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)