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Precipitation in Double Replacement Reactions

Adapted from Addison-Wesley Small Scale Experiment #10

Objectives:

·  Observe precipitation reactions by mixing aqueous solutions of cations and anions

·  Write and balance complete chemical equations to describe precipitation reactions

Introduction:

In chemistry, the term precipitation does not refer to meteorological phenomena such as rain or snow. Rather, precipitation occurs in solution when two chemicals react to form a product that is insoluble in water and falls out of solution like rain or snow. A precipitate is a solid substance that separates from solution during a chemical reaction. A precipitate can be identified by the cloudy, milky, gelatinous or grainy appearance it gives to the mixture. The solid might even settle to the bottom of the container.

A barium sulfate precipitate can be produced by the reaction of barium chloride and sodium sulfate. A chemical equation to describe the reaction is written and balanced like this:

BaCl2(aq) + Na2SO4(aq) à 2NaCl(aq) + BaSO4(s)

Barium sulfate, BaSO4, is a common precipitate used as an X-ray contrast medium because it is insoluble in water and opaque to X-rays. Typically a patient drinks an aqueous slurry of barium sulfate just before he is X-rayed. The precipitate coats his stomach and intestines. These organs then show up on the X-ray film in vivid contrast, aiding the doctor’s diagnosis.

Notice that the reaction that forms BaSO4 is a double replacement reaction in which the cations and anions of the reactants trade partners to form the products. Also notice that the ratios in which cations and anions combine to form reactants are different from the ratios for products. For example, Na+ combines with SO42- in a ratio of 2:1 in sodium sulfate, Na2SO4; whereas Na+ combines with Cl- in a ratio of 1:1 in sodium chloride, NaCl. Recall that formulas for ionic compounds must be written so that the charge of the formula is zero.

Purpose:

Lead and silver compounds commonly undergo reactions that form precipitates. Silver bromide, AgBr, for example, is a light-sensitive precipitate used to coat photographic papers. In this lab you will mix solutions of lead and silver compounds with other compounds in solution. You will observe and describe the precipitates that are formed. Finally, you will write and balance complete chemical equations to describe precipitation reactions.

Safety:

·  Wear your safety goggles and aprons at all times.

·  Use full small-scale pipets only for the carefully controlled delivery of liquids.

Materials:

Small-scale pipets of the following solutions:

Lead(II) nitrate (Pb(NO3)2) Silver nitrate (AgNO3)

Potassium iodide (KI) Sodium chloride (NaCl)

Sodium hydroxide (NaOH) Iron(III) chloride (FeCl3)

Sodium sulfate (Na2SO4) Sodium phosphate (Na3PO4)

Sodium carbonate (Na2CO3) Copper(II) sulfate (CuSO4)

Calcium chloride (CaCl2)

Equipment:

Empty pipet for stirring

Small-scale reaction surface

Procedure:

  1. Put 1 drop of AgNO3 on each X in the top row.
  2. Add 1 drop of the solution at the top of each column. Stir by blowing air.
  3. Repeat with Pb(NO3)2 on the second row and CaCl2 on the bottom row.
  4. Record your results in Table 10-1.
  5. Avoid contamination by cleaning up in a way that protects you and your environment. When you have finished answering the questions, carefully clean the file protector by absorbing the contents onto a paper towel. Rinse the file protector with a damp paper towel and dry. Dispose of the paper towels in the waste bin.

Data:

KI / NaCl / NaOH / FeCl3 / Na2SO4 / Na3PO4 / Na2CO3 / CuSO4
AgNO3
Pb(NO3)2
CaCl2

Analysis: Complete and turn in with your data by the end of class.

  1. What two cations commonly form precipitates?
  1. How can you tell if a precipitate forms when you mix two solutions?
  1. Which of the mixtures give no visible reaction?
  1. Which solutions react to form precipitates with CaCl2?
  1. Write chemical equations for 4 more precipitation reactions of AgNO3 you observed (you cannot repeat the examples given in a and b). A complete chemical equation for a precipitation reaction can be written as a double-replacement reaction. Notice that the precipitate is designated with an “(s)” to denote that it is a solid.
  2. AgNO3 + KI à AgI(s) + KNO3
  3. 3AgNO3 + FeCl3 à3AgCl(s) + Fe(NO3)3

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KI / NaCl / NaOH / FeCl3 / Na2SO4 / Na3PO4 / Na2CO3 / CuSO4
AgNO3
Pb(NO3)2
CaCl2