known as a precipitation reaction.

Question 3

What is a precipitation reaction?

Answer 3

A precipitation reaction is a chemical reaction in which an insoluble substance forms and separates from the solution as a solid.

Question 4

What is the best way to predict the identity of a precipitate?

Answer 4

How to Predict Precipitates When Solutions of Two Ionic Compounds Are Mixed

• Step 1: Write the reactants as they actually exist before any reaction occurs. (Remember that when a salt dissolves, its ions separate.)
• Step 2: Consider the various solid that could form. To do this, simply exchange the anions of the added salts.
• Step 3: Use the solubility rules to decide whether a solid forms and, if so, to predict the identity of the solid.

Question 5

What happens when an ionic compound dissolves in water?

Answer 5

In virtually every case when a solid containing ions dissolves in water, the ions separate and move around independently.

Ex: Ba(NO₃)₂(aq) does not contain Ba(NO₃)₂ units. Rather, it contains separated Ba²⁺ and NO₃^- ions, and the solution contains two NO₃^- ions for every Ba²⁺ ion.

Question 6

When each unit of a substance that dissolves in water produces separated ions, the substance is called a ____________.

Answer 6

When each unit of a substance that dissolves in water produces separated ions, the substance is called a strong electrolyte.

Question 7

What is an ionic solid?

Answer 7

An ionic solid is the precipitate that forms in a reaction involving aqueous solutions of ions.

Question 8

What is a soluble solid?

Answer 8

A soluble solid is a solid that readily dissolves in water.

Question 9

What is an insoluble solid?

Answer 9

The terms insoluble solid and slightly soluble solid are taken to mean the same thing: a solid where such a tiny amount dissolves in water that it is undetectable with the naked eye.

Question 10

What are the general rules for solubility of ionic compounds (salts) in water at 25ºC?

Answer 10

1. Most nitrate (NO₃^-) salts are soluble.
2. Most Na⁺, K⁺, and NH₄^<em>+</em> salts are soluble.
3. Most chloride salts are soluble.
   Notable exceptions are AgCl, PbCl₂, and Hg₂Cl₂.
4. Most sulfate salts are soluble.
   Notable exceptions are BaSO₄, PbSO₄, and CaSO₄.
5. Most hydroxide compounds are only slightly soluble.
   Important exceptions are NaOH and KOH.
   Ba(OH)₂ and Ca(OH)₂ are only moderately soluble.
6. Most sulfide (S^2-), carbonate (CO₃^2-), and phosphate (PO₄^3-) salts are only slightly soluble.

Question 11

Types of Equations for Reactions in Aqueous Solutions

Answer 11

1. The molecular equation shows the overall reaction but not necessarily the actual forms of the reactants and products in solution.
2. The complete ionic equation represents all reactants and products that are strong electrolytes as ions. All reactants and products are included.
3. The net ionic equation includes only those components that undergo a change. Spectator ions are not included.

Question 12

Molecular Equation

Answer 12

An equation representing a reaction in solution and showing the reactants and products in undissociated form, whether they are strong or weak electrolytes.

Question 13

Complete Ionic Equation

Answer 13

An equation that shows as ions all substances that are strong electrolytes.

Question 14

Net Ionic Equation

Answer 14

An equation for a reaction in solution, representing strong electrolytes as ions and showing only those components that are directly involved in the chemical change.

Question 15

spectator ions

Answer 15

Ions present in solution that do not participate directly in a reaction.

Question 16

Two classes of compounds: _____ and _____.

Answer 16

Two classes of compounds: acids** and **bases.

Question 17

acids

Answer 17

An acid is a substance that produces H⁺ ions (protons) when it is dissolved in water.

Question 18

strong acids

Answer 18

Substances that are strong electrolytes that produce H⁺ ions are called strong acids.

Studies show that when HCl, HNO₃, and H₂SO₄ are placed in water, virtually every molecule dissociates to give ions. This means that when 100 molecules of HCl are dissolved in water, 100 H⁺ ions and 100 Cl^- ions are produced. Virtually no HCl molecules exist in aqueous solution.

Question 19

bases

Answer 19

A base is a substance that produces hydroxide ions (OH^-) in water.

Question 20

strong bases

Answer 20

Hydroxide compounds that are strong electrolytes containing OH^- ions.

Question 21

What happens when strong acids and strong bases (hydroxides) mix?

Answer 21

When strong acids and strong bases (hydroxides) are mixed, the fundamental chemical change that always occurs is that H⁺ ions react with OH^- ions to form water.

H⁺(aq) + OH^-(aq) → H₂O (l)

Question 22

_______ is always a product of a reaction involving an acid and OH^-.

Answer 22

Water is always a product of a reaction involving an acid and OH^-​.

Question 23

salts

Question 24

Summary of Strong Acids and Strong Bases

Answer 24

1. The common strong acids are aqueous solutions of HCl, HNO₃, and H₂SO₄.
2. A strong acid is a substance that completely dissociates (ionizes) in water. (Each molecule breaks up into an H⁺ ion plus an anion.)
3. A strong base is a metal hydroxide compound that is very soluble in water. The most common strong bases are NaOH and KOH, which completely break up into separated ions (Na⁺ and OH^- or K⁺ and OH^-) when they are dissolved in water.
4. The net ionic equation for the reaction of a strong acid and a strong base (contains OH^-) is always the same: it shows the production of water.
   * H⁺(aq) + OH^-(aq) → H₂O (l)*
5. In the reaction of a strong acid and a strong base, one product is always water and the other is always an ionic compound called a salt, which remains dissolved in the water. This salt can be obtained as a solid by evaporating the water.
6. The reaction of H⁺ and OH^- is often called an acid-base reaction, where H⁺ is the acidic ion and OH^- is the basic ion.

Question 25

Oxidation-Reduction Reaction

Answer 25

A reaction that involves a transfer of electrons.

• Can involve a metal reacting with a nonmetal
• Electron transfer reactions can also take place between two nonmetals

Question 26

The reaction of a _______ with a _______ to form an ______________ involves the transfer of one or more electrons from the former to the latter.

Answer 26

The reaction of a metal with a nonmetal to form an ionic compound involves the transfer of one or more electrons from the metal (which forms a cation) to the nonmetal (which forms an anion). This is the third driving force for reactions.

Question 27

One sure sign of an oxidation-reduction reaction between nonmetals is the presence of ____________.

Answer 27

One sure sign of an oxidation-reduction reaction between nonmetals is the presence of oxygen, O₂(g).

Question 28

Characteristics of Oxidation-Reduction Reactions

Answer 28

1. When a metal reacts with a nonmetal, an ionic compound is formed. The ions are formed when the metal transfers one or more electrons to the nonmetal, the metal atom becoming a cation and the nonmetal atom becoming an anion. Therefore, a metal-nonmetal reaction can always be assumed to be an oxidation-reduction reaction, which involves electron transfer.
2. Two nonmetals can also undergo an oxidation-reduction reaction. At this point we can recognize these cases only by looking for O₂ as a reactant or product. When two nonmetals react, the compound formed is not ionic.

Question 29

Classification of Reactions

Answer 29

1. Precipitation Reactions
2. Double-displacement Reactions
3. Acid-Base Reactions
4. Oxidation-Reduction Reactions

Question 30

Precipitation Reactions

Answer 30

A reaction in which an insoluble substance (the precipitate) forms and separates from the solution as a solid.

Question 31

Double-displacement Reactions

Answer 31

A double displacement reaction, also known as a double replacement reaction or metathesis, is a type of chemical reaction where two compounds react, and the positive ions (cation) and the negative ions (anion) of the two reactants switch places, forming two new compounds or products.

Question 32

Acid-Base Reactions

Answer 32

Reactions in which water is formed when a strong acid is mixed with a strong base. We can recognize acid-base reactions by the H⁺ ion that ends up in the product water, H₂O.

Question 33

The third driving force for chemical reactions is _____________________.

Answer 33

The third driving force of chemical reactions is electron transfer. Evidence of this driving force is seen particularly in the affinity for metals to donate electrons to nonmetals.

2Li(s) + F₂(g) → 2LiF(s​)

This process of electron transfer is called oxidation-reduction.

Question 34

The fourth driving force for chemical reactions is ______________________.

Answer 34

The fourth driving force for chemical reactions is formation of a gas. A reaction in aqueous solution that forms a gas (which escapes as bubbles) is pulled toward the products by this event.

Zn(s) + 2HCl(aq) → H₂(g) + ZnCl₂(aq)

*Note: This example involves an electron transfer process, so the reaction can be classifiefd as an oxidation-reduction reaction.

or

2HCl(aq) + Na₂CO₃(aq) → CO₂(g) + H₂O(l) + NaCl(aq)

*Note: Because this reaction involves H⁺ that ends up in the product water, we classify it as an acid-base reaction.

Question 35

Single-replacement Reaction

Answer 35

A single-displacement reaction, also named single-replacement reaction, is a type of oxidation-reduction chemical reaction when an element or ion moves out of one compound and into another - that is, one element is replaced by another in a compound.

• This will most often occur if A is more reactive than B.
• A and B must be either:
  
  • Different metals (hydrogen’s behavior as a cation renders it as a metal here), in which case C represents an anion; or
  • Halogens, in which case C represents a cation.
• In either case, when AC and BC are aqueous compounds (which is usually the case), C is a spectator ion.

Question 36

The most commonly used classifications of chemical reactions

Answer 36

• Precipitation Reactions
• Acid-Base Reactions
• Oxidation-reduction Reactions

Question 37

Combustion Reactions

Answer 37

Vigorous and exothermic oxidation-reduction reactions that take place between certain substances (particularly organic compounds) and oxygen.

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)

This reaction produces the flame of the common laboratory burner. This reaction of methane with oxygen is both an oxidation-reduction reaction and a combustion reaction.

Combustion reactions, in fact, are a special class of oxidation-reduction reactions.

Question 38

Synthesis (Combination) Reactions

Answer 38

When a given compound is formed from simpler materials.

• Synthesis of water
• 2H₂(g) + O₂(g) → 2H₂O(l)*
• Synthesis of carbon dioxide
• C(s) + O₂(g) → CO₂(g)*
• Synthesis of nitrogen monoxide
• N₂(g) + O₂(g) → 2NO(g)*

The first two reactions are also commonly called combustion reactions because they produce flames.

The reaction of hydrogen with oxygen to produce water can be classified three ways: as an oxidation-reduction reaction, as a combustion reaction, and as a synthesis reaction.

Synthesis reactions can also occur without oxygen:

• Synthesis of sodium chloride
• 2Na(s) + Cl₂(g) → 2NaCl(s)*
• Synthesis of magnesium fluoride
• Mg(s) + F₂(g) → MgF₂(s)*

Synthesis reactions in which the reactants are elements are oxidation-reduction reactions as well. So, we can think of these synthesis reactions as another subclass of the oxidation-reduction class of reactions.

Question 39

Decomposition Reactions

Answer 39

A decomposition reaction is a type of chemical reaction in which a single compound breaks down into two or more elements or new compounds. These reactions often involve an energy source such as heat, light, or electricity that breaks apart the bonds of compounds.

• Decomposition of water
  (with application of an electric current)
  2H₂O(l) → 2H₂(g) + O₂(g)
• Decomposition of mercury (II) oxide
  (with application of heat)
  2HgO(s) → 2Hg(l) + O₂(g)

Because O₂ is involved in the first reaction, we recognize it as an oxidation-reduction reaction.

In the second reaction, HgO, which contains Hg²⁺ and O^2- ions, is decomposed to the elements, which contain uncharged atoms. In this process, Hg²⁺ gains two electrons and each O^2- loses two electrons, so this is both a decomposition reaction and an oxidation-reduction reaction.