Solutions in Chemistry PDF

SOLUTIONS GENERAL CHEMISTRY 2 Q3,WEEK 5-6 ROMINA D. MEDIANA  SOLUTIONS Solution vs Mixtures  SOLUTION is a special type of homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in...

SOLUTIONS GENERAL CHEMISTRY 2 Q3,WEEK 5-6 ROMINA D. MEDIANA  SOLUTIONS Solution vs Mixtures  SOLUTION is a special type of homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. The mixing process of a solution happens at a scale where the effects of chemical polarity are involved, resulting in interactions that are specific to solvation (hydration) The solution usually has the state of the solvent when the solvent is the larger fraction of the mixture, as is commonly the case. One important parameter of a solution is the concentration, which is a measure of the amount of solute in a given amount of solution or solvent. The term "aqueous solution" is used when one of the solvents is water.  DEFINITION OF Solute TERMS – Substance being dispersed Solvent – Dispersing medium Solubility – Is the maximum amount of solute that dissolves in a fixed quantity of a particular solvent at a given temperature. Miscibility -is the property of two substances to mix in all  TYPES OF SOLUTION SOLUT SOLVENT SOLUTIO EXAMPLES E N Gas Gas Gas Air (N2, O2, and other gases) Gas Liquid Liquid Carbonated drinks (CO2 in water Liquid Liquid Liquid Vinegar (acetic acid in water) Liquid Solid Solid Amalgam (mercury in silver) Solid Solid Solid Bronze How solutions are formed? A solution is made when one substance called the solute "dissolves" into another substance called the solvent. Dissolving is when the solute breaks up from a larger crystal of molecules into much smaller groups or individual molecules. This break up is caused by coming into contact with the solvent. In the case of salt water, the water molecules break off salt molecules from the larger crystal lattice. They do this by pulling away the ions and then surrounding the salt molecules. Each salt molecule still exists. It is just now surrounded by water molecules instead of fixed to a crystal of salt. SPONTANEITY OF THE DISSOLUTION PROCESS Two major factors affect dissolution of solutes 1. Change of energy content or enthalpy of solution, Hsolution – If Hsolution is exothermic (< 0) dissolution is favored. If Hsolution is endothermic (> 0) dissolution is not favored. 2. Change in disorder, or randomness, of the solution Smixing If Smixing increases (> 0) dissolution is favored. If Smixing decreases (< 0) dissolution is not favored. 4 SPONTANEITY OF THE DISSOLUTION PROCESS What factors affect Hsolution? – Solute-solute attractions such as ion-ion attraction, dipole-dipole, etc. Breaking the solute-solute attraction requires an absorption of energy (ENDOTHERMIC) – Solvent-solvent attractions such as hydrogen bonding in water. Breaking the solvent-solvent attraction also requires absorption of energy  SPONTANEITY OF THE DISSOLUTION PROCESS Solvent-solute attractions, solvation, releases energy (Bond formation, ∴EXOTHERMIC) solution = Hsolute-solute + Hsolvent- H solvent + Hsolute-solvent – If solvation energy (meaning more negative) > solute-solute + solvent- solvent attractions the dissolution is exothermic, Hsolution < 0. – If solvation energy < solute-solute and solvent-solvent attractions (more positive), the dissolution is endothermic, DISSOLUTION OF IONIC SOLIDS IN LIQUIDS Dissolution is a competition between: 1. Solute -solute attractions crystal lattice energy for IONIC solids 2. Solvent-solvent attractions IMFA of liquid 3. Solute-solvent attractions Solvation or hydration (if water is solvent) energy  DISSOLUTION OF SOLIDS IN LIQUIDS Solvation is directed by the water to ion attractions as shown in these electrostatic potentials.  DISSOLUTION OF In an SOLIDS IN LIQUIDS exothermic dissolution, energy is released when solute particles are dissolved. – This energy is called the energy of solvation or the hydration energy (if solvent is water). DISSOLUTION OF LIQUIDS IN LIQUIDS Most polar liquids are (MISCIBILITY) miscible in other polar liquids. In general, liquids obey the “like dissolves like” rule. – Polar molecules are soluble in polar solvents. – Nonpolar molecules are soluble in nonpolar solvents. For example, methanol, CH OH, is very soluble in DISSOLUTION OF LIQUIDS IN LIQUIDS Nonpolar molecules essentially “slide” (MISCIBILITY) in between each other. – For example, carbon tetrachloride and benzene are very miscible. DISSOLUTION OF GASES INareLIQUIDS Polar gases more soluble in water than nonpolar gases. – This is the “like dissolves like” rule in action. Polar gases can hydrogen bond with water Some polar gases enhance their solubility by reacting with water. 19 DISSOLUTION OF GASES IN LIQUIDS A few nonpolar gases are soluble in water because they react with water. Because gases have very weak solute- solute interactions, gases dissolve in liquids in exothermic processes. A. Predict whether or not these substances will mix. 1. Paint thinner and oil 2. Paint thinner and water 3. Oil and salt 4. Oil and sugar 5. Salt and water B. Which compound would dissolve naphthalene better-water, benzene or ethanol? C. How do you think acid rain is formed? RATES OF DISSOLUTION AND SATURATION Finely divided solids dissolve more rapidly than large crystals – Compare the dissolution of granulated sugar and sugar cubes in cold water. VS The enormous increase in surface area helps the solid to dissolve faster. RATES OF DISSOLUTION AND SATURATION Saturated solutions have established an equilbrium between dissolved and undissolved solutes – Examples of saturated solutions include: Air that has 100% humidity. Some solids dissolved in liquids. RATES OF DISSOLUTION AND SATURATION Symbolically this equilibrium is written as: In an equilibrium reaction, the forward rate of reaction is equal to the reverse rate of reaction. Supersaturated solutions have higher- than-saturated concentrations of dissolved LE CHATELIER’S PRINCIPLE AND SOLUBILITY According to LeChatelier’s Principle when stress is applied to a system at equilibrium, the system responds in a way that best relieves the stress. – Since saturated solutions are at equilibrium, LeChatelier’s principle applies to them. Possible stresses to chemical systems include: 1. Heating or cooling the system. 2. Changing the pressure of the system. 3. Changing the concentrations of reactants or products. 4. Stirring the solution  EFFECT OF TEMPERATURE ON SOLUBILITY What will be the effect of heating or cooling the water in which we wish to dissolve a solid? – It depends on whether the dissolution is exo- or endothermic. For an exothermic dissolution, heat can be considered as a product. Warming the water will decrease solubility and cooling the water will increase the solubility. Predict the effect on an endothermic dissolution like this one. 26 EFFECT OF TEMPERATURE ON SOLUBILITY For ionic solids that dissolve endothermically dissolution is enhanced by heating. For ionic solids that dissolve exothermically dissolution is enhanced by cooling. Be sure you understand these trends. EFFECT OF PRESSURE ON SOLUBILITY Pressure changes have little or no effect on solubility of liquids and solids in liquids. – Liquids and solids are not compressible. Pressure changes have large effects on the solubility of gases in liquids. – Sudden pressure change is why carbonated drinks fizz when opened.  EFFECT OF PRESSURE ON SOLUBILITY Can be explained using Henry’s law Henry's law is one of the gas laws formulated by William Henry in 1803 and states: "At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid." Effect of Pressure on Solubility 31 WAYS OF EXPRESSING CONCENTRATIONS 1. % by mass of solute 2. Molarity 3. Molality - number of moles of solute per kilogram of solvent.  WAYS OF EXPRESSING CONCENTRATIONS Calculate the molarity and the molality of an aqueous solution that is 10.0% by mass glucose, C6H12O6. The density of the solution is 1.04 g/mL. 10.0% glucose solution has several medical uses. 1 mol C6H12O6 = 180 g Ways of Expressing Concentrations Ways of Expressing Concentrations 4. Mole fraction is the number of moles of one component divided by the moles of all the components of the solution In a two component solution, the mole fraction of one component, A, has the symbol XA. WAYS OF EXPRESSING CONCENTRATIONS What are the mole fractions of glucose and water in a 10.0% w/w glucose solution?  WAYS OF EXPRESSING CONCENTRATIONS What are the mole fractions of glucose and water in a 10.0% glucose solution?  Ways of Expressing 5.Concentrations Parts per Million (ppm) This is a way of expressing very dilute concentrations of substances. The key point of ppm tells how many units of substance you have for every million units of the whole solution. In contrast, a percentage tells how much of something you have “per hundred,” and so it ’s a very similar measure to ppm. General Formula: ppm = mass of solute (g) x 1,000,000 mass of solution (g) Derived Formula: Amount of solute= (ppm) (amount of solution) 1,000,000  SAMPLE PROBLEM The solubility of NaCl (Table Salt) is 284 g/100 grams of water. What is the concentration in ppm? Given: mass of solute = 284 g of NaCl mass of solvent = 100 g of water Required: mass of solution= ? (grams) ppm= ? Solution: mass of solution = mass of solute + mass of solvent = 284 g NaCl + 100 g H O 2 = 384 g of solution ppm = amount of solute x 1,000,000 amount of solution = 739,583 ppm of NaCl solution = 284 g of NaCl x 1,000,000 384 g of Solution SAMPLE PROBLEM SOLVING: SAMPLE PROBLEM SOLVING: SAMPLE PROBLEM SOLVING: