EXPERIMENT 5: COMMON ION EFFECTChem 18.1 AB2

Puyaoan, Rio Joana; Ngo, Lester Lloyd Vinz | Prof. Kreza Ligaya

INTRODUCTION• KEY WORDS:

– Common ion effect

– Buffers

– Soluble salt

RESULTS AND DISCUSSION• Part A: The Effect on the Ionization of

Acids and BasesThe following results were obtained from mixing 10ml at 2 ml of the respective reagents. pH values of each solution were tested using pH papers. The summary of the data gathered could be found below.

Reagents pH

A.) 10 mL 0.1M HCl + 2mL H2O

1

B.) 10 mL 0.1M HCl + 2 mL 0.1M NaCl

1

C.)10 mL 0.1M HOAc + 2mL H2O

3

D.) 10 mL 0.1M HOAc + 2mL 0.1M NaOAc

4

E.) 10 mL 0.1M NaOH + 2mL H2O

12

F.) 10 mL 0.1M NaOH + 2mL 0.1M NaCl

13

Table 1. shows the effect of adding a common ion to strong and weak electrolytes.

RESULTS AND DISCUSSION• In Part A, the common ion effect is an application of

Le Chatelier’s Principle.

Strong Electrolyte: irreversible, complete dissociation (solution A, B, E, F)

Weak Electrolyte: reversible, partial dissociation (solution C, D)– Contain HOAc, a weak electrolyte that partially

dissociates into:

HOAc H+ + OAc-

So like, NaOac, that dissociates into Na+ + OAc-, the concentration of OAc is increased and in turn the H concentration is decrased, and thus a shift to the left will occur

RESULTS AND DISCUSSION• Part B: Buffering Effect

A. 10 ml of 0.5 M HOAc + 10 ml of 0.5 M NaOAc

B. 10 ml of 0.5 M HCl + 10 ml of 0.5 M NaCl

C. 10 ml of 0.5 M HNO3 + 10 ml of 0.5 M NaNO3

D. 10 ml of 0.5 M NaH2PO4 + 10 ml of 0.5 M Na2HPO4

E. 10 ml of 0.5 M NH4OH + 10 ml of 0.5 M NH4Cl

RESULTS AND DISCUSSION

Solution pH pH after

HClpH after NaOH

Experimental

Conclusion

Theoretical

Conclusion

A 6 5 9Non-

BufferBuffer

B 1 1 1 BufferNot

Buffer

C 1 1 1 BufferNot

Buffer

D 6 3 7Non-

BufferBuffer

E 7 8 9 Buffer Buffer

Table 2. presents

the pH of the solution

as well as the pH

when 6M of HCl and 6M

of NaOH were added

to the original

solution. It also gives

the conclusion

whether the solutions

exhibit buffering effect as

well as the theoretical

answers.

RESULTS AND DISCUSSION• A buffer consists of an acidic and a

basic component which does not consume each other in a neutralization reaction. Its behavior is based on establishing excesses of both the original acid or base, and its conjugate. The presence of excess the "common ion" causes a shift in the equilibrium of the first reaction and sets up the required condition for buffering behavior.

• Buffers are used to minimize the change in the pH of the solution when an acid or base is added to it.

RESULTS AND DISCUSSION• Part C: Effects of Common Ion on the

Solubility of a Slightly Soluble Salt

To be able to calculate the solubility of the benzoic acid in a solution with a common ion, a comparison was made against the solubility of benzoic acid in water. The data was used to calculate the solubility of benzoic acid in sodium benzoate. This can be illustrated in the following calculations:

RESULTS AND DISCUSSION• Volume of 0.1 M NaOH = 1.3ml

• Solubility of benzoic acid in water = 1.5 x 10-2 M

• Solubility of benzoic acid in sodium benzoate solution= 0.0013 M

MacidVacid = MNaOHVNaOH

Macid(10ml) = 0.01M (1.3ml)

Macid = 0.0013 M

RESULTS AND DISCUSSIONS• When benzoic acid crystals were added to the sodium

benzoate solution, the concentration of C6H5COO- is increased, causing a shift to the left. It follows then that the solubility of benzoic acid is decreased or reduced. This is in relation with the Le Chatelier’s principle which states that the presence of a common ion (C6H5COO-) influences the equilibrium of a slightly soluble salt system and theoretically reduces the solubility of C6H5COOH, shifting the solubility equilibrium to the left. This reduction in solubility is also due to the common ion effect.

NaC6H5CO2 (s) Na+(aq) + C6H5COO-

C6H5COOH (s) H+ + C6H5COO- 

• Generally speaking, the presence of a second solute that gives a common ion decreases the solubility of a slightly soluble salt. And it is evident that base on the data gathered, if you will compare the solubility of benzoic acid in sodium benzoate solution, it is smaller compare to the solubility of benzoic acid in water, which is the right thing to occur.

CONCLUSIONSOne of the special case of the Le

Chatelier’s Principle is the common ion effect. The common ion effect can generally be seen in weak electrolytes, wherein the partial dissociation of weak electrolytes gives a reversible reaction that is in equilibrium state. It is a shift in equilibrium induced by an ion which is the same with one of the species in the equilibrium.

Buffered solutions contain a weak conjugate acid-base pair which can resist drastic changes in pH upon the addition of small amounts of strong electrolytes. A buffer resists changes in pH because it contains both acidic and basic species to neutralize OH- and H+ ions, respectively.

(1)The presence of a common ion suppresses the dissociation of weak acid / base but the effect in a strong electrolyte is negligible because strong electrolyte dissociates completely making it irreversible.

(2)When adding common cations, the concentration of the salt increases slightly because the anions are consumed, consequently when you add common anions to the buffer solution will cause the salt to dissociate more and thus the concentration of the anion increases.

(3)Addition of common ions decreases the solubility of slightly soluble salts.

THESE ARE THE EFFECTS OF COMMON ION EFFECT:

RECOMMENDATIONS• To minimize experimental errors, the

reagents should be sealed properly after use so that it would not be contaminated.

• The instruments to be utilized should be properly washed before and after the experiment so that it could give accurate measurements.

• To reduce human mistakes, students should learn to strictly follow the steps and procedures as indicated in the laboratory manual.

WORKS CITED

http://www.citycollegiate.com/commonion_effect.htm

Chang, R. Chemistry. McGraw-Hill Companies, Inc. Boston, Massachusetts. 

Brown,T. et.al. 2004. Chemistry: The Central Science, 9th ed. Prentice Hall.

Petrucci and Wismer. General Chemistry with Qualitative Analysis. McMillan Publishing Company. New York.