chemistry

Laboratory 6. Double Replacement Reactions

In double replacement reactions, the cations and anions of compounds exchange partners.

AB + CD AD + CB

For these types of reactions to occur, there must be a net change of the ions present in solution between reactants and products.

The driving force of a double replacement reaction is at least one of the following:

1. Formation of a precipitate (insoluble solid).

Pb(NO3)2 (aq) + 2 NaCl(aq) PbCl2(s) + 2 NaNO3(aq)

2. Formation of a non-ionizable compound (such as water, or a weak acid)

2 HCl(aq) + Ca(OH)2 (aq) 2 H2O(l) + 2 CaCl2 (aq)

HCl(aq) + NaC2H3O2(aq) HC2H3O2(aq) + NaCl(aq)

3. Formation of a gas that escapes solution. (like CO2, SO2, and others)

Na2S(aq) + 2 HCl(aq) 2 NaCl(aq) + H2S(g)

There are special classifications of double-replacement reactions:

Precipitation reaction is a double replacement that results in the formation of an insoluble solid (precipitate)

Acid-Base reactions is a double replacement that results in the formation of water (H2O) and a salt (ionic compound). Usually release heat.

Observable signs that a double replacement reaction could be occurring are·

· a solid appears (precipitate), cloudy appearance of solution

· bubbles appear, indicating the formation of a gas

· a color change of the solution occurs

· a temperature change occurs

Keep in mind that to observe a temperature change, we either need to be in contact with the reaction vessel or measure the temperature during the experiment using a thermometer.

Chemical Equations of Double Replacement Reactions

Molecular equation: shows the complete formulas of all reactants and products

Cr(NO3)3(aq) + 3 KOH(aq) Cr(OH)3(s) + 3 KNO3(aq)

Complete ionic equation: all substances that are strong electrolytes (see next page) are represented as ions.

Cr3+(aq) + 3 NO3–(aq) + 3 K+(aq) + 3 OH–(aq) 3 K+(aq) + 3 NO3–(aq) + Cr(OH)3(s)

Net ionic equation: includes only compounds and ions directly involved in the reaction. Spectator ions are not included.

Cr3+(aq) + 3 OH–(aq) Cr(OH)3(s)

Spectator ions (K+ and NO3–) are not participating in the precipitation reaction.

Which compounds should I separate into ions in a complete-ionic equation?

1. Soluble ionic compounds. See “Solubility Guidelines” to determine if the ionic compound is soluble or insoluble (not soluble).

Solid sodium sulfate (Na2SO4) is put into water

Na2SO4(s) 2 Na+(aq) + SO42–(aq)

Solid iron(III) chloride (FeCl3) is put into water

FeCl3(s) Fe3+(aq) + 3 Cl–(aq)

2. Strong acids. Only separate strong acids into ions. Do not separate weak acids

There are seven (7) strong acids: HCl, HBr, HI, HNO3, HClO3, HClO4, H2SO4

HCl is a strong acid HCl(aq) H+(aq) + Cl–(aq)

HNO3 is a strong acid HNO3 (aq) H+(aq) + NO3–(aq)

Formation of a gases during double replacement reactions

If H2CO3(aq) or H2SO3(aq) form in a double replacement reaction, they decompose to form H2O and CO2, and H2O and SO2, respectively (see below).

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

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

For example, look at the reaction of Na2SO3(aq) with HCl(aq), in which you would predict the products in the reaction to be:

Na2SO3(aq) + 2 HCl(aq) 2 NaCl(aq) + H2SO3(aq)

instead, we write:

Na2SO3(aq) + 2 HCl(aq) 2 NaCl(aq) + H2O(l) + SO2(g)

In another example:

CaCO3(s) + 2 HCl(aq) CaCl2(aq) + H2CO3(aq)

instead, we write:

CaCO3(s) + 2 HCl(aq) CaCl2(aq) + H2O (l) + CO2 (g)

In other cases, if the double replacement results in the formation of NH4OH, which decomposes to NH3, a gas that escapes the solution.

NH4OH(aq) NH3(g) + H2O(l)

For example:

NaOH(aq) + NH4Cl(aq) NaCl(aq) + NH4OH(aq)

instead, we write:

NaOH(aq) + NH4Cl(aq) NaCl(aq) + NH3(g) + H2O(l)

Also, in the double replacement, reactions can form H2S, a gas at room temperature.

Solubility Guidelines for Ionic Compounds in Water

Soluble Compounds	Important Exceptions (Insoluble)
Most salts of Group 1A cations and NH4+ are soluble	---
Most nitrate (NO3–), acetate (C2H3O2–), chlorate (ClO3–), and perchlorate (ClO4–) salts are soluble	---
Most halide (Cl–, Br–, I–) salts are soluble	Insoluble exceptions: Ag+, Pb2+, Hg22+
Most sulfate (SO42–) salts are soluble.	Insoluble exceptions: Ca2+, Sr2+, Ba2+, Ag+, Pb2+, and Hg22+
Insoluble Compounds	Important Exceptions (Soluble)
Most hydroxide (OH–) salts are insoluble.	Group 1A hydroxides are soluble. Ca(OH)2, Sr(OH)2, and Ba(OH)2 are moderately soluble
Most sulfide (S2–), carbonate (CO32–), phosphate (PO43–), and chromate (CrO42–) salts are insoluble.	Group 1A and NH4+ compounds are soluble

Chemical reactions (double displacement reactions): chem 121 ( 8 min video)

Ten pair of reactions

https://www.youtube.com/watch?v=jSm0F8EMZn8

Lab 6. Report. Double Replacement Reactions

1. What is the driving force in a double replacement reaction?

2. What are four (4) observable signs that a chemical reaction is occurring?

3. For each of the reaction in the video (10 reactions):

Look for any observable signs (clues) that a chemical change is occurring, and record the if any of the following happens:

· Did a solid form? (indicate the color of the solid)

· Did a color change in the solution occur? (indicate initial and final color)

· Did bubbles form? (indicate if bubbles were observed)

· If no observable signs were noted, indicate “no observable signs”

Write the balanced chemical molecular equation, the complete-ionic equation and the net-ionic equation for the reactants provided.

Make sure to include the correct states for reactants and products. (s) (l) (g) (aq)

If there is no net-ionic equation, make sure to write “no reaction occurs”

Finally, comment if the net-ionic equation matched your observations?

Example:

Reaction X.

Observable signs:

Formation of a precipitate (brown)

Equations:

FeCl3(aq) + 3 NaOH(aq) Fe(OH)3(s) + 3 NaCl(aq)

Fe3+(aq) + 3 Cl–(aq) + 3 Na+(aq) + 3 OH–(aq) Fe(OH)3(s) + 3 Na+(aq) + 3 Cl–(aq)

Fe3+(aq) + 3 OH–(aq) Fe(OH)3(s)

Comments:

The net-ionic equation matches my observations. The brown precipitate is Fe(OH)3(s)