AP Chemistry 4.3-4.4: Representations and Physical vs Chemical Changes

Precipitation reaction

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

When aqueous solutions of ionic compounds are mixed, how can you predict whether a precipitate forms?

List the ions present, exchange possible cation-anion partners, and use solubility rules to determine whether any product is insoluble. If an insoluble product forms, write it as a solid in the net ionic equation.

What precipitate forms when aqueous $\mathrm{AgNO_3}$ and $\mathrm{NaCl}$ are mixed?

Silver chloride precipitates. The net ionic equation is $\mathrm{Ag^+(aq) + Cl^-(aq) \rightarrow AgCl(s)}$.

What is the net ionic equation for mixing aqueous potassium sulfate and barium nitrate?

$\mathrm{Ba^{2+}(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s)}$.

Acid-base reaction

A reaction involving transfer of a hydrogen ion, $\mathrm{H^+}$, from one chemical species to another.

In the reaction $\mathrm{HCl + H_2O \rightarrow Cl^- + H_3O^+}$, which species donates and which accepts the proton?

$\mathrm{HCl}$ donates $\mathrm{H^+}$ and acts as the acid; $\mathrm{H_2O}$ accepts $\mathrm{H^+}$ and acts as the base.

What is the balanced molecular equation for neutralizing hydrochloric acid with magnesium hydroxide?

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

What is the net ionic equation for the neutralization of any strong acid by a soluble ionic hydroxide?

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

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

The carbonate reacts with stomach acid, consuming hydronium ions. The overall reaction produces a salt, water, and carbon dioxide: $\mathrm{CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2}$.

How does baking soda cause batter to rise?

Sodium bicarbonate, $\mathrm{NaHCO_3}$, reacts with an acid to form carbonic acid, which decomposes to produce $\mathrm{CO_2(g)}$. The carbon dioxide bubbles expand the batter.

Oxidation-reduction (redox) reaction

A reaction in which at least one element changes oxidation number. Oxidation is an increase in oxidation number, while reduction is a decrease.

What do oxidation and reduction mean in terms of electrons?

Oxidation is loss of electrons and reduction is gain of electrons. The mnemonic OIL RIG summarizes this relationship: Oxidation Is Loss, Reduction Is Gain.

Reducing agent and oxidizing agent

The reducing agent donates electrons and is itself oxidized. The oxidizing agent accepts electrons and is itself reduced.

How can you determine whether a chemical equation represents a redox reaction?

Assign oxidation numbers to relevant elements in reactants and products. If any element's oxidation number increases and another's decreases, the reaction is redox.

Why is $\mathrm{H_2 + Cl_2 \rightarrow 2HCl}$ a redox reaction even though it forms a covalent product?

Hydrogen changes from 0 in $\mathrm{H_2}$ to +1 in HCl, while chlorine changes from 0 to −1. The oxidation-number changes establish that oxidation and reduction occurred.

What happens when copper metal is placed in aqueous silver nitrate?

Copper is oxidized and silver ions are reduced: $\mathrm{Cu(s) + 2Ag^+(aq) \rightarrow Cu^{2+}(aq) + 2Ag(s)}$. The solution develops a blue color from dissolved $\mathrm{Cu^{2+}}$, while silver metal deposits.

Why is $\mathrm{BaCl_2(aq) + K_2SO_4(aq) \rightarrow BaSO_4(s) + 2KCl(aq)}$ not a redox reaction?

It is a precipitation reaction, but no element changes oxidation number. The ions exchange partners while retaining their original oxidation states.

How do molecular and net ionic equations represent an aqueous reaction differently?

A molecular equation writes soluble compounds as intact formulas, whereas a net ionic equation shows only the ions and species that undergo the chemical change, omitting spectator ions. For example, $\mathrm{NaCl(aq) + AgNO_3(aq) \rightarrow AgCl(s) + NaNO_3(aq)}$ becomes $\mathrm{Ag^+(aq) + Cl^-(aq) \rightarrow AgCl(s)}$.