AP Chemistry 4.7-4.8: Types of Reactions and Acid-Base Intro

Precipitation reaction

A reaction in which dissolved species react to form one or more solid products. Many precipitation reactions involve ion exchange between aqueous ionic compounds and are also called double-displacement or metathesis reactions.

Solubility

The maximum concentration of a substance that can dissolve in a solvent under specified conditions. A substance with relatively high solubility is soluble; one with relatively low solubility is insoluble.

What condition causes a dissolved ionic compound to precipitate from solution?

A precipitate forms when the concentration of a substance exceeds its solubility under the specified conditions.

Which cations generally make ionic compounds soluble, regardless of the anion?

Ammonium, $NH_4^+$, and group 1 cations—$Li^+$, $Na^+$, $K^+$, $Rb^+$, and $Cs^+$—form soluble compounds.

Which common anions generally form soluble ionic compounds?

Acetate, $C_2H_3O_2^-$; bicarbonate, $HCO_3^-$; nitrate, $NO_3^-$; and chlorate, $ClO_3^-$, generally form soluble salts.

What are the solubility rules for chloride, bromide, and iodide salts?

Most $Cl^-$, $Br^-$, and $I^-$ salts are soluble. Important exceptions contain $Ag^+$, $Hg_2^{2+}$, or $Pb^{2+}$.

What are the solubility rules for fluoride salts?

Most $F^-$ salts are soluble, except compounds containing group 2 cations, $Pb^{2+}$, or $Fe^{3+}$.

What is the general solubility rule for sulfate, $SO_4^{2-}$, salts?

Most sulfates are soluble, but sulfates of $Ag^+$, $Ba^{2+}$, $Ca^{2+}$, $Hg_2^{2+}$, $Pb^{2+}$, and $Sr^{2+}$ are exceptions and are treated as insoluble or sparingly soluble.

What is the general solubility rule for carbonate, chromate, phosphate, and sulfide salts?

Most $CO_3^{2-}$, $CrO_4^{2-}$, $PO_4^{3-}$, and $S^{2-}$ salts are insoluble. Their major soluble exceptions contain group 1 cations or $NH_4^+$.

What is the general solubility rule for hydroxide salts?

Most hydroxides are insoluble. Hydroxides of group 1 cations and $Ba^{2+}$ are soluble exceptions.

How can you predict whether mixing two aqueous ionic solutions will produce a precipitate?

List the ions present, pair each possible cation with each possible anion, and use solubility rules to identify any insoluble product. If an insoluble product forms, write it as a solid in the net ionic equation.

What is the net ionic equation for precipitation of lead(II) iodide?

$Pb^{2+}(aq)+2I^-(aq)\rightarrow PbI_2(s)$.

What is the net ionic equation for mixing aqueous silver nitrate and sodium chloride?

$Ag^+(aq)+Cl^-(aq)\rightarrow AgCl(s)$. The sodium and nitrate ions remain aqueous spectator ions.

Which solution—sodium chloride, sodium hydroxide, or sodium sulfate—can precipitate $Ba^{2+}$ from water, and what solid forms?

Sodium sulfate is effective because it supplies $SO_4^{2-}$, forming insoluble $BaSO_4$: $Ba^{2+}(aq)+SO_4^{2-}(aq)\rightarrow BaSO_4(s)$.

Acid in the aqueous Arrhenius-style definition used here

A substance that dissolves in water to produce hydronium ions, $H_3O^+$. It does so by transferring a proton, $H^+$, to water.

Base in the aqueous Arrhenius-style definition used here

A substance that dissolves in water to produce hydroxide ions, $OH^-$. Ionic hydroxides dissociate directly, while some molecular bases react with water to generate $OH^-$.

What is the Brønsted–Lowry definition of an acid and a base?

A Brønsted–Lowry acid donates a proton, $H^+$, and a Brønsted–Lowry base accepts a proton. This definition applies even when the reaction does not occur in aqueous solution.

In $NH_3(aq)+H_2O(l)\rightleftharpoons NH_4^+(aq)+OH^-(aq)$, identify the Brønsted–Lowry acid, base, and conjugate pairs.

$H_2O$ is the acid because it donates $H^+$, and $NH_3$ is the base because it accepts $H^+$. The conjugate acid-base pairs are $NH_4^+/NH_3$ and $H_2O/OH^-$.

What is the difference between a strong acid and a weak acid?

A strong acid reacts essentially completely with water, whereas a weak acid ionizes only partially and establishes an equilibrium. Strong-acid reactions are represented with a single forward arrow; weak-acid ionization uses an equilibrium arrow.

Which common acids are classified as strong acids in aqueous solution?

The common strong acids are $HCl$, $HBr$, $HI$, $HNO_3$, $HClO_4$, and $H_2SO_4$.

Write the aqueous reaction of hydrochloric acid with water.

$HCl(aq)+H_2O(l)\rightarrow Cl^-(aq)+H_3O^+(aq)$.

Write the equilibrium for acetic acid ionizing in water.

$CH_3CO_2H(aq)+H_2O(l)\rightleftharpoons CH_3CO_2^-(aq)+H_3O^+(aq)$. Acetic acid is weak because only a fraction of its molecules ionize.

What is the difference between a strong base and a weak base?

A strong base produces hydroxide essentially completely in water, while a weak base reacts only partially with water and establishes an equilibrium.

How do soluble ionic hydroxides produce hydroxide ions in water?

They dissociate rather than chemically react with water. For example, $NaOH(s)\rightarrow Na^+(aq)+OH^-(aq)$.

Write the equilibrium reaction that shows why ammonia is a weak base.

$NH_3(aq)+H_2O(l)\rightleftharpoons NH_4^+(aq)+OH^-(aq)$. Ammonia accepts a proton from water, but the reaction is incomplete.

Neutralization reaction

An acid-base reaction between an acid and a base that commonly produces a salt and water: acid + base $\rightarrow$ salt + water.

Write the molecular equation for neutralization of hydrochloric acid by magnesium hydroxide.

$Mg(OH)_2(s)+2HCl(aq)\rightarrow MgCl_2(aq)+2H_2O(l)$.

What is the net ionic equation for neutralization between a strong acid and a soluble ionic hydroxide?

$H_3O^+(aq)+OH^-(aq)\rightarrow 2H_2O(l)$.

Why can calcium carbonate be used as an antacid, and what additional product can form?

The basic carbonate ion reacts with stomach acid, neutralizing $H_3O^+$. The overall reaction can produce water and carbon dioxide: $CaCO_3(s)+2HCl(aq)\rightarrow CaCl_2(aq)+H_2O(l)+CO_2(g)$.

How does sodium bicarbonate cause baked goods to rise when mixed with an acid?

$NaHCO_3$ is a base that reacts with acids; the resulting carbonic acid decomposes to produce $CO_2(g)$, whose bubbles expand the batter.

What is a synthesis reaction, and what is its general form?

A synthesis reaction combines two or more reactants to form a single product: $A+B\rightarrow AB$.

What is a decomposition reaction, and what is its general form?

A decomposition reaction breaks one compound into two or more simpler substances: $AB\rightarrow A+B$.

What is a combustion reaction in the context of common reaction types?

A combustion reaction occurs when a substance reacts rapidly with oxygen, usually releasing heat and often light. Combustion of a hydrocarbon commonly produces $CO_2$ and $H_2O$.

What is the general pattern for a single-displacement reaction?

An element replaces another element in a compound: $A+BC\rightarrow AC+B$. In many aqueous examples, a more reactive metal displaces a less reactive metal ion or hydrogen ion.

Single-displacement reaction as a redox subclass

A reaction in which an elemental metal is oxidized and displaces an ion from solution. For example, $Zn(s)+2HCl(aq)\rightarrow ZnCl_2(aq)+H_2(g)$.

What is the general pattern for a double-displacement reaction?

Two ionic compounds exchange ions to form two new compounds: $AB+CD\rightarrow AD+CB$. A reaction proceeds when it forms a precipitate, water, or a gas.