ASRJC 2024 H2 Chem Solubility Equilibria Notes

2024 JC1 H2 Solubility Equilibria 2024/ASRJC/Chemistry 1 Anderson Serangoon Junior College 2024 JC1 H2 Chemistry Solubility Equilibria Content Resources 1 Introduction 2 Salts and saturated solutions 2.1 Types of salts 2.2 Difference between a soluble salts and a sparingly soluble salt 2.3 Saturated solution 3 Solubility and solubility product (Ksp) 3.1 Solubility of solute (in water) 3.2 Solubility product (Ksp) 3.3 Relationship between solubility and solubility product 3.4 Calculations 3.5 Calculations (continued) 4 Effect on solubility of ionic salts when equilibrium is disturbed 4.1 Common ion effect Lecture 1 • Lecture notes: Pg 1 to 10 • SLS quiz: Qn 1 and 2 4.2 Effect on solubility of ionic salts by chemical reactions 5 Ionic Product and Precipitation 5.1 Ionic product 5.2 When and why does precipitation occurs? 5.3 Calculations 6 Further Applications of Ksp on solubility of silver halides Lecture 2 • Lecture notes: Pg 11 to 18 • SLS quiz: Qn 3 and 4 Learning Outcomes Students should be able to: (a) show understanding of, and apply, the concept of solubility product, Ksp (b) calculate Ksp from concentrations and vice versa (c) discuss the effects on the solubility of ionic salts by the following: (i) common ion effect (ii) formation of complex ion as exemplified by the reaction of halide ions with aqueous silver ions followed by aqueous ammonia (See also – An Introduction to the Chemistry of Transition Elements) References 1. Chemistry for Advanced Level, Cann and Hughes, Murray 2. Chemistry, The Molecular Nature of Matter and Change (Fourth Edition), Silberberg, McGraw Hill 3. Chemistry & Chemical Reactivity (Sixth Edition), Kotz, Treichel and Weaver, Thomson 4. Chemistry The Central Science (Ninth Edition), Brown, LeMay, Bursten, Prentice Hall 5. Chemistry (Sixth Edition), Zumdahl and Zumdahl, Houghton Mifflin 6. Understanding Advanced Physical Inorganic Chemistry, The Learner’s Approach, Jeanne Tan, Kim Seng Chan, WS Education

2024 JC1 H2 Solubility Equilibria 2024/ASRJC/Chemistry 2 1. INTRODUCTION Solubility is a very important phenomenon that impacts our daily life, environment and health. Water with high content of calcium or magnesium ions (known as hard water) are present in some regions of the world. Mineral deposits can be found on cooking dishes or in bathtubs. Sometimes, you may also have felt like there was a film of residue left on your hands after washing your hands with soap. Mineral deposits are formed due to the formation of insoluble precipitates by calcium or magnesium ions in the hard water. These deposits can make hard water unsuitable for many uses. They can reduce the lifespan of equipment, raise the costs of heating the water, lower the efficiency of electric water heaters, or clog pipes. A variety of methods can be used to minimise the hardness of water. For example, mineral deposits in pots can sometimes be removed by running vinegar (an acid) through the pot. At the end of this topic, try to make sense of why precipitation occurs, and why the above method works in removing the mineral deposits. In this topic, we will find out the answer to the following questions:  What is a saturated solution?  What is solubility and solubility product? How are they related?  When and why does precipitation take place?  What are the effects on the solubility of ionic salts when its equilibrium is disturbed? Hard water can leave a film on glasses coming out of the dishwasher. (http://water.usgs.gov/edu/hardness.html)

2024 JC1 H2 Solubility Equilibria 2024/ASRJC/Chemistry 3 2. SALTS AND SATURATED SOLUTIONS 2.1 Types of salts Ionic compounds can be classified as soluble or sparingly soluble salts. We often use the term “insoluble salt” to describe ionic compounds that have low solubility in water. However, these are more appropriately described as sparingly soluble salts. Recall from ‘O’ levels that all ionic compounds containing Na+, K+ and NH4+ are soluble. The table below shows a general classification of salts as either soluble or sparingly soluble. Solubility in water Soluble Salts Sparingly Soluble Salts All nitrates (NO3–) – Nitrites (NO2–) Only AgNO2 Halides (Cl–, Br–, I–) Only halides of Ag+, Pb2+, Cu+. PbCl2, PbBr2, PbI2 are soluble in hot water Sulfates (SO42–) Only sulfates of Ba2+, Ca2+ and Pb2+ Only sulfides of Na+, K+ and NH4+ Sulfides (S2–) Only carbonates of Na+, K+ and NH4+ Carbonates (CO32–) Only oxides of Na+, K+, NH4+ and larger Group 2 cations (e.g. Ca2+, Sr2+, Ba2+) Oxides (O2–) Only hydroxides of Na+, K+, NH4+ and larger Group 2 cations (e.g. Ca2+, Sr2+, Ba2+) Hydroxides (OH–) When you add a sparingly soluble salt (or “insoluble salt”) into water at a fixed temperature, it will dissolve in water until a point when, to the naked eye, no more solute can dissolve. This is when a saturated solution is obtained (See section 2.3 – page 4). 2.2 Difference between a soluble salt and a sparingly soluble salt • When a soluble salt is added to water, it dissociates completely to give separate hydrated ions in the solution. e.g. NaCl(s) Na+(aq) + Cl–(aq) • When a sparingly soluble salt is added to water gradually, it dissolves slightly and produces a mixture consisting of a very dilute solution of the ions in equilibrium with the undissolved solid. e.g. AgBr(s) Ag+(aq) + Br –(aq) The solution is said to have become saturated and any excess solid would remain undissolved and settle to the bottom. Ag+(aq) Br–(aq) Ag+(aq) Br–(aq) undissolved AgBr(s) unsaturated solution saturated add AgBr(s) water add AgBr(s) Source: https://manoa.hawaii.edu/exploringourfluidearth/chemical/ properties-water/comparison-water-other-liquids

2024 JC1 H2 Solubility Equilibria 2024/ASRJC/Chemistry 4 2.3 Saturated solution A saturated solution is one that contains the maximum amount of solute in a given amount of solvent such that the ions are in equilibrium with the solid. At equilibrium in a saturated solution, the rate of dissolution (from solid to hydrated ions) is equal to the rate of crystallisation or precipitation (from the hydrated ions to the solid). 3. SOLUBILITY AND SOLUBILITY PRODUCT (Ksp) 3.1 Solubility of a solute (in water) The solubility of a substance (salt) is the maximum amount of solute that dissolves in 1 dm 3 of water to form a saturated solution at a stated temperature. • The units for solubility of a salt is either in g dm–3 or mol dm–3. • Example • If the solubility of PbCl2 is 44.1 g dm–3 at 25oC, it means that a maximum mass of 44.1 g (0.1585 mol) of PbCl2 can dissolve in 1 dm3 of water forming a saturated solution at 25oC. • At saturation, PbCl2(s) Pb2+(aq) + 2Cl–(aq) • This implies that 0.1585 mol Pb2+(aq) and 0.3170 mol Cl–(aq) are formed in 1 dm 3 of the saturated solution. • The value of the solubility of a sparingly soluble salt, at a stated temperature, is o determined experimentally and o used to calculate the solubility product, Ksp, of the salt. • To determine the solubility of a sparingly soluble salt (at a given temperature). 1. Prepare the saturated solution – by dissolving the salt until no more solid can dissolve, and undissolved solid remains in the solution. 2. Filter to remove the undissolved solid. 3. Perform a titration to determine the concentration of the salt that dissolved in the solution. dissolve crystallise / precipitate MxAy(s) x Mm+(aq) + y Aa–(aq) Source: http://wps.prenhall.com/wps/media/ objects/3312/3391718/blb1302.html

2024 JC1 H2 Solubility Equilibria 2024/ASRJC/Chemistry 5 3.2 Solubility product, Ksp In a saturated solution of sparingly soluble salt, MxAy, the following equilibrium exists: MxAy(s) x Mm+(aq) + y Aa–(aq) The equilibriu