0.8 Metathesis: to exchange or not?

Lab 4: metathesis: to exchange or not

Objectives

• To give practice writing equations for metathesis reactions, including net ionic equations
• To illustrate the concept of solubility and the effect of temperature and crystallization

 Grading

You will be determined according to the following:

• Pre-lab (10%)
  • Must attach graph
• Lab Report Form (80%)
  • Must include detailed observations for each reaction
• TA Evaluation of lab procedure (10%)

Before coming to lab…

• Complete the pre-lab exercise, including the plot (due at the beginning of lab)
• Read the instructions for the lab and refresh your memory on anything that isn’t clear by reading your textbook

Introduction

In molecular equations for many aqueous reactions, cations and anions appear to exchange partners. These reactions conform to the following general equation:

Equation 1: $\text{AX}+\text{BY}\to \text{AY}+\text{BX}$

These reactions are known as metathesis reactions. For a metathesis reaction to lead to a net change in solution, ions must be removed from the solution. In general, three chemical processes can lead to the removal of ions from solution, comcomitantly serving as a driving force for metathesis to occur:

1. The formation of a precipitate2. The formation of a weak electrolyte or nonelectrolyte3. The formation of a gas that escapes from solution

The reaction of barium chloride with silver nitrate is a typical example:

Equation 2: ${\text{BaCl}}_2(\text{aq})+{\text{2AgNO}}_3(\text{aq})\to \text{Ba}({\text{NO}}_3{)}_2(\text{aq})+\text{2AgCl}(s)$

This form of the equation for this reaction is referred to as the molecular equations. Since we know that the salts ${\text{BaCl}}_2$ , ${\text{AgNO}}_3$ , and $\text{Ba}({\text{NO}}_3{)}_2$ are strong electrolytes and are completely dissociated in solution, we can more realistically write the equation as follows:

Equation 3: ${\text{Ba}}^{2+}(\text{aq})+{\text{2Cl}}^{-}(\text{aq})+{\text{2Ag}}^{+}(\text{aq})+{\text{2NO}}_{3^{-}}(\text{aq})\to {\text{Ba}}^{2+}(\text{aq})+{\text{2NO}}_{3^{-}}(\text{aq})+\text{2AgCl}(s)$

This form, in which all ions are shown, is known as the complete ionic equation. Reaction occurs because the insoluble substance AgCl precipitates out of solution. The other product, barium nitrate, is soluble in water and remains in solution. We see that ${\text{Ba}}^{2+}$ and ${\text{NO}}_{3^{-}}$ ions appear on both sides of the equation and thus do not enter into the reaction. Such ions are called spectator ions. If we eliminate or omit them from both sides, we obtain the net ionic equation:

Equation 4: ${\text{Ag}}^{+}(\text{aq})+{\text{Cl}}^{-}(\text{aq})\to \text{AgCl}(s)$

This equation focuses our attention on the salient feature of the reaction: the formation of the precipitate AgCl. It tells us that solutions of any soluble ${\text{Ag}}^{+}\text{salt}$ and any soluble ${\text{Cl}}^{-}\text{salt}$ , when mixed, will form insoluble AgCl. When writing net ionic equations, remember that only strong electrolytes are written in the ionic form. Solids, gases, nonelectrolytes, and weak electrolytes are written in the molecular form. Frequently the symbol (aq) is omitted from ionic equations. The symbols (g) for gas and (s) for solid should not be omitted. Thus, Equation 4 can be written as

Equation 5: ${\text{Ag}}^{+}+{\text{Cl}}^{-}\to \text{AgCl}(s)$

Consider mixing solutions of KCl and ${\text{NaNO}}_3$ . The ionic equation for the reaction is

Equation 6: $K^{+}(\text{aq})+{\text{Cl}}^{-}(\text{aq})+{\text{Na}}^{+}(\text{aq})+{\text{NO}}_{3^{-}}(\text{aq})\to K^{+}(\text{aq})+{\text{NO}}_{3^{-}}(\text{aq})+{\text{Na}}^{+}(\text{aq})+{\text{Cl}}^{-}(\text{aq})$