Chemistry 20 Solutions (15) Definitions: Solution: a homogenous mixture of two or more compounds that are uniformly distributed Heterogenous Mixture: observable segregation of the components of the mixture Solvent: substance in a solution in greater quantity Solute: substance in a solution in lesser quantity Solutions Solutions can be solid, liquid, or gaseous. o Ex: Gaseous solution: air o Ex: Solid solution: alloy (metal mixture) – brass In liquid solutions, if water is the solvent in a liquid solution, the solution is called aqueous A solution that can dissolve more solute is called unsaturated At some point, a solution cannot dissolve any more solute and the solution is called saturated o At this point, any excess solute will remain undissolved – this means an equilibrium or balance has been reached. We represent it like this: Solvent + solute solution + solute (forward reaction balanced by reverse reaction) Ex. Saturated salt solution: NaCl Na+ + Cl- The limit of specific solute that will be dissolved in a given amount of solvent is known as solubility o Ex. At 0.0°C, 35.7 g of NaCl will dissolve in 100.0 g of water The rate at which a solution is created (when a solid is dissolved in water) can be increased by: o Raising the temperature of the solution o Stirring the solution o Increasing the surface area of the solute, or crushing the solute This happens because: o Dissolving takes place at the surface of the solute; increased surface area = increased dissolving o Increased temperature = increased energy of particles (increased molecular motion) o Stirring brings fresh solute to the surface = increased solution rate There are some liquid-liquid solute/solvent combinations that form solutions in any proportion. There is no limit to how much of each you can add. o These solutions are termed perfectly miscible Ex: Ethanol and water Substances that do not dissolve in one another are immiscible o Ex: Oil and water Concentration Remember: Molarity = moles of solute/litres of solution In chemistry it is important to know the concentration of individual ions. o Ex: K2SO4 2K+ + SO4-2 So each mole of potassium sulfate produces 2 mole of potassium ions and 1 mole of sulfate ions. By convention, we express the concentration of a substance in mol/L by placing square brackets around each formula. o Ex: If we have 0.5 M solution of H2SO4 it looks like this: [H2SO4] = 0.5 M Example Question: o Calculate [Cu+2] and [Cl-] if 10.0 g of CuCl2 dissolves in 500.0 mL of solution. Factors Affecting Solubility a) Nature of Solvent and Solute Solvation: interaction between solvent and solute particles Ex: Hydration: surrounding an ion by water molecules (polar) Ex: Dissociation: decomposing a crystal into ions in solution (KCl K+ + Cl-) Solvation also occurs between polar compounds and polar solvents BUT oil (non-polar) will not interact with water (polar) while salt (ions) will not interact with oil (non-polar) On the other hand, non-polar substances readily dissolve non-polar substances This leads to the saying; LIKE DISSOLVES LIKE b) Effect of Temperature Gases dissolve in liquids more at decreased temperature than at increased temperature Liquids and solids dissolve more at increased temperature A solution becomes supersaturated when a solution dissolves more solute than normal at an increased temperature c) Effect of Pressure Change in pressure has practically no effect on the solubility of solids and liquids Gases are more soluble in liquid as the pressure is increased (ex carbonated beverages) d) Heat of Solution Energy involved in the solvation process is called heat of solution Exothermic: heat is released as a solute dissolves Solute + solvent saturated solution + heat Endothermic: heat is absorbed as a solute dissolves Solute + solvent + heat saturated solution If heat of solution is negative, the solution is exothermic If heat of solution is positive, the solution is endothermic Precipitation and Net Ionic Equations Net ionic equations show only the species of reactants and produces that produces a visible produce (precipitate) To write an ionic equation, you must know which compounds disassociate into ions in solution and which product forms a precipitate. Solubility can be determined from the Solubility Table To write ionic equations, 3 basic steps are involved: o Write the balanced equation showing all reactants and products o Show the reactant and product ions in the equation o Eliminate the spectator ions – those ions that remain unchanged during the reaction (they don’t form a precipitate) – and write the net ionic equation Qualitative Analysis A process used to determine what substance is present in a certain solution Using reactants, you can determine what ions are present and which ions can be removed by precipitation Need to use the Solubility Table for this.
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