lab 4

Unknown Solution Determination

by Precipitation and Acid–Base Reactions

Introduction to Chemical Reactions:

1) Most chemical reactions are placed in three categories:

a. Precipitation reactions

i. Two ionic solutions are mixed and produce an ionic solid.

ii. The most common type of precipitation reaction is a double displacement reaction or metathesis reaction.

1. This reaction can also be referred to as an exchange reaction.

2. Example: 2KBr(aq) + Pb(NO3)2(aq) ( PbBr2(s) + 2KNO3(aq)

iii. To predict precipitation reactions, the solubilities of reactants and products must be assessed.

1. If all reactants and products are soluble in water, no product results.

b. Acid–base neutralization reactions

i. An acid and base are combined to form an ionic compound (i.e. salt) and possibly water (if the base is a hydroxide).

ii. A Brønsted–Lowry acid is defined as a proton donor.

1. Example: HBr or H2SO4

iii. A Brønsted–Lowry base is defined as a proton acceptor.

1. Example: LiOH or NH3

iv. An example reaction: LiOH(aq) + HBr(aq) ( LiBr(aq) + H2O(l)

v. Some ionic salts, when reacted with acid, form gases.

1. The most common gases produced are CO2, SO2, and H2S.

2. Example: K2CO3(aq) + 2HBr(aq) ( 2KBr(aq) + H2O(l) + CO2(g)

a. The H2O(l) + CO2(g) result from the spontaneous decomposition of H2CO3(aq).

c. Oxidation–reduction reactions

i. These reactions involve electron transfer where one species is oxidized and another species is reduced.

1. Oxidation: loss of electrons or an increase in the oxidation number.

2. Reduction: gain of electrons or a decrease in the oxidation number.

ii. The most common types of oxidation–reduction reactions are:

1. Combination (or synthesis) reactions

2. Decomposition reactions

3. Single displacement reactions

4. Combustion reactions

2) There are a number of driving forces for reactions to occur:

a. Formation of molecular compounds

i. Example: H2O

b. Formation of a gas

i. Examples: H2S, CO2, SO2

c. Formation of an insoluble precipitate

3) You will be given seven aqueous solutions to identify.

a. You will identify the acids and bases using pH paper.

b. Next, you will mix each solution with another unknown and determine if a reaction has taken place.

The Reactants:

A) AgNO3 (0.1 M)

B) BaCl2 (0.1 M)

C) Ca(OH)2 (0.024 M)

D) HCl (1.0 M)

E) MnSO4 (0.1 M)

F) Na2CO3 (0.1 M)

G) Na3PO4 (0.1 M)

Substances to include in the Table of Chemical and Physical Properties:

Please include in table as well as work on the balanced equations table.

The Experimental Procedure:

1) Obtain seven test tubes. Clean and dry if necessary.

a. Label them with the numbers 1–7.

2) Obtain approximately 1.5 mL (about a centimeter and half in a regular test tube) of each unknown solution using pipette and place them into the appropriately labeled test tube.

a. In addition, obtain seven plastic disposable pipettes.

i. Make sure each pipette is used only for one solution.

3) Observe the color and clarity of the solution.

a. Record this physical information into a data table like the one attached.

i. If the solution has a color, report the color.

ii. If the solution has no color, it is referred to as colorless.

iii. If you can see through the solution, it is referred to as clear.

4) Acquire wide range pH paper (pHydrion) and a stirring rod.

5) Immerse the stir rod into the first three unknown solutions and test for the presence of an acid or base.

a. Report the color of the pH paper following the addition of a drop of the unknown solution and indicate whether the solution is acidic, basic, or neutral.

b. Document results in data table.

c. After testing each solution, clean and dry the stir rod before analyzing the next solution!

6) Obtain 21 clean and dry test tubes.

a. Label the test tubes with all of the 21 possible unknown combinations.

7) Mix each solution combination in the appropriately labeled test tube.

a. Record the initial temperature of the first solution before adding the second solution.

b. Mix between 0.5–0.75 mL* of the aqueous solutions with each other.

i. *This is only an approximation. Add enough of first solution to cover the bulb of thermometer. Next, add roughly the same volume of second unknown.

c. Record the final temperature following mixing.

d. Report the presence or absence of gas formation.

e. Report the presence or absence of a precipitate.

i. If present, indicate the color.

Clean – Up:

8) Place all combinations in the appropriately labeled waste containers.

9) Do not discard any materials down the sink; especially the precipitates.

Unknown Combination	Heat Evolution: Initial & Final Temp (oC)	Gas Formation (+ or –)	Precipitation & Color (+ or – w/ color)
1 + 2	Initial: _________ Final: _________
1 + 3	Initial: _________ Final: _________
1 + 4	Initial: _________ Final: _________
1 + 5	Initial: _________ Final: _________
1 + 6	Initial: _________ Final: _________
1 + 7	Initial: _________ Final: _________
2 + 3	Initial: _________ Final: _________
2 + 4	Initial: _________ Final: _________
2 + 5	Initial: _________ Final: _________
2 + 6	Initial: _________ Final: _________
2 + 7	Initial: _________ Final: _________
3 + 4	Initial: _________ Final: _________
3 + 5	Initial: _________ Final: _________
3 + 6	Initial: _________ Final: _________
3 + 7	Initial: _________ Final: _________
4 + 5	Initial: _________ Final: _________
4 + 6	Initial: _________ Final: _________
4 + 7	Initial: _________ Final: _________
5 + 6	Initial: _________ Final: _________
5 + 7	Initial: _________ Final: _________
6 + 7	Initial: _________ Final: _________
The Reactant(s):	Yields	The Product(s):
__AgNO3(aq) + __BaCl2(aq)
__AgNO3(aq) + __Ca(OH)2(aq)
__AgNO3(aq) + __HCl(aq)
__AgNO3(aq) + __ MnSO4(aq)
__AgNO3(aq) + __ Na2CO3(aq)
__AgNO3(aq) + __ Na3PO4(aq)
__BaCl2(aq) + __Ca(OH)2(aq)
__BaCl2(aq) + __HCl(aq)
__BaCl2(aq) + __ MnSO4(aq)
__BaCl2(aq) + __ Na2CO3(aq)
__BaCl2(aq) + __ Na3PO4(aq)
__Ca(OH)2(aq) + __HCl(aq)
__Ca(OH)2(aq) + __ MnSO4(aq)
__Ca(OH)2(aq) + __ Na2CO3(aq)
__Ca(OH)2(aq) + __ Na3PO4(aq)
__HCl(aq) + __ MnSO4(aq)
__HCl(aq) + __ Na2CO3(aq)
__HCl(aq) + __ Na3PO4(aq)
__ MnSO4(aq) + __ Na2CO3(aq)
__ MnSO4(aq) + __ Na3PO4(aq)
__ Na2CO3(aq) + __ Na3PO4(aq)

Chemical Formula	Solution Number:
AgNO3 (0.1 M)
BaCl2 (0.1 M)
Ca(OH)2 (0.024 M)
HCl (1 M)
MnSO4 (0.1 M)
Na2CO3 (0.1 M)
Na3PO4 (0.1 M)

Sample Data Table for Unknown Color & pH Test:

Unknown #	Solution Color	pH Color & Approximate pH	Acid, Base, or Neither
1
2
3
4
5
6
7

Banerjee

General Chemistry-I Lab

Sample Data Table for Unknown Mixture Reactions:

Sample Data Table for the Balanced Reactions:

Sample Data Table for the Identities of the Unknown Solutions: