I-5. 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 pre-equilibrium concentrations of reactants and products after an initial reaction occurs when mixing two or more chemical compounds. The subscripts in the reaction equilibria refer to pure solids: AgCl(s), liquids: H2O(l), gases: H2 (g), or aqueous solution: Cl-(aq).


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 change in energy that occurs when a reaction take place is described by thermodynamics, and the rate or speed at which a reaction occurs is described by kinetics.


Gas-Phase Reactions

Gases can react with other gases, liquids, or solids to form new products. Some examples are:

Combustion of hydrogen gas: 2 H2 (g) + O2 (g) ---> 2 H2 (l)

Combustion of methane: CH4 (g) + O2 (g) ---> CO2 (g) + 2 H2O(l)

Heating calcium carbonate (limestone) to make calcium oxide (lime): CaCO3 (s) + heat ---> CaO(s) + CO2 (g)


Acid-Base Reactions

The Bronsted-Lowry definition describes acids are proton donors, and bases are proton acceptors. Mixing acids and bases results in neutralization because the base will accept the proton that the acid donates.

HNO3 + NaOH ---> H2O(l) + Na+(aq) + NO3-(aq)

The spectator ions are usually left out of the reaction and the reaction is:

H+(aq) + OH-(aq) ---> H2O(l)


Precipitation Reactions

Many compounds have limited solubility in aqueous solution (water). When the concentrations of the ions in solution rise above the solubility limit, the ions combine to form solid particles that precipitate from solution. The concentrations of the ions remaining in solution are governed by the equilibrium constant (Ksp, also called the solubility product).

Example: When chloride is added to a silver solution, solid silver chloride precipitates from solution. The resulting equilibrium is always written in the direction of the solid dissolving:

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


Other Reactions

Reduction-oxidation (redox) reactions involve the transfer of electrons from one species to another. One species will be oxidized, and the other will be reduced. Redox reactions are beyond the scope of this equilibrium tutorial.

Example: Zn(s) + Cu2+(aq) ---> Zn2+(aq) + Cu(s)

In this reaction two electrons are transferred from each zinc atom to each copper ion. The zinc metal is oxidized to zinc ions, and the copper ions are reduced to copper metal.


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