Understanding Colligative Properties: A Chemistry Lesson | Lecture notes Chemistry

LESSON: CHEMISTRY

Stoichiometry of Reactions in Solutions

The numerical relationship derived from balanced

reactions are referred to as stoichiometric ratios. The

term is derived from the ancient Greek words stoicheion

(element) and metron (measure). Recall that the

coefficients in the equations represent the number of

moles in the reactants or product. The number of mole of

a substance is related to its molar mass and number of

molecules, and is directly affected by the volume at a

standard temperature and pressure (STP). Stoichiometric

calculations also includes reactions using different

measures of concentration of the substances involved.

For laboratory analysis, the most common measure is

used in molarity. The relationship among mole, mass,

molecule, and volume is given in the diagram below.

(Stoichiometric Ratios, Page 4)

Stoichiometric Calculations for

Dilution and Titration

Dilution is commonly practiced in the preparation of

solutions from concentrated or stock solutions with higher

concentrations. A few key points on dilution are:

oDilution is the addition of water to the solution

of known volume and concentration to reduce

its concentration and prepare a dilute solution.

oThe number of moles of solute does not change

in dilution because the amount of solute does not

change. Only the volume and concentration of

the solution changes.

Summarization of the main foundation of dilution

calculations:

Titration is the process where the concentration of an

unknown solute is determined by reacting with a solution

of known concentration or a standard solution.

Here are some essential points on titration:

oA useful application of this is the determination of

the concentration of ions in samples of river

water, such as minerals like Pb+2, Hg+2, and

Cd2+.

oThe amount of the standard solution to be added

is determined by the end point which is indicated

by a color change brought about by an

indicator. The end point indicates that enough

of the standard solution has reacted with the

unknown solution.

oThe standard solution should be well prepared

and must be come from a stock solution.

Preparation uses dilution method. Only solutions

of lower concentration can be prepared from

stock solutions or solution of higher

concentration.

(Dilution, Page 8. Titration Concept, Page 9.)

Colligative Properties of Solution

Colligative properties are properties of a solution that

depend only on the number and not on the identity of a

solute particles. Thus, these depend on the collective

effect of the concentration of solute particles present in an

ideal solution. Colligative properties include vapor

pressure lowering, boiling point elevation, freezing point

depression, and osmotic pressure.

oEffect of solute concentration on the colligative

properties of solution.

Sugar and NaCl (table salt) are examples of nonvolatile

solutes, presence of these in a solution have an effect on

the colligative properties of solution. The ratio of the

number of particles of solute and solvent in the solution

has an effect on the colligative properties not on the

nature of the solute.

oEffects of electrolyte and nonelectrolyte on

colligative properties of solutions.

Since a nonelectrolyte solute does not ionize in solution,

the number of solute particles will be less in a solution

containing nonelectrolyte solute than in solution

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Stoichiometry of Reactions in Solutions

The numerical relationship derived from balanced reactions are referred to as stoichiometric ratios. The term is derived from the ancient Greek words stoicheion (element) and metron (measure). Recall that the coefficients in the equations represent the number of moles in the reactants or product. The number of mole of a substance is related to its molar mass and number of molecules, and is directly affected by the volume at a standard temperature and pressure (STP). Stoichiometric calculations also includes reactions using different measures of concentration of the substances involved. For laboratory analysis, the most common measure is used in molarity. The relationship among mole, mass, molecule, and volume is given in the diagram below. (Stoichiometric Ratios, Page 4)

Stoichiometric Calculations for

Dilution and Titration

Dilution is commonly practiced in the preparation of solutions from concentrated or stock solutions with higher concentrations. A few key points on dilution are: o Dilution is the addition of water to the solution of known volume and concentration to reduce its concentration and prepare a dilute solution. o The number of moles of solute does not change in dilution because the amount of solute does not change. Only the volume and concentration of the solution changes. Summarization of the main foundation of dilution calculations: Titration is the process where the concentration of an unknown solute is determined by reacting with a solution of known concentration or a standard solution. Here are some essential points on titration: o A useful application of this is the determination of the concentration of ions in samples of river water, such as minerals like Pb+2, Hg+2, and Cd2+. o The amount of the standard solution to be added is determined by the end point which is indicated by a color change brought about by an indicator. The end point indicates that enough of the standard solution has reacted with the unknown solution. o The standard solution should be well prepared and must be come from a stock solution. Preparation uses dilution method. Only solutions of lower concentration can be prepared from stock solutions or solution of higher concentration. (Dilution, Page 8. Titration Concept, Page 9.)

Colligative Properties of Solution

Colligative properties are properties of a solution that depend only on the number and not on the identity of a solute particles. Thus, these depend on the collective effect of the concentration of solute particles present in an ideal solution. Colligative properties include vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure. o Effect of solute concentration on the colligative properties of solution. Sugar and NaCl (table salt) are examples of nonvolatile solutes, presence of these in a solution have an effect on the colligative properties of solution. The ratio of the number of particles of solute and solvent in the solution has an effect on the colligative properties not on the nature of the solute. o Effects of electrolyte and nonelectrolyte on colligative properties of solutions. Since a nonelectrolyte solute does not ionize in solution, the number of solute particles will be less in a solution containing nonelectrolyte solute than in solution

containing an electrolyte solute. Thus, the effect of an electrolyte solute will be significant than a nonelectrolyte solute in a solution. o Vapor Pressure Lowering The Vapor pressure is a direct measure of escaping tendency of molecules. A pure liquid (solvent) in a closed container will establish equilibrium with its vapor. And when the equilibrium is reached, the pressure exerted by the vapor pressure. A substance that has no measurable vapor pressure is nonvolatile, while one that exhibits a vapor pressure is volatile. The escaping tendency of the molecules is high, and the vapor pressure is high if the substance is volatile. However, a low vapor pressure occurs in nonvolatile substance because it has a low escaping tendency. If the solution contains a nonvolatile solute, the molecules of solute occupy the space at the surface thus, hindering the solvent molecules to escape. Interaction between the solute and solvent is greater than the solvent-solvent interaction in a solution, thus, making molecules difficult to escape. Lower vapor pressure of the solution becomes lower than pure solvent. The greater the concentration of solute present, the greater the vapor pressure lowering. The number of solute particles is greater in a solution containing an electrolyte solute, then the vapor pressure of a solution containing an electrolyte solute will be lower than the vapor pressure of a solution containing a nonelectrolyte solute. It is important to take into consideration whether the solute is an electrolyte or non-electrolyte. Ionic compounds like sodium chloride, NaCl, are strong electrolytes that separate into ions when they dissolve in solution resulting in an increased number of dissolved particles. Consider two different solutions of equal concentration: one is made from ionic compound NaCl, while the other is made from the molecular compound glucose (C6H12O6). The equations below show what happens when these solutions dissolve: C6H12O6 (s) ————-> C6H12O6 (aq) 1 dissolved particle NaCl(s) ———> Na+ (aq) + Cl - (aq) 2 dissolved particles Glucose does not dissociate into ions while sodium chloride, NaCl dissociate into 2 ions. The same concentrations of each solution will result in twice as many dissolved particles as in the case of NaCl. In the solution (electrolyte) the vapor pressure of the solvent is lowered twice as much as that of the solvent in the glucose (nonelectrolyte) solution. More solute particles covered the surface of salt solution preventing the solvent to evaporate, thus lowering the vapor pressure of solvent. o Boiling Point Elevation Boiling point of a liquid is the temperature at which its vapor pressure equals to applied pressure on its surface. For liquids in open container, this is atmospheric pressure. The vapor pressure of a solvent at a given temperature is lowered by the presence in it of a nonvolatile solute. Such a solution must be heated to a higher temperature than the pure solvent to cause the vapor pressure of the solvent to equal to atmospheric pressure. The boiling point elevation (ΔTb) is the amount by which the boiling point temperature of a solvent is raised. It is the difference between the boiling point of a solution and the boiling point of a pure solvent. The lowering of the vapor pressure in a solution causes the boiling point of the solution to be higher than pure solvent. In accord with Raoult’s Law, the elevation of the boiling point of a solvent caused by the presence of a nonvolatile, non-ionized solute is proportional to the number of moles of solute dissolved in a given mass of solvent. Mathematically, this is expressed as: ΔTb= Kbm Where: Kb= molal boiling point elevation constant m = molality of solute ΔTb = boiling point elevation o Freezing Point Depression The freezing point of a substance is the temperature at which the solid and liquid phases coexist, and their vapor pressures are the same. If a nonvolatile solute is added to a solvent, the freezing point of the solvent is lowered and the reduction in the freezing point of the solvent is lowered. The reduction in the freezing point depends on the number of moles of solute present. The effect of electrolytes as solutes is greater than non-electrolytes because electrolytes ionize in solution and such contain a greater number of particles. The greater the number of particles, the greater the effect on the reduction of the freezing point. When the vapor pressure is lowered in a solution it causes the boiling point of the solution to be higher than the solvent. Thus, adding solute will decrease the freezing point of solvent. The magnitude of the freezing point depression is directly proportional to the molality of the solution Thus: Tf = Kf m Where: Kf – is the molal freezing - point depression constant. A constant that is equal to the change in the freezing point for a 1 molal solution of a nonvolatile molecular solute Tf – freezing point depression m – Molality of solute

When a non-volatile solute is added, the vapor pressure of the solution lowers as stated by Raoult’s Law (Lower 2020). This is because solvent molecules at the surface of the liquid are replaced by solute molecules. In effect, fewer molecules evaporate causing the vapor pressure to decrease. See Figure 1. For the solution to boil, the vapor pressure must match the atmospheric pressure. Thus, a much higher temperature is needed consequently leading to higher boiling point of the solution (Lumen, n.d.). Figure 1. Illustration of the molecular appearance of adding non-volatile solute to a volatile solvent As shown in the phase diagram (pressure vs. temperature graph) in Figure 2, point A indicates the normal boiling point of the pure solvent at 1 atm vapor pressure (violet lines). Adding a non-volatile solute to the solvent decreases the vapor pressure (sky blue lines). Thus, the heat supplied to the solution must be increased, for the vapor pressure to go back to 1 atm. The resulting boiling point of the solution is indicated by point B. How do you compare the boiling points of the pure solvent and that of the solution? The solution has higher boiling point than the pure solvent at 1 atm. This change or difference in temperature is the boiling point elevation (ΔTb). Figure 2. Effect of lowering vapor pressure to the boiling points of the pure solvent and solution. The elevation of boiling point is directly proportional to the molal concentration of the solute (Lower 2020). Mathematically, it is expressed as: Where: i - Van’t Hoff factor of solute Kb - boiling point elevation constant m - Molality Consider however, that the molal boiling point elevation constant (K mol-1 kg) has a specified value depending on the solvent’s identity. See Table 1. Meanwhile, the Van’t Hoff factor is usually not included in the formula if the solute is non-electrolyte in some references (Belford

and the equation becomes: ∆Tb = Kb ∙ m. Sample Problem Solving: Negros is known as the sugar bowl capital of the country because of its vast sugarcane plantation. Sucrose (C12O11H22), a product of sugarcane manufacturing process, is highly soluble in water like many sugars; almost 2000g will dissolve in 1 kg of water, used as syrup in taho. Calculate the boiling point of sugar solution.

Freezing Point Depression

Freezing point depression is a phenomenon that lowers the freezing point of the solvent as a result of adding a solute. (BYJU’S The Learning App, n.d., Lumen, n.d.). Generally, equilibrium exists between the solid and liquid states of the solvent as it reaches its freezing point. As it freezes, the particles are also becoming ordered (Chemistry LibreTexts, 2020). Consequently, both phases also have equal vapor pressures. Adding a non- volatile solute disrupts the state of equilibrium. This disturbs the state of orderliness of the particles (Chemistry LibreTexts, 2020) causing the vapor pressure of the solution to be lower than that of the pure solvent

(BYJU’S The Learning App, n.d.). Thus, more energy must be released to return to the state of equilibrium for the solution to freeze (Chemistry LibreTexts, 2020).Figure 3 illustrates the phase diagram of how a pure solvent changes when adding a solute. Point A indicates the freezing point of the pure solvent at 1 atm vapor pressure (violet lines). Adding a non-volatile solute to the solvent decreases the vapor pressure (sky blue lines). To regain the state of equilibrium, more energy must be removed for the solution to freeze . The freezing point depression of solutions is directly proportional to the molality of solute. It is expressed in an equation as: Where: Kf – is the molal freezing - point depression constant, a constant that is equal to the change in the freezing-point for a 1 molal solution of a non- volatile molecular solute Tf – freezing point depression m – Molality Sample Problem Solving: Ethylene glycol (C2H6O2) is a molecular compound that is used in many commercial antifreezes. A water solution of ethylene glycol is used in vehicle radiators to lower its freezing point and thus prevent the water in the radiator from freezing. Calculate the freezing point of a solution of 400 g of ethylene glycol in 500 g of water.

First Law in Thermodynamics

Change always involves energy. Both physical and chemical changes involve either the absorption or release of energy. Physical processes like vaporization and melting resulted from the absorption of energy by the liquid or solid phase of a substance. On the other hand, energy is released as gas condenses or as liquid turns to solid. System and Surroundings To describe the role of energy in physical processes or in chemical reactions, isolate what is changing and refer to it as the system. Everything else that is in the immediate vicinity or in direct contact with the system is referred to as the surroundings. For a tube of ice, the system is the ice while the surroundings include the plastic holding the ice, the jug or bucket holding it, and the air around it. In the same way, for the reaction of vinegar and baking soda in a beaker, the system refers to the vinegar and baking soda, while everything else – the beaker or flask, the air, as well as the table, are part of the surroundings. Endothermic and Exothermic Processes When a substance undergoes a phase change such as vaporization, energy flows from the surroundings to the system. In this case, the system absorbs energy from the surroundings, which is referred to as an endothermic process. So when alcohol evaporates from your hand, the liquid alcohol absorbs energy undergoing an endothermic process. Conversely, when a liquid such as water freezes, the liquid system releases energy to the surroundings. This is an exothermic process. The same is true for chemical Ice bag in a container reactions. A reaction that releases energy to the surroundings is an exothermic reaction, while endothermic reactions absorb energy from the surroundings. Monitoring the temperature or observing the system and surroundings enable us to determine in which direction energy is flowing. Chemical Reactions and Energy When a chemical reaction occurs, old bonds are broken and new bonds are formed. Both processes involve energy. Energy is required to break chemical bonds while the formation of new bonds generally releases energy. The study of the energy involved in chemical changes is called thermochemistry. When the focus of the study is on the different transformations and movement of energy, it is called thermodynamics. When you eat a bar of chocolate, your body converts the chocolate in a series of reactions that ultimately gives you

Understanding Colligative Properties: A Chemistry Lesson, Lecture notes of Chemistry

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Download Understanding Colligative Properties: A Chemistry Lesson and more Lecture notes Chemistry in PDF only on Docsity!

Stoichiometry of Reactions in Solutions

Stoichiometric Calculations for

Dilution and Titration

Colligative Properties of Solution

Freezing Point Depression

First Law in Thermodynamics