Double Displacement Reactions

 

Objectives

 

The objectives of this lab are:

a)   To perform and observe the results of a variety of double displacement reactions,

b)   To become familiar with some of the observable signs of these reactions,

c)   To identify the products formed in each of these reactions,

d)   To write balanced chemical equations for each double displacement reaction studied.

 

Background

 

During a chemical reaction both the form and composition of matter are changed.  Old substances are converted to new substances, which have unique physical and chemical properties of their own.  Some of the observable signs that a chemical reaction has occurred include the following:

 

·     A metallic deposit appears

·     Bubbles appear

·     A temperature change occurs

·     A color change occurs

·     A precipitate (cloudy, tiny particles) appears

 

Note that there are many other observable signs for chemical reactions, but these are the ones most likely to be encountered in this lab.

 

Double Displacement Reactions

 

All double displacement reactions have the general form:            AB  +  CD  ®  AD  +  CB

 

Reactions that can be classified as double displacements include precipitation reactions, neutralization reactions and gas forming reactions.

 

Precipitation Reactions

 

Here AB and CD are usually aqueous ionic compounds (or acids) consisting of aqueous ions (A+ and B-, C+ and D-).  When a double displacement reaction occurs, the cations and anions switch partners, resulting in the formation of two new ionic compounds AD and CB, one of which is in the solid state.  This solid product is an insoluble ionic compound called a precipitate.  To determine whether a product ionic compound will be soluble or insoluble, consult the Solubility Rules provided at the end of the Background section.  Note that if both of the predicted products are soluble, a precipitation reaction will not occur.

 

Example1:       aqueous lead(II) nitrate + aqueous sodium chloride

                                                           

The predicted products are lead(II) chloride (insoluble) and sodium nitrate (soluble).  Since one of the predicted products is insoluble, a precipitation reaction is will occur.

                                                                                               

Reaction Equation:  Pb(NO3)2 (aq) + 2 NaCl (aq) ® 2 NaNO3 (aq) + PbCl2 (s)

 

 

Neutralization Reactions

 

Here AB is an acid (consisting of H+ and X- aqueous ions) and BC is a base (consisting of M+ and OH- ions).  When a double displacement reaction occurs, the cations and anions switch partners, resulting in the formation of water and a new ionic compound (or salt), which is usually soluble.  Neutralization reactions are exothermic, and are generally accompanied by a noticeable release of heat.

 

 

Example 2:       sulfuric acid + aqueous lithium hydroxide

 

The predicted products are water and lithium sulfate. 

                                                                                               

Reaction Equation:  H2SO4 (aq) + 2 LiOH (aq) ® Li2SO4 (aq) + 2 H2O (l)

 

 

Gas Forming Reactions

 

In these reactions one of the products (AD or CB) after the double displacement is in the gaseous state, such as hydrogen sulfide (H2S) or ammonia (NH3).  One of the products could also be carbonic acid (H2CO3) or sulfurous acid (H2SO3).  Both carbonic acid and sulfurous acid are unstable and will decompose to form carbon dioxide and sulfur dioxide gases, respectively:

                       

Carbonic acid

H2CO3 (aq)      ®        H2O (l) + CO2 (g)                               

 

                        Sulfurous Acid

H2SO3 (aq)      ®        H2O (l) + SO2 (g)

 

 

Example 4:       nitric acid + aqueous sodium sulfite

           

The predicted products are sulfurous acid and sodium nitrate.  However sulfurous acid decomposes to sulfur dioxide and water:

 

Reaction Equation: 2 HNO3 (aq) + Na2SO3 (aq) ® 2 NaNO3 (aq) + H2SO3 (aq)

                                                                                                                                     decomposes

 

Final Equation: 2 HNO3 (aq) + Na2SO3 (aq) ® 2 NaNO3 (aq) + H2O (l) + SO2 (g)

 

 
Writing Equations for Reactions

 

·     Write the correct formulas for each reactant and place a yield arrow (®) after the last reactant.

·     Identify the reaction type – precipitation, neutralization or gaseous, using the guidelines above.

·     If you determine that a reaction will occur, write the correct formula(s) of the products after the arrow.  If you determine that a reaction will not occur, simply write “no reaction” after the arrow.

·     Balance the equation (to ensure mass conservation).

·     Be sure to include the physical states of all reactants and products in your final equation.

 

 

 

 

Solubility Rules and Activity Series

 

SOLUBILITY RULES

 

1.      Alkali metal compounds, acetates, nitrates, and ammonium compounds are all soluble

 

2.      Hydroxides of alkali metals and NH4+1, Ca+2, Sr+2, and Ba+2 are soluble.  All others are insoluble.

 

3.      All halides (chlorides etc.) are soluble except for those containing Ag+1, Pb+2, and Hg2+2.

 

4.      Most sulfates are soluble, except for BaSO4, SrSO4, Ag2SO4, PbSO4, and CaSO4.

 

5.      Most phosphates, carbonates, chromates and sulfides are insoluble (except those of the alkali metals and ammonium). 

 

6.      In addition, all acids are soluble!

 

 

Procedure

 

Safety

 

Be especially cautious when using the 6M HCl, 3M H2SO4 and 6M NaOH as they can burn your skin.  Also be aware that skin discoloration will result from contact with AgNO3.  If you feel any tingling sensations or see any color changes on your skin, flush with water immediately for a minimum of 15 minutes.  Inform your instructor of any chemical contact as soon as possible.

 

Materials and Equipment

 

Solids: solid sodium hydrogen carbonate

Solutions: 6M sodium hydroxide, 3M sulfuric acid, 6M hydrochloric acid; all other solutions are 0.1M and include silver nitrate, sodium chloride, iron(III) chloride, ammonium hydroxide, sodium carbonate, cobalt(II) nitrate, sodium phosphate, copper(II) sulfate, potassium nitrate, nickel(II) nitrate, barium chloride.

Equipment: 9 small test tubes, plastic test tube rack

 

Instructions for Performing Reactions

 

·     Use approximately 3-mL quantities of all solutions.  A good estimate is to use three full dropper squirts of each chemical.

 

·     Perform the following reactions, and record your observations for each on the data sheet.  If results are not obtained immediately, give the reaction some time.  Some reactions take longer than others.  All waste is to be disposed of in the plastic container in the hood!

 

  1. Aqueous sodium chloride + aqueous silver nitrate
  2. Aqueous sodium phosphate + aqueous copper(II) sulfate
  3. Hydrochloric acid + solid sodium hydrogen carbonate (just a small scoop)
  4. Aqueous nickel(II) nitrate + aqueous sodium hydroxide
  5. Aqueous barium chloride + sulfuric acid
  6. Hydrochloric acid + aqueous sodium hydroxide
  7. Aqueous sodium carbonate + aqueous cobalt(II) nitrate
  8. Aqueous sodium chloride + aqueous potassium nitrate
  9. Aqueous iron(III) chloride + aqueous ammonium hydroxide

 

 

·     When finished, complete the data sheet by writing the balanced equation for each reaction.

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