4.8
Chemical Reactions in Aqueous Solutions
Chemical substances interact in many different ways. Certain chemical reactions exhibit common patterns of reactivity. Due to the vast number of chemi…
Many chemical reactions occur in aqueous solutions. These reactions fall into 3 main categories: precipitation, acid–base, and oxidation–reduction.
In a typical precipitation reaction, dissolved cations and anions come together to form an insoluble ionic compound that precipitates from the solution. For example, the reaction between aqueous potassium chloride and aqueous silver nitrate leads to the precipitation of insoluble silver chloride.
In an acid–base reaction, the acid and base neutralize each other, such as when a proton (H+ ion) from an acid and a OH− ion from a base react to form water. The counterions typically react to form a salt. For example, hydrogen bromide reacts with potassium hydroxide to give water and potassium bromide.
In an oxidation–reduction, or redox, reaction, electrons are transferred from one reactant to another. For example, an atom of potassium loses one electron to an atom of chlorine when they combine to form potassium chloride.
When a chemical reaction occurs in an aqueous solution, water-soluble molecular solids dissolve as intact molecules, whereas water-soluble ionic solids exist as separated ions. Ionic solids that are water-insoluble remain undissolved.
While writing equations for reactions that occur in aqueous solution, the nature of the dissolved substance must be indicated.
Consider the precipitation reaction between aqueous lead(II) nitrate and aqueous sodium iodide to form solid lead(II) iodide and aqueous sodium nitrate.
The balanced equation is called the molecular equation because the complete neutral formulas of the compounds are written as if they existed as molecules or whole units in solution.
Water-soluble ionic compounds such as lead(II) nitrate, sodium iodide, and sodium nitrate exist as ions in solution, whereas water-insoluble lead(II) iodide exists as an ionic solid.
The molecular equation can, therefore, be rewritten as a complete ionic equation, which shows soluble ionic species as free ions in solution.
The ions that appear in identical forms on both sides of the complete ionic equation are called spectator ions. Canceling out spectator ions leaves the net ionic equation, which includes only the ions and molecules that actually participate in the reaction.
View the full transcript and gain access to JoVE Core videos
Q1: What are the three main types of chemical reactions that occur in aqueous solutions?
Chemical reactions in aqueous solutions fall into three main categories: precipitation, acid-base, and oxidation-reduction. In precipitation reactions, dissolved ions combine to form insoluble compounds that settle out. Acid-base reactions involve neutralization where protons and hydroxide ions form water. Oxidation-reduction reactions involve electron transfer between reactants, where one substance loses electrons and another gains them.
Q2: How do ionic compounds behave differently when dissolved in water?
Water-soluble ionic compounds dissociate into separated ions that disperse throughout the solution, while water-insoluble ionic compounds remain as solid precipitates. Molecular solids dissolve as intact molecules without breaking apart. This distinction is crucial when writing equations for aqueous reactions because it determines how substances are represented in chemical equations.
Q3: What is the difference between a molecular equation and a complete ionic equation?
A molecular equation shows complete neutral formulas of compounds as if they existed as whole units, providing stoichiometric information for calculations. A complete ionic equation explicitly represents all dissolved ions as free species in solution. The complete ionic equation reveals spectator ions—ions appearing identically on both sides that don't participate in the actual reaction.
Q4: What are spectator ions and why are they removed from net ionic equations?
Spectator ions are ions that appear in identical forms on both sides of a complete ionic equation. They are neither chemically nor physically changed during the reaction and are required only to maintain charge neutrality. Removing spectator ions produces the net ionic equation, which shows only the ions and molecules that actually participate in the reaction.
Q5: How does a precipitation reaction work in aqueous solution?
In a precipitation reaction, dissolved cations and anions from separate ionic compounds exchange partners to form an insoluble ionic compound. For example, aqueous potassium chloride and aqueous silver nitrate react to produce insoluble silver chloride that precipitates from solution. The net ionic equation shows only the ions that combine to form the solid precipitate.
Q6: What happens during an acid-base reaction in aqueous solution?
In an acid-base reaction, a proton (H+ ion) from an acid reacts with a hydroxide ion (OH−) from a base to form water. The counterions from the acid and base combine to form a salt. For example, hydrogen bromide reacts with potassium hydroxide to produce water and potassium bromide, with the acid and base neutralizing each other.
Q7: How does electron transfer occur in oxidation-reduction reactions?
In oxidation-reduction reactions, electrons transfer from one reactant to another. The reactant that loses electrons is oxidized, while the reactant that gains electrons is reduced. For example, when potassium and chlorine combine to form potassium chloride, a potassium atom loses one electron to a chlorine atom, resulting in electron transfer between the two elements.