AP Chemistry 4.1-4.2: Chemical Equations and Net Ionic Equations

Chemical equation: What do the left and right sides of the equation represent, and what symbols separate them?

Reactants are written on the left and products on the right. Plus signs separate substances on the same side, and an arrow separates reactants from products.

Coefficients in a chemical equation

Coefficients are numbers placed immediately before chemical formulas to show the relative amounts of each species. A coefficient of 1 is usually omitted.

How should the coefficients in a balanced chemical equation be interpreted?

They represent ratios of particles, moles, or any other amount unit. For example, $CH_4+2O_2\rightarrow CO_2+2H_2O$ represents a $1:2:1:2$ ratio of methane, oxygen, carbon dioxide, and water.

What does it mean for a chemical equation to be balanced?

A balanced equation has the same number of atoms of every element on both sides. This reflects the law of conservation of matter.

How is the number of atoms of an element determined from a chemical formula in an equation?

Multiply the equation coefficient by the element's subscript. If the element appears in multiple formulas on one side, calculate and add all of its contributions.

Why may coefficients be changed when balancing an equation, but subscripts may not be changed?

Changing coefficients changes the relative amount of a substance, whereas changing a subscript changes the substance's identity and therefore changes the reaction itself.

Balancing by inspection

Balancing by inspection is the process of adjusting coefficients while comparing atom counts on both sides of an equation. The final coefficients must be the smallest possible whole numbers.

What is the preferred form of a balanced chemical equation?

Its coefficients are the smallest possible whole-number integers. If all coefficients share a common factor, divide them by that factor.

When can a fractional coefficient be useful during equation balancing, and how is it removed?

A fraction can temporarily balance an element when an even-numbered diatomic molecule must produce an odd number of atoms. After balancing, multiply every coefficient by the denominator to obtain whole numbers.

Balance the decomposition of water: $H_2O\rightarrow H_2+O_2$.

$2H_2O\rightarrow 2H_2+O_2$

Balance the formation of dinitrogen pentoxide from nitrogen and oxygen: $N_2+O_2\rightarrow N_2O_5$.

$2N_2+5O_2\rightarrow 2N_2O_5$

Balance the decomposition of ammonium nitrate into nitrogen, oxygen, and water.

$2NH_4NO_3\rightarrow 2N_2+O_2+4H_2O$

Balance the combustion of ethane: $C_2H_6+O_2\rightarrow CO_2+H_2O$.

$2C_2H_6+7O_2\rightarrow 4CO_2+6H_2O$

Reduce $3N_2+9H_2\rightarrow 6NH_3$ to the preferred balanced equation.

$N_2+3H_2\rightarrow 2NH_3$

Physical-state symbols in chemical equations

$(s)$ denotes a solid, $(l)$ a liquid, $(g)$ a gas, and $(aq)$ an aqueous solution in which the substance is dissolved in water.

What does the symbol $\Delta$ above a reaction arrow indicate?

The reaction is carried out with heating.

Write the balanced equation for sodium reacting with liquid water to form aqueous sodium hydroxide and hydrogen gas.

$2Na(s)+2H_2O(l)\rightarrow 2NaOH(aq)+H_2(g)$

Molecular equation

A molecular equation represents aqueous ionic compounds using their formula units rather than explicitly showing the ions in solution. It identifies the compounds used as reactants and products.

Complete ionic equation

A complete ionic equation shows strong, soluble ionic compounds in aqueous solution as their separated ions. Insoluble solids, liquids, gases, and substances not dissociated in solution remain intact.

Net ionic equation

A net ionic equation shows only the species that undergo a chemical change, after spectator ions are canceled from the complete ionic equation.

Spectator ion

A spectator ion is present in the same form on both sides of a complete ionic equation. It does not undergo the chemical change and is omitted from the net ionic equation.

Why must ionic equations conserve both atoms and electric charge?

Chemical reactions conserve matter, so each element must have the same atom count on both sides. Because electrons are not created or destroyed overall, total charge must also be equal on both sides of an ionic equation.

Double-displacement (metathesis) reaction

A double-displacement reaction involves exchange of ions between two ionic compounds, often in aqueous solution. If one exchanged product is insoluble, the reaction is also a precipitation reaction.

Precipitation reaction

A precipitation reaction occurs when dissolved substances react to form one or more solid products. Many are double-displacement reactions in which ions exchange partners.

Solubility

Solubility is the maximum concentration of a substance that can dissolve in a solvent under specified conditions. A precipitate forms when the concentration exceeds the substance's solubility.

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

Identify all ions present, pair the possible cations and anions, and apply solubility rules to the possible products. A reaction is predicted if at least one product is insoluble.

Which classes of ionic compounds are generally soluble in water?

Salts containing $NH_4^+$, Group 1 cations, $NO_3^-$, $C_2H_3O_2^-$, $HCO_3^-$, or $ClO_3^-$ are generally soluble. Chlorides, bromides, iodides, fluorides, and sulfates are also generally soluble but have important exceptions.

What are the major exceptions to the solubility of chloride, bromide, and iodide salts?

$Cl^-$, $Br^-$, and $I^-$ salts are generally soluble except those containing $Ag^+$, $Hg_2^{2+}$, or $Pb^{2+}$.

What are the major exceptions to the solubility of fluoride salts?

Fluoride salts are generally soluble except those containing Group 2 metal cations, $Pb^{2+}$, or $Fe^{3+}$.

What are the major exceptions to the solubility of sulfate salts?

Sulfates are generally soluble, but compounds containing $Ag^+$, $Ba^{2+}$, $Ca^{2+}$, $Hg_2^{2+}$, $Pb^{2+}$, or $Sr^{2+}$ are exceptions and may be insoluble.

What are the general solubility rules for carbonates, chromates, phosphates, and sulfides?

$CO_3^{2-}$, $CrO_4^{2-}$, and $PO_4^{3-}$ compounds are generally insoluble except with Group 1 cations or $NH_4^+$. Sulfides are generally insoluble but are soluble with Group 1 cations and $NH_4^+$.

What is the general solubility rule for hydroxides?

Hydroxides are generally insoluble except those containing Group 1 cations or $Ba^{2+}$.

For the reaction $CaCl_2(aq)+2AgNO_3(aq)\rightarrow Ca(NO_3)_2(aq)+2AgCl(s)$, write the net ionic equation.

$Ag^+(aq)+Cl^-(aq)\rightarrow AgCl(s)$

In the precipitation of AgCl from $CaCl_2$ and $AgNO_3$, which ions are spectators?

$Ca^{2+}(aq)$ and $NO_3^-(aq)$ are spectator ions because they appear unchanged on both sides of the complete ionic equation.

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

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

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

Sodium sulfate can precipitate barium ions: $Ba^{2+}(aq)+SO_4^{2-}(aq)\rightarrow BaSO_4(s)$.

Predict the precipitate and write the net ionic equation when lead(II) nitrate and ammonium carbonate are mixed.

The precipitate is $PbCO_3(s)$. Net ionic: $Pb^{2+}(aq)+CO_3^{2-}(aq)\rightarrow PbCO_3(s)$.

Write the balanced molecular, complete ionic, and net ionic equations for carbon dioxide reacting with aqueous sodium hydroxide to form aqueous sodium carbonate and liquid water.

Molecular: $CO_2(aq)+2NaOH(aq)\rightarrow Na_2CO_3(aq)+H_2O(l)$. Complete ionic: $CO_2(aq)+2Na^+(aq)+2OH^-(aq)\rightarrow 2Na^+(aq)+CO_3^{2-}(aq)+H_2O(l)$. Net ionic: $CO_2(aq)+2OH^-(aq)\rightarrow CO_3^{2-}(aq)+H_2O(l)$.

Write the molecular, complete ionic, and net ionic equations for electrolysis of aqueous sodium chloride.

Molecular: $2NaCl(aq)+2H_2O(l)\rightarrow 2NaOH(aq)+H_2(g)+Cl_2(g)$. Complete ionic: $2Na^+(aq)+2Cl^-(aq)+2H_2O(l)\rightarrow 2Na^+(aq)+2OH^-(aq)+H_2(g)+Cl_2(g)$. Net ionic: $2Cl^-(aq)+2H_2O(l)\rightarrow 2OH^-(aq)+H_2(g)+Cl_2(g)$.

Acid-base reaction

An acid-base reaction involves transfer of a hydrogen ion, $H^+$, from one chemical species to another.

Acid in the aqueous Arrhenius/Brønsted context used here

An acid is a substance that reacts with water to produce hydronium ions, $H_3O^+$. This occurs through transfer of a proton, $H^+$, from the acid to a water molecule.

What reaction demonstrates that HCl acts as an acid in water?

$HCl(aq)+H_2O(l)\rightarrow Cl^-(aq)+H_3O^+(aq)$. HCl transfers a proton to water.

Strong acid versus weak acid

A strong acid reacts essentially completely with water, producing hydronium ions from nearly all dissolved acid molecules. A weak acid reacts only partially, so much of the dissolved acid remains un-ionized.

How does the amount of hydronium produced distinguish strong and weak acids at comparable concentrations?

A strong acid produces a relatively large hydronium concentration because it ionizes nearly completely. A weak acid produces less hydronium because only a fraction of its molecules transfer protons to water.