Understanding Chemical Reactions


Introduction

This tutorial describes the different types of chemical reactions that are used in the equilibrium practice problems. The important thing to remember from the following sections is what happens when certain reactants are mixed. Many of the equilibrium problems require determining the initial conditions after two or more substances react.


What is a Chemical Reaction?

A chemical reaction occurs when substances (the reactants) collide with enough energy to rearrange to form different compounds (the products). Although not covered in this tutorial, the energy changes that occur when reactions take place is described by thermodynamics, and the rate at which reactions occur is kinetics.


Gas-Phase Reactions

Gases can react with other gases, liquids, and solids to form new products.


Acid-Base Reactions

We will use the Bronsted-Lowry definitions for acids and bases:
Acids are species that donate a proton (H+) and bases are species that accept a proton.

Acid example:
HNO3 (aq) + H2O NO3-(aq) + H3O+(aq)     K = a very large number

In this example, HNO3 is an acid and H2O is acting as a base.
NO3- is called the conjugate base of the acid HNO3, and H3O+ is the conjugate acid of the base H2O.

Base example:
NH3 (aq) + H2O NH4+(aq) + OH-(aq)     K = 1.8x10-5

In this example, NH3 is a base and H2O is acting as an acid. NH4+ is the conjugate acid of the base NH3, and OH- is the conjugate base of the acid H2O.

A compound that can act as either an acid or a base, such as the H2O in the above examples, is called amphiprotic.

Now, what happens when we mix acids and bases? They react to form water and salt.

Example: What happens when we mix nitric acid (HNO3) and sodium hydroxide (NaOH)? Note that we have a strong acid and a strong base, and both of these compounds will completely dissociate in water. The reaction is:

H+(aq) + NO3-(aq) + Na+(aq) + OH-(aq) H2O + Na+(aq) + NO3-(aq)

Note that we can simplify the reaction equation by removing the spectator ions from each side of the equation, and write only the net ionic reaction:
H+(aq) + OH-(aq) H2O


Precipitation Reactions

Many substances are insoluble in water. When a substance exceeds its solubility limit, it forms solid material, which we call a precipitate. Mixing two solutions of soluble salts, for example, silver nitrate and sodium chloride forms insoluble . The sodium and nitrate ions remain in solution.

Ag+(aq) + NO3-(aq) + Na+(aq) + Cl-(aq) AgCl(s) + Na+(aq) + NO3-(aq)

Note that we can simplify the reaction equation by removing the spectator ions from each side of the equation, and write only the net ionic reaction:
Ag+(aq)+ Cl-(aq)) AgCl(s)


Other Reactions

Reduction and oxidation reactions (redox reactions) involve the exchange of electrons. When a chemical species loses electrons we say that it is oxidized, and when a chemical species gains electrons we say that it is reduced.


Chemistry Practice Problems
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