Double-Replacement Precipitation Reactions

There is one type of chemical reaction that affects your hfe each and every day, no matter who you are, no matter where you live, from the day you are bom to the day you die. This reaction type is the one found in the preceding section, the complete oxidation, or burning, of organic compounds. 

If you live in an urban area, your home is probably heated with natural gas, which is mostly methane. The reaction that occurs in your furnace is 

If you live in a rural area where there are no natural gas pipelines, you probably use liquid propane as a source of energy  

Even in the summer when you don t use your furnace, you probably use one of these reactions to heat the water in your hot-water heater. Both reactions are the oxidation of an organic compound. 

Every time you drive a car, gasoline, diesel fuel, or, less commonly, other forms of carbon-hydrogen or carbon- 

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Double-Replacement Precipitation Reactions COAL 7 Predict whether a precipitate will form when known solutions are combined if a precipitate forms, write the net ionic equation. (Reference to a solubility table or a solubility rules list may or may not be allowed.)... 

Given the reactants in a double-replacement precipitation or neutralization reaction, write the equation. 

As the solutions are mixed, positive A and C ions exist in solution, as do negative B and D ions, and these oppositely charged ions attract each other. A reaction takes place if a compound forms that removes ions from solution. Products that remove ions from solution in a double-replacement reaction are a precipitate, a gas, or a slightly ionized material, such as water. 

In general, a double replacement results if one combination of ions leads to a precipitate, a gas or an un-ionized or very slightly ionized species such as water. In all of these reaction classes, it is very often possible to use your physical senses to observe whether a chemical reaction has occurred. The qualitative criteria may involve the formation of a gaseous product, the formation of a precipitate, a change in color, or a transfer of energy. 

When aqueous solutions of barium chloride and sodium chromate are mixed, a precipitate is formed. The balanced equation for this double replacement reaction is... 

Which double replacement reaction forms an insoluble precipitate ... 

When a double replacement reaction occurs between AgN03(aq) and KCl(aq), the products formed are AgCl and KN03. AgCl is insoluble in water and forms a white precipitate. 

One of the key characteristics of double-replacement reactions is the type of product that is formed when the reaction takes place. All double-replacement reactions produce either a precipitate, a gas, or water. An example of a double-replacement reaction that forms a gas is that of potassium cyanide and hydrobromic acid. 

When two aqueous solutions that contain ions as solutes are combined, the ions may react with one another. These reactions are always double-replacement reactions. The solvent molecules, which are all water molecules, do not usually react. Three types of products can form from the double-replacement reaction precipitate, water, or gas. You can observe a precipitate forming when you do the miniLAB for this chapter. 

Some reactions that occur in aqueous solutions produce precipitates. For example, when aqueous solutions of sodium hydroxide and copper(II) chloride are mixed, a double-replacement reaction occurs in which the precipitate copper(II) hydroxide forms. 

Another type of double-replacement reaction that occurs in an aqueous solution produces water molecules. The water molecules produced in the reaction increase the number of solvent particles. Unlike reactions in which a precipitate forms, no evidence of a chemical reaction is observable because water is colorless and odorless and already makes up most of the solution. For example, when you mix hydrobromic acid with a sodium hydroxide solution, a double-replacement reaction occurs and water is formed. 

The term metathesis literally means to transpose. The term double replacement is often used to describe these reactions because the cations switch places with each other. These are chemical reactions that involve an exchange of positive ions between two compounds and that generally take place between two ionic compounds in an aqueous solution. The driving force that causes these reactions to occur is a decrease in the number of ions from the reactants to products. This will occur when a stable product forms from the ionic reactants. The three types of stable products that can form are a precipitate (or an insoluble solid), a gas (gaseous materials will bubble out of the solutions and leave the reaction mixture), or a stable molecule (a weak electrolyte, or nonelectrolyte, such as water). A general equation for a double-replacement reaction is... 

Precipitation reactions, such as the ones we will see in this section, belong to a general class of reactions called double-displacement reactions. (Double displacement reactions are also called double-replacement, double-exchange, or metathesis reactions.) Double displacement reactions have the following form, signifying that the elements in two reacting compounds change partners. 

In the equation, AB and CD are the original compounds and AD and BC are the new compounds. Some important double-replacement processes do not occur in solution, but most do, and those that do either (a) produce a precipitate (an insoluble solid), (b) produce a gas, or (c) produce water. These are the three categories of double-replacement reactions. 

Double-replacement reactions that form a precipitate. 

How you can tell whether an equation is describing a double-replacement reaction There are two reactants (both compounds) and two (or three) products (all compounds). If one of the reactants is a carbonate or hydrogencarbonate salt, there are three products. In addition, one of the products in double-replacement reactions is a precipitate, a gas or water. The following is a summary of the three types of double-replacement reactions ... 

Double-replacement reaction two compounds exchange ions. A precipitate is formed. 

Completing equations of double-replacement reactions is done in a similar way. There are always two reactants, which exchange partners to give two (or three) products, one being a precipitate, a gas or water. Neutralization reactions produce a salt and water. You might first want to examine the formulas of compounds and separate them into ions. This might make it easier to see the exchange. Here are several incomplete equations for double-replacement reactions. 

Chemical reactions, whether they involve the formation of precipitate, reaction with oxygen, acids and bases, or oxidation-reduction, generally follow one of a few simple patterns combination, decomposition, and single- or double- replacement. Recognizing the underl)dng pattern will improve your ability to write and understand chemical reactions. 

Double-replacement reactions occur between substances in aqueous solutions and produce precipitates, water, or gases. 

Predicting precipitates Suppose equal volumes of O.IOM aqueous solutions of iron(III) chloride (FeCla) and potassium hexacyanoiron(II) (K4Fe(CN)6) are combined. Will a precipitate form as shown in Figure 17.19 The following double-replacement reaction might occur. 

Aqueous solutions of ionic compounds contain dissolved positive, and negative ions. When two such solutions are mixed, the ions may take part in a double-replacement reaction. One outcome of a double-replacement reaction is the formation of a precipitate. By writing ionic equations and knowing the solubilities of specific ionic compounds, you can predict whether a precipitate will be formed. 

How do the concentrations of the ions of a solution change when a double-replacement reaction occurs and a precipitate forms ... 

A metathesis (double-replacement) reaction involves the exchange of two groups or two ions among the reactants. Remember that in a single-replacement reaction, there is only one group or ion being switched. A metathesis reaction can often result in an insoluble product from soluble reactants, and the insoluble compound formed is called a precipitate. 

When solutions of two compounds are mixed, a positive ion from one compound may combine with the negative ion from the other compound to form a solid compound that settles to the bottom. The solid is a precipitate the reaction is a precipitation reaction. In the equation for a precipitation reaction, ions of the two reactants appear to change partners. The equation, and sometimes the reaction itself, is a double-replacement equation or a double-replacement reaction. The general equation, with bridges to show the rearrangement of ions, is... 

In Section 8.9 we showed how to write the double-replacement equation for a precipitation reaction between solutions of two ionic compounds. Such a reaction occurs when a solution of lead(II) nitrate is added to a solution of sodium chloride ... 

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