Experiment 4 2y6u19

Experiment 4: The solubility of some salts of group II elements

Objective To demonstrate the trends in solubility of the Group II hydroxides, sulphates, sulphites and carbonates.

Introduction Group 2 elements are of the following order: beryllium, magnesium, calcium, strontium, barium and finally, radium. The Group 2 elements are classified as alkaline earth metals, and they can form salts with various anions such as hydroxides, sulphates, sulphites and carbonates which were tested out on the specific Group 2 cations in this experiment. The Group 2 cations involved are magnesium (Period 3), calcium (Period 4), strontium (Period 5), and barium (Period 6). In this experiment, the solubility of the Group 2 hydroxides, sulphates, sulphites and carbonates were tested and taken into and the trend may be explained through certain aspects in Chemistry. The solubility of the Group 2 salts can be noticed and compared by looking at the presence of precipitate after the addition of the anions (OH-, SO 4 2-, SO 3 2-, and CO 3 2-), drop by drop, to the 1 mL Group 2 cation solution. For each salts, the solubility is proportional to the number of drops of anion added. If precipitate is present after the addition of a type of anions, the Group 2 salt formed is considered as partially soluble or insoluble, depending on the intensity or the state of the precipitate. If precipitate is absent even after the excessive addition of a type of anions, the Group 2 salt which was formed is considered as soluble.

Apparatus 8 test tubes, dropper, teat pipette with 1 mL mark, test tube rack.

Chemicals/Reagents 0.1 M solutions of the following cations: Mg 2+ , Ca 2+ , Sr 2+, Ba 2+ ; 1.0 M solution of OH-, 0.5 M solutions of SO42-, and SO32- ions, 0.05 M solution of CO32-.

Procedure Two rows of four test-tubes each were set up. The first test tube on the first row was labelled Mg2+, the second test tube was labelled Ca2+, the third test tube was labelled Sr2+, and the fourth and final test tube for the first row of test tubes was labelled Ba2+. 1 mL of the appropriate cation solution was added to each test tube using a teat pipette with a 1 mL mark. The second row of test-tubes were labelled OH- for the first test tube, SO42- was labelled on the second test tube, the third test tube was labelled SO32- , and the fourth and final test tube was labelled CO32- . OH- was added drop by drop to each cation solution in the first row with shaking, test tube by test tube. The number of drops of solution used was recorded in a copy of Table 1. The precipitate was classified as slight (s) or heavy (h) if a precipitate appeared suddenly. The salt was regarded as soluble if there is absence of precipitate after the addition of forty drops of the anion. Repeat the produce with replacing OH- to SO42-, SO32-, and CO32-. If there is no precipitate after forty drops, then write ‘40+’ and regard the salt as soluble

Results Table 1 Cation Solution Mg 2+ Ca 2+ Sr 2+ Ba 2+

Number of drops of anion solution added to give a precipitate OHSO42SO32CO322 (s) 40+ (soluble) 40+ (soluble) 25 (s) 2 (s) 40+ (soluble) 4 (s) 5 (s) 28 (s) 1 (s) 1 (h) 2 (s) 6 (s) 1 (h) 1 (h) 2 (s)

Table 2 Singly-charged anions Compound Solubility / mol per 100g of water MgCl2 5.6 x 10-1 CaCl2 5.4 x 10-1 SrCl2 3.5 x 10-1 BaCl2 1.5 x 10-1 Mg(NO3) 4.9 x 10-1 Ca(NO3) 6.2 x 10-1 Sr(NO3) 1.6 x 10-1 Ba(NO3) 0.4 x 10-1

Doubly-charged anions Compound Solubility / mol per 100g of water MgCO3 1.8 x 10-4 CaCO3 0.13 x 10-4 SrCO3 0.07 x 10-4 BaCO3 0.09 x 10-4 MgSO4 3600 x 10-4 CaSO4 11 x 10-4 SrSO4 0.62 x 10-4 BaSO4 0.009 x 10-4

Mg(OH)2 Ca(OH)2 Sr(OH)2 Ba(OH)2

0.020 x 10-3 1.5 x 10-3 3.4 x 10-3 15 x 10-3

MgCrO4 CaCrO4 SrCrO4 BaCrO4

8500 x 10-4 870 x 10-4 5.9 x 10-4 0.011 x 10-4

Question 1. For Group II, what are the trends in solubility of the salts listed below: (a) Hydroxides = The hydroxides become more soluble as you go down the Group. (b) Sulphates = The sulphates become less soluble as you go down the Group. (c) Sulphites = The sulphites become less soluble as you go down the Group. (d) Carbonates = The carbonates tend to become less soluble as you go down the Group.2. Use Table 13.2 to answer the following questions (a) Explain the trends in solubility for each type of salt for Group II elements as listed in Table 13.2. The magnitude of both will decrease as the Group is descended - because of the increase in the size of the cation. If, on descending the Group, the magnitude of the hydration enthalpies decreases quicker than the magnitude of the lattice enthalpies then the salts will become less soluble on descending the Group and vice versa. (b) Do the solubilities give above for the carbonates, sulphates and hydroxides agreewith your findings in this experiment. Yes. I do agree that the solubility given for sulphates, carbonates and hydroxides were similar with the findings. All nitrates are soluble. Practically all sodium, potassium, and ammonium salts are soluble. All chlorides, bromides, and iodides are soluble except those of silver, mercury(I), and lead(II).All sulfates are soluble except those of strontium, barium, and lead(II), which are insoluble, and those of calcium and silver which are moderately soluble. All carbonates, sulfites, and phosphates are insoluble except those of sodium, potassium, and ammonium. All sulfides are insoluble except those of the alkali metals, the alkaline earth metals, and ammonium. All hydroxides are insoluble except those of the alkali metals. The hydroxides of calcium, strontium, and barium are moderately soluble. Ammonium hydroxide does not exist; ammonium hydroxide is a misnomer for aqueous ammonia, NH3(aq). (c) So singly- or double-charged anions give the more soluble compounds? In group II, the generally smaller size, together with the double positive charge, enables soluble than the compounds of group II elements with singly charged anions. Group II elements are highly electron positive. Due to this they quickly dissociate in water at room temperature, the double charged ions soluble more than single charged ions

Precaution steps The equal amount of the group 2 ions are used (1mL) to prevent unbiasedness in the result of the experiment. The test tube was shaken after adding every drop of the anion concerned to observe the presence of precipitate in the test tube, hence provide accurate result for the experiment.

Conclusion The solubility of group 2 hydroxide increases (from magnesium hydroxide to calcium hydroxide) and decreases (from calcium hydroxide to strontium hydroxide) and lastly increases (from strontium hydroxide to barium hydroxide) down the group. This may be due to experimental error as the solubility of the group 2 hydroxides should increase down the group because the difference in sizes between the group 2 cations and the anions increases which result in stronger attraction of water. The solubility of the group 2 sulphates, group 2 sulphites and the carbonates decreases down the group due to the decrease in size differences between the group 2 cations and anions (SO42-, SO32-, and CO32-) which result in the weaker and less attraction of the water molecules.

Reference 1. Clark, J. (2002). Solubility of the hydroxides, sulphates and carbonates of the Group 2 elements in water.[online]Chemguide.co.uk.Availableat:http://www.chemguide.co.uk/inorganic/group2/solubility.ht ml [Accessed 28 Feb. 2018]. 2. Clark, J. (2013). The Solubility of the Hydroxides, Sulfates and Carbonates-Chemwiki. [online] Chemwiki.ucdavis.edu. Availableat:http://chemwiki.ucdavis.edu/Core/Inorganic_Chemistry/Descriptive_Chemistry/Elements_O rganized_by_Block/1_sBlock_Elements/Group__2_Elements%3A_The_Alkaline_Earth_Metals/1Group_2%3A_Chemical_Rea ctions_of_Alkali_Earth_Metals/The_Solubility_of_the_Hydroxides,_Sulfates_and_Carbonates [Accessed 28 Feb. 2018]. 3. Clark, J. (2002).EXPLANATIONS FOR THE TRENDS IN SOLUBILITY OF SOMEGROUP 2 COMPOUNDS. [Online] Chemguide.co.uk. Available at: http://www.chemguide.co.uk/inorganic/group2/problems.html [Accessed 28 Feb. 2018]. 4. Daintith, J. (2008). A dictionary of chemistry. 6th ed. New York: Oxford University Press, p.19.